Proceedings of the 8Th. International Emis Conference

INSTITUTE OF PHYSICS

PROCEEDINGS OF THE 8TH. INTERNATIONAL EMIS CONFERENCE

LOW ENERGY ION ACCELERATORS AND MASS SEPARATORS

SKOVDE, SWEDEN, 12-15 JUNE 1973

EDITORS: G.ANDERSSON AND G. HOLMEN

AUGUST 1973 Preface

The conference held at Skovde, 12-15 June 1973, has been recognized as number 8 in a series of international meetings with electromagnetic isotope separation (EMIS) as the common theme. The scientific contents, however, have changed markedly with time. Whereas mass spectrometry formed an essential part of the first conference (Harwell 1955), the next one (Amsterdam 1957) confronted the electromagnetic process with other methods of isotope separation, and the Vienna symposium in 1960 dealt rather exclusively with radioisotope separation. All aspects of EMIS were covered at Orsay (1962), including its application to solid state research as a novel feature. The same was true of the Aarhus meeting in 1965, where on- line radioisotope separation first appeared on the program.

Ey that time the field was becoming inconveniently extensive, largely because of the rapid growth of ion physics, that is, the application of mass separators and other low energy ion accelerators to atomic and solid state problems. Consequently, in 196*7 this part was given a conference of its own at Chalk River, immediately following the 6th EMIS conference at Asilomar.

This splitting up according to type of application, however, seemed to cause some confusion among machine people. For the next conference (Marburg 1970) a new approach was tried, concentrating on apparatus and methods and stressing in particular the technical problems common to classical separation work, ion physics research (whether using mass discrimination or not) and radioisotope separation. This fitted well with the organization close in time of a conference at Leysin, covering the more and more dominating branch of nuclear physics applications: on-line studies of nuclides far from beta stability.

As it turned out5 the remaining topics sufficed well to attract a fair number of participants and contributions and to ensure stimulating discussions, Thus it vas natural to accept the same general program philosophy for the 8th conference. In retrospect it can "be said that the expectations were, on the whole, fulfilled also this time. Admittedly not very many contributions contained spectacular news, and one may certainly regret the necessity of abandoning reports on the results of applications. But the positive impressions prevail. The exchange 01" information was lively throughout. In fact, most participants were present at all sessions, ofLen adding fuel to discussions on subjects apparently far from their own specialities. The conclusion is that EMIS conferences of this type still serve a meaningful purpose.

The present record of the proceedings has been prepared by direct photo- offset reproduction of the submitted manuscripts. The order of the papers is that of the final program and not necessarily the one actually followed in the sessions. In a few cases contributions have been included which were not presented at the conference. It is a pleasure to thank a majority of the authors for their cooperation in following the typing recommendations issued. Needless to say, the way of reproduction relieves the editors of any responsi- bility for possible typographical or other errors.

A number of people who made major contributions towards the realization of the conference ai 3 listed with gratitude on the next page. If some name should be mentioned in particular, it is that of Gillis Holmen, who took on the hard task of coordinating all secretarial work. The assistance given by various administrative personnel at Chalmers University of Technology is also gratefully acknowledged.

During the conference the International Organizing Committee met and decided a continuation of the series, the next meeting to be held in Israel, tenta- tively at Easter time 1976. It seems appropriate to end this preface by wishing the new chairman of the Committee, Saadia Ami el, good luck with the arrangements.

Goteborg, August 1973 Goran Andersson These Proceedings are dedicated to the memory of J^rgen Koch and Rene Bernas, whose contributions are implicit throughout the work.

PROCEEDINGS OF THE 8TH INTERNATIONAL EMI3 CONFERENCE on Low Energy Ion Accelerators and Mass Separators Skovde, Sweden, 12-15 June 1973

Organized by the Institute of Physics , Chalmers University of Technology and University of Gothenburg

Financially supported by International Atomic Energy Agency Swedish Natural Science Research Council Swedish Atomic Research Council Swedish Board for Technical Development

Editors: G. Andersson and G. Holmen

August 1973 Preface

The conference held at Skovde, 12-15 June 1973, has been recognized as number 8 in a series of international meetings with electromagnetic isotope separation (EMIS) as the common theme. The scientific contents, however, have changed markedly with time. Whereas mass spectrometry formed an essential part of the first conference (Harwell 1955), the next one (Amsterdam 1957) confronted the electromagnetic process with other methods of isotope separation, and the Vienna symposium in 1960 dealt rather exclusively •with radioisotope separation. All aspects of EMIS were covered at Qrsay (1962), including its application to solid state research as a novel feature. The same was true of the Aarhus meeting in 19&5, where on- line radioisotope separation first appeared on the program.

By that time the field was becoming inconveniently extensive, largely because of the rapid growth of ion physics, that is, the application of mass separators and other low energy ion accelerators to atomic and solid state problems. Consequently, in 1967 this part was given a conference of its own at Chalk River, immediately following the 6th EMIS conference at Asilomar.

This splitting up according to type of application, however, seemed to cause some confusion among machine people. For the next conference (Marburg 1970) a new approach was tried, concentrating on apparatus and methods and stressing in particular the technical problems common to classical separation work, ion physics research (whether using mass discrimination or not) and radioisotope separation. This fitted well with the organization close in time of a conference at Ley sin, covering the more and more dominating branch of nuclear physics applications: on-line studies of nuclides far from beta stability.

As it turned out. the remaining topics sufficed well to attract a fair number of participants and contributions and to ensure stimulating discussions. Thus it was natural to accept the same general program philosophy for the 8th conference. In retrospect it can be said that the expectations were, on the whole, fulfilled also this time. Admittedly not very many contributions contained spectacular news, and one may certainly regret the necessity of abandoning reports on the results of applications. But the positive impressions prevail. The exchange of information was lively throughout. In fact, most participants were present at all sessions, often adding fuel to discussions on subjects apparently far from their own specialities. The conclusion is that EMIS conferences of this type still serve a meaningful purpose.

A number of people who made major contributions towards the realization of the conference are \isted with gratitude on the next page. If some name should be mentioned in particular, it is that of Gillis Holmen, who took on the hard task of coordinating all secretarial work. The assistance given by various administrative personnel at Chalmers University of Technology is also gratefully acknowledged.

Goteborg, August 1973 Goran Andersson INTERNATIONAL COMMITTEE S. Amiel, Soreq, Israel G. Andersson, Goteborg, Sweden J. Camplan, Orsay, France- J.H. Freeman, Harwell, UK L.O. Love, Oak Ridge, USA N.I. Tarantin, Dubna, USSR H.E. Wagner, Marburg, BRD

LOCAL ORGANIZING COMMITTEE G. Andersson, Chairman K.-H. Eklund G. Holmen, Secretary General K.A. Johansson L. Loostrom, Adminstrative Secretary R. Skoog

CHAIRMEN OF THE SESSIONS W. Walcher, Marburg, BRD H. Wagner, Marburg, BRD 0. Almen, Goteborg, Sweden S. Amiel, Soreq., Israel R.A. Naumann, Princeton, USA J.H. Freeman, Harwell, UK J. Camplan, Orsay, France W.L. Talbert, Jr, Ames, USA G. Rudstam., Studsvik, Sweden O.B. Nielsen, Copenhagen, Denmark H. Ewald, Giessen, BRD KEY TO CONFERENCE PHOTOGRAPH

1. M. Menat 53. P. Johnson 2. J. de Raedt 54. D. Aitken 3. C. Ristori 55. C.W.A. Maskell 4. S. Peterstrom 56. Mrs G. Fiebig 5. E. Kugler 5,7. W. Zuk 6. G. Dumont 58. H. Lawin 7. B. Grapengiesser 59. 0. Almen 8. K. Augenlicht 60. P. Paris 9. Mrs C. Martel 61. E.H. Spejewski 10. M. Thuresson 62. 0. Hoick 11. L. Loostrom 63. C. Martel 12. G.H. Debus ? 64. CM. Truong 13. R.A. Naumann 65. E. Roeckl 14. G. Andersson 66. J. Denimal 15. R. Wagner 67. W.D. Schmidt-Ott 16. M.E.J. Wigmans 68. L.0. Love 17. H.R. Ihle 69. H. Pavn 18. G. Fiebig 70. J.P. Zirnhelci 19. A.G. Waddock 71. J. Jastrzebski 20. M.J. Nobes 72. W. Walcher 21. K.A. Johansson 73. R. Lotti 22. R. Skoog 74. G. Cembali 23. G. Nyman 75. M. Asghar 24. K-H. Eklund 76. R.W. Fink 25. G. Holmen 77. E.J. Dropesky 26. L. Jacobsson 78. H. Wilhelm 27. A. Hoglund 79. H. Wollnik 28. H-E. J^rgensen 80. J.H, Freeman 29. 0. Skiil.reid 81. S. Amiel 30. V. Toft 82. I. Chavet 31. D.C. Santry 83. O.B. Nielsen 32. F. Meunier 84. A. Lindahl 33. M. Ferrari 85. P.G. Hansen 34. C. Lejeune 86. F.R. Krueger 35. K. Valli 87. J. Camplan 36. J. 'Ayst'6 88. E. Hechtl 37. V. Rantala 89. W.L. Talbert, Jr 38. A. Balanda 90. U. Scheu 39. K. Freitag 91. T. Liljeiors 40. B.0. ten Brink 92. G. Rudstam 41. A. Fontell 93. O.C. Jonsson 42. A. Lehto 94. S. Sundell 43. G. Dearnaley 95. M. Braun 44. A. Appelqvist 96. B. Emmoth 45. H. Wagner 97. R. Kirchner 46. V. Kornahl 98. K. Fransson 47. D. Enider 99. R» Buchta 48. G. Kromer 100. I. Bergstrom 49. A.K. Mazumdar 101.-R. Eir 50. C. Ekstrcm 102. E. Pasztor 51. A. Arnesen 103. H. Pattun 52. H. Ewald 104. L. Forraan •^-"^•^

CONTENTS

page Chapter 1: Invited papers

I. Bergstrom (Stockholm): Introductory remarks (Excerpt) 1 L.O. Love (Oak Ridge): Developments in the ORNL electromagnetic separation program € G. Gautherin and C. Lejeune (Orsay) : Production of intense ion beams with moderate charge states (Presented by C. Lejeune) 17 A.G. Maddock (Cambridge): Chemical studies of ion implantation 57 G. Dearnaley (Harwell): New trends in the use of accelerators in solid state physics 75 B.J. Dropesky and B.R. Erdal (Los Alamos): Expectations and problems of an on-line mass separator project on a high intensity accelerator (LAMPF) (Presented by B.K. Dropesky) 93 P.G. Eansen (CERN): Trends in the study of nuclei away from stability 102

Chapter 2: Stable isotope separation L.O. Love: Process efficiencies in Calutron separations 128 D. Brilning, H.R. Ihle and H. Lipperts: A new method of internal metal oxide reduction in electromagnetic, isotope separation 137 R. Meunier, J. Camplan, M, Ligonniere and G. Morov: Preliminary results concerning the separation of rare gases in a closed loop circuit 1 A3 R. Meunier, J. Camplan, M. Ligonniere and G. Moroy: Simultaneous collection of all mercury isotopes 147 1 }R A, Neubert and R. Wagner: Enrichment of La from natural isotopic abundance 150

Chapter 3:, Ion production

J. Aubert, G. Gautherin and C. Lejeune: The triplasmatron: New positive and negative ion source 155 R. Meunier and J. Camplan: A new ion source for electruraagnetic isotope separator 163 W. 2uk, D. Majczka and A. Wasiak: Formation of multiply charged ions in arc discharge of magnetron source 168 M. Menat, I. Chavet and M. Kanter: The lens effect of the hole or slit in the extraction electrode of high current ion sources 174

Chapter 4: Machine development

F. Brown: The high voltage ma P .separator at the Chalk River Nuclear Laboratories (Presented by D.C. Santry) 1B0 page

J. Camplan, B. Meunier and C. Fatu: P.A.R.I.S. The new isotope separator at Orsay I^Chavet.v.M.^Kanter, I. Leyy and H.Z.. Sar-El: Description and performance! of the MEIPA E.M.I, separator lyl G. Sidenius and 0. Hoick: An universal range ion accelerator and separator P. Abrahamsen, H.E. Jdrgensen and 0. Skilbreid: A miniature electromagnetic isotope separator G. Holmen: Improvements of the isotope separator at GSteborg for ion- solid interaction studies in ultra high vacuum 214

Chapter 5: Recoil ion analysers

H. Ewald et al.: Velocity filter for the separation of projectiles and reaction products behind the target of a heavy ion accelerator 220 U.A. Arifov: Fission fragments separation according to mass, energy and effective charge in electrical and magnetic fields (Not presented at the conference) 226 G. Fiebig: Computer simulation of the motion of heavy ions passing through gas filled electromagnetic separators 232 H. Lawin et al.: The Jiilich gasfilled separator "JOSEF" 239 E. Moll et al.: Optical performance of the recoil mass spectrometer Lohengrin 249 H. Hammers et al.: High voltage performance of an electrostatic sector field for unslowed fission products 255 M. Oron and Y. Paiss: A dynamic spectrometer for the study of plasma bursts 262 F.R. Krueger, B.I. Persson and E. Kankeleit: Velocity separation of heavy ions by high frequency deflection 268

Chapter 6: ion physics techniques

J. Chaumont, F. Lalu, M. Salome and C. Seide: Progress report on the ion implantation facility at Orsay 271 J.S. Colligan, J.H. Freeman, W.A. Grant, M.J. Mobes and G.W. Lewis: A versatile isotope separator system for ion implantation and ion-' surface interaction studies m, A. Arnesen and T. Noreland: Ion ranges in evaporated targets 297 D.C. Santry: A study of dose effects on the retention of ions implanted at keV energies

G.K. Wolf: High energy chemistry with low energy accelerators 307

Chapter 7: General descriptions of ISOL facilities

E.H. Spejewski et al.: The UNISOR project 318 page E. Roeckl and W. Dumanski: Plans for an on-line mass separator at the heavy ion accelerator UNILAC at Darmstadt 324 K. Fransson, M. af Ugglas and A. Engstrom: PINGIS-- ^n JSOL-system at a cyclotron for heavy ions 330 S. Sundell et al.: The reconstructed ISOLDE facility at CERN 315 R. Foucher, P. Paris and J.L. Sarrouy: The Orsay on-line separator 341 A. Piptrowski, P. Klepacki, A. Sulik, J. Ludziejewski and J. Jastrzebski: ZBIR; A project of an ISOL system on the reactor 346 E.Ye, Berlovich, E.I. Ignatenko and Yu.N. Novikov: Nuclear spectroscopy using on-line mass-separator in Leningrad Synchrocyclotron (Not presented at the conference) 349

Chapter 8: ISOL beam handling and collection techniques

G. Andersson: A beam switching principle for ISOL facilities 356 S.J. Balestrini and L. Forman: Developments in areas of on-line fission-yield and direct mass measurements at the Los Alamos Scientific Laboratory 359 CM. Truong, J. Fournet-Fayas, G. Levy and J. Obert: On the coupling of an E.S. quadrupole doublet with post-acceleration or retardation collector systems at the first Orsay separator 365 B. Grapengiesser and G. Rudstatn: Rapid chemical separation techniques combined with an ISOL system 371 P. Hornsh^j, B. Jonson, H.L. Ravn and L. Westgaard: On-line element separation at the collector end 379 E. Kugler: The beam handling system for ISOLDE 2 382 V.L. Mikheev: Simultaneous determination of Z and A for nuclei with A up to~50 by combination of the magnetic analysis and the A E, E method (Presented by A. Balanda) 389 W.L. Talbert, Jr., J.R. McConnell, J.K. Halbig and G.A. Sleege: Novel ion beam handling and collection techniques at the TRISTAN facility 397 C. Ekstrom and I. Lindgren: Plans for on-line atomic-beam experiments at the ISOLDE facility 405 A. Hoglund: A simple source-collecting system for off-line work 410

Chapter 9: ISOL target - ion source techniques

S. Amiel, Y, Nir-El, M. Shmid, A. Venezia and I. Wismontsky: A versatile integrated target-surface ionization source for on-line isotope separations 412 H. Feldstein and S. Amiel: On-line study of gaseous fission products; rapid transfer techniques 420 F. Hansen, A. Lindahl, O.B. Nielsen and G. Sidenius: The application of ceramic oxides at high temperatures as ISOL-targets 426 page H.L. Ravn, S. Sundell and L. Westgaard: New molten-metal targets for ISOLDE 432 W.-D. Schmidt-Ott and R.L. Mlekodaj: He-jet ion source of the UNISOR mass separator H.E.J. Wiginans et al.: Production of pure short-lived Te and Sb sources with and isotope separator semi-on-line system 451 G.K. Wolf, T. Fritsch and J. Dreyer: Experience with chemical compounds as targets for the production and isotope separation of carrier free nuclei 456 Ch. Andersson, B. Grapengiesser and C. Rudstam: New target arrangement for the OSIRIS facility 463 J.P. Zirnheld and L. Schutz: On line separation of fission fragments at the Strasbourg reactor 469

Chapter 10: The Asterix He-jet project

H. Wollnik et al.: The on-line separator Asterix 477 H.G. Wilhelm et al.: A Helium-Jet system designed to feed an ion source 481 D.F. Snider et al.: Connection experiments between an helium jet and an ion source 490 A.K. Mazumdar and H. Wagner: Measurements of the influence of the Penning effect on the ioniaation efficiency in gas mixtures 498

Addendum: Press stop news

R.A. Naumann: The Princeton on-line separator 505

List of participants ,-,,- Chapter 1: Invited papers -1-

Excerpt from Introductory remarks by I. Bergstrom, Research Institute for Physics, Stockholm

IT SO HAPPENED

that

In The Beginning of the very first day, there were only two kinds of Particles. And God found the Universe which he had made rather dull, very dark and somewhat uninteresting! The Particles he had created were at that time nude and undressed but did not seem very interested in each other anyhow. They were thus unable to feel attraction as well as repulsion. God looked upon his Particles and concluded that he had to make a more dynamic Universe. A few minutes after midnight on the very first morning God therefore decided to plug in THE GRAVITATIONAL FORCE. To his great surprise God then found that all his Particles became collected in a very small, spherical ball, but the Particles still did not take any initiative to marry each other. God then became a little bit irritated about his Particles and wanted them strictly to obey his will. God therefore decided to plug in also THE STRONG INTERACTION which he thought would force the particles to marry. Having decided this, he watched with pleasure how millions and millions of marriages took place within a fraction of a fraction of a second. After a while, God got rather scared, because the temperature became unpleasantly high even to him. -2-

He concluded, however, that HEAT later on could be very useful if decently controlled, and properly located. THE STRONG INTERACTION was now unplugged for a short while because of the observation of poor Particle morel. Like Particles were namely found to marry as frequently as unliKe Particles. It was decided that a legalization of such marriages would be postponed until a later occasion. Fifteen minutes after midnight, on that very first morning, God therefore decided to plug in also THE ELECTROMAGNETIC INTERACTION. And God named the Particles which he charged up PROTONS and the other ones he called NEUTRONS. And now a Proton had real difficulties to marry another Proton, and the Neutrons would sometimes repel each other by their magnetism. God now had reasons to relax for a while and looked upon his happy Particle family and found that everything started out rather promisingly in his Universe. Once in a while, God discovered, however, tendences of improper approaches between like Particles. God then became very angry and decided to forbid any like Particles to occupy any spot of the Universe simultaneously. -3-

Thereafter, life in the Universe looked pretty peaceful for a while. But after some time

NEUTRONS began claiming that they were superior to Protons because they were bigger and heavier. Since God wanted a DEMOCRATIC universe he decided to quench the neutronic superiority complex. God therefore found that it was high time to plug in also THE WEAK INTERACTION through which a Neutron suddenly could become a Proton without any notice ahead of time. Having done this, God became fed up with interactions and decided that there now wers enough of them. He was tired and fell asleep for a few seconds. Thereafter God suddenly was wakened by the sound of a BIG BANG and he found to his horror that the Particle intermarriages had run entirely out of his control. All kinds of babies like photons, electrons and neutrinos were born. But it was impossible to trace their proper parents because Neutrons and Protons now liked to live in group marriages. God first became furious\ith ,;his. sinful Particles and considered, for a few milliseconds, to go back to his starting point by plugging in, in addition, THE TIME REVERSAL. However, the Universe now had become brighter and after all it was not stationary any longer. -4-

God therefore decided to forgive his Particles and legalized the Neutronic-Protonic group marriages. And God decided to call these families ISOTOPES. And some families were his favourate ones and to them he promised an eternal life. Other families were not so obedient and God decided to kill them off as soon as they did not behave. It was now twenty minutes after midnight on the very first morning in the Universe and God started having difficulties in remembering all his families. God now decided to register all his Particles properly according to their origin. He therefore made a graph through which he could find how many Neutrons and Protons there rfere in each family. And God ordered his favourate families close to a line which he called THE LINE OF BETASTABILITY. And this line was the line of eternal life - the Isotopic Heaven reserved for his favourate families. But at large distances from this line Life became very short. And God placed all the illbehaved families far from the stability linr so that they could watch the edge

of the Isotopic Hell4 where Particle families disintegrated as soon as they were formed. And all these events happened during a few minutes after midnight on that very first morning. At the end of the week God invented the human being. And God now intended to tell the human being about all his secrets. -5-

Exhaup'.,e 1 as he was, God however decided to postpone his lectures about the Universe. "They will probably find it out by themselves one of these days", he concluded and retired to rest.

* 3 -6-

DEVELOPMENTS IN THE ORNL ELECTROMAGNETIC SEPARATION PROGRAM* L. 0. Love Oak Ridge National Laboratory Oak Ridge, Tennessee 37830

When one contempts the tap-, t of the nuclear age upon mankind, three great divisions emerge. The first is the atomic weapons; whether w; like them or not, we will have to agree that they are important m the affairs of men. The second is of cour,-, the advent of nuclear power, which will emerge more and more in the next few decades. The third is in the Held of isotopes - a Held that has a special appeal to many of us because of its selflessness, its importance in the growth of our knowledge, and its beneficence when one thinks of applications, particularly in the medical field. In this third of the great divis.ons the electromagnatic separation of the isotopes, both radioactive and stable, takes a position of eminence. Its impact is diffuse but pervasive. An example of the growing interest in electromagnetic stable 1SotopeS is the increasing number of experiments that use them. Figure 1 is a tabulation of the percentages of papers reported in the journal Nuclear Physics, using electromagnetically enriched isotopes, and shows an increase over the past few years of from less than 40% in 1966 to more than 60% by 1972. I feel that it is appropriate at this time to review the background of this third division, as it has related to the ORNL electromagnetic separation program, and to bring you up-to-date oa our recent improvements. As we all know, the electromagnetic separation of isotopes had its first large-scale application during the last World War when, in the USA, the first large-scale isotope separation effort began in 1943 when the Manhattan Engineer District contracted with the Tennessee Eastman Corporation to operate the electromagnetic plant located in the newly created town of Oak Ridge, Tennessee. The purpose was, of course, to obtain large quantities of enriched "SU. By 1945, this separations program was functioning at its peak with nearly 25 thousand people supporting the operation of more than 1100 separators, and supplying kilogram quantities of uranium isotopes. However, by mid-1945 another separation process (Gaseous Diffusion) had demonstrated that there was a much cheaper way of obtaining 235U, and

•Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation, Oak Ridge, Tennessee 37830.

- (61.7)

, —' o (57.0) 50 (52.4) (54.4) i (50 7) h- (45.5) 2 tr bj (39.1)

30 1969 t970 1971 1972 YEAR

Fig. 1. Percentage of experimental papers in Nuclear Physics using enriched targets. -7- subsequently the electromagnetic plant was declared obsolete and a complete shutdown of the facilities was initiated. It was during this period that representatives of the Manhattan Engineer District and members of the calutron Process Improvement Group met to discuss the possibility of using a part of this two-year-old, but by then obsolete, electromagnetic plant to supply usable quantities of enriched isotopic materials for basic and applied research. A modest program started, and soon afterward there was an exchange of correspondence between E. P. Wigner of the Clinton Laboratories (now operated by Union Carbide Nuclear Division for the T1.S. Atomic Energy Commission) and A. V. Peterson of the Manhattan Engineer District (now the USAEC). Said Wigner in reference to the isotope work, "I am writing to you to review some of the things which were brought out in our conference with you and Drs. McDaniel and Aebersold .... In our opinion the work now being done ... at the Y-12 Plant is and promises to continue to be scientifically one of the most important projects now underway in this country. We should have, as the very baris of future work in nuclear physics and chemistry, knowledge of the various cross-sections of pure stable isotopes. Eventually separated isotopes of the elements may provide invaluable raw material for the production by pile or other irradiations, of radioisotopes of value in science, medicine and industry. Since we believe that (he stable isotope program at Y-12 is today scientifically more important and soon will be more important on every count than the uranium isotope separation, we wish that greater emphasis could be placed on it...." Replied Peterson, "As you no doubt are aware the District has held discussions, in line with the comments made in your letter, and ... you will be glad to know that Dr. Clarence Larson has been placed in charge of the overall stable isotopes program at Y-12. Both he and Dr. Keim, who supervises the electromagnetic phase, are enthusiastic about the program and will promote it as fully as possible...." Based on this type of support from responsible administrators, the pilot plant, which contained four (two "alpha" and two "beta")1 of the more than 1100 separators2 still in existence, was devoted to processing the stable elements with multiple nuclidic species, while Building 9204-3, containing 72 beta separators, was held for improving the process and for making special uranium separations such as the enrichment of 234U. Thus, when the salvaging operation was finished, only two of the nine buildings housing calutrons were retained intact the pilot plant and one production building. Today the program has available 66 separators located in four major magnetic volumes. The two larger magnets arc modified to contain seven semi-independent systems. With this arrangement of magnetic fields, at least nine different elements can be processed simultaneously. Three types of instruments are used in the program (the calutron, the 255°, and the 180° sector). The types of focusing and the arrangement of the 66 separators in the magnets are shown in Fig. 2. The beta calutron, with the collector positioned 180° from the ion source, is the machine most used for separating isotopes in large quantities. Using the four calutrons in the pilot plant, all elements with naturally occurring stable isotopes had been processed at least once by the mid-1950's (except for osmium, which was done in April 1960). In about 1959 the USAEC decided that the need for enriched stable isotopes had increased to the point that quantities required could not be supplied by the four separators then in operation. To increase the availability of isotopes the 72 beta-type separators in Building 9204-3 were then made available for their

1. The "alpha" and "beta" referred to identified the machines with first and second pass. The alpha calutrons (48-in. radius) enriched the 23SU from its normal abundance of 0.7% to -15%, and the beta (24-in. radius) from this purity to about 90%. 2. Usually called calutron(s) from the contraction of California University and cyclotron; locally referred to as "tanks due to the large-volume vacuum chamber in which are situated the ion source and receiver. -8-

.PROPOSED YOKE SEPARATOR

MAGNET YOKE ' MAGNET EXCITATION CO'L © ©I

SEPARATORS IN USE: (SJSTABLE ISOTOPE SEPARATIONS ••H 180°; RADIUS 6! cm i 1 (80° ; RADIUS (22 cm @RADIOACTIVE ISOTOPE SEPARATIONS ;iii.,.i!ii 255°; RADIUS 51cm i 1 180° , n = 0.8; RADIUS 61 cm

SEPARATORS IN STANDBY CONDITION . 255° 180°, n = 0.9 ;___3 180°. RADIUS 61cm

Fig. 2. ORNL calutron magnet facilities.

original purpose electromagnetic isotope separation. These separators were positioned in two groups, called tracks, each track containing 36 separators. Physically, the track consists of two 100-ft-long solenoids placed parallel to each other, the magnetic circuits at their ends being closed with two 30-ft-long, 160-ton yokes. Each solenoid has 19 excitation coils on 4'/2-ft centers, leaving 24-in. gaps in which the separators are located. One entire magnet complex weighs about 3000 tons. At the time of being assigned to stable isotope work, the tracks were subdivided into segments containing either 6, 8, 10, or 14 separators. The modification was made by removing one separator assembly on each side of the track, installing an auxiliary yoke of iron (78 100 tons, shown in Fig. 3), and connecting a motor generator to each of the semi-independent magnetic segments. In addition to providing magnetic segmentation, this arrangement provides for the control panels of the separators for a given segment to be seen from a central position, thus reducing the number of people required to operate that segment. Such innovations have resulted in significant reductions in operating costs as indicated by the fact that in 1945 over 1000 people were assigned to support the operation of this particular building to process one element for one isotope. Today approximately 100 people process over 50 elements for about 250 isotopes, and, although only part of the separators are being operated at present, all of them could be put into operation with less than 100 additional employees. Since the program's inception, some of the elements have been processed only once; other separations have been repeated as many as 19 times. One-mass-unit isotope impurities on special occasions have been confined to less than one part per million and, in some cases, to the parts-per-billion range. Enrichment factors for various isotopes ftom the calutron range from about 30 up to as high as 80 thousand in a single pass through the machine. One of the problems in the calutron processing of uranium arose from the fact that the vapor of any element finding its way into the arc chamber was ionized. These unwanted ions made up a part of the beam spectrum and limited the uranium output from the ion source; they also cut the operating equipment to pieces. The intensity of these extraneous beams prompted remarks that the machine would be fine for separating elements other than uranium. This thought eventually led to our first stable isotope collection - the isotopes of copper in late 1945. Fig. 3. Modification of beta calutton track.

A subsequent publication3 based on the use of these copper isotopes states: "The availability of enriched copper isotopes in the Manhattan Project has now made possible a positive assignment or the 2.6 h nickel isotope to a mass number of 65." This investigation provided tangible evidence that materials from the calutron were unique for physical investigations and further supported the idea of saving a portion of the electromagnetic plant to provide separated isotopes. For example, medical interests have developed for specific isotopes like 1J8Mo and ' 'J6Hg, and since 1965 we have distributed over 1800 grams of 9"Mo and almost 5.3 grams of l96Hg. These enriched materials, when exposed to neutrons in nuclear reactors, result in radioactive 9""Tc and I97Hg, .which are extremely valuable in the early diagnosis of malignant tumors (especially brain tumors). The collection of mercury provides a good example of how savings can be realized when a proper technique is discovered. In 1948 we retained isotopic mercury by allowing the ions to strike a silver collector pocket, the mercury being held by amalgamation. The trouble was that the silver did not show sufficient preference for the separated mercury ion over the normal mercury vapor in the calutron tank and held it also. The simplest method to reduce the mercury vapor background was to refrigerate the calutron

3. J. A. Swartout, G. E. B« yd, A. E. Cameron, C. P. Keim, and C. E. Larson, "Mass Assignment of 2.6 h 65Ni," Phys. Rev. 70, 232 (1946); E. E. Conn, A. R. Brosi, J. A. Swartout, A. E. Cameron, R. L. Carter, and D. G. Hill, "Confirmation of Assignment of 2.6 h Ni to a Mass Number of 65," Phys. Rev. 70,768 (1946). -10-

liner walls. A cooling system was designed to use Dry Ice. The magnitude of the refrigeration effort is better appreciated when it is understood that 288,000 pounds of Dry Ice were ussd to get the milligram amounts of 98.3% 2O2Hg then needed. Today we would make a mercury collection by allowing the ions to strike a th;n film of oil; in the process, the ions carbonize the oil, the carbon absorbs the isotope, and the thin layer of oil protects it from the normal mercury vapor. For a collection of similar magnitude to the one above, the oil cost would probably be less than one dollar and the isotopic purity of the lesser-abundant 'Hg would be, for example, increased from about 5% to ~50%. A means for handling alpha-active materials in a modified calutron was devised in 1954, and a pilot facility was constructed for separating transuranium isotopes. Since then, facilities have been expanded, and isotopes in purities and quantities not initially considered possible have been made available. This work is performed in'the portion of the track shown in the background at the upper right of Fig. 4. The enclosure contains eight calutrons that are used regularly for processing alpha-active elements such as thorium, uranium, and plutonium, and less frequently for americium and curium separations. The entire area, including separators and labs, occupies 8000 square feet and is maintained at minus 0.1 in. water pressure. At this writing, approximately 200 kilograms of stable and radioactive isotopes of more than 250 nuclides ranging from mass 6 lo 248 have been made available from the separators at ORNL. Quantities of individual elements and processing time are shown in Fig. 5. Improvements in component design and in high-purity techniques have resulted in the continued reduction of isotopic impurities. Additional services such as target fabrication, ion deposition, and ion

Fig. 4. Beta tracks in the production building. -11- implantation have become a normal consequence of the separation effort, and, to date, over IS thousand targets have been prepared in the separators. Watching the ORNL separation program grow over the past quarter century has been an exciting and rewarding experience for the people who have been involved in it. During the past three years there have been special collections made in addition to the scheduled series of separations; some are summarized in Table 1. The quantities collected range from milligrams to hundreds of grams; some we have done before, others are new. The enrichment of radioactive 59Ni listed in Table 1

H He

Li Be B C N 0 F 2985 0.003 65 150 He 35.9 1.3 9.5 10.3 ft No Mg Al P Ar 28B9 5757 3447 01 45, 47.7 146 127 26., Sc V Mn N 2200 2561 186 ' 4088 ,1,106 45.5 3S5 41.5 5.8 55.3 Part U Zn 8 As Br Kr 3517 1620 713 1663 * 784 572 43.6 18.2 14.6 | 22.1 31.0 22.8 Rb Sr Y |Zr Nb M Tc Ru Rh Pd 1 658 8985 2314 ° 9603 64 97 15.2 78.8 1 59.7 150 7., 12.21 Ag In Sb I Xe 801 Cd 531 4932 384 2271 12.4 m 7.1 109 5.4 27.9 Cs To Re P 1963 602 1427 9576 370 05 48 lf 6.7 40.9 8.6 24.5 28.7 92.9 6.7 2.8 3.7 Au Hg Bi Po At Rn " 648 2539 52B0 51 122 17.0 105 0.6 ,Fr Ra Ac

P Pm Sm Eu Gd II Tb Dy Ho Tm 2666 627 1911 2.1 1178 |Yb2487 376 61.1 58.9 39.5 14.7 8041| OB 32.2 II 4oi 1 63.2 9.7 Th Pt u Np 1 Pu Am Cm 35 6653 1506 , ^^-TOTAL ESTIMATED WEIGHT (grams) 1.5 24.3 51.7 0.1 0.I "^THOUSANDS OF TANK-HOURS

Fig. 5. Processing time and grams of isotopes collected in ORNL caluIrons (1946-1972).

Table 1.

SPECIAL COLLECTIONS

Charge Weight Assay PE Isotope Amt. Assay (mg) (%) (%) (g) (%)

59Ni 4.2 4.3 35.8 95.35 20 210 Bi 4.3 0.12 0.53 0.78 6 4 0.70 1.32 14 244pu 37 25 1200 >99 15 2 3 9pu 23 99.9 4800 99.9985 20 241 Pu 600 14.5 4200 94 16 I4*Pu 1100 79 140 (g) 99.5 17 84Sr 10 54 ~500 >99.8 12 10Be 100 0.075 »"Dy 0.056 0.63 99.5 3 8 " Sn 23 93 1900 99.975 Q -12-

S=r£25 nLe in a ca>utron outside the a.pha-contained area and provided 30 milligrams of metal assay ng 95 35% »NL We were pleased that the highest process efficiency recorded for a nickel separate (20%) was VZ7Z> * S4% "Sr which the geo,ogists .anted enriched to >*,%. This enriched charge (0 56% to 54%) was worth over S25.000, so it was important to get the highest process efficiency possible^ Normally, strontium metal is the preferred charge, but reducing a smal. amount of SrO to Sr metal w.th ,00% conversion was not a routine lab task. The alternative was to make the metal in the separator. Th.s was done by pressing SrO and lanthanum metal powder into pellets and heating them in the source oven to produce the Sr metal at a controlled rate. The separation was better than expected, the major cost being the loss of charge material. Even with an efficiency of 12% and a collection of 500 milligrams, the unit cost amounted to about $60 per milligram. A ' °Be collection provided a surprise as we were preparing for the separation. It was important to get the spacing between mass 9 and 10 accurate, so when we were making a trial run to evaluate the use of BeO

+ CCl,, we also introduced BC13 to verify the location of the mass-10 collector. Watching the output climb

from 9 mA to 32 mA was unbelievable. Repeated trials verified that the introduction of the BC13 did indeed increase the Be+ ion output by more than 3 times. During the real collection we repeated the use of BC1., and obtained an increase from 30 mA to 40 mA. We have speculated on the reason for these effects

and have concluded that the increase in output between the trial and collection runs using CCI4 in ihe separation series was due to the larger amount of charge used in the separation series. The reason for the

increase when BC13 was used in either instance is still a matter of speculation.

Table 2.

TYPICAL IMPLANTS AND TARGETS PREPARED IN THE 180° OAK RIDGE SECTOR ISOTOPE SEPARATOR

Semiconducting Implantation Dosage Substrate Temperature Dopant Material Energy (keV) (ions/cm2) during Implantation (°C)

PbSnTe Sb 40 IX 10'5 110 GaP Al 40 IX 1017 400 GaAs In 40 IX 10'7 400

Collected Substrate Collection Depositional Isotope Material Energy (keV) Density (fig/cm2)

Ni,C 0.2 25 17tHf Ni 0.850 500 l78 Hf Ni 0.850 250 + 250 1' 5Sn C 0.2 25 ""Si! SS 0.050 -1000 -13-

The ls6Dy collection was made in the 180° sector separator, and this sample is associated with the highest enrichment factor ever obtained in one of our separators, namely, 350,000. In this we used electrostatic deflection near the receiver to move the ion beams out of the path of energetic neutrals. One of our objectives in the target program is to develop techniques that will permit us to form thick deposits of material. Table 2 shows typical specifications of some of the targets prepared recently. By decelerating ions by use of positive target voltage in the geometry shown in Fig. 6, we are able to make 2 2 targets containing V2 mg/cm of material quite regularly and have made one with ~1 mg/cm . In considering the automation of separators, a general agreement was reached that the goal expected will be an increase in the number of separators operated rather than a saving in the present total operating cost. Since electromagnetic separation is a batch process, we plan for an operator to participate in all start-ups; additionally, personnel are required for services which cannot be automated. Automation couid at most reduce the work performed by only 15% of the personnel; thus the chances of saving total dollars are remote. Other costs such as those for mechanical services for source and receiver preparation, equipment handling, and cleaning; costs for chemical activities such as charge preparation, isotope recovery, and purification; and costs for the analytical services necessary for establishing purity levels - these all reduce separator control to only a small fraction of the total. With these criteria in mind we plan to add components to the existing sector to achieve computer-assisted operation, and when operation of the sector is routine, we plan to establish what can be done with one calutron. Once this is done, judgments will be made about what is practical for computer-assisted operation of a calutron track. Plans for partially computerizing the 180° sector predated 1969, but it was in that year that funds were approved for equipment that would permit such an evaluation. By late 1970, data collection began, and in mid-1971 a degree of computer-assisted operation was being realized. By the end of 1972, computer control of all operational parameters was effected, and Fig. 7 shows the direction of effort and status of the electromagnetic separator computer program. Major problems are due to transients and radio-frequency noise being fed through data channels back to the scanner module. When this happens, the scanner becomes unstable and control is interrupted. Additional shielding of the control wires is being added in an attempt to correct this problem which must be solved before calutron automation is attempted. The operational problems associated with the calutron are more complex than those of the 180° sector instrument due, in part, to the fact that all separator components are located within the magnetic field, the beam intensity is increased, sparking is more prevalent, and the

MONITOR PROBE ^SUPPRESSOR OR / DEFLECTOR ELECTRODE

BEAM POSITIONER PROBES-

^-COLLECTION BOX n FOR DIFFUSED OR ION BEAM LINE (1.5 mm) IMAGE

L®~ 40 TO +40kV 0 TO - 40 kV

Fig. 6. 18(P ORSIS decel receiver. -14- beam dispersion in the calutron is only one-seventh that of the sector. Moreover, the control program for a group of calutrons requires the flexibility to process over 50 elemenls in differing types of equipment. For these reasons we believe that it is necessary to have the basic difficulties resolved with the sector before making an attempt to automate the calutrons. One of the more important separation techniques developed recently is the internal fluorination of the platinum group of elements. When we learned of the phenomena! success of the Russian group with the fliiorination of Pt, we changed our thoughts about fluorination and designed the source seen in Fig. 8.

COMPUTER CONTROL MANUAL CONTROL

ARC PARAMETERS PUMP DOWN ION EXTRACTION START-UP

SCANNER PROCESS INTEGRATING DATA VOLTMETER INTERFACE

COMPUTER

INPUT OUTPUT UNITS

Fig. 7. Calutron and 180° ORSIS computer control.

HIGH-VOLTAGE TERMINAL

Pt METAL AND/OR SPONGE

Fig. 8. Fluorinsition system for calutron ion source. -15

Figure 9 is a beam-spectrum scan made about 1947, and shows why we were discouraged with the use of fluorine. At that time we were using ion sources made primarily of copper, and in other tests comparing the halides we could see no appreciable improvement over chlorine as an internal halogenating agent. In fact, we could see no advantage in an internal system until we tried it with the rare-earth elements. However, after it was demonstrated that one could get an improvement in process efficiency by using it, we revised our thinking and now use the technique whenever we can. We have been successful in separating the

isotopes of Pt, Ir, and Os, using C1F3 as the fluorinating agent, and think Pd and Ru will be separated equally well. Table 3 compares the increase in enrichment from natural abundance of the platinum and iridium

isotopes by the electron-bombardment heated source with those from the ClF3 method. The increase in performance' justifies the precautions required in using this compound. Like fluorine, it is extremely hazardous and care is taken to avoid its release. With adequate precautions the compound can be handled safely, and the quality of the isotopes provided by the method is no longer of substandard purity. Without doubt the internal fluo.ination technique must be designated as one of the major accomplishments of recent years.

I«9M

r r * • r n a i

,__.. ^..^y

f?'':

Fig. 9. Beam scans comparing number of side bands obtained from solid charge and Jutogeniting agent.

H-- -16-

Table 3.

COMPARISON OF ASSAYS USING ELECTRON

BOMBARDMENT SOURCE AND FLUORINATION WITH C1F3

NA "Bombardment" "C1F3" - -(.%) (%) (»)

"°pt 0.012 0.8 9 l"pt 0.78 14 69 194pt 32.9 65 98 "5pt 33.8 60 96 "6pt 25.3 66 98 "8pt 7.2 61 96 RUN DATA: Time 4800 hr 3700 hr Collection 65 g 185 g 19Ilr 37.3 94.6 98 "3!r 62.7 98.7 99 RUN DATA: Time 1800 hr 300 hr Collection 3g 50 g -17-

PRODUCTION OF INTENSE ION BEAMS WITH MODERATE CHARGE STATES

G. GAUTHERIN and C. LEJEUNE Institut d'Electronique Fondamentale, Laboratoire associe au CNRS Universite Paris-XI, Batiment 220 - 911+05 ORSAY (France)

INTRODUCTION

The gas discharge sources have been shown to be efficient to deliver important currents of multiply-charged ions of charge state z 4 10 when the discharge conditions are optimized by a proper choice of the operating and geometric parameters. However, the number of these parameters is high in such magnetized discharges ; thus the logical improvement of the performances of a given source needs a detailed analysis of the physical processes governing the discharge behaviour and the ion extraction. These processes will be examined in this paper. Technological problems will be mentionned briefly as they are similar to those already solved for intense monocharged ion sources.

- The elementary processes leading to the production and the destruction of multiply-charged ions are reviewed and discussed. New concepts for the production of very highly charged heavy ions are presented briefly.

- The fundamental mechanisms leading to the equilibrium of a low pressure discharge are discussed. The existence of a minimum pressure in order to obtain stable operation with a given arc current is then demonstrated. An elementary approach of the step-by-step ionization in a magnetized discharge gives an approximate charge distribution in the extracted ion beam ; the necessary discharge conditions for optimizing a given moderate charge state are then derived.

- The previous conclusions are illustrated by a review of the experimental performances of the most currently discharge sources now in use : calutron, magnetron, duoplasmatron and PIG-sources. 1• ION PRODUCTION AND DESTRUCTION PROCESSES

1.1. Ionization by electron collisions In all the compact sources -as opposed to acceleration and stripping - the multiply-charged ions are created by electron collision. Multiple ionization may occur either by a single electron impact or as a result of repeated collisions producing successively higher charge states as one electron is removed at a time (stepwise ionization). The requirements of these two ba^ic -1H- mechanisms to take place are quite different. 1.1.K IonizatioR_b^_sinRie_eIectron_impact

i|_Oeneral_consideration

Single electron impact followed by multiple electron ejection is the process : A° + e —* AZ+ + (z + 1)e where A0 is a neutral atom, AZ+ a a-timescharged ion and e an electron. The orbital electrons of an atom are grouped within consecutive shells (each r,einK devided in subshells), respectively cdled K, , M and so on are they art: more external.

In the ground state of an atom, the electrons fill the different shells from the inner one. An electron belonging to an external shell is only weakly bound to the positive nucleus owing to the screening effect of the internal shell electrons. Thus a low energy is sufficient to remove such an electron : the corresponding ionization potential (IP) is of the order of a few eV. IP are higher for electrons belonging to internal shells, and increases progressively from external shells to K-shell ; ionization in K-shell require:-, an energy of 900 eV for Ne ; 3.2 keV for Ar ; I'l.0. keV for Kr and is • renter than 100 keV for U. 1 2 ;;pectroscopic values of II may be found ' ; approximate expression for the binding energies up to very highly charged ions is givc-n in , where 1, 5 a:: experimental values are given in .

Thr1" Agh a single electron impact, two basic processes rr.ay lead to multi- pi;,-ionized species. First the direct ejection of two or more electrons from the external r.hell (optical electrons) - process 1 ; second the ionizaticn or excitation of an inner-shell electron followed by one or more Au^er processes, occasionnably combined with shake-off transitions - process 2. Expe- rimental evidence of these two processes results from the comparison of the energy dependence of tne ionization cross-sections for high electron energy to the theoretical variations. According to the Bethe-Born approximation (electron energy E much higher than IP) the cross-section for an optically allowed transition, involving any one of the orbital electron is fiver, by :

a = A E ' Ln c E and for an optically

o = b E*"1 -19- where A, B and c are constants related to spectroscopic parameters. The best way to compare data with theory is to plot them in graphs of az x E versus Ln E. Such a graph is shown for Ne in Fig. 1 . At high energy we see for a, a 1 straight line with a positive slope, in agreement with theory for an allowed transition. The curves for a and o are straight horizontal lines : hence Ne and Ne are produced predominantly by the direct ejection of 2 or 3 electrons - process corresponding to a disallowed transition. 5 10 The curves for a, and Fig.1 - Plot for neon of a.E/Uir 4- start, more or less horizontal- versus In E for the partial cross sections ly but increase strongly above for single and multiple ionization (ref.6) a threshold energy and they are again straight lines with positive slopes : Ne and Ne ions are formed partly by direct ionization and partly by inner shell ionization.

The previous conclusions may be generalized to all other atoms : - for the weakly-charged ions (say z .$ h) the direct ionization is predominant ; - the relative importance of process 2 increases with the charge multiplicity We shall discuss this latter process.

ii2_Rearrangement_of_inner_shell_ionized_atoms

The formation of an inner shell vacancy is a common event in nature, taking place, for example, in X-irradiation by way of photoelectron emission, or when such nuclear events as electron capture or internal conversion occur. A systematic study of the atomic readjustement to an inner shell vacancy have been developed both theoretically and experimentally. A good agreement is obtained 7. We shall consider two phenomena playing a predominant role in this readjustement leading to multiple ionization : Auger process and electron shake- off.

Augerjsrocess

An atom with an inner shell vacancy is strongly excited ; there is two ways for its de-excitation (fig.2) : -20-

Auger •e electron

(photon)

2+ Auger transition K - L^^j^Ne" X-ray line K - Lm (K c^) =£> Ne

Fig.2 - Readjustment following K-shell vacancy In Ne. - either a radiative decay with emission of the fluorescence X-ray, the photon taking away the energy excess ; - either a non radiative process : according to the excitation energy, the atom may be then self-ionized 2, 3, ** or z times. The energy excess being given to the ejected electrons, the energy distribution of which is independent of the impiging particle energy. This Auger process may be described as follows for an initial K-shell vacancy : a Lj. electron fills the K-shell hole ; if the energy giv.en up by the transition L_ —>• K is greater than the binding energy of an electron of the LJJJ subshell, this latter is ejected with a kinetic energy Ec = (EK - ELl) - Ei? ; where E^, EL are the binding e- energies. At this time the ion is doubly-charged and possesses two Oi.n.m vacancies in its L-shell. In the case of an atom of higher atomic number, both L vacancies can themselves be filled by Auger transitions from M-shells so that four vacancies are produced and so on the vacancies (and charge multiplicity) increasing as they proceed outward through the electron structure. This phenomenon , known as an Auger or vacancy cascade is illustrated schematically on fig.3 , where a K-electron is removed from Xe ; then the most likely step K will be a K —^ Lm radiative tran- 7- Typical vacancy cascade in Xe sition, which in turn be followed following K-shell ionization with ulti- by an Auger cascade with ultimate mate loss of all electrons of the O-shell -21- losses of all electrons of the O-shell (Xe ).

Of course, as shown in the previous example, a competition occurs, at each stage or the vacancy cascade, vith the radiative transition. But with exception of the K and L-shells of the heavy atoms, Auger processes are much more probable than radiative transitions and the vacancy cascade gives rise to highly charged ions.

Normally they are many different routes possible within a vacancy cascade and the end result will be a distribution-in-charge . A number of works have been devoted to predict the charge spectra following a vacancy cascade 8 9 . This has been done for noble gases with a fair degree of success ; but such calculations require knowledge of transition rates and energy of atom with multi-vacancies in inner shell that are not always available. However, in each case with the energy conservation, an upper limit of the charge multiplicity may be proposed ; furthermore Carlson et al suggest a recipe for estimating the average charge resulting from an inner shell vacancy that provide a reasonable guess for all atoms.

However, the Auger process is inadequate to explain all experimental data resulting from an initial inner-shell vacancy : both in the ejected electron and ion charge spectra. Most of the additional ionization (or extra electrons) can be explained "by the so-called shake-off.

Experimental evidence of the phenomenon non appeared from the study of nuclear & - decay such as : Ar S- K . One only expects the presence of K ; in fact K , K. ... exist although with decreasing abundances. But the doubly charged ions are not negligible ; for noble gases their relative abundance is : Ne2+ : 17.5$ ; Ar2+ : 12.55? ; Kr2+ : 10.9? ; Xe2+ : 855.

In the same time, the shake-off was predicted theoretically , caused by a "sudden" change of the nuclear charge. It is a perturbation theory, treated in the sudden approximation : the electron has to be ejected with velocity higher than the orbital velocities of the bounded electrons. Then, adiabatic readjustement of the electron structure occurs in 10 s or less which can result in transitions to excited states or to continuum. The transition probabilities may toe computed . An interesting particularity of the shake-off theory : it does not mention the specific cause of the change in effective charge or screening. Therefore it should be applicable to the ioni zation of an electron from any shell by either photon, electron or heavy -22- particle.provided the process occurs in periodg of time that are small compared with the orbital period of the electrons that undergo shake-off excitation. This condition is fulfilled if the energy of the incident particle is somewhat above the ionization threshold of the electron to be ejected : thus such "sudden" rearrangement will be a common event. The initial Auger charge spectrum is always modified towards higher charge states by electron shake- off.

The probability for creation of extra-electrons by shake-off in all shells of an atom varies as Z~ , Z being its atomic number ; and the probability increases from internal to external shells.

An example of the distorsion of the charge spectrum by shake-off following photoemission primarily from L-shell of Ar may be taken from :

Ar ions 1+ 2+ 3+ h+ 5+ 6+

Auger process (%) 9. 1 39. 5 51. k 0 0 0

Auger + shake-off process (%) 9. 0 3U.h k9. 9 6.2 O.k 0.05

Auger process and shake- I 1 B i off are predominant in deter EXF 20 -Q CAI c- mining the charge spectrum re-

sulting from inner-shell va- 10 .. cancy. However, a gratifying

s il l - agreement between experimental -, and calculated charge spectrum 2 as shown on fig.U is only — - - -

tij obtained if more complex me- ( 1 ! I , ,. | i I ! 1 . ,1 , | chanisms are taken into ac- Mil l 1 | - - —— count such as : direct colli- OS

- Il l I sion, double Auger process - - - -

and radiative Auger process . 0.2 -

In the case of electron Of 1 ! M 11 23456789 10 II 12 impact on neutrals, the final CHARGE OF ION charge spectrum is made up ol Charge distribution resulting from contributions from each of the photoionization in the K-shell of Krypton, individual shell vacancies pro- Comparison of experiment with calculation perly weighted. But unfortuna- which is based on vacancy cascade model. -23- teljr the partial cross-sections for ejection of one internal electron are not well known. This explains the large discrepancy between theoretical and experimental results for multiple ionization through single electron impact.

Experimental results obtained till 1966 may be found in a survey paper by Kieffer and Dunn . Systematic errors in absolute measurements are discussed.

A great deal of data will be found in recent papers published by specia- 16 1 19 20 21 lized laboratories located in Amsterdam > » 7)18> orsay ' ' and Tokyo 22,23_

As may be seen from fig. 1 , the PICS decreases strongly as the charge multiplicity increases. The resultant charge spectra which may be expected from single electron impact in Xe - with different values of the electron energy are plotted on fig,5 from Such a process is not efficient for optimizing a charge state higher than unity.

The final charge state may be achieved by several successive electron impact as :

Z+ 2e

Highly stripped ions may then be obtained with low energy electron beam because the IP to remove only one electron from a z- times charged ion increases relatively slowly as the charge multiplicity increases , as compared to IP by single electron impact on neutrals.

Ionization energies in eV for different charge states z of Xenon : a) by 9 single electron impact ; b) for the Fig. 5 - Charge spectrum through removal of only one electron, from spectro- single electron impact scopic values : in Xenon (from ref.16). -24-

6 7 8 9 z 1 2 3 h 5 362 535 820 a 12.1 33 65 110 172 (eV) 102 126 218 b 12.1 21.2 32.1 »*5 57 89

However, the ions have to remain in the ionizing region for a sufficient ti me to allow the stepwise ionization to take place. 21). 25 Experiments in sources using an ion trap performed by Redhead et al have shown the possibility to produce Xe10+ and Cs10+ with an electron beam energy of 250 eV. Provided the parameters of the ionizing electrons and the cross-sections are known, an upper limit of the time required to obtain z- 26,^7 times charged ions may be calculated If we consider the ionization by a monoenergetic electron bean, the"life time" T. of a i-times charged ion - to be ionized to (i + i) is given by :

T. = e/je ol (1)

where je is the current density and a- the value of the cross-section. To obtain a z-times charged state, the ion containment time must be higher than : (z-1) z-1 . Vz= Z *-=£- E — (2) O ® O Oj This value is surrestimated, since single impact collisions have a no negligible probability for lower charge states and furthermore no account is taken of processes in which 2 or 3 electrons are removed from an ion.

However, the main cause of error in the calculation of the necessary containment time is the poor knowledge of the ionization cross-sections and of their energy dependence .

So far only a few measurements have been done to determine the ionization cross-sections for moderate charge states ; except for some values of transition from 1 to 2 there is not much known . For this reason a great deal of theoretical work have been devoted to the computation of

An approximate formula is given by Bethe30 :

i j Lnf

where EK is the IP of the ^electron and E the electron energy (in eV) ; the summation is extended over all electrons K< i. But this formula gives values which are about ten times lower than experimental values, studied either in ,2k trapped ion sources (Redhead^ , Donetz^or by crossed-beam methods32. -25-

Thus only a rough estimation of the ion containment time is possible in the J(Xe2"*)/J(Xe*) statement of the knowledge. The discrepan- 10 cy between experimental and theoretical values of ICS is attributed to the important role played by excited or metastable ion states '

This influence is illustrated on 32 fig.6 . 2 Experimental cross-sections a for E,-eV exp single ionization of some ions a?e listed in table I for Ei = 150 eV and E2 corres- Fig.6 - Ratio of secondary to pa- ponding to the maximum cross-section ; rent ion current as function of they may be compared to theoretical values energy E^ of the primary electrons a^ (ref.32). E1 is the energy of the creating the monocharged ions in primary electrons creating the mono- a crossed-beam method: the energy charged ions in a crossed-beam method ; Eg of the testing electron is cons- Ep the testing electron energy. tant (from ref.32) Table I

+ + 2+ 2+ Ions Xe Kr+ Ar+ Ne Xe Ar Kr 2 10-16 cm 5.6 1.k 0. It It 1.6 2.0 exp 7 5. 10-16 cm 2.0 1.1 0. 0. 8 0.6 °th 1-5 5 0.5

From these latter works, the following practical conclusions may be said : - the experimental cross-section of the heavy ions is larger than theoretical value owing to existence of met as table levels with life-times in excess of

_o '• " • • ••.••••.-.-• 10 s ; furthermore ionization appears below the 3pectroscopic value of IP ; - the effective cross-section for single ionization depends little on the charge multiplicity in the case of ions with many electrons such as Hg, Kr and Xe ;

- in the case of light ions (He , Ar ) the experimental value is close to the theoretical one, in relation with the shorter life-time of the metastable states •; • - -26-

- the curves for multiple ionization of an ion by single electron impact are similar to the ionization curves of neutral atoms ; the ejection of only one electron is the most probable event.

The charge spectrum following stepwise ionization in gas discharge sources will be derived in section 2. 1.2. Ion destruction processes

In order to be able to write the creation-destruction balance for multiply -charged ions, it is necessary to know the cross sections for the creation an'd destruction reactions. We therefore consider below the most probable reactions for the destruction of multiply-charged ions, both in the interior of the discharge and in the transport of the beam. To show the total balance, it is convenient to take into account the "leak" terms to the walls ; we shall do so further along in this article. We shall successively look at the following reactions:

z (z—1) A + e -»• A + hv radiative recombination

XY +e+X Y + (z - i - j)e dissociative recombination

A + e + e -+ A +e 3-body recombination A + X -»• A +XJ+(j-i) e charge exchange

Quantitatively, the electron-ion recombination process is usually described by a rate coefficient, 0 , uefined so that

dn + Z+ -T~ = - o (z , ne, Te) n .ne

This process is the inverse of photoionization. It proceeds at a slow rate and is normally of no significance except in very tenuous plasmas. In a range of electron energies defined by

Te where E(Z,g ) is the ionization potential of an ion of charge z in the ground state g, the coefficient a has the value33 • -27-

2.05x10~12 -1. g) (Te, z, g) cm s

where Te is in degrees Kelvin and E in eV. For thermal (= 300°K) electrons the radiative recombination coefficient is calculated to have values in the range 10 to 10 cm s for various positive ions (monocharged) and va- 1/2< ries as Te"

We neglect three body recombination of the type

(z-1) + e + A + X

even though no value for the coefficient a for multiply-charged ions is found in the literature. However, for the case of helium, Massey and Burhop estimate this coefficient to be 10 P cm s~ (P in torr). For the pressures normally found in a discharge we neglect this term and only consider the three body recombination. We limit the discussion to the domain kT « E^. Gurevich and Pitaevskii *' have calculated a using the Fokker-Planck equation. They obtain

8 3 2 9/2 .8x1O~ z log (Viz + 1)ng Te~

where n is in cm and Te in °K.

For the values of the parameters ne and Te normally found in discharge-

type ion sources a is in the range 10 to 10 cm s , as shown in the following table.

Three body recombination coefficient in cm3 s-1

Charge state ; z = 1 • ... ,," z = 10

10 1U """•\^ii cm"3 ho 1O1U : •• io10 1O

1 • : • Te (eVV^-^^

13 1 6x1O-13 8xio" 8x10"9

-2dd2 18 100 6X10"22 8x10 8X1O"

•••"•••••

Dissociative recombination take place when a radiationless transition occurs to some state of thCi molecule XY in which the atoms recede one from the other and,gain kinetic energy under the action of their mutual repulsion so that the neutralization is made permanent by the requirement im- -28- posed by the Franck-London principle.

There has been, to the best of our knowledge, no work done on this subject for multiply-charged ions. The only known results are for molecular ions such as N+, 0* and Xe*3T . The dissociative recombination rate is very

7 3 1 large (a » 10~ cm s" for Te * 300°K). This rate varies with the temperature, and the dependence is a function of the nature of the ion.

1.2.2. Charge_exchange_

in discussing charge transfer, it is convenient to distinguish between symmetric resonance reactions such as Z+ + X -* A + X (Z + A - A

and asymmetric reactions (unlike systems) such as IZ+ + X - I + XZ+

Z+ (z i} i+ A + X * A ' + X The variation of the cross-section as a function of energy is quite different for the two groups of reactions because of the energy dsfect between the two sides of the reaction (Massey criterion for an adiabatic collision ) It is known that when the electron transition time is approximately equal to the collision time, the transition probability increases rapidly. This condition can be written as

(collision time) a/v = (transition time) h/|AE|

where a is the so-called "adiabatic parameter", a distance of the order of atomic dimensions. The "adiabatic maximum rule" is thus a convenient method of estimating the energy of a cross-section function maximum, apparently valid over a large range of impact energy.

As concerns resonant charge transfer, the cross-sections obtained vary as a function of impact velocity v of the incident ion, the relationship having the form39 1/2 a = c - d Log v where c and d depend strongly on the ionization potentials.

We discuss below several theoretical or experimental conclusions concerning the rare work found in literature on multiply-charged ions. It is to be noted that there has been very little research on ions other than the rare -29- gases. Resonant_charge_exchange kc 80 I.K. Fetisov and O.E. Fusov Fig: 7 60 have computed the cross-section \

40 • for resonant charge exchange of \ ^. doubly-charged ions in the adia- v cm/sec batic approximation. Results are 6 6 7 8 shown in fig.7, where the coef- 1.10 5.10 no' 5.10 1.10 ficient a is related to the two Computed resonant charge exchange ionization energies. cross-section 0 (ref.UO). a gives a 1 EMi + normalization for all gases (see text), a = 2E. where E,. + E? is the total energy, E is the energy of the H atom and a is o the Bohr radius, (a = 1.7i/a for He).

It is interesting to note that the cross-section thus obtained is related to both processes 20/02 and 20/11 ; more precisely .2002 Fig-8 ° * °20/02 "c "20/11 but each of one may not be deduced. Xe However, they may be found in the 20N vm/sec experimental work of M. Islam et al 105 3.5.1 ;they are shown in fig.8 for the case of Xenon. The behaviour is identical for all the rare gases : Experimental charge exchange cross- a at low preponderance of ?o/op section 0 (ref.M) versus impiging energy. particle velocity. Non resonant_charge_exchange_ As an example, we present in table II data for multiple charge transfer for Xe ions accelerated through potential differences of 520 and 1600 eV for various gases (Ne, Ar, Kr and Xe) ". The reactions considered are of the type

Xe i+ B Xe f+

Absolute cross-section values for single charge transfer a. ^_1 are approximately 1.5x1O~15 cm2, except for Xe3+-Ar and Xe3+ - Kr, with values -30- of about 10 cm2 and 5x10~1^ cm respectively.

In the third section we shall see that the creation rates (ionization) are, for the plasma parameters normally encountered in discharge ion sources, much larger than the destruction rates. As a result recombination phenomena play a minor role in the operation of the source, and the properties are much more sensitive to the conditions at the limit (radial or longitudinal leaks).

Table II

Ratio of a.,f/a. . for multiple charge transfer of xenon ions in neon, argon, krypton and xenon within scattering angle of -0.6° < a < +0.6° (values in %) ; i : initial state ; f : final state.

E, eV i ; i Ke Ar Kr Xe

520 X i 3 1 < 0.1* 280 68 19-5 k 2 1U-5 32.1+ 3** 31.8 U 1 - 1.0 2.3 5-1* 5 i 6 31.fc *»9.3 63.3 5 2 ^ 0.2 6.2 10.0 17.1 5 1 - ^ 0.6 * 0.3 2.3 6 k 28.6 5U.5 26.6 27-5 6 3 1.7 17.8 7-5 15-5 6 2 - 2 2.1 2.2 1600 X i 3 1 < 0.3 133 116 28.9 U 2 U.8 3U.7 37.8 31*.6 k 1 < 0.1 2.3 2.8 6.1* 5 3 10.3 1*5-8 58.2 82.2 5 2 1 10.3 16.2 22.3 5 2 - 1.5 * 0.5 3.0 6 1» l»0.3 62 3U.8 33.0 6 3 1.5 30 17.2 20.1* 6 2 3.3 0.2 3.6

—" 1

The increasing interest of multiply-charged ions has stimulated the ela- boratxon of new concepts for the production of very highly stripped heavy -31-

ionss in compact ion sources. According to the predominant ionization process these sources belong to three categories. Furthermore ve must remenber the already working but most costly method of acceleration and stripping.

sources This process does not require ion confinement but implies the presence of highly energetic electrons. The hot electrons (T£ = 10 keV) are created and confined in a magnetic mirror container through the coupling of a high power microwave source to the cyclotron resonance of electrons in the container's magnetic field (Elmo at Oak Ridge , Pleiade at Grenoble ). It3 . _p 12+ ? Postma has estimated that 10 u A.cm of Xe from one of the 300 cm mirror throats should be obtained from Elmo.

i2_Laser ion_source 21 —3 2

A very dense plasma (n = 10 cm ) with a high temperature (Te = 10 eV) is obtained in a few nanoseconds by irradiation of a solid target with the light beam deliver from a higher power solid state laser. The properties of such a plasma are correctly described by a "corona model'1, the step by step ionization being balance mainly by radiative recombination : the time required to reach a steady state equilibrium (=10 s) is smaller than the interaction time between laser beam and plasma* It is predicted that with a laser intensity of 10 W.cnT , Al , Fe and U could be obtained . But the problems of the pulse length and extraction without losses must be overcome before a source becomes a practical reality. ii2_Ion_containment_sources A high degree of electron stripping is obtained through the long confinement time of the ions trapped in the potential well of a dense electron beam (100 A. cm" ; E = 3 keV) either magnetically focused (Ebis at Dub a ) or contained in a toroidal magnetic field (Chipac at Oak Ridge ). Vacuum pressures of about 10 Torr are required for the containment times (= 6 s) necessary to reach U even with electron current flux of nearly 100 A cm"2. These projects are promising, but the technological difficulties clearly appear from the previous requirements. !i3»3« Discharge ion_sources Both processes may occur, depending on the discharge conditions as will be discussed in the next section. Up to now, discharge sources have been the -32- basis of nearly all the multiply-charged ion sources, the new concepts sources being still under development. More details may be found in review papers ' .

2. LOW PRESSURE DISCHARGE PROPERTIES

In the discharge sources, currently used for multiply-charged ion production, this charges are emitted from a magnetically confined plasma created between the cathode and the anode. The extraction is either axial and parallel to the magnetic field lines or radial. Both the ion dwell time and the step-by-step ionization frequency depend on the mechanisms leading to the discharge equilibrium. They will be remembered in the case of a typical magnetized discharge, that operating in the calutron . In this discharge, the main parameters are (fig.9) :

- operating parameters : arc current p I.; arc potential Vft j magnetic ^ field strength B ; neutral pressure p and cathode heating power II n Pv which may be equal to zero.

- geometric parameters : the length L and the diameter D of the arc column ; this latter depends both /A Arc column \ Ext f on the source geometry and the magnetic field. Fig.9 - Calutron source - K : heated

Relations between these parame- cathode i A : anode ; Ext : extractor D L are ters are imposed in order to obtain > column diameter and length",; a steady state equilibrium of the B : axia-l magnetic field . discharge.

2.1. Principal discharge mechanisms50'51>52

2i^i_1i_Primai2_electron source

The creation of a plasma is a simple method to carry a high electron current from a cathode to an anode : the plasma insures the space charge z neutrality (n_ - Z , n ) in the whole volume, excepted near the walls where thin space-oharge sheaths are set up. As a result of charge neutrality the plasma is almost equipotential, the maximum possible variation being determined by the mean thermal energy of the electrons : this variation over the plasma length must be smaller than k Ve. On the contrary -33-

the rapid variations of potential occur over small distances which are of the order of a Debye length \^ , with an appreciable departure from charge equilibrium : -. ,, m 1/2

e As example : n = 101 cm"3 ; T = 5 eV ; •+• A = 1.5x10*** cm.

The primary electrons are extracted from K by the potential jump of a double layer set up between K and the plasma. The electron emission is then governed :

- either by the ion current density issued from the plasma : it is the "space charge limitation", the bipolar flux being related by the "Lang- muir sheath criterion" : 1 /2 Je = a j+ (^) < jes (3a)

a : 0.3 - 0.6 depending on the cathode material ; m, M are the respective masses of electron and ion ;

- either by the electron emission processes from the cathode ; it may be for instance a "temperature limitation" ; then

1/2 Je - 0es <« J+ (|) (3b)

2i^±21_Arc_conduction_and_charge_production As required by the sheath criterion, only a few primary electrons have to sufferionizing collisions in the discharge volume. Furthermore with the usual operating pressure the mean free path for elastic collisions is much larger than the column length. Hence the beam losses are weak ; the primary electron beam insures most part of the arc conduction through the plasma and most part of the ionization through impact with neutrals. Owing to the much smaller thermalenergy of the secondary electrons, their contribution in general is estimated to be less than one third of total ion production. As the electron beam gains its energy ell in the thin sheath and is then mo- nokinetic, the production rate is almost constant over the whole volum of the confined electron beam :

where n is the neutral density, a., v the ionization cross-section and x a correcting factor for secondary ionization (y. = 3/2). -34-

2.1.J. Charge trans£ort_and_iou_dwell_time

The radial motion of the charges is greatly reduced by the strong magnetic field and the charges have to escape axially. Due to the greater axial mobility of the secondary electrons,a slight excess of positive ions is set up which gives formation of a maximum of the potential , between A and K, in the middle plane of the arc column. This maximum of potential is of the order of kTe/e as compared to the potential at the plasma-sheath boundaries ; it insures two effects : l) it traps the secondary electrons and adapts their flow in order to compensate the weak beam losses and insure arc continuity on the anode (the existence of a "negative" anode fall) ; 2) it accelerates the ions both towards the cathode in order to satisfy the sheath criterion and towards the anode, from where they may be axially extracted. Typical axial variation of potential is shown on fig.10

With the usual values of the dischar- .Sheaths. ge parameters, volume recombination and Plasma charge exchange are negligible ; the ions losses are dominated by charge escape and consecutive recombination on the walls : it is the "free-fall approximation " of the plasma as initially studied by Langmuir. The following properties may then be derived from the "plasma equations"'' :

- the axial profile of electron density presents a maximum ne in the middle plane of the column ; the mean axial density over the column length is ne = 0.8 neo ; - the ion current density is equal at Fig-10 - Axial variation of poten- both ends of the arc column ; it is tial from cathode to anode : U : related to neQ by the relation, valid cathode fall ; vM : negative anode for monocharged ions : fall.

1/2 j+ = O.k e neQ(2k Te/M) (5)

- within one half of the column the ion production and loss equilibrium leads to the relation :

~ Y e G L/2 Q (6) -35- : where y $ 1 is a coefficient taking into account the influence of radial ion losses ;

- the confinement time T of the ions in each point is defined by the equa- lity of production rate G^ and escape rate n+/x . Thus T depends on the ion creation point ; a mean value of this time over a half column length is obtained by the extension of the previous balance equation :

T 2 " o 2 ~ Ye ^' From eqs.5,6 and 7,one obtains for monocharged ions :

T = Y L (M/2k Tj1/2 (8) The ion dwell time depends mainly on the arc length, the ion mass and the radial losses. In the case where the charge neutrality is mainly insured by z-times charged ions (n ~ z n .T) the mean dwell time of these ions is :

t. • t, ."'« <«

Typical value of t- in a PIG-source will be : Argon ; Te = k eV ;y = 1/2 ; L = 100mm + T = 12 ps. 1'his value is in agreement with experimental values

- In a axial extraction source, the beam current density is equal to j at the column boundary, which furthermore owing to tne sheath criterion (eq.3)

j+ a e (10) - In a side extraction source, the current density is much smaller, as the ion density suffers in the diffusion type plasma - all around the ionizing electron beam - an exponential decrease. The current density is then

i/2 3+ x = e n+ v+ = e n+ (k Te/M) (11) n being the ion density in the vicinity of the emissive meniscus. 2.2. Minimum pressure or maximum arc current for stable operation 2.2^1. Variation of nsutral_densit3f_vitliin_arc_colUTnn

Owing to the efficient ionization process in the discharge, the neutral density n inside the arc column may be smaller than the density n outside the column. The relation between the two quantities is obtained from the

total ionization rate R+ in the vhole column from two independent ways : -36-

- First, from eq.U extended to the whole volume :

(12) R+ = i ai(u) x n L/e

where the ion contribution to IA has been neglected (eq.3).

- Second, from the calculation of the probability I of ionization of the neutral flux,4 entering the electron beam ; P is approximated by the expres-

sio. n 52

R u « ,p = 1 - exp - (T /T-> = 1 - exp - (a) 03) + »O Ox

where the "ionization index" a is the ratio of the mean dwell time TQ of

the neutrals in the electron beam, to theirionizing time ti - (Je

If the neutral flux is maxwellian (temperature To, mean velocity SQ), the integration over all paths crossing an electron beam of diameter D, gives

TQ = D/oi0 ; thus

1/2 a = IAaiX /eD (2M/ir k TQ) (U*) The comparison of eqs.12 and 13 gives : n = n (1 - exp - (a))/a (15) o This relation shows the decrease of the neutral density within the arc column as long as the ionization index increases, consecutively to the arc current growth.

- The ion current density 3 to the column boundary must satisfy the sheath criterion in order to obtain stable operation of the arc. Thus from eqs.3, U and 6,one obtains the inequality : 1/2 ff (m/M) i(u) » 2/o Y X nL =am , (16)

which defines a lower limit of the ionization cross-section and hence a

lower limit of the electron energy e Um, mainly dependent on the neutral pressure frr a given geometry. Physically this limit corresponds to : " U ~ Um : the cathode emission is space-charged limited which occurs if the cathode is over-heated : the arc potential has then its lower value ;

. U > Um : the emission is temperature limited, all the potential excess being localized in the cathode sheath.

- But a physical limitation of the ionization cross-section of the neutrals

exists : Oi has an upper limit aM only dependent on the gas when the ener-

gy is % Um. When put in eq.16 this upper limit require a minimum neutral -37-

density n in the column to obtain a stable operation :

1/2 n > n~ = (m /M) 2/a Y x Oyi L (1?)

This limit is attained either with decreasing the density n outside the o column at a given arc current, or as seen from eq.15 with increasing I at a

density: jnQ. From, this^atter point of view,-the maximum possible value of the ionization index will be calculated from eq.15 with n n

n* =no(i-exp- (18) The values of a as a function of normalized neutral density n /ri* is plotted on fig.11. From this graph it clearly appears :

- no < n * : a stable arc cannot exist ; this corresponds to an absolute minimum pressure for any 3 y / arc existence ; y - n » n* : a., is high and the pro- f bability P of ionization of the neutrals ,1 , Ik entering Lhe electron beam is close to • unity, but this condition is only fulfil- o i led if the arc current is increased up Fig. 11 - Maximum value of the ioniz*- M to its maximum value I corresponding tion index a versus relative neutral to ; from eqsilt and 18, one obtains density

1/2 XI = e nQ a DL To/8m) if Q >, 3n* (.19) Beyond this value, the discharge is either extinguished or converted in- to an other mode depending on geometry and magnetic field circumstances. In particular a sudden growth of the arc diameter may appears, the electron beam being no more confined by the magnetic field in relation with instabilities (and consecutive drain diffusion), the so-called "hash" which generally is set up somewhat below the maximum arc current. The hash may include high frequency oscillations (beam-plasma interaction) and relaxation phenomenon between two possible arc modes .

Inversely eq.19 gives the relative minimum density no (or pressure) compatible with a given arc current I. , with a given set of the geometric -470-

Table 1

Capillary tube length 10cm 20cin 40cni 100cm 200cm 400cm

Transmission 96 63 23 (i.d. 0.5 nua) 97 87 85 Transmission 92 74 13 (i.d. 0.3 nun) 97 85 51 Table 1 shows the exeprimental values of the transmission for two series of teflon capillary tubes. The carrier gas is helium under 2 bar pressure .

Ill - MOLBCULAR JST SYSTEM With the molecular jet system, we can remove most of the

T the second chamber carrier gas and retain the greater „.—_ Skimmer 1 mm part of the aerosols. • •— • Skimmer 0,5mm Skimmer 0,3 mm Two quantities are to be determined : the pressure at the level of the second chamber and the ratio of aerosols transfe- red through the skimmer. Experimen-

tally (£ig.1)r this pressure has been measured versus the distance between the capillary and the skimmer.

P0.3bcr

-2bor Dislancenozzle sltimmw 7 (mm) Fig.1 -471-

To measure the transmission through the skimmer, we applied the same method as above. Ono determines by 3-counting t.'.o quantities of gold which are deposited on two targets, ono situated downstream from the skimmer, the other upstream (for identical experimental conditions). The results show, within the limits of the fluctuations, that the transmission is generally about 50 % for a skimmer diamter of 0.5 mm..

IV - ION SOURCE The hollow cathode ion source has been described in a previous paper (1). The results shown in fig.2-3 have been obtained by systematically examining ion current and arc voltage variation as a function of pressure inside the ion source, A net- work of curves has been plotted, taking as parameters arc current and mangetic Field (they were varied respectively between 5 and 10 A and between 0 and 500 G) . Helium, nitrogen, neon, argon, krypton and xenon have been used. The curves obtained for all theses gases have similar shapes. Specifically, the ion current presented a minimum at a .source pressure of about 0.2 torr. Nevertheless helium is distinguished from other gases in that ion current and arc voltage were observed co be 50 % more important.

Arc w*OTfvolB) k?_iUrr","'If Al

1 7 CUT curr*nl 5A 2.B. arc currert &fc 3 9 ore eurrtrt 'A A 10 an. currtotBA 5 11 ore currwiW 6 12 ore cuT-yit 10A

(lorrj

Fig.: Fig. 3 -472-

IVL order to sec if the ion source could breal: up the ao. ,j-;ol cher.ical bounds and. ionise metallic elements we have uood the cluster production device. The gold current (fig.4) collected at the focus of a sector magnet has been measured as a function o£ the arc chamber pressure ("magnetic field created by coils around the source was chosen as a parameter). It is to be noticed that the maximum gold current corresponds to a mini- rr.ur.i in the argon carrier gas current. It is lively, that the plasma created by a gas jet is not completely tliemalised. It has a definite structure ; actually one notes (fig.5) that the currents due to gold and argon ions depend on the cat'node-anode distance, their maximum corresponding to a gap of about 2 rvun.

Argon ion current^A) Gold ion currenl^iA) Argon ion currentAjA) Gold ion current/hA) Cathode dion «»er 3mnr> Cathode lengl i 25mm

1-Argon ion current 300 150 300- 2-Gold ion current 150

2O0 1C30 200 100

1-Viouthoul 'tic field 2-W,th 125dauss" njiagnetic Field 3-Vilh 27 Igouss moqnetiliec field Ogaussn agnetic field

1CC 50 100 50

10 05 [on source pressure Cathode-anode distance V) (mmj -473-

Thr- whole- of the o:i-liuo cv.iia.-.o.Ll: Is ~,\:C."TL ir. fi>,J. !u-v. I'.udorgoAc ovcr^l r.o •••.!: 3 'co Ictcr:..;..:•• : th;; LOtal tran;;:..ission the overall transit tir.'G the nuclei liable to be isolated the possibility of optimising the yield of certain i'iOto. tho effect of adding an easily polymerisable gas.

"IB

.1 a I r) « S ».

/ }

liji

Fig. 6

A plastic scintillator situated on the beam axis facing a plastic collector detects £3-rays. This ?-counting for 20 s irradiation periods shows' that the storage rate is 5 x 10 per second for 39xe. The uranium target emits 1.5x1O7 of these nuclei, ,-4 so that the total yield is then about 3 x 10 -474-

b) Overall transit_time A source transport syste-a has been set up at the focus ulane of the separator. After their irradiation, the collectors are withdrawn and within 0.1 s are Placed in front of different 142 counters. In this way, the Xe has been identified by means of its Ge(Li) y-ray spectrum. Its period is 1.4 s. In other tests, inspection of the 3-activity decreasing with time after dropping the reactor control rods, shows that between the creation and"the collection of the isotopes there is about 5s. c) IiH2lSi_Ii§^i§_i2_^§_iS2lS£S^ Their identification has been made in two steps. In order to have a general Fission Yields knowledge of the col- p activity lection efficiency (fig.7), p-activity 1 - Fission Yields 2-p activity corresponding to dif- -1 ferent masses has 1000 10 been plotted. Irra- diation times of 200s have been arbitrarily 100 10"' chosen. In a second step, the identit}' of the nucleus has been determined by a 10- 10 systematic y spectroscopic study, some y spectra are presented 10" in fig.8 . 50 100 150 Mass

Fig.7 -475-

P number/s (5 Activity of the n\ass KO (Onumber/iOs)

2- 200 Helium pressure 3bor 20000

Fig.9 Fig. 10 1- Introducfton ofC2H 15C 1SOO0 2-5»opof iheccihode

10C 1000G-

5C 1 - P Acliviy of the moss 1<0 5000 2 - p Acliwly cf ihr ma» 67

10 20 500 1000 Distance between uranium Time (s*condj end cap.'Sary lube (cm) -476-

d) Possibility_of 0£timisin2_the_yield__of_certain__isotO£es It would be interesting to be able to select a given group of fission fragments, by talcing advantage of their slowing dorm properties. From this point of view, the distance between the uraniiy.i target and the entrance of the capillary tube has been varied. The fig. 9 shov/s the corresponding variations of the 3-activity for the masses 140 and 87. It is possible, in this way, to optimise the relative yield of rare earth fission products by choosing a distance of 3 cm. The uranium deposit is about (1.5 mg/cm ).

In order to improve the transport through the capillary tube and the collection efficiency some trial runs using a small proportion of ethylene in the helium have been made. In this way, it is likely that one increases both the number and size of the aerosols (the greater their size, the less is their diffusion and absorption on the walls). So the ^-activity variation with time has been examined after the introduction of ethylene (fig. 10).

VI - CONCLUSION

Investigation of the principal characteristics of this on-line apparatus has 'shown the possibility of isolating short- lived nuclides in the rare earth region. With the inclusion of a second skimmer in the molecular jet system to enable an increased upstream pressure, the introduction of ethylene in the helium and an augmentation in the neutron flux, an improvment by a factor of 10 on the fission product number and a decrease by a factor of ten on the transit time should be obtained.

R5F5RENC5

i/i) G.GAUTHE3IN, J.P.ZIRNH3LD, L.SCHUTZ, Proc.of the 2nd Int. Conf.on ion Sources, Vienna, (1972) 639. Chapter 10: The As.terix He-jet project -477- ThE ON-uIUE SEPARATOR ASTERIX

H.Wollnik+, R.Brandt++, H.Ewald+, H.Jungclas++, G.Kornahl+++, D.Snider+, H.Wagner+ + +, W.Wai cher+++, H.Wilhelm"1"

+Fachbereich Physik, Justus Liebig-Universitat, Giessen, Germany

^Fachberei ch Physi ks Chemi e , Philipps Uni versi fat, Marburg, Germ. Fachbereich Physik, Philipps Universitat, Marburg, Germany

We will report here on the progress in the preparation of an on-line ion source separator for heavy ion reaction products as we hope to obtain them in the GSI in Darmstadt starting at tha end of next year. (GSI=Gesel lschaft flir Schweri onenforschung). We in this case stands for a group of physicists and nuclear chemists from the Universities in Giessen and Marburg which have formed the ASTERIX-Collaboration hoping thet in due time an QBELIX-group will be created within the GSI.

ASTERIX stands for: Analyzing System for Transferred E/cited Recoils In the £SI so that the name describes the project already.

The basic idea behind the project is to create in Darmstadt heavy ion reaction products, thermalize them, ionize and accelerate them, send them through a mass separator, and finally investigate their decay properties. Naturally such a system works only if each step has a high efficiency and long term stability.

We follow two ways to achieve this goal. In the one we will stop fission fragments or other recoils in a pressurized He-gas chamber and then sweep them by a He-jet transport system to a thermal or a plasma ion source followed by a magnetic separator. In the other we stop fission fragments or heavy ion reaction products in hot graphite and ionize them thermally.

The latter experiment is advanced further already. In detail we have coated the inner surfaces of porous carbon by Uranium 90c oxide. The obtained 0.5g of U of are exposed to a thermal 7 ? neutron flux of 10 n/cm sec at a neutron guide tube of the reactor in Garching. Due to the temperature of 170n C the fission fragments diffuse quickly to the surface of the graphite where the isotopes of Rb and Cs are ionized efficiently. Ac- celerated to 15 keV energy the particles are separated by a com- -478- pact on-line separator and finally impinge on a moving tape system. For different tape speeds we then determined the y- spectra of each isotope. In order to reduce the background only Y-B coincidences were recorded with surface barrier Si-detectors for the |3-particles and large Ge (Li) detectors for the Y~ radiation. The instrument wor-s more or less reliably since the fall of last year usually 24 hours a day. The obtained mass spectrum behind the mass separator is shown in fig.l. The ions Mass spectrum of tfiwrmoUy ionized fission fragmanls were not counted directly fig.l The intensity distribution in order to avoid back- of the thermally ionized fission ground. We used the 3-ra- fragments. diation emitted from the fission fragments collected behind an exit slit on a moving tape. Since our magnetic field scan was rather quick compared to the time in which 3-particles were counted into one channel, the resolving power in fig.l approximately a factor two less than in reality. As one can see the Rb-isotopes can be recorded with reasonable intensities up to mass 97 and the Cs-isotopes up to mass 145. We have recorded half lifes and -y-spectra of these isotopes in the meantime. I will not go into details here, however. Besides tries to investigate more neutron rich Rb and Cs isotopes we hope to be able to measure Qjj-values by B-y coincidences. In the future we hope to investigate also other than Rb and Cs isotopes of fission fragments or transfer reaction products.

To investigate heavy ion fig.2 Sketch of the He-jet ion reaction products at the source on-line separator for heavy ion accelerator in heavy ion reactic.i products. -479-

Darmstadt is the declared goal of our second approach the He-jet ion source on-line separator indicated in fig.2. The heavy ion reaction products here shall be created in some target, stopped in some He-chamber of about one or two atmospheres pressure and then swept through a thin capillary to an ion source. After acceleration the particles then shall be mass analyzed in a magnetic separator and impinge on some moving tape system. An electrostatic seflector finally shall transport particles of one isobar to a low background area.

One of the big problems of this system is the construction of the tar- igl liir cine Ht-jtt-Tcrgtlun.TCdnung |schnnnli;.ch) get and of the stopping chamber |see fig.3a|. In the quite intense heavy ion beam even a very well constructed target should be heated to average temperatures between 1000° and 2000°C. Furthermore we expect temperature variations of 500° to 1000° having a frequency of 50 cycle as the macro structure of the primary ion beam. Additionally we probably will find rather pronounced sputtering effects even for targets and chamber windows that are covered by thin layers of light elements as carbon for instance. Such a FcralcrMie He-jet target arrangement could look like in fig.3a. Rnglorgc! The tungsten windows of fig.3a-b He-jet target arrangements the stopping chamber in this case are calculated to be at temperatures of above 2000 C for the highest beam currents we expect to obtain in Darmstadt. -480-

Due to the mechanism of heavy ion reactions the reaction products leave a target always in forward directions. The angle of divergence is about one degree for compound nuclei, about 10 degrees for fusion-fission products and about 60 degrees for transfer reaction products. In the latter two cases therefor a target construction as shown in fig.3b could be used which looses the center cone of the products but on the other hand avoids to be hit by the bulk of the primary beam and thus can be operated at much lower temperatures. The next big problems are efficiency of the He-jet transport system and the efficient coupling of this system to an ion source. The following three papers will comment on these questions. Let us assume for the moment that the construction of a magnetic and electrostatic mass separator stage as shown in fig.2 does present problems which can not be overcome within a year or two provided the available funds are sufficient which they are not for the time being. After all this effort has been spent we will have to ask ourselves which problems we then will tackle first.

One application will be: Qfl-value measurements by direct mass determinations. We will try to solve this problem by designing a mass separator that achieves a mass resolving power of better than 30 000 with reduced transmission for a mass range of only one isobar. Besides such measurements I think we will mainly try to investigate fusion-fission and transfer reaction products. The very interesting compound nuclei we will investigate only in special cases since a recoil separator as described by Ewa'id in this conference normally is much better suited for the investigations of the only slightly diverging compound nuclei. -481-

A HELIUM-JET SYSTEM DESIGNED TO FEED AN ION SOURCE

+ ++ + H.G.Wilhelm ,H.Jungclas ,HeWollnik , D.F. Snider*, R.Brandt++,G.Robig+

Fachbereich Physik,Justus Liebig Universitat.GieBen,Germany ++Fachbereich Physik.Chemie,Philipps Universitat,Marburg,Germ.

(1) Introduction: A Helium-Jet transport system consists of a thermalisation chamber,where recoils from a nuclear reaction are slowed down to thermal velocity by collisions with the helium.From this chamber the activity is transported through a capillary to a vacuum chamber by the helium gas flow.There the radioactive products can be collected.Such a transport system is very useful for accelerator experiments or other unaccessible sources of radioactivity,which have a high underground radiation. The exact transport mechanism of a He-Jet is still unclear. However,it has been shown that small amounts of impurities in the helium lead to the formation of clusters in the strongly reactive atmosphere of an accelerated particle beam or the intense radiation of an UV light source '.The radioactive particles are attached to these clusters which can 8 3) be as heavy as 10 u . To separate the helium transport gas from the clusters carrying the recoils,it is customary to introduce a second vacuum chamber with a skimmer between the two.The skimmer is formed by a hollow cone with an orifice at the top.The helium leaving the nozzle of the capillary expands very fast.The clusters stay near the axis of the system because of their big momentum and pass through the orifice of the skimmer.In the second vacuum stage with its lower pressure are now better conditions for<*-spectroscopy,a MAGGIE-system,or the coupling of an ion- source. -482-

(2) Experimental Set Up: The central part of our He-Jet system (FigJ) is a separate cluster breeder which consists of a vapor mixer and an UV light source.This system He-Jet Transport systen allows the production of a desired Source chornber Collection chamber vapor mixture with *J'Cf soui the helium gas.The mixture than enters Capilnry (01mm) a reaction chamber in which an intense f 1 FU*vmeter UV radiation is generated by a low • ' 1 1 • -Hi pressure mercury lamp. Cluster breeder The UV light causes break up reactions of Fig.1 Helium-Jet Transport System the vapor molecules and initiates presumably the formation of large clusters. The clusters are carried by the helium stream to a source chamber containing 0.7 microgram of ^ Cf.The Cf is covered 2 by a 1.3 mg/cm aluminium foil in order to prevent self- sputtering of the source.In the source chamber of 2000 cur volume the fission products are thermalized by helium at a pressure of one atmosphere and become attached bo the clusters. The gas and the clusters are swept from the source chamber through a 5 to 10 m long capillary with 1 mm inner diameter to a collection chamber which is pumped with a 2000 nr/h roots blower.The transported fission products are collected on a mylar foil.The resulting y-activity is monitored by a Nal(Tl) scintillator.Exact transport efficiency measurements were performed off-line with a methane flow counter,for /S -particles. -483-

(3) Experimental Results for UV-produced Clusters: In our experiments we tried to find more information about the nature of the mysterious clusters which are necessary for good transport efficiency.As Jungclas et al.2'^ have shown,the clusters range in mass from 10 to 10 u/

Using pure helium of technical grade we were not able to collect fission products from the Cf on the mylar foil (<.1%). The combination of additives and UV light gave a significant increase in transport efficiency,which reached some kind1 of saturation after a few minutes.As additives we used water,carbon tetrachloride,trichlorethylene,and ethanol. the best results were reached with carbon tetrachloride.In this case the helium contained about 5% CC1, per volume i.e. one- half the saturation pressure of carbon tetrachloride under these conditions.For further results see Tab.1. The interesting fact that even tobacco smoke injected into the helium caused an increase in transport efficiency,gave us the hint for later experiments with aerosols.

Tab.1 The effect of additives on the transport efficiency 252, of '"'""Cf fission fragments.

Additive Number of trials Transport efficiency maximum average

(pure helium) (3) (o.l %) water 5 39 % (21+10)% carbon tetrachloride 27 Sh (50+10) trichlorethylene 7 53 (29+11) ethanol 18 86 (32+ 8) - 4 0 4 -

The transport efficiencies were determined by collecting fission fragments on a mylar foil for 15 minutes.The/S -activity of the foils was then determined with a methane flow counter.The number of counts in the fourth, minute after the collecting period was monitored (Fig.2).As reference we took the counts in the fourth minute from a i^Ofiir. thick aluminium catcher which had been placed next to the Cf-source for 15 mi- | nutes. l In order to test whether i different elements were transported with different efficiencies,we compared 15 20 25 Time [min] the KX-ray spectra from the directly collected sample Fig.2 Standardized procedure and from the others we get from the He-Jet.The spectra were taken for 1000 sec starting with the seventh minute after the end of the collecting period (see Fig.2) using a Si(Li) detector with 250 eV FWHM at 20 keV energy resolution.In Fig»3 two such spectra are shown.The upper spectrum is taken from our reference sample,the lower from a sample collected with the He-Jet system.Each' elemnt has been identified by its K

Element number Z J5 «0 45 50 4S XI I I I I I I I I I I I 111. 1 i i I 1.5 L_

NORM

100 -

0- I ••! I I I I I I I I I I I I I- •—1—|—|—|—|- Ir Rb V Kb Tc Rh Ag In St) J Cj Lo Pr Pm tu It, 100' Kr » Zf Mo Ru Pd Ct Sn 7e Xt Ba C. N« Sm Od

12 16 20 24 28 32 36 40 U K x-ray energy [KeV]

c o * 50- o mn. Tc- Ag Te La Nd c o Ru Sb J Pr

ft '. i i i ••j—r i • I | 1 1 • | u i i i I F i » ) i i i i i i 50 55 60 65 35 40 45 Element number of fission product. Z=Z(Kto)

252 X-ray snectra of collected Cf fission product:: nc;uir.Gci for 1000 seconds starting six Minuter, nfter the end of irradiation (see tpij.4). r.: The recoiling fission fragments strike in aluminium catcher foil. The corresponding x-r-.y spectrum is called .'.'or:.. D: X-ray oroctrur.i of fission fragments which are collected v/ith the He-jet transport method. c: The ratio of corresponding peak areas of the two i,r, :••:. tra a and b is plotted as a function of the x-ray ener-y indicatins approximately the transport efficiency for different elements Z. Further details are given in the text. -486-

(/|) Skimming experiments with IV produced clusters: In our experiments using the set up shown in Fig.4 about 20 to 40 % of the activity leaving the capillary passed through the skimmer and 1 . could be collected.The skimming efficiency decreased with larger distance between nozzle and j -t- ] •' L - —> fij skimmer.On the other hand >—3 i^"-—~-^ ' p—J we obtained a better se- c~l—r1"3 C~J—T- "" paration of the activi- L'2' r ,^' ty and the helium carri- Fig.4 Skimming set up er gas at greater distances.So we have to make a compromise for reaching the right helium pressure behind the skimmer to operate an ion source and having still enough activity.

(5) Aerosol particles as clusters: Earlier He-Jet systems used either the strongly ionising atmosphere of an accelarator beam or intens UV light -as we did too- for the production of clusters.Assuming that the physical state of the tobacco smoke we had tried as cluster material is an aerosol,we tried to use other aerosols for the attachment and transportation of fission products. For these experiments we replaced the separate cluster-breeder by a LaMer aerosol generator.Such a LaMer generator is shown schematically in Fig.5.The helium gas passes at first the condensation nuclei source,where small sodiumchloride crystals are evaporated from an electrically heated wire.In the central bulb,called heater,the substance which shall form the aerosol is heated by a thermostat. The sodiumchloride condensation nuclei effect the condensation of the vapor in the heater bulb.The aerosol consisting of helium and the little droplets is then stabilized by passing a reheater column. We connected the aerosol generator to the source chamber of -487- our He-Jet system by a 1Om long PVC hose of 6mm inner diameter. At first stearic acid was used as aerosol substance.Although we collected small white spots + no activity was present. We /"••<=» believe that this effect is \ due to the fact that stearic acid forms solid particles at room temperature.If the attachment of the recoils to the aerosol particles is an effect of adhesion,it should be better to have liquid droplets.Therefore we choose "Diffelen N",that is an oil for diffusion pumps. It has a very low vapor pressure ( 6-10~9Torr at 20°C) and has no highly reactive chemical components as e.g. Fig.5 Aerosol generator carbon tetrachloride. (System LaMer) Using a very simple set up for the aerosol generator with no temperature controlled heating we reached 75 % overall efficiency for Cf fission products.The results of a 24 hours run are displayed in Fig.6.The mean efficiency was 55 %.The large fluctuations (+20%) in activity seems to be caused by the temperature instability of our aerosol generator.With a new proper temperature and gas flow stabilisation we hope now to be able to limit the variations in the transport and skimming efficiency to about 10%.

+We repeated this experiment a few days ago and found an overall efficiency of about 30 %;but this may be caused by small rests of Diffelen in the LaMer generator. -4B8-

For coupling an ion source to a He-Jet it is important to have a beam of activity with a small angle of divergence behind the skimmer.In Fig.7 the diameter of the collected spots of UV produced clusters and of aerosol particles behind the skimmer is shown as a function of Fig.6 Longtime stability nozzle skimmer distance.The full of the He-Jet system. line gives tne spot diameter assuming an isotropic point source at the nozzle and the skimmer orifice acting as aper- 0 ture.lt can be seen from the measured diameters that the jet beam is better focused, than the optical model allows. The difference in spot diameter between carbon tetrachloride and Diffelen additivs is very interesting.For a nozzle skimmer distance of 5mm Fig. 7 Spot diameter as a function of distance we found the angle of divergence to be 6 for CC1, clusters and 3 for Diffelen aerosol. The smaller angle for Diffelen aerosols seems to be caused by a greater momentum even though the exit speed of all particles from the end of the capillary should be the same.Con- sequently Diffelen aerosol particles seem to be heavier than carbon tetrachloride clusters by a factor of 2. Aerosol particles swept through a long capillary suffer loss because of two different effects.Small particles stick to the walls of the tube by diffusion and large particles will touch the walls when there are bendings in the capillary because of their momentum or because of gravity.Therefore we are presently studiing the transport efficiency of aerosols,to find the proper particle size for good transmission. -4B9- geferances

1) K.Wien,Y.Fares, and R.D.Macfarlane, Nucl.Instr. and Methods 10^,(1972) 181 2) H.Jungclas,R.D.Macfarlane, and Y.Fares, Radiochim. Acta 16,(1971) 1M 3) H.Jungclas, R.D. Mac far lane,, and Y,Fares, Phys.Rev. Lett. 27_,(1971) 556 aoc\s -490-

"C-o;-; octi-- Exr.orimeitc Botv/een an Helium Jet and. an Ion Source"

&D. F. Snider, 3U. Wagner, %. Jtmyclas,8^. Wollnik, §A. K. Mazumdar,?H. G. Wilhelm

. Physikalisches Institut, Justus Liebig Univernitttt, S3 Giessen, West Germany ^Inrtitwt fir K^rnchemie, Philippe U:.]ivcrr:itc.t, 31T- M:-.r":-urc» Vest Gory-fan y ^Physikalischea Institut, Philippe Universitat, 35? Marburg, Vent Germany

In the T.>r?coding report, we have cLL-jcussoci. QV.T .. liclium Jet. We should liks to proneat ov. oro^recc in the development ox an ion courcc rsuitablo for coupling to a liolium jet. The helium jet work and ion source c;:p3rincnta are intimately tound tcgethor through the cluster material. The choice of cluster material seems central not only for good transport efficiency Tntt also for desirable performance of the ion r.ource. We shall compare variour, civ.:?tor tic-ieri^lE: later in thin report, aid we shall begin by discussing our early work with water. From the fall of 1972, the helium jet work had developed GO that the transport of activity became possible using a cluster breeder, a water admixture, and a recycling nystem. At this time we had a hollow cathode ion ooivroe at our disposal, and so we inserted a connecting tube between the back of the skimmer and the entrance t"> the ion source to guide the clusters into the ion source. A block diagram of the apparatus used in the measurements is shoTOi in Pig. 1. Helium BLOCK DIAGRAM FOR "WATER CLUSTER" MEASUREMENTS

IOM SOURCE SnlMMER PVC

CAPILLARY Pi J Pi | \TUaNG I »6rr, 4 = I mm

OUSTER MtEDEN

RECYClMO SYSTEM

Fig. 1 gas v/as recycled through tiie cluster Lroeder ."•••>'". r~r- a. Cf o-5Uj oo o" a^irnyiniately 0.7 fig. The recoil :t£3 attached to clusters were blovm tsrov',°;h >:lo •j :\nto •;ho changer before the skimmer. The ski: unor -.vat; located alon,^ the capillary axis, an'! f-luntorn na.'T~.j.i" through the j'.tiriii&er could eater tho extensible tubi-~. The hollow cathode io;i 'j ">urce v/an ^or.nectod to the chamber following the nkinner throii/jli a length, of polyvinyl tubiug. Tliis tuLi.'^ (.:.>;v'-.:-?.c.i.-lly isolated the cha:absr • from the ion s-.ov.ri?i=*, rxino thora could be an arc potential of 600 volts between the helium jot OIK! ica roiu-ce. A valve directly before the ion source ancle it ronsible for the heliun jet to run continuously iut for the foil to be activated -.."or 3. fi::--d time interval. Particles passing through Lhe ion sourcs ;j-;-,ruclt a collection foil centered on the axis. The total distance between the skimmer and ion source r/ns 1.3 -.labors. The cluster transport of activity through s. capillary for a haliura jet at a presnuro o° F.OVOVP! Inincred Tor-r nay be related to the laminar flow of the t?-•--' r,-r.;:t Q-ac. Ilo'vev'-r, Tor a -srissixre lo^-s V\n~i n. To^r, t"\n ••?,-• - 4 9 ? -

flow lies in a traiuiition region bctv.-ce;: la.:.... *Tow and ciolfjular flow and the transport ef ficiow.v o. .. M vc r.:rir,unly affected. Our firot measurement was to 1 Cct-.-vTiir.':. uhethsr trap:'" crt to the ion GOU~'C;3 v.ra3 "o:-"j'-]o ii ti_ : low m-essure region at v/lrich tnir im source could operate. Proof of transport far-rj.^h t! <•„ j.r,n :-nurcc v^ cairicd out using a simiLr-i ;- • hly \ el\ vie.:: OT>cr?tcd without the extraction arertnre, T'r.G rtv3?'.ltiv " r-êctrv_T. i::o.ic?ted i'e Iin' co]].poted r.ot oily 220Pn p.n.-; 21So, lut also212Bi and "12Po. Trsnn- ir.i;r,irr. cf the 1-tter tvo ppeciee v/ould ljJcely come n aboiit orly tlirough cluster attachment, Sub.7.:q vvntlyf a Cf source fcocrxie available, said because it war advsuitaêous to use the Cf rather than the Th source, r.ll ->crLc7.-i:r; ricczix? events v;ere performed usii-g Cf. A preliminary ncanurement asinr; the Cf source ivas made to r- rnvc hhr activity tranrmorted through thf: Pkir.'jncr. The extensible tubi:is v/as removed, and the ?.ctiv'ty â--iif; through the nltiraer was collected on a taps an-1 '•'rrr/;.red after a collection tiaa of ?.O mir-utcr u:-i-,~ a methane flow counter. Tho collected activity v.'&s 3*000 counts per minute cjid thj I-.T.C'--- ;:roaic. level v/as 9 counts per minute. In ths follov-ong experiments vie used the system shovi in Tfic. U Clitsters with attached fission fr£\g- nentc vrere blov.-n for 20 minutes throuijli the ion source and t>? extraction aperture oato tho foil acunt^d :V.-. the ion collector. The p activity v/as aeaaui-ed UHin^; a latlir-io flow counter. Wo made throe different tjr'.-r, of ncnoTireiusntsi 1) Transmission through a cold ion r.ou-rr.e vrLtb- out r "colcrati.i^ voltage; 2) Collection with cathode hoc tin •: and arc tut -493-

without accelerating voltage; 3) Collection v/ith cathode heating, arc and accelerating TRANSMISSiON AND voltage. ©NIZATION Of "WATER CLUSTERS" The results are displaysd in Fig. 2. The upper curve shows the transmission measurements. At a pressure of 0.? Torr, the transmission curve has a maximum, and at this value thirteen percent U) 2J0 3JO PRESSURE BEHIND SKIMMER of the activity transported through the skimmer was deposited on the Fig. 2 foil, despite the 0.4 mm hole in the extraction aperture through which the clusters had to pass. The lower curve shows the ionizotion results, which also peak at the same pressure as the transmission curve. At the peak, we ionized several percent of the activity passing through the skimmer. With just the cathode heatjjig and arc {type 2 measurements), the observed activity was never greater than the background. This can be explained either by valorization of the clusters or breaking of the bonds between the clusters and recoil products. For particles leaving, the ion cource with isotropic distribution of velocity, the collection probability would be less than two per mil. Although the ionization efficiency for this syr.tem was high, the absolute activity which was ionised v/as low. We assumed that v/e could increase the activity by shortenlag the connecting tube between the dinner and the ion source. Another desire was to eliminate the chamber after the skimmer by fastening the inn nource -494-

directly to the back cf the skimmer and reevicting thf> wofsure in the ion source by chaaîn.i; biic dista-ice betv;e>,n the capillary arid skinunero Consequently, TO built another ion source which is shorn in. Fig1. 3» The connec- ti:;^ tube v;as forced Capillary Skimmer Ion Source Collector u jvfi in G t t li e back of the forming a __«_ relatively tight serl, The entire assembly var. nounted on the capil- Fig, 3 l^ry-.'hirr/.cr axis. In the tranr- mir.eion measurements, tne activity struck a Mylar berzA fr,-trîrsn over the collector cage. The '-elium jet was operated continuously between runs, and to ensure that no activity reached the foil whon the filament v/as cooling, the capillary v;as yithdravm ten cantinetors before the ski:ancr entrance and a plate turned perpendic- ularly to the capillary end. Measurements indicated that no activity reached the collecting foil undsr these condition?. The new ion source had better transmission properties and a different variation with capillary-skimmer roarcti ••" than the old ion source. The transmission results through the cold ion source are shown in Fig. k» The Biifelen traasmission. results through the coir1, ion source appaar as curve (e), and, for comparison, the earlier transmission results for water ivith the old ion scarce as curve (d) . Note that the transmission with -495- the new ion source ha;:, no •êak, as -.7ith the old source; the naxi- UUK for tuc old sourc-j 0.001 about be- caune of 5 B 15 the larje CiPILLAR* -SKIMMER SEMUTION neâration between the Rkimraor Curve (a): Skimmer transmission (field of values) for Diffolon and collec- (b): Skimmer transmission tion foil. (field of values) for CC1. ' (c): Skimmer transmission for H?O The activity or hydrocarbons after -Tun gc las et» al. with Dif- Cold ion source transmi-iiTn felen ap- for H20 (e)t Cold ion soxirce transmission pearc to for Diffelen* Background in (e) fall off is about 30 cpm. exponentially until eight millimeters, and remains cn-tcrt thoro- aftar. This curve is similar in appearance to curve (c). After examining the transmission cnaracteristics through the cold ion source, we proceeded to look iuto the ionization behavior. Our preceding experience v/ith water clusters iadicated that they vaporised rith r moderate arc current, but that the transported activity vdth \nati?.T clusters was low and subject to fluctuations. With the new ion source we investigated the other materials used in the helium Jet, CCl^ and Dlffelen. We found that while carbon tetrachloride transported activity quite well, it was not especially suited for the ion source. The ionization of CCl^ required a large -496-

filcr.cnt current and caused the rapid destruction of the filr.T.ent. Further, the minimum transport time usinr: CCl^ is severely limited by the high concentration necessary as may he •'shown using the follcrrin/s argument. The evacuation time ~Q of the gas quantity (pV). ^ . in tl:« target chamber is given by *"£ = (PvHarget^Q1» where Q^ is the amount of gas per unit time evacuated by the pump in the chamber before the skimmer. If skiiruning of the helium gas were corapletely effective,

QIS, the aaount of gaB entering the ion source -oer unit tin?, wouli be just the amount of cluster material in the stream. If k is the Initial fractional concentration of clusters, the amount of cluster material is

co tliat QIS^ isQ^. For a 53s concentration of CC1, , _ has the value of about

1 Torr-l/nec, and (?V)target probably lies in the region between 50 and 500 Torr liter. Thus, "{/ is between 2.5 secondr. and 25 seconds for thir concentration of CC1,. Of the three materials, Diffelen is the bc-t suited for use with the ion source, Diffelen is highly efficient in the trans- LOSS OF ACTIVITY ° port of activity, and IN CONNECTING TUBE bocauro the initial g concentration is 10"^ or less, the transport timo can be shorter than for CC1, . Further, Diffelsn does not at- o s n is DSTMCC FROM SKM«R BACK ALONG TaFQL tack tho filament. Con- sequently, our present ueasurcments are being carried out with Dif- feln as a cluster' W-6. 5 -497-

material. Ionisation of Diffelen with the new ion source is currently in progress. Despite the high transmission in the cold ste.te, we have observed no activity transport with the ion source in operation. Consequently, we have measured the deposition of activity on a Ta strip inserted in the ion source connoctin,.; tube. The activity is plotted as a function of distance from the back of the skimmer and is shown in Fig. 5. As the distance to the hot ion source decreases, the deposited activity increases, suggesting that the clusters arc decomposed before reaching the arc. The integrated intensity shown here is on the order of magnitude of the total activity passing through the skinnier. The reason for the large loss in the new ion source ae opposed to the old ion source may be attributed to the temperature distributions. The old ion source differed from the new ion source through a thermal contact between the discharge chamber and an external flange and through better thermal insulation between the cathode and connecting tube through which the cluster stream flowed. Consequently, the entry tube in the new ion source was probably hotter than in the old source, resulting in the premature vaporization of clusters. For our forthcoming experiments, we have constructed an ion source with a short connecting tube which can be cooled. Thus, we should be able to maintain the connecting tube to within one centimeter of the discharge at a sufficiently low temperature to avoid cluster decomposition or activity separation in the connecting tube.

Reference: H. Jungclas, E. D. Macfarlane, and Y. Fares, Radiochimica Acta 16. 141-147 (197D- -498-

Measurements of the Influence of the Penning Effect on the Ionization Efficiency in Gas Mixtures

A.K. Mazumdar, H. Wagner Fachbereich 13, Philipps-Universitat Marburg

The production rate of nuclear reactions is, in most cases, insufficient to run a plasma ion source. The use of a support gas will be necessary. In other cases, in transporting the reaction products from the target into the ion source, a transport gas is often helpful. Furthermore, a stopping gas may be employed for slowing down the recoils of the nuclear reactions as by the He-gas-jet method.

In all these cases, the nuclear reaction products form only a small component of the charge fed into the ion source. The ionization efficiencies of a plasma ion source may differ by several orders of magnitude for the different components [1].

In addition to the interactions taking place between one component, the wall and the electrons in the discharge, interactions between the two components take place. Of special interest is the collision between a He-atom excited to a metastable state (He being support gas) and an alien atom X:

He"1 + X + He + X+ + e which leads to ionization of the additive X. This process is called ionization by collision of the second kind or ionization by Penning effect. Penning ionization (PI) is only possible when the ionization energy needed is smaller than the excitation energy of the metastable He-atom. Table 1 gives the data.

We see that PI of He is not possible by metastable He-atoms. All the other noble gases, however, may be ionized by this effect. Thus the comparison of the ionization efficiencies of the noble gases should give a hint on the importance of PI. lonization Excitation F. ru r Energy £V of meta- Time of Li{o E. /eV eV statle state %/s Element V

He 24,58 19,82 2 3S 6-105 20.6 2 lS 3.8-10 o

Ne 21,55 16,62 lO-2 16,72 3p 2 o ID"

Ar 15,76 11,55 11,72 2 o lO-

2 Kr 14,00 9,92 ID" 10,56 ID"2 o

ye 12,12 8,32 ID "2

Table 1

The ion source used is shown in fig. 1.

Haiifadm ZMtfandt / .nodi

: Fig. 1: The ion source,

/The source is of the hollow cathode type described by Sidenius. The Jions are extracted from • a circular aperture of 0,U mm diameter at the -500-

cathode side. It was connected to a small separator shown in fig. 2.

Fig. 7: The Separator.

It has a bending radius of about 45 cm and a usable cross section of the magnstic field of Ix2cin . It will be used at Gielien for the connection to the He-gas-jet. The acceleration gap and the ion source regime were adjusted to the small acceptance of the magnet. Transmission lay between 20% and 50%. It was determined for each point of measurement from the total beam current collected in front of the magnet and the collector currents behind the magnet. Secondary electron emission was prevented by Faraday cap collectors. In order to reduce the number of discharge parameters, we always used, with the exception of fig. 3, a constant filament current of 5A and an arc current of 0,5A. The arc current was regulated with the help of the voltage of the arc supply. The resulting arc voltage varied from 80V to 135V.

In this range, the source efficiency changed little as shown in fig. 3. -501-

40 80 120 »0

The source efficiency is by a factor of 1 to 1.5 smaller than the ionization efficiency, since there are losses of ions to the acceleration electrode.

Fig. 4 gives the source efficiencies for the ncble gases as a function of the gas flow through the source. The curves

no%

Fig. '•*: Source efficiency as function of gas flow through source.

-3 -502-

may be roughly described by a relation «^oc 1/Q, The highest efficiency is obtained for the lowest throughput. From the measured points the figures of table 2 are derived.

Table 2

Element Imax 1 -Torr

Ne 2,4% 7.5-1O"3 Ar 15% 2.0-10"3 Kr 7% 1.9-10"3 Xe 25% 2.3-10"3

The relative efficiency, i.e. the source efficiency for one noble gas divided by source efficiency for He is given in fig. 5.

W3-

Fig. 5: The relative efficiency for the different noble gases as a function of the gas flow.

Jkk, T«rl -503-

Again we observe a difference in the behaviour of Ne compared to the other noble gases which have relative efficiencies lying about one order of magnitude higher than those of Ne. The difference may be caused by the fact that Penning ionization is impossible only for Ne. Thus ionization efficiency is reduced for Ne and a large gas flow is necessary for the running of the arc. The efficiency for Ne is still by a factor of about 100 higher than for Helium. This phenomena cannot be effected by PI for energetic reasons as shown. The ionization by electrons u from the filament ( arc«* 80V) should differ by less than one order of magnitude for the two elements. Charge exchange He + Ne -* He + Ne seems impossible since the first excited state of Ne spectrum is too high.

The following processes may be responsible:

He* + Me -• Ne * + He Ne*-* Ne m , i •> and Nem + He* -* Ne+ + He + e Nem + Nem -* Ne+ + Ne + e Nem + e -• Ne+ + 2e

Fig. 6 gives the source efficiency for Xe as a function of the Xe-concentration in H*». for two different gas flows. The efficiency is nearly proportional to the inverse of the concentration for large parts of the two curves.

As a result of our measurements we conclude •••hat PI plays an important role in ionizing small amounts of alien atoms in Helium, that a small concentration of additives and the minimum gas flow to run the source should be used to obtain the 4 best efficiency. In addition to PI other interactions between j the two components are at least for the Ne admixture important. -504-

Fig. 6: The influence of concentration on efficiency

[1] ORON, H and S. AMIEL in Proc. of the international Conference on Electromagnetic Isotope Separators, Marburg, 1970: BMBW-FB K70-28, pg. 87 Addendum: Press stop news THE PRINCETON ON-LINE SEPARATOR R.A. NAUMANN

The activity levels of the short-lived radioisotopes available for study at an on-line isotope separator system are strongly dependent on the hold-up time of the target- ion source system in use. For liquid metal and solid targets one often finds that a single effective residence half-life is operative as evidenced both by observations of single component decay curves for transport of activity from the target as well as the simple exponential dependence of this residence time on the target Absolute Temperature.^1' Assuming there is one limiting transport process with associated half-life, T± res, for steady state operation one may relate the production rate, P, of some radioisotope in the target and the corresponding disintegration rate, R, occurring at the separator collectori T. (decay) R=P (res> * Tt (decay) where T. (decay) is the intrinsic half-life of this species and 6 the efficiency of target system and isotope separator! Thus, for example, it is possible to observe 0.1 second activities with a 10 second residence time. However, for this case, if the residence time could be made negligible in comparison with the intrinsic half-life an increase in the collector activity level by two orders would be realized. At Princeton our isotope separator is now installed at the sector-focus cyclotron in the Physics Department. This accelerator can furnish 4-5-Mev protons, 60-Mev alpha particles and 75-Mev helium 3 particles at external beam currents in excess of io microamperes. Our interest is in the beta decay processes of mirror nuclei lying to the left of the N = Z line. For such nuclei -superallowed" positron decay connectB analogue states so that very short half-lives are encountered. For example, the known simple mirror positron emitters range from carbon 11 with a half-life of 20,4 minutes and positron endpoint of 0.960 Mev to 0.49 sec titanium 43 with endpoint 5.84 Mev. Accordingly the availability of target systems having the shortest residence times ia of great importance to us. Figure I shows the layout of the present isotope separator system at Princeton. The zero degree deflected beam of the cyclotron can be brought to a focus on a target-ion source system of the separator by means of a quadrupole triplet. In order to maintain the 60 kilovolt accelerating potential of the separator, two Pyrex glass insulating sections have been installed in the beam line on either side of the target region. Each insulating section is approximately 15 centimeters in length. rigure II shows a detail of the target-ion source system. A molybdenum target tube and associated components are shown in the foreground. This target tube is 12 centimeters in length and threads into the back of the ion source which is shown at the left of the photograph. The cyclotron beam passss through a thin tantalum window at the end of the molybdenum tube and intercepts the target. The molybdenum target tube is heated by means of the resistance heating jacket and is surrounded by a heat shield. On the right side of the photograph, the pneu- matically operated beam shutter with fluorescent screen can be seen. This screen is viewed through the Pyrex tube by a TV monitor. The water cooled cross-pipe also carries electrical and gas feed-throughs for operation of the ion source. Since the heated target tube is supported from the back structure of the ion source, cool spots where vaporized reaction products may condense are avoided. This target system ia used in cases where * more volatile radioactive product is produced by bombardment of some less volatile target material. We have tested this system using a mixture of minus 30O- mesh nickel powder mixed with powdered graphite as a target for 30-Mev helium 3 particles. Figure III is a radioautograph made from a collector plats from the separator showing the zinc radioisotopes produced. The collection time was five minutes and the tube waa held at approximately 900°C. The shortest-lived activities observed are 07-second zinc 61 and 2,4-minute line 60. In order to determine the residence time of this target system, successive 90 second collections of the zinc 60 were made using a moving tape collector. As shown in Figure IV, Bteady state operation was achieved and then the beam was turned off. Within 180 seconds after beam termination the activity level from the target decreased approximately two orders of magnitude. Analysis of this curve indicates a target residence half-life of 28 seconds for this high temperature system. Presently we are undertaking investigation of higher temperatures and finer nickel powders in attempts to reduce further the retention time. When penetrating projectiles are available, it is desirable to use thick targets of consequently larger volume to insure efficient beam utilization. However, with these larger volumes, longer target residence times can be expected. One attractive technique for releasing highly volatile products such as inert gases from large volume liquid targets is offered by boiling. Two problems with this method can be immediately anticipated when using a target liquid of low boiling point 1 first the equilibrium pressure of the target liquid may be so high that the separator ion source discharge cannot be maintained and second since the liquid is being boiled under low pressure /severe "bumping" can rapidly eject target liquid and vapor from the target chamber into the rest of the separator system. -r-0R-

To test, the feasibility of boiling liquid targets for use with on-line mass separators we are using a system schematically shovm in Figure V. The beam passes through a window cell equipped with double tantalum foilB cooled by flowing helium gas. This cell provides window cooling and protection for the cyclotron beam line system in case of window failure. The beam then enters along the axis of a cylindrical target cell which is 3/4 filled with approximately 300 mis of the volatile liquid target. This liquid vigorously boils when several hundred watts are deposited by the beam, the bulk of the released vapor is refluxed back to the target as liquid by the parallel condensers cooled with iced water. The remaining vapor and gaseous reaction product is then transported by hydrodynamic flow to a large liquid nitrogen trap equipped with shelves. Essentially complete condensation of the target vapor is accomplished by this trap which is pumped through a 10 centimeter diameter tube by an oil diffusion pump backed by the ion source. Thus the inert gas product passes from the trap to the ion source under conditions of molecular flow. Figure VI is a photograph of the system. We have tested this system using 30 Mev-proton bombardment of diehloroethylene ( C% Cl^ , boiling point 121° C at N.T.P.) to generate 1.8 second argon 35. Under vacuum without beam, the liquid target cools and remains quiescent. When beam intercepts the target the TV monitor shows immediate boiling of the liquid near the entrance window. High levels of argon 35 activity are simultaneously observed at the collector of the mass separator. The system operates continuously for three to four hours before the trap must be emptied. In order to determine tha residence time of the liquid target system, a Nal (Tl) detector was placed at the mass 35 position of the collector and the argon activity allowed to build to saturation. At this point the cyclotron beam was turned off and the decay of the activity recorded during successive OA second intervals using a multiscaler. As shovm in Figure VII, the decay curve deviated from simple exponential form during the first two seconds after beam termination due to the additional residual target activity transferred from the target. Detailed analysis of this curve indicates a retention time of less than 0.7 seconds. The successful demonstration of a boiling liquid target encourages us to suggest that similar techniques are applicable using chemically reactive target systems. Here the product activities would be released in the form of gaseous compounds for transport to the ion source of an on-line separator. It is a pleasure to acknowledge the many contributions of my Princeton colleagues Drs. E. Ansaldo, G. Garvey, and Messrs. F. LoeBer and J. Lind in realizing the results here reported. -•;i it-

Reference i (1) E. Hageb«S, A. Kjelberg, P. Patzelt, G. Rudstam and S, Sundell CERN Report ?0-3 January 1970. ;//////

^BOMBARDMENT BOX ill

% 0' BEAM FROM CYCLOTRON

BEAM LINE FOR ISOTOPE SEPARATOR

Figure It Lay-out of the on-line separator .it ^aiT.et-ion source su,semfcly Jneluuinf- '.on ECU.?; heated target tube, beam shutter and lyre A an inft secti-n

M, (3He, xn ) Z

HI] Hndiop.utograph of the active z.lnc isotopes ."oj lectci during 70 VPV Ke ' boiibaniti.e'st: of -513-

Beam off

50 MeV He on Ni-graphite target A =60 position, all gammas 10' -

in 2

to2 J i 100 200300400500600700800900 TIME (sec)

Figure IV« Retention curve for zinc observed for the nickel-graphite target. Bars indicate time interval length.

Figure V, Plan of the boiling liquid target system -514-

Figure VIi Photograph of the boiling liquid target system

Beam off Beam on (tape moves! I ..,-

511 keV gammas

_i i _ 10 15 20 25 30 :i!/t ( sec i

Figure VIIi Retention curve for argon 35 observed for the boiling perchloroethylene target -515-

LIST OF PARTICIPANT?

Aitken, D. ten Brink, E.O. Lintott Engineering Ltd., Foundry Free University, Department of Lane3 Forsham, Sussex, U.K. Physics, De Boelelaan 1081, Amsterdam, The Netherlands Almen, 0. Department of Physics, Chalmers Buchta, R. University of Technology, Fack, Research Institute for Physics, A02 20 Goteborg 5, Sweden 104 05 Stockholm 50, Sweden

Amiel, S. Camnlan, J. Soreq Nuclear Research Center, Laboratoire Rene Pernas, Batiment Yavne, Israel 108, Boite postale 1, 91406 Orsay, France Andersson, G. Department of Physics, Chalmers Cembali, G. University of Technology, Fack, Consiglio Nazionale Delle Ricerche, 402 20 Goteborg 5, Sweden Laboratorio di Chimica e Technolop.in dei Materiali e dei Componenti per Appelqvist, A. l'Elettronica, Via De' Castagnoli, Department of Physics, Chalmers I, 40100 Bologna, Italy University of Technology, Fack, 402 20 Goteborg 5, Sweden Chavet, I. Soreq Nuclear Research Center, Arnesen, A. Yavne, Israel Department of Physics, Uppsala University, Box 530, 751 21 Dearnaley, 0. Uppsala, Sweden Atomic Energy Research Establishment, Nuclear Physics Division, Bid. 8, Asghar, M. Harwell, Didcot, Berkshire, U.K. Tnstitut Max von Laue - Paul Langevin, Cedex 156, 38 - Debus, G.H. Grenoble-Gare, France C.R.N.M. (Euratom), Steenweg Na.ir Retie, 2400 Heel, Belgium AugenTicht, K. Department of Physics, Chalmers Denimal, J. University of Technology, Fack, C,R.N., Separateur d'Isotopes, P.P. 402 20 Goteborg 5, Sweden 20 CRO, 67200 Strasbourg, France

Balanda, A. Dolnicar, J. Institute of Physics, Jagiellonian International Atomic Energy Agency, University, Cracow, Poland P.O.Box 590, A-1011 Wien, Austria

Bergstrom, I. Dropesky, B.J. Research Institute for Physics, Los Alamos Scientific Laboratory, 104 05 Stockholm 50, Sweden P.O.Box 1663, Los Alamos, New Mexico 87544, USA Bir, R. DGI/SET - Bat 121, CEN Saclay, Dumont, G. BP no 2, 91190 Cif-Sur-Yvette, Universiteit Leuven, I.K.S. - France Celestunenlaan 200 D, 3030 Heverlee, Belgium Braun, M. Research Institute for Physics, Dynefors, B. 104 05 Stockholm 50, Sweden Department of Physics, Chalmers University of Technology, Fack, 402 20 Goteborg 5, Swedan -516-

Eklund, K-H. Grapengiesser, B. Department of Physics, Chalmers The Swedish Research Councils' University of Technology, Fack, Laboratory, 611 00 Nykoping, 402 20 Goteborg 5, Sweden Sweden

Ekstrom, C. Hansen, P.G. Department of Physics, Uppsala Division NP, CERN, CH-1211 Geneve University, Box 530, 751 21 23, Switzerland Uppsala, Sweden Hechtl, E. Emmoth, B. Physik-Department, E 18, Technische Research Institute for Physics, Universitat Miinchen, 8046 Garching, 104 05 Stockholm 50, Sweden BRD

Ewald, H. Hellborg, R. II. Physikalisches Institut, Department of Physics, University Justus Liebig-Universitat, of Lund, Solvegatan 14, 223 62 Lund, 63 Giessen, BRD Sweden

Ferrari, M. Hoick, 0. Institut de Physique Nucleaire, The Niels Bohr Institute, 17, 43, Bd du 11 Novembre, 69 Blegdamsvej, DK-2100 K^benhavn, Villeurbanne, France Denmark

Fiebig, G. Holmen, G. Kernforschungsanlage Jiilich, Department of Physics, Chalmers Institut fur Neutronenphysik, University of Technology, Fack, D-517 Jiilich, Postfach 365, BRD 402 20 Goteborg 5, Sweden

Fink, R.W. Hbglund, A. School of Chemistry, Georgia Department of Physics, University Institute of Technology, Atlanta, of Stockholm. Vanadisvagen 9, Georgia 30332, USA 113 46 Stockholm, Sweden

Fontell, A. Ihle, H.R. University of Helsinki, Department Institut fur Nuklearchemie der of Physics, Siltavuorenpenger 20 E, Kernforschungsanlage Julich GmbH, 00170 Helsinki 17, Finland 5170 Julich, Postfach 365, BRD

Forman, L. Jacobsson, L. Los Alamos Scientific Laboratory, The Swedish Research Councils' P.O.Box 1663, Los Alamos, New Laboratory, 611 00 Nykoping, Mexico 87544, USA Sweden

Fransson, K. Jastrzebski, J. Research Institute for Physics, Institute for Nuclear Research, 104 05 Stockholm, Sweden SWIERK near Warsaw, Poland

Freeman, J.H. Johansson, K.A. Atomic Energy Research Establish- Department of Physics, Chalmers ment, Chemistry Division, Building University of Technology, Fack, 10.30, Harwell, Didcot, Berkshire, 402 20 Goteborg 5, Sweden U.K. Johnson, P. Freitag, K. Lawrence Livermore Lab., Radio- Lehrstuhl fur Kern- u. Neutronen- chemistry Div. L-515, Livermore, physik der Universitat Bonn, D-53 California 94550, USA Bonn, Nussallee 14-16, BRD -517-

Jonsson, O.C. Liljefors, T. The Swedish Research Councils' Research Institute for Physics, Lahoratory, 611 00 Nykoping, 104 05 Stockholm 50, Sweden Sweden Lindahl, A. J^rgensen, H-E. The Niels Bohr Institute, 17, Danfysik A/S, Jyllinge, DK-4000 Blegdamsvej, DK-2100 KcSbenhavn, Roskilde, Denmark Denmark

Kirchner, R. Loostrom, L. Institut fiir Kernchemie, Information Office, Chalmers Universitat Mainz, D-65 Mainz, University of Technology, Fack Postfach 3980, BRD 402 20 Ceteborg 5, Sweden Lotti , R. Kjellman, J. Consiglio Nazionale Ilelle Ricerche, Nucletec S.A., 34 Ave. Tronchet, Laboratorio di Chimica e Tecnologi ;i CH-1226 Millesulaz, Geneve, dei Material], e dei Componenti per Switzerland 1'Elettronica, Via Pe' Cnstagnoli 1, 40100 Bologna, Italy Kornahl, K. Fachbereich Physik der Love, L.O, Universitat Marburg, D-355 Oak Ridge National Laboratory, Marburg, Deutschhausstrasse 32, Bldg. 9731, Y-12 Plant, P.O.Box Y, ERD Oak Ridge, Tennessee 37830, USA

Krueger, F.P. Lonsici, 0. Institut fur Kernphysik, T.F., Department of Physics, P.O.Box 1048, 61 Darmstadt, Schlossgastenstr. Blindern, Oslo 3, Norway 9, BRD Haddock, A.C. Kromer, G. The Chemical Laboratories, I.ensfield Fachbereich Physik der Road, Cambridge, U.K. Universitat Marburg, D-355 Marburg/Lahn, Renthof 5, BRD Martel, C. Gamma Industrie, 52, Rue de Dunkcrque, Kugler, E. 75009 Paris, France Division NP, CERN, CH-1211 Geneve 23, Switzerland Maskell, C.W.A. Atomic Energy Research Establishment, Lawin, H. Room 1-48, Bldg. 424, Harwell, Didcot, Kernforschungsanlage Jiilich, Berkshire, U.K. Institut fur Neutronenphysik, 517 Jiilich, Postfach 365, BED Mazumdar, A.K. Fachbereich Physik der Philipps- Lehto, A. Universitat Marburg, 355 Marburg/Lahn, University of Helsinki, Department Renthof 5, BRD of Physics, Siltavuorenpenger 20 C, 00170 Helsinki 17, Finland Menat, M. Israel Atonic Energy Commission, Lejeune, C. Soreq Nuclear Research Center, Yavne Institut d'Electronique Israel Fondamentale, Laboratoire associe au CNRS, Batiment 220, Meunier, R. A Universite Paris XI, 91405 Orsay Laboratoire Rene Bernas, Batiment France 108, Boite postale 1, 91406 Orsay, France -518-

Mollerberg, R. Frokopets, G. HAFO, Faek, 162 10 Vallingby 1, Swedish Atomic Energy Company, Sweden Studsvik, 611 00 Nykcping, Sweden

Kaumann, R.A. de P.aedt, J. Jadwin Laboratory, Princeton Research Institute for Physics, University, Princeton, New 104 05 Stockholm, Sweden Jersey 08540, USA Rantala, V. Nielsen, O.B. University of Jyvnskyla, Department The Niels Bohr Institute, 17, of Physics, Nisulankatu 78, 40720 Blegdamsvej, DK-2100 K^benhavn, Jyvaskyla 72, Finland Denmark Ravns H. Nobes, N.J. Division NP, CERN, CH-1211 Geneve Department of Electrical 23, Switzerland Engineering, University of Salford, Salford M5 4WT, Rieder, R. Lancashire, U.K. Hockegasse 59, A-1190 Wien, Austria

Noreland, T. Ristori, C. Department of Physics, Uppsala Centre d'Etudes Nucleaires de University, Box 530, 751 21 Grenoble, Laboratoire de Chimie Uppsala, Sweden Nucleaire, B.P. 85 Cente de Tri, 38041 Grenoble, Cedex, France Nyman, G. Department of Physics, Chalmers Roeckl, E. University of Technology, Fack, GSI Darmstadt, D-61 Darmstadt, 402 20 Goteborg 5, Sweden Postfach 541, BRD

Oron, M. Roesch, H. Laboratory for Laser Energetics, IBM Germany, Dept. 0135 (German University of Rochester, River Mfg. Technology Center), D-7032 Campus, Hopeman 110, Rochester, Sindelfingen, P.O.Box 266, BRD N.Y. 14627, USA Rudstam, G. Paris, P. The Swedish Research Councils' Centre de Spectrometrie Nucleaire Laboratory, 611 00 Nykoping et de Spectrometrie de Masse du Sweden C.N.R.S., BP no 1, 91406 Orsay, France Santry, D.C. Atomic Energy of Canada Ltd., Pasztor, E. Chalk River, Ontario, Canada Hungarian Academy of Sciences, Central Research Institute for Scheu, U. Physics, H-1525 Budapest 114, Research Institute for Physics, P.O.Box 49, Hungary 104 05 Stockholm 50, Sweden

Pattun, E. Schmidt-Ott, W.D. Universiteit Leuven, I.K.S. - .. Bldg. 6000, Oak Ridge National Celestunenlaan 200 D, 3030 Laboratory, Oak Ridge, Tennessee Heverlee, Belgium 37830, USA

PeterDtrom, S. Skilbreid, 0. Department of Physics, Chalmers Danfysik A/S, Jyllinge, DK-4000 University of Technology, Fack, Roskilde, Denmark A- 2 20 Goteborg 5, Sweden -519-

Skoog, R. Walcher, W. Department of Physics, Chalmers Physikalisches Tnstitut der University of Technology, Fack, Universitat Marburg, 355 Marburg/ 402 20 Goteborg 5, Sweden I.a'nn, Renthof 5, BPD

Snider, D. Wiedling, T. IT. Physikalisches Institut, Swedish Atomic Energy Company, Universitat Giessen, 63-Ciessen, Studsvik, 611 00 Nykopin, Sweden Arndtstrasse 2, BRD

Spejewski , E.H. Wigmans, M.E.J. Bldg. 6000, Oak Ridge National Free University, Department of Laboratory, P.O.Box X, Oak Ridge, Physics, De Boelelaan 1081, Tennessee 37830, USA Amsterdam, The Netherlands

Sundell, S. Wilhelm, H. Division NP, CERN, CH-1211 Geneve II. Physikalisches Tnstitut, 23, Switzerland Justus Liebig-Universitat, 63 Giessen, Arndtstr. 3, RRD Talbert, Jr, W.L. Physics Department, Iowa State Wolf, G.K. University, Ames, Iowa 50010, USA Lehrstuhl fiir Radiochemie der Universitat Heidelberg, 75 Thuresson, M. Karlsruhe, Postfach 3640, RPD Department of Physics, Chalmers University of Technology, Fack, Wollnik, H. 402 20 Goteborg 5, Sweden II. Physikalisches Institut der Justus Liebig-Universit.-it, 63 Toft, V. Giessen, Arndtstrassp 2, HRD Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Zuk, W. Denmark Department of Nuclear Physics, Institute of Physics, Maria Truong, CM. Curie-Sktodowska University, Institut de Physique Nucleaire, Lublin, Nowotki 10, Poland Universite Paris-SUD, 91400 Orsay, France Zirrheldj J.P. Universite Louis Pasteur, Service Tykesson, P. Pile Universitaire, 17, rue Institute of Physics, University Becquerel, 67037 Strasbourg, of Aarhus, DK-8000 Aarhus C, Cedex, France Denmark Ays to. J. Valli, K. University of Jyvnskyln, Department University of Jyvaskyla, Department of Physics, Nisulan^atu 78, 40720 of Physics, Nisulankatu 78, 40720 Jyvaskyla' 72, Finlar ! Jyvaskyla 72, Finland

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Wagner, R. Institut fiir Nuklearchemit J?r Kornforschungsanlage Jiiiich GmbH, 5170 Jiilich, Postfach 365, BRD Publisher: Dr G. Holmen Institute of Physics Chalmers University of Technology P.O. Box, U02 20 Gothenburg 5. Sweden

Price: Sw.kr. 75 -38- parameters (y , D,a, L). il+mA The phenomenon which have been described here in a special and simple / discharge is general in 20 « 2 low pressure discharges ; \ 4.10 \ it is illustrated on fig. \ 12 in the case of the /A 2 duoplasmatron source : 10 A \ 3.10 beyond I. >\, p, the ex- '/ \ 2 tracted ion beam decrea- 2.10 torr ses in relation with the f IA increase of the column 0 5 ' 15A diameter ; at the regime Fig.12 - Ion extracted current versus arc current change over point the in duoplasmatron ; argon ; 3 = 5 anode ionization efficiency .52 pressure parameter. is close to unity' However depending on the particular mechanisms of each source, the quantitative relation 19 has to be re-examined.

2i2il3i._Charge_exchange_2robabilitx The previous phenomenon is very important for multiply-charged ion sources : near the regime change over point (or arc extinction) - the arc potential must be high (> I),,) ; - the neutral density in the arc is minimum (n*), which involves a minimum charge exchange rate. An order of the charge exchange probability is given by the ratio of charge exchange mean free path X = (n a ) to the half column length ; from eq.17 :

2 (M/m) ' (20) h a y $ ex ex Argon y 1A i; a = 1/2 ; x 3/2 2 a =3.10 cm ; a I M ex

This example shows that charge exchange will be negligible only when the arc operates near the regime change over point. Furthermore radial losses have to be reduced when possible, as seen from the influence of y , in eq.20. -39-

2.3« Multiply-charged ion production

According to the value of the ratio of ion dwell time T to the mean ionizing time T' by stepwise ionizatinn, the ion production will be dominated either byLsingle electron impact ( TA '<< 0 or by step by step ionization (T/t ' » 1). We have seen that this latter process is required in order to obtain a distribution in charge presenting a maximum for a z-state higher than unity. Thus the necessary conditions for optimizing the charge spectrum on a given charge state will be derived from the influence of the discharge parameters which are favourable to such a process.

iurthermore it is interesting to derive the expected charge spectrum for a given set of these parameters. A rigorous calculation would require : 1) the analysis of all the processes which lead to the production and loss equilibrium for each charge state ; 2) the analysis of the influence of multiply- charged ions on the discharge equilibrium equations. However, the interest of such a model would be limited because of the uncertainties in the knowledge of the atomic parameters. Hence the objective will be restricted to present a simple approach of the problem as already proposed by Pigarov and Morozov Sit 55 and Bennet for PIG-sources. It will be helpful in understanding the first order behaviour of such discharges.

^j ionization

In order ot simplify we assume that :

- the ionization is insured by primary monoenergetic electrons, the energy of which allows stepwise ionization up to z* ;

- the cross-section o . to remove only one electron is independent of the charge multiplicity owing to the influence of metastable levels (section 1.2); hence the time T' between two collisions is independent of z ;

- single impact ionization, charge exchange and volume recombination are neglected ;

- the ions remain in the discharge up to time T , but charge escape is not explicitely taken into account in the equilibrium equations.

The variation of the population N (t) of the z-times charged ions in the whole arc column as a function of time, :.iay then be derived from the &et of equilibrium equations between production and loss of a charge state : (21) dN /dt = (N -NZ)7T' with the initial conditions : -40-

N (t = 0) = 0 if z >, 1 It (22) N (t = 0) = N (0) = ,J,0(1 " exp - (a))

The solution is :

z-1 H.(0) (23) N (t) « —' (~ exp - (-,) (z - 1)! T T The maximum of this population is obtained when t = ( z - 1) T' as shown on the graph of fig. 13. The charge distribution in the particle flux emitted from the discharge at time T is calculated from this graph (fig.\k) ; the most 0 13 5 probable charge j . 13 - Variation of the z-times charged ion popula- state is : K tion vs relative time, by stepwise ionization *.. =T/T' + 1 j tho ^ owing to the decrease of the ion dwell ti- / / me as the charge multiplicity increases, / » T the most probable charge state will lie \ >\ 7,( =^ within the limits : \ \ < z,, - 1; T./T (2"y) \ \ where t is the dwell time of mcnochar- 1 ged ions (eq.8). In the case T «T • 4— —N single electron impact and stepwise ionization occur simultaneously, the f ! ( relative contribution of each process J b J 1 depending on T /T ' and on the ratio Fig. 11* - Calculated charge spectrum of the single electron impact cross- by stepwise ionization for two va- sections ; as example : N^ - N (0).T / lues of the ratio of the dwell ti- irom eq.23 to be compared to me T to ionization time T'. N 2 1 1 N"2 - 1(0).aQ/ao resulting from T/T = 2:the 3-times charge ions single electron impact. are optimized. -41-

2i3_L21_0gtimum_conditions_for_ste2wise_ionization

The discussion is straigh-forward from the previous relation and the two characteristic time variations as function of the discharge parameters. The charge -spectrum will be shifteditoihigher values as l ,1/2 . the ion dwell time T..= Y (M/2kT^ - enlarging the discharge length L

increases with - decreasing the radial loss (y -*1)

- increasing the ion mass

1 . the ionizing time x'=e/je a- - increasing the arc current I.

decreases with - decreasing the column diameter D

- increasing the ion mass.

Furthermore, we must remember that with each set of the previous parameters :

- the arc potential which fixes the electron energy must be higher than the threshold reaction of the maximum expected charge multiplicity ;

- the pressure must be adjusted to its minimum value compatible with the arc stability : charge exchange processes are then negligible ; this minimum value is derived from eq. 19

I. = e n o kT (19) A o (* o/8m) ; Thus the optimum pressure increases with increasing the arc current ; when these conditions are fulfilled, if we adopt the less favourable case in

eq.25, the optimized charge state zM is given by : k ¥ 1/2 T2 , *MT r, 1/2 (ZM - > ±-a ( 2-•) 2 kT M ire o D x m Te (26) where appears the influence of the main geometric and operating parameters of the discharge. Stepwise ionization will be efficient only with high arc current, in sources having a large discharge length. Furthermore these conditions are favourable to large extracted total ion beam (eq..22) with an ionization efficiency close to unity, particularly in axial extraction source, owing to the decrease of neutral density within the arc column. However, such operating conditions require either an improvement of the cooling system of the source for continuous operation or pulsed operation.

The conclusion of this discussion about multiply-charged ions production in discharge sources are in agreement with experimental data as will be ex- -42-

posed in the next section.

3. SURVEY OF EXPERIMENTAL RESULTS

We shall limit the discussion to sources which function continuously, excluding threfore those which operate in a pulsed mode which correspond to the search for high charge states for injection in accelerators. We single out, in particular, the calutron, the PIG source, the duoplasmatron and the magnetron in discharge operation, vie shall compare the results for these sources with the simplified model of the preceeding paragraph. We then present results obtained for various configurations : rf source, magnetron with electron cloud,...

3.1. Calutron

Camplan et al have studied this type of source at Orsay using krypton gas . The following conclusions can be discuss from their results:

- Th>» ratio I /I varies approximately linearly with V (fig.15) and arc with the magnetic induction -

- The ratio I /I decreases with increasing pressure (fig.16).

Kr++,v 20' Kr + %

15 Fig: 15 -3 3 4.75.10 cm/s ID Fig: IB

12.8.103cm3/s Varc D ernes'1 •* I 1 I I 100 120 12 3 4 5 10 15

FJR.15 - Relative abundance of Kr F1P-1^ ~ Relative abundance of Kr ions ions vs arc potential in calutron vs pressure in calutron (ref.56). (ref.56).

3 For an arc current of 2 A, a pressure of 5x1o" torr, an induction of ++ + 220 gauss and an arc voltage of 80 V, the ratio I /I = 12%. If the arc current is increased to ,0 A (arc voltage :00 V), the ratio increases to Uh% -43-

In order to compare these results with the model of §2, we calculate the lifetime of Kr ions in the discharge in comparison with the ionization time in order to judge the importance of cumulative ionization. The following parameters are used :

p = 2x10 cm D = 5 mm 5 eV L = kO mm

T (weak magnetic induction)

2+ + Theoretical I /I Exp 1 ps h p a /af •*• Direct Step by step 2+ + o o I /I

2 3 80 1055 2055 7.5 % 12 % 10 3 16 10% 2055 38 % kk %

Cumulative ionization is therefore only of minor importance for 2+ + weak intensities, and the ratio I /I are comparable with the cross-section

ratio 2a /a' (single collision). This agrees well with the experimental re- ?+ + suits, and, furthermore, explains the variation of I /I with V (fig.15). arc As concerns the behaviour as a function pressure, the mean free path for charge exchange gives a value x « ~~Z~ for a pressure of 10 torr (minimum pressure for operation), which is in agreement with formula 17. Any further increase in pressure diminishes the mean free path and therefore causes 2+ + a reduction of the ratio I /I . This is indeed observed experimentally. 3.2. PIG source AXIAL MAGNETIC FIELD| 3i2iJi_Characteristics

The geometry of a PIG source CATHODE ANODE CATHODE is shown in fig.17. The magnetic induction, which can reach or depass 0.5 teslas, plays an essential role in the discharge. The relevant mechanisms are extremely complex, and no theory describes them in a completely satisfaction manner . In the Fig.17 - Schematic diagram of the Reflex high pressure mode (10 torr) arc or PIG geometry a plasma forms and the arc voltage is concentrated in asheath near the cathode. Because of the magnetic 58 induction, the motion of electrons toward the anode is slowed. Backus -44- suggests that plasma oscillations help not only the electrons to reach the anode, but also the ions. This explains the capability of the PIG source for ion extraction both radially and axially, from the ionization volume. The large number of variables makes comparison of experimental results difficult. It is, however, possible to distinguish

- Penning with cold cathode ; - Penning with hot cathode (direct or indirect heating (electron bombardment or heating by the discharge )). These two types can themselves be divided into axial extraction and radial extraction. Very complete review articles55 have been published on this subject. We limit the discussion to continuous operation and make the following comments - axial extraction provides an ion beam much more intense than radial extraction (one order of magnitude), but the average charge states is lower ; - sources with a cold cathode are better adapted to provide a beam of "solid" ions than are sources with a hot cathode, for technological reasons (interest of a sputtering source 59) ; - the charge state of sources in continuous operation is lower than for pulsed operation ,because of the lower electron density. Comparison of charge distribution 6? from hot cathode and cold cathode source is shown in fig.18. Table III from Bennett'' summarizes the performance of multiply-charged heavy ion sources. t cathode (62) 3i2i2i_Discharge_mechanisms The electronswhich lose energy on cathode (63) the first traversal of the plasma are Xe able to oscillate along the anode chamber. These electrons "trapped" in Fig-18 the discharge can make many more col- 0.1 1 3 5 7 9 11 lisions before finally reaching the : Fig-18 - Charge spectra from : anode. Bennett found that about 02 cathode source .. = 600 V; Ift=13A 63 7 to 8 ions per electron are produced 2) cold cathode source ; V = 2000V • A in a cold cathode discharge under arc I = 1 "5 A A ' Table III - Performance of multiply charged heavy ion sources with Nitrogen and Xenon (from ref.55)

Moan ABC COMITIONS OF IOH CUHKSHT IN CHAHOK STATS Pulse S»j>stitloj] Charge CAS soracs APTHOR Potential Currant Fovar Length Bate 10 12 State mB c/a 580 4.6 2.67 Continuous 25.2 39.9 29.0 5.5 •

Bide extraction . Paa/uk at »1 600 1J 7.8 1.8 5.5 14.2 16.0 17.5 16.0 13.7 11.0 2,7 .9 .2 .O6(.1=A)4.3 I Indirectly heated f Faayuk k Kutner,50 950 13 12.4 (Currant in BA 12 12.5 10-5 10 11 11 5 2.8 .9 .12) oathoda aide extraction aelf heated Bennttt35 660 2.5 1.65 Continuous) .8 7.1 20.9 21.0 19.s 14.6 11.2 4.9 4.0 XWOK cathode 42f 3 aide extraction Ohlorao at al * 2000 1.5 12 6.7 11.4 12.2 13.8 16.5 12.7 1? 0 6.9 4.2 ?,5 .4(26^*) 3.5 cold cathode "*• end. axtraotlon Isalla 4c Prelec 6000 .1 6 .1 20 22 6.5 1.9 1.3 .65 1.2 oold cathoda Iaalla & Pralec 4000 .8 3.2 ,1 20 22 30 15 10 2.5 1.0 (at and of pulaa) 2.4 -46-

drop of 2 kV with N2. The crossed magnetic and electric fields also generate a large number of instabilities which can possibly create a population of fast electrons (to 2 keV) with a density of the order of 10 cm . Unli- ke the calutron discharge, the ion current in the interior does not repre-

a'negligible^portion,because; ^constitutesrfrom-30% (hot cathode : rid^MorbzpV5'1) to 80% (cold cathode : Bennett55) of the arc current.

As concerns the influence of discharge conditions on multiply-charged ions,' the following conclusions can be drawn from the work of Pigarov and Morozov for a hot cathode source with radial extraction :

- there is an optimum pressure for the intensity of the extracted beam and for the average charge state ;

- the extracted current increases 2000 slowly with increasing arc current, but the charge state increa- Fig: 19 ses rapidly (K /K = h for

+ + IA = U0 A; while N /N < 0.02 for IA = 10 A (fig.19) ; 50 - extracted current increases i slowly with increasing dischar- Fig.19 - Variation of multiply-charged ion ge voltage, while the average currents vs arc current in hot cathode PIG charge state increases rapidly 5 e^ ; V = 335V ;p=2.8x10~i torr; 1) 5+ + source^ (K /N 1.5 for V_ = 800 V, 3 while N5+/N+ < 0.003 for V = Si 200 V) owing to increasing threshold of the successive ionizations. The work of Pigarov and Morozov clearly illustrates the ionization mechanisms and sec especially the influence of cumulative ionization. It can be seen in fig.20 that the charge states increase, one •FiK-20 - Variation of I (1).. V. (2) end after the other, to reach an output of multiply-charged K ions (3-N ; equilibrium for t > 10 us. k-N ;5-N ;6-N ) as a function of time According to the experimental during the pulse (ref 5I,). p = 2x10"^ torr values provided concerning the -47- discharge conditions, we have tried to determine the average charge state using the model presented in §2.

Let us consider the experimental conditions of the fig.7 of ref.54 : a discharge arc current corresponds to an electron current of about 8A..We. can estimate that an electron makes 30 oscillations before arriving a.t the anode which is equivalent to a current of 250 A, thus an electron density of about 2 2 6.10 A/cm . For argon, under these conditions, ve obtain

T = 10 Us (with L = 10 cm and = ^ ; T = '+ eV)

i6 2 T2 = 1.6 ps (with a g = 1.6x10~ cm ) Therefore the ionization is indeed cumulative, and the maximum charge state is given by 1*.3 < z^ < 7 . Experimental optimized charge state is z^ = 3 , but we must take into account recombination phenomena compared with the creation term.

Creation term for r Je 1 ,m,1/2 Z . 19 -3 -1 u { 2x1 0 cm s u „ . o " T f W I • monocharged ions -, Creation term for „ Je , x ,,,19-3-1 Gi*+ Kr{ax) n3+ : 10 cm s multicharged ions Radiative recombi- 2 .05x10 E nation rate a = (z-1,g) T 1/2 e Leading to a radiative k+ lT 3 1 = a n n~ 10 cm" s- destruction rate Three-body destruction 3 term (with Te = 1+ eV) 2.10 0 cm"cm s At the optimum pressure, the density of neutrals in the column is too low to lead to loss of charge state by charge transfer. It is clear therefore that the volume loss processes begin to have the same order of magnitude as the creation terms for the high charge states. The model presented can 61 therefore accound for creation and destruction processes. However, Fuchs has obtained theoretical results in very good agreement with experiment by considering cumulative ionization based on a maxwellian election distribution Necessary electron temperatures are between 70 and 700 eV, which seems to the authors, to be uncommonly high. The temperature determination could be perturbed by the population of fast electrons. -48-

^.?. Duoplasroatron ion source

This source is extensively used as a proton source of accelerators. But a few theoretical and experimental works have been devoted to the improvement of the source for multiply-charged ion production. The ions are extracted from a plasma created by a low pressure discharge (5.10~13 - 5.1O"-3 torr ) set up between a hot cathode K and an anode A. (fig.21). The emissive aperture is bored in the anode on the discharge axis. A high density (n =10 cm ) and highly ionized plasma is obtained in front the anode from a dual compression of the nrc, by the geometrical action Fig.21 - Duoplasmatron structure - K, of a third electrode IE and then heated cathode; IE, intermediate elec-

the action of strong magnetic field trode; A, anode ; Ext, extractor; Bz= set up between the IE and the A. A f(z) axial profil of magnetic field, systematic study of the discharge mechanisms and plasma properties has permitted the elaboration of a qualita- 52 tive model of the discharge similar to that exposed in 2. A fair agreement is obtained in the interpretation of the emissive properties of the source, The main facts are :

- The electron energy of the primary ionizing electrons is not an independent parameter of the source. This is related to the emission of these electrons from the cathode plasma - thus always in a space-charge mode (eq. 3a). The electron energy is a function of the neutral density n in the arc eq.16) : it depends mainly on the neutral pressure inside the arc and is affected by the starvation of the column as I. increases (eq.15).

- When the starvation mode occurs, the arc properties are :

. the floating potential of IE is close to the value corresponding to the maximum of the total ionization cross-section ;

. the extracted beam intensity is maximum (lM ^ p ) (fig.12) ; -49-

. the ionization efficiency is close to unity (n - 1) ; . the neutral density within the arc is minimum (n = n* : eq.17).

- Beyond the regime change-over-point, the arc potential increases abruptly where as the arc current density decreases, in relation with the increase of the arc diameter through drain diffusion.

The discharge conditions (low voltage and high current density) of this source seems favourable to step-by-step ionization. However, in the most commonly used duoplasmatron, the arc length is small (L~ 3 mm) and the ion dwell time is not long enough for this process to be efficient. Improvement of the source performance may be obtained by a proper choice of the length of the anode column and a consecutive matching of the operating parameters according to eq..19 - as demonstrated experimentally by the Heidelberg group 6k 05 'in agreement with the previsions of the model . In table IV are given typical arc conditions at regime change-over-point - and corresponding values of the ratio T /T ' ; then the calculated charge multiplicity from eq.26. Table IV

Arc conditions argon krypton B kG L mm p \ \ (A) 5 3 2 3. 6 0.1 1 0.2 1 5 U 12 0.6 1. 6 1.2 2 C J 10 2 12 1.2 2. 1 2.1* 2.8

3.6 8 U 18 1.5 2. 3 3 3 (ref.64)

The strong influence of the arc column length, clearly appears in this table ; the arc column length is slightly larger than the distance between A and IE.

Small_arc_column_length This geometry permits to obtain a high ionization efficiency with a low arc current at regime change-over-point, allowing for continuous duty operation. Fig.22 from ref.52 shows the influence of arc current and magnetic field or relative abundances of argon. But, stepwise ionization is not very efficient and the multiply-charged ion abundances corresponds approximatively to the partial cross-sections for -50-

;AT < 10 i •So.5 • / ! c !V £ 3 02 '/ ! \ "S u 01 0) "o U 05 \

02 ft Fig-23 3 D I 2 "0 'iV100 200 30r0 400 500 Pressure in 10 Torr Fig. 22 - Influence of arc current and Fig. 23 - Extracted argon ion magnetic field on relative abundances current vs pressure in duo- of argon. plasmatron (ref.23)

direct ionization. In the case of argon at regime change-over-point one ob- 62 tains : A+ 78% ; A2+ 20? ; A3+ 2%. A previously said the pressure affecting the charge spectrum as shown on fig 23 from ref.66. Below the minimum pressure, the abundances of doubly and higly charged ions increase, but the presence of instabilities may limit the 66 optical properties of the beam

The maximum arc current is then higher and stepwise ionization is predominant , if the parameters are properly matched. In the last line of table IV , are reported arc conditions from ref. 6k . Ar and Kr where then optimized in good agreement with the calculation. Where as the magnetic has only a slight influence on multiply-charged ions in PIG sources, it has a strong influence in the duoplasmatron because it determines the column diameter and thus the electron beam current density. A further shift of the spectrum towards higher charge states seems possible with a further increase of arc column length, however the matched arc current is then very high and the source have to be pulsed. An advantage of duoplasmatron over PTG source is the long lifetime of the heated cathode, i. method for direct operation with metal ions has been studied by Illgen et al , especially for multiply-charged ion production : a ring shaped vaporizer oven is placed around the magnetized arc column between the "intermediate electrode" and the anode. Good performances are obtained. -51- qt cathode 3.U. Magnetron This source, studied especially by B j the group at Belgrade is shown in fig. 2Ui. The cylindrical anode has on its axis a-hot filament cathoder Tiie DC r 1 Extractor anode voltagerproduces^theIradial;electric field in the interior of the cylinder. Fig.2U - Magnetron source geometry The source is set up in the axial magne- from the group at Belgrade (ref.66 tic field, sjnder the action of the elec- trie and magnetic forces, the electron trajectories can be represented by

B < Be, by spirals . Bc is the cut-off field and is given by 2 <8 n a -1 B ( \ ) c = e a o with r anode radius a : cathode radius anode voltage. If the induction B is greater than B , the electrons cannot reach the anode. Two distinct operating modes can be established : the Brillouin mode, corresponding to circular trajectories around the cathode ; and the bidromic mode, where all the electrons return to the cathode, ihis analysis is correct as: long as the Debye length of the plasma is of the same magnitude as the diameter of the structure. Beyond that, the plasma shields the potentials of the electrodes, and the analysis becomes much more complicated. None the less, the experimental results of Cobic et al show the existence of a cut- off, which is however at a much higher value than the theoretical cut-off. Solid materials -were introduced in the discharge by the cathode sputtering of metallic poles (Cu, Ag, Ni, Ke, Ti, i-to, W and Ir). The collector ion currents ranged from 100-500 uA. The fraction of multiply-charged ions was approximately X2+ : 25$ and X3+ : 3%- This seems to indicate that the predo- minate ionization mode is single collision.

A new structure of the magnetron source is actually developed at Orsay . The cathode is localized at one end of the cylindrical anode and the emissive aperture is bored in it. At the opposite end a reflector acts on the electrons (fig.25). Furthermore the operating pressure is low (- 10 torr) ; hence the production rate of secondary electrons is smaller than electron current Fig:25 emitted from the cathode. Beyond the

critical magnetic field Bc the electron FiB..2? - Magnetron source geometry path is very large ( > 10* m) and the from the group at Orsay (ref.67) -52- electron energy is fixed by the anode potential ( 2000 V). A high value of emitted ion current is then obtained (fig.26). Without ion trap- ping (which will be studied in the near future) the relative abundances of the multiply-charged ions Fig = 26 are proportional to the direct partial ionization cross-section. These operating conditions are very t interesting as the total consumed 1 2 power is smaller than 20 W. FJK.26 - Electron and extracted ion current vs magnetic field.

3.5. RF source

The rf source operates with a gas (10-10 torr) in a chamber to which an rf field is applied capacitively or inductively (20 to 200 MHz). The walls are non-conducting (quartz or glass) in order to limit loss by recombination. In a certain number of sources, an axial or radial mapnetic induction is applied in order to increase the density of the ionized medium . Komarov et R.1 have tried to adapt the rf source to the production of "solid" ions (fig.27) . The ion source operates on the vapor of fusible elements (P, Sb), solid compounds (AgCl, AlBr^), liquids (B Br,, SiCl^,

etc) and, of course, on gases (Ar, 02, Cl2, etc) with a current greater than 1 mA at a consumed energy of 1.5 kW, r.nd with a gas yield preater than 10%. Unfortunately, the authors do no give the yield of multicharped ions obtained.

The only work on rf sources from the point of view of multiply-charged ions concerns He and XeT°' 71. Valyi71 has obtained a He++ beam of 1.5 VA with a He /He ratio > 2% for a relatively low power (100 W). On of the most interesting aspects of this work is that the percentage varies from 5x1O~3 to 2.2x10 for oscillator frequencies from 3 to 120 MHz at constant oscillator power. It is difficult to reach a conclusion concerning cumulative ionization because the power injected in the plasma is not known. -53-

Fig.27 - rf source for " ions" (ref.69)

CONCLUSION

In this paper, we have discussed the main physical processes governing the behaviour of the magnetized discharge used in source for moderate charge states production. An elementary approach of the stepwise ionization has permitted the derivation of the necessary conditions - for operating and geometric parameters - in order to optimize a moderate charge state in the emitted ion beam.

The conclusions are in agreement with experimental results concerning the most currently discharge sources now in use. Whilst PIG sources are at present the most convenient sources for charge states up to 10, the choice is much larger for doubly and triply charged ions. However, it is necessary that thorough investigations of the mechanisms occuring within these latter sources are made in order to obtain a better understanding and a better description of the source properties such as : intensity, efficiency, nstabilities, energy dispersion, brillance and lifetime. Large improvements of the sources and separators performances will then result. -54-

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CHISMICAL STUDIES OF ION IMPLANTATION

A. G. Maddock

The Chemical Laboratories, Lensfield Road, Cambridge, England.

The environment, chemical and electronic states of ion implantated atoms have been investigated in a variety of ways. In principle, one would prefer in situ methods, such as electron spin resonance, angular correlation and Mtissbauer emission spectra.

The low concentrations of implanted atoms that can be achieved without gross modification of the matrix exclude all but the most sensitive of such procedures. In practice, most of these methods present

some difficulties in the interpretation of the results. Thus 57 although the Mtissbauer emission spectra of a Co implanted matrix provides useful information about the electronic environment of

the \Fe it produces by orbital electron capture, conclusions

regarding the environment of the cobalt require information, not at

present available, about the effect on the electronic state of the

cobalt of the electron capture events.

A cruder, but still useful, approach to the problem is to

explore the state of oxidation and chemical combination of the

implanted atom by appropriate conventional chemical methods. The

difficulties due to the small concentrations of the implanted species

are readily mitigated by implanting reasonably long-lived radioactive

species. Obviously using suitable etching or sectioning techniques

one can obtain both the geometrical distribution and the chemical

state of the implanted species.

The ion implanted solids can be regarded as a probably tneta-

stable solxd solution of the implanted atoms in the matrix and its -58- bohaviour .should reflect that^a very dilute atomic despersxon of

tlic implanted element in the matrix material. It is important

to note that the concentrations of the implanted clement in the

niutri* will generally be of comparable, or even lower, order of

magnitude to those of the natural radiation generated defects

in the matrix.

It is therefore not surprising that most of the chemical work

.so far published on ion implantation has been carried out by

chemists who had been studying the closely related systems that

can be Generated by nuclear transformations in solids. In many

nuclear reactions, including particularly the radiative thermal

neutron capture reaction, the affected atom suffers a sufficient

mechanical recoil to break all the chemical bonds, by which it was

previously bound, and to inject it into the surrounding lattice.

It seemed reasonable to expect that ion implanted systems should

closely resemble the related systems produced by nuclear transformation.

The larger part of the extensive body of results on the behav-

iour of radioactive atoms, or perhaps sometimes molecular fragments,

released in a crystalline matrix by nuclear recoil, related to atoms

produced by radiative thermal neutron capture. The effects could

only be studied radiochemically in matrices of some chemical .

complexity. For example, a potassium chromate crystal is about as

simple a system as can be used. For this reason some of the ion.

implantation studies have also been carried out in such complex

matrices. However, detailed physical models for such structured

polar insulators are not available and there are considerable

attractions in clioosing a better understood kind of matrix.

Fortunately there is also a lot of radiochomical information on the

32 behaviour of ^s and P, generated by the (n,P) and (n,a) reactions, -59- respectively, in alkali chloride crystals. For such crystals there is an adequate framework of theoretical knowledge of the matrix and single crystals of these materials are readily available.

I shall begin therefore with a brief account of the correlation between the behaviour of ion implanted and nuclear recoil generated atoms in complex solids and then consider the behaviour of sulphur and phosphorus in. the alkali chlorides in rather more detail. Ion implantation in complex solids I do not want to get involved in a lengthy account of the effects of neutron capture in crystalline solids; numerous reviews (l 2) arc available on the subject^ ' '. The radioactive product is isotopic with one of the atoms of the molecules of the target matrix. Very often one can represent the target species as ML , which may be either a neutral or ionic species. The atom M yields the radioactive *M on neutron capture. The neutron irradiated solid is subjected to radiochemical analysis to determine the state or states of chemical combination and oxidation of the product *M. The overwhelming majority of analytical procedures require dissolution of the irradiated crystals. This confronts one with the major difficulty of this technique of investigation: the relation of crystal precursors to the species found after solution. Some of the chemical fprms in which *M is present in the lattice are species of very high intrinsic reactivity and will probably react with the solvent at the moment ,of dissolution. So that the products found may not be identical with those present in the crystal, although they may be expected to bear a unique relation to them. For example,.with, a permanganate salt a substantial part of the •• radioactive, manganese ,is found after,-solution as manganous ions. But both atomic manganese, and the singly charged cation arc -60-

sufficiently reactive to reduce water and yield nionKanous ions, .so that some uncertainty exists regarding the identity of the crystal precursor of the radioactive raanganous ions.

The problem of reactions on solution is further aggravated by the very low concentration of the species *M and by the possibility of reactions with point defects at the moment of solution.

It is true that with some ingenuity, for example by the use of non-aqueous solvents and other means, a more rigorous identification of the crystal processes may be possible, but the chemist has been rather slow in obtaining a solution of this essentially chemical problem.

Some proportion of the radioactive product is xnvariably found in the form of the matrix species and for this part there is hardly any doubt that the crystal precursor must be the same chemical

species. Since solution and gas phase studies show that the nuclear

event is almost always followed by molecular rupture any appreciable yield of the radiocictive matrix species must be due to rapid

reformation following the fragmentation.

The characteristic and chemically interesting aspect of these

neutron irradiated crystal systems is that the reformation reactions,

yielding the radioactive matrix compound, can be induced after the

neutron irradiation by heat, light, ionising radiation, pressure

and other means. These reactions have been shown to be a type of

exchange reaction that can be represented *M + ML —> M + *ML and n n

which are practically unknown in solution. The exchange process

must involve both transfer of ligands and one or more electronic

processes because the oxidation state of *M is generally lower than

that of M in MLn. Perhaps because of the redox^these reactions are

usually very sensitive to the concentration and nature of the -61- dofects in tlie crystal matrix.

The kinetic aspects oi" these reactions are also unusual and rather reminiscent of the annealing of other kinds of radiation damage. The processes are first order, but the energy of activation, and, possibly, also the frequency factor, appear to have a spectrum of values.

A number of ion implantation experiments have therefore been designed to explore how far the implanted systems resemble the ( o\ neutron irradiate materials. The first reportK ' used ion implantated Cr in a potassium chromate matrix because a part- icuarly extensive range of data is available on the behaviour of the neutron irradiated materials. Implantation energies of from

5 - ^50 keV were used with Cr+ and Cr + beams. High specific to , » activity chromic oxide was convertedichloride in the source* .

Irradiations were conducted at room temperature and the implanted crystals contained from 10~^ to lf^Ci of 51Cr. About 80$ of the

Cr was found in the implanted crystals as chromate compared with about 60$ in a neutron irradiated sample of the same crystals. The

remainder Was faund in the solution in the form of the monomeric aquated chromic ion and the various related polymeric aquo cations

formed by CrIII. The latter indicated the formation of Cr-0-Cr

linkages in both systems.

The ion implanted and the neutron irradiated material showed

qualitatively, but not quantitatively similar annealing reactions.

The *Cr was also found as chromate after solution following

implantation in KC1O. , KgSO. and KgBeF, , although in decreasing

yields.

A more recent comparison of the annealing of ion implanted

and recoil generated Se in potassium and sodium selenates has -62- shown that the fine structures of the annealing isochronals obLained are the same Tor the two treatments. From the chemists point of view these are still reasonably simple substances and although the reaction of the recoiled 'DSe atom with its surroundings

= to reform "^SeO,4 is unusual it is by no means incredible. But the much more remarkable annealing reactions found to follow neutron irradiation of much more complex substanced are also duplicated by ion implanted systems. Curiously, two independent groups nave verined such correspondan.ee with practically the same system ' ' .

One of tne surprising features is the stereospecificity of the reaction following neutron irradiation. The same characteristics have been found for ion implanted systems. Thus ion implanted

Co in trans Co(C_H, N.,11. ) ,,C1 .N0_ becomes incorporated as the 2 4 2 i( 2 id 3 trans complex, as does the radiocobalt formed by nuetron capture.

With the salt of cis configuration the radioactive cobalt, introduced in either way, is preferentially incorporated as the cis complex. In all cases there is some formation of the other isomeric form, but the preferences of the trans for incorporating in the trans form is much more marked than that of the cis as cis complex. (Table l). Experiments with different energies of ion beam and different doses of deposited kinetic energy in the matrix showed that the stereospecificity decreased as the energy deposited increased. -63-

TABLE 1 ION IMPLANTED

Target Compound Ion Energy Labelled trans Labelled cis

t rans[Co engC±2]NO 20 keV 33.3 1.8

60 keV 23.3 i+.O 20 keV 11.7 18.5 60 keV 12.1 12.0 NEUTRON IRRADIATED*

After lh at 100°C

Target Compound Labelled trans Labelled cis trans cis trans[Coen2Cl2]N0 7.3 0.2 48.0 0.2

0.1 3-1 0.1 2t2.9

* Data from II.H. Rauscbci", N. Sutin and J.M. Miller, J.

Nuclear Chem. 12., 378 (i960) .

In the same paper it was shown that ion implantation with radioactive copper of the p crystalline form of copper phthalocyanine gave a much greater yield of the labelled complex than did the a modification, an analogous result to that found on neutron irradiation. Implantation of Mn in Cr(CO)^ has shown^ ' that a small proportion of Mn(CO) and Mn(CO). radicals are produced. The yields are only one or two percent of the implanted activity. Very possibly a mixed manganese chromium carbonyl is also produced, but the analytical procedures used might not reveal such a product.

These observations serve to confirm that intruding foreign atoms or ions react in a crystal matrix in much the same way however they are introduced into the lattice. Although such studies -64- have revealed a great deal of interesting qualitative information about the curious behaviour of such systems they seem too complex for really satisfactory quantitative study.

Ion Implantation in Simple Polar Solids

The simplest, type of solid for which much data existed on the behaviour of foreign atoms produced by nuclear reactions was the 35 alkali chlorides. Reactor neutron irradiation produces both S and -*2P in such crystals. Experiments made very many years ago

showed that both these species appeared in a variety of chemical

forms after solution of the irradiated crystals, including their

oxyanions, especially sulphate and phosphate. 'ihesc observations

started a controversy about the identity of the crystal precursors

of the entities found after solution, and, indeed, the answer to

this question is still incomplete, particularly in the case of the

phosphorus species.

For the Cl(n,p) "'S reaction if the proton can be considered

to travel far enough away that its eventual neutralisation does not

affect the electronic balance in the vicinity of the sulphur atom,

a chloride ion might be expected to yield a sulphide ion, S~. By

the same argument for the Cl(n,a) P reaction a chloride ion 32 3- /

might yield a P . ^ But the charge state of the recoiled atom

once it reaches a metastable lattice position will probably depend

on the nature of this site. Electrostatic balance will tend to

ensure that cationic lattice sites will be occupied by cationic or

neutral phosphorus or sulphur. For similar reasons, as well as

size considerations, interstitial species are likely to be cationic

or neutral, while anionic species are likely to be restricted to

anionic sites. The situation will, however, bo further complicated by covalent -65- bond formation between the sulphur or phosphorus and the chlorine in the lattice. Because phosphorus and sulphur are not very- different in size from chlorine one might expect that the phosphorus ? and sulphur containing analogues of the V centres, such as, SC1~ and PCI ~, might be formed, particularly if hole donors are available in the matrix.

Even, the question whether formation of some P-0 or S-0 bonds can be formed in the crystal matrix needs some detailed consideration. Unless special precautions are taken in the preparation of the alkali chlorides crystals (fusion under an atmosphere of HC1 gas), they will contain a small amount of 0H~ and under the influence of the ionising radiation, almost inevitably accompanying the neutron irradiation, these ions yield oxygen species which might react with the phosphorus or sulphur atoms or ions. In addition alkali chloride crystals have been shown to absorb oxygen and form super-

oxide ions, Cl + 0 ^Cl_ + 0 , which can migrate in the lattice at moderately elevated temperatures. Fortunately both these possibilities can be avoided by the use of material use first under HC1 gas and then an inert gas. One might suppose therefore that the foreign atoms would be present as simple monatomic anionic, neutral or cationic species of very low charge or, perhaps, as analogous species to the V centres found in these crystals with one chlorine atom replaced by phosphorus or sulphur.

But the reactions of these species with an aqueous medium still

presents problems. Neutron irradiation is always accompanied by

ionising radiation and the creation in the matrix of F and V centres

and their associates. These may react with the phosphorus or

sulphur species at the moment of solution, possibly at the crystal- -66- solution boundary. One .night expect oxidation of the more reducing phosphorus or sulphur species by the defects formed by the radiation during solution. To avoid this complication the composition of the aqueous phase chosen for solution of the crystals should be such that it reacts more rapidly with the defects.

Further the reaction of atomic sulphur and phosphorus with aqueous solution are unfamiliar. One can indeed calculate the free energy changes for most of the possible reactions, but one has little information about kinetic factors.

it is desirable to choose a reactant solution that, converts

-^s", "^S", 55S°, ^5S+ and the 35S containing; V centres to 32 separately identifiable species; similarly for the P species.

For sulphur a suitable solution which goes some way to reaching this (q) objective has been developed '. It consists of a strong solution

of potassium cyanide containing carrier S~, CNS , SO," and SO.

Interferring effects due to the F and V centres are eliminated.

The 'S~ appears as sulphide, atomic "S as CNS~ and two of the V 3 5 _ T r centre analogues hydrnlyse to yield SO and J'S0 ~. The precise

identity of these two precursors is still uncertain and the fate 35 - of S is debatable; probable it exchanges with and appears in the sulphide fraction. 32 The analysis for the P is on a less satisfactory basis.

It generally appears as phosphine, hypophosphite, phosphite and

phosphate. Recently my group has published data, based on analyses

in non-aqueous solvents, showing that the accepted distributions

amongst the above species overemphasises the proportion of

oxidized products and that much of the 32P is probably really

present in the crystal as anionic or neutral phosphorus.

However, for the purpose of comparison of ion implantation -67- and generation by nuclear reaction data obtained by the earlier analytical procedures is still significant.

The first results reported by Andersen and S^rensen^10' showed that there was considerable similarity between the behaviour of op ion implanted and neutron generated P in KC1 (Table II). TABLE II 35Cl(n,oc)32P 70 keV P+ in KC1 PV 43 .'16

27 P1 30 34 Auout the same time a similar study of the distribution of S species in ion implanted and neutron irradiated NaCl was reported^ ' (Table III) . TABLE III Proportion of S Mode of Production S= CNS~ SC " SO.= 3 h 35Cl(n,p)35S 46.1 9.6 21.5 22.8 Ion implanted 46.4 26.4 17-7 9-5 Having shown that there is at least a qualitative similarity between the two kinds of system it becomes useful to examine the quantitative differences more closely. The local concentration of sulphur or phosphorus atoms is generally three or four orders of magnitude greater in the ion implanted than the neutron irradiated material. In both cases the matrix suffers radiolytic damage. The total dose received is greater in the case of the neutron irradiation, because of the ionising radiation accompanying reactor irradiation. The recoil of the nascent sulphur or phosphorus atom in the neutron irradiated material is of the same order of magnitude as the implantation -6 8- enerey. So that the local radiation damage to the system should be- proportional to the concentration of foreign atoms and thus be hi/jher in the ion implanted material. However, in neither case will there be a great deal of overlap of the tracks of the recoiling or implanted species so that the local environment of the thermalized atoms or ions should not be very different for the two kinds of systems. But the most deeply penetrating of the ion implanted species should reach regions of coniparitively low radiation damage and this will be still more marked for those ions penetrating the matrix very deeply by a channeling process, with an appropriately orient-

.»ILH1 matrix crystal.

In either case the concentration of the foreign atom used is less than, or at most comparable, with that of the less common defects in the matrix crystal, such as the anionic vacancies.

First we shall consider the overall behaviour of the foreign atoms and then we shall treat the range effects.

PL Inferences in Behaviour of Ion Implanted find Neutron Generated Species 35 32 l'or both the S and P, ion implantation produces a smaller proportion of the oxidised species than is produced by neutron

In both processes the distribution of the sulphur or phosphorus

is very dependent on the purity and content of imperfections of the

crystal matrix used. Reproducibility is very difficult to achieve

with different preparations of crystals and most of the conclusions

have been drawn from comparison experiments using mirror image

pieces of cleaved crystals.

Pre-irradiation of the matrix crystal with ionising radiation

lias a much greater effect 011 the neutron irradiated than on the

ion implanted crystals. ('fable IV) . -69-

TABLE 35 32 Cl(n,a) p 60 keV 32p in KC1 V pill 1 Dose P P PV ] P1 0 Mrads 50 15 35 30 16

20 " 19 12 69 13 13 57 100 " 8 Ik 78 15 10 75 Similar results have been obtained for the ^"'s.

The effect of heating the JJS implanted crystals is quite different from that on the neutron irradiated materials. Results are shown in Table V for NaCl„ TABLE

35S Distribution s Treatment S~ CNS— v 1. (n,p) produced 50.8 10. 7 27.7 10.7 2. " annealed Ih at 205°C 65.0 1.9 10.3 22.7 3. Ion implanted 68.6 21. 3 7.7 2.3 h. " annealed In at 2O5°C 22.3 57- 0 15-9 4.8 32 But both kinds of P impregnated KC1 seem to behave in the same way.

In both systems evidence for reactions between the foreign atoms and F and V type (electron or hole donating) centres in the matrix can be obtained. Thus if the electrons trapped at P centres are released by optical or thermal bleaching the proportion of S or

P appearing in low oxidation states (S~ and P , hypophosphite) increases:(10,12-15 ') An ionising irradiation of an ion implanbed 35S in NaCl crystal produces a comparatively small modification of the 35S distribution, but on thermal annealing of' such, a crystal a substantial increase

35 = 35 = in the SO and SO; yield occurs (Table VI). -70-

TAULE VI

35S Distri but i on s s~ 3NS~ so - Troatmcnt 3 v 1. Ion implanted JJS/NaCl crystal 55. 7 25.9 15- 3 3.0

2. " annealed lh at 205°C 8. 2 40.4 46. 0 5.6

3. " irradiated 10 Mrad. 45. 0 32.9 18. 2 3-8

4 . " " and annealed Ih 7. 6 32.4 48_ 2 11.7 at 2O5°C

This suggests that the reaction of the S~ and CNS precursors, presumably S=, S~ and S°, with the V type centres requires

thermal activation. 32 (l3 15)

However the data for P implanted crystalsv ' seems difficult to accommodate with this explanation of the orvigin of

the phosphate and sulphate fractions (v. infra).

The apparatus that was used for the S implantation^ ' had previously been used with various chlorine compounds and it proved 3 5 impossible to decontaminate it completely from a CT beam. This meant that the effect of the "^Cl/ S ratio in the beam used had

l.o be explored and siifyfyon ted the idea of other experiments in which two species were implanted. Results for various -^^

ratios and for pre-implantation of chlo-ine, oxygen and 32S before 3 5 the •'^S are shown in Table VII. -71-

TABLE VII (-U)

S Dis t ri but Lon = Treatment s~ CNS~ so3 ih 1. 35C1/135S 1.1.102 71. 9 12.2 12. 4 3 .3 2 2 . " 5.5.10 63. 6 15.3 13. 7 2 .3 3 3. " 8.1O 54.5 25.5 16. 5 3 .3 annealed lh at 2O5°C 8. 2 40.4 46. 0 5 .6 5- Pre-implanted Cl 45- 5 21.2 25. 8 7 • 5 6. " " " annealed lh at 2O5°C 8. 9 30.2 46. 8 ik .2

7. Pre-implanted 0 37. 0 46.2 11. 7 5 .0 8. " " " annealed In at 21. 0 6k. 1 10. 3 4 .6 205°C 23. 4 63.5 10. 9 2 .0 9. Pre-implanted S

10. " " " annealed lh at 9. 3 75.1 13. 1 2 .6 2O5°C 35C1, 1 0 and 32S 101 atoms/cm2 crystal.

These results showthat only a very large excess of chlorine atom implantation has much effect on the 35S distribution. With such excess, a greater proportion of SO ~ and SO. ~ precursors form on thermal annealing.

Neither pre-implanted 0 or S much affects the proportion 35 __ of S as the SO,, and SO. precursors,but both treatments increase

the proportion of S° (CNS~ fraction) at the expense of other

S=, This might be due to the 0 and S occupying nearly all the sites which can accommodate 35S" =« Range Effects A particularly interesting feature of this technique of investigation is the possibility of exploring the way in which the -72- distribution of forms of 35S or 32P change with the distance t.he iniplnntnd ion has penetrated the crystal. Results have boon reported both for 'J5S and j2P in NaCl and KC1, respectively. The phosphorus work^ used an improved etching-stripping technique to remove successive layers of the chloride crystal for analysis. In essential features the results Tor the two systems are in agreement and they raise a number of interesting questions. It is clear that the ranges of3j S and ' P in these crystals are considerably larger than those normally predicted„ It is found that the penetration of the ions (usually of between 20 and 60 koV) into the crystal is complicated by both diffusive effects and,probably movement along dislocations. Indeed a small but easily 35 measurable proportion of implanted S, using a Zj.0 keV beam, penetrated a one millimetre thick KC1 single crystal^ + ' ( ~ 0.2$)„ The geometrical distribution of implanted phosphorus in KC1 changes with storage time even when the implanted samples are stored at liquid nitrogen temperature (13)'. The depth within which the TO deepest penetrating 40$ ofJ P is found at first increases, then decreases and finally increased again during storage at room temperature. The distribution between the different species also changes with the depth of penetration of the ions. With both 35S and ^2P there appears to be an increase in the proportion of the more oxidized species, the sulphate and phosphate precursors, for the most penetrating ions. There also appears to be some correlation between the mean penetration of the ions and the proportion of the PY precursor. This suggests that this species must be very mobile, even at rather low temperatures, which is hardly compatable with its identification as a covalently bonded V centre like species and -73-

Ls perhaps more in keeping with an interstitial cationic phosphorus

ion.

A lot more experimental work seems desitable on these effects. In conclusion it would appear that chemical studies of ion implanted studies can yield interesting information about the behaviour of low concentrations of foreign atoms in a crystalline matrix and that much more will be learnt from further work of thxs kind.

Insert an page no 64

A However shake-off following the nuclear event will reduce this charge by one, or occassionally more, units. -74- REFERENCES

1. H. Muller, Angow. Chem, , 6, 13J 2. A.G. Haddock and R.H. Wolfgang, in "Nuclear Chemistry" Ed. L. Yaffe, Academic Press, N.Y., 19^8 Vole XI, Chap. 8. 3. T. Andersen and G. S/irensen, Trans. Faraday Soc, 62, 3427, (1966) k. T. Andersen and G. S^rensen, Nucl. In.st. Methods, _3_8, 204 (1965) . 5. M. Cogneau, G_ Duplatre and .T.I. Vai^gas , J. Inorg. Nucl. Chem,

2h., 3021 (1972). 6. T. Andersen, T. Langvad and G. Sj4rensen, Nature (London) 218, 1158, (1968). Corr. Nature (London) .219, 5hk, (1963) . 7. G.K. Wolf and T. Fritsch, Radiochira. Acta, la, 194 (l96"9) •

8. G.M. Jenl

9. A.G. Maddock and A.JO Mahmood, Inorg. Nucl. Chem. Lett., _9_,

509 (1973). 10. T. Andersen and G. S^rensen in "Interaction of Radiation with Solids" Ed. A. Bifhay, Plenum Press, N.Y., 1967 p.373. lie J.H. Freeman, M. Kasrai and A.G. Maddock, Chem. Coram., 1967, 979. 12. A.G. Maddock and R.M. Mirsky, Proc. l.A.E.A. Conf. Vienna 1965 STI/PUD, Vienna 1966, Volume II p. hi.

.13. T. Andersen and A. Ebbcsscn, Trans Faraday Soc., 67_, 35ifO (1971) . l/l. M. Kasrai, A.G. Maddock and J.H. Freeman, Trans. Faraday Soc, 67, 2108 (1971).

15. To Andersen and J.L. Baptista, Trans. Faraday Soc., §2_, 1213, (1971).

16. T. Andersen and A. Ebbssen, Radiation Effects, 11., 113, (1971). -75-

NEW TRENDS IN THE USE OP ACCELERATORS IN SOLID STATE PHYSICS

C-. Dearnaley,

Harwell, England.

ABSTRACT The new developments which will be discussed will include (1) the growth of ion implantation in areas other than semiconductor devices, and (2) the use of ion accelerators as increasingly sophisticated probes of solid surfaces. Ion implantation of semiconductors has been carried out with currents ranging from a few (jA to several mA, and equipment capable of producing these beams is now commercially available. Most other materials require substantially larger doses of impurities in order to modify their corrosion behaviour, mechanical properties, magnetic properties, etc. New demands are being placed on the accelerators with which to carry out research and practical applications in this field. These requirements will be discussed in the light of experimental results obtained in work on the implantation of metals and alloys. Ion backscattering and ion-induced characteristic X-ray measurement have become established probing techniques in solid- state physics. Because of the quantitative nature of the data, and the depth resolution which can be achieved, these methods are increasingly widely used. It has now become possible to carry out these experiments with highly-focused ion beams, sometimes only 4 (im in diameter. Applications of such techniques in solid-state physics and micro-electronics will be discussed. -76-

TNTRODUCTION The number of ion accelerators being marketed for solid state physics research and its applications, such as ion implantation, now exceeds that for all other purposes. According to manufacturers• figures world-wide sales will soon surpass 100 per annum. This development has come about rather rapidly, as a result of an earlier phase in which research was carried out using existing accelerators and isotope separators, employed for new purposes. It is therefore timely to examine the trends taking place in this research field in order to discern the types of development which may follow in the accelerator design of the future. For this purpose I shall not restrict the energy range considered to that of conventional mass separators, but will also give some consideration to accelerators capable of operating at a few MeV. The term 'physics' will likewise be interpreted somewhat broadly to include the technological development of ion implantation which has, at any rate in the case of silicon-baaed semiconductor devices, reached the state of vigorous industrial exploitation. The objective in such work is the modification of the composition of a target specimen, near its surface, by the chemical doping effect of an accelerated beam of appropriate ion: . As such, it is not so very different from the use of a mass separator to produce targets containing specific stable isotopes for nuclear physics experiments: this application was discussed by M.L. Smith in the very first of this series of conferences, held in 1955. Rather more understanding of the process is demanded in ion implantation, of course, in order to achieve the the desired electrical or other properties of the solid, but experimentally there is a close parallel. The investigation of radiation damage in reactor materials has been going on for about three decades in many laboratories, but the development of fast reactors raised new problems due to the greater effectiveness of fast neutrons in displacing atoms. An important use of ion accelerators has proved to be the rapid simulation of damage processes, such as void formation, which would occur over a period of three decades in a power realtor. -77-

ION IMPLAUTATION OF SEMICONDUCTORS We have now reached the point where the techniques of ion implantation in silicon device fabrication are very well understood, the advantages of the method from both a technological and economic point of view are becoming firmly established, and equipment for carrying out the process on an industrial scale is commercially available (figure 1). It is now clear that

Pig. 1. An 80 keV ion implanter for industrial use. (courtesy of Lintott Engineering Ltd., Horsham,England.

the precision, versatility and controilability of ion implantation^ ' are valuable not only in MOS transistors and integrated circuits, but also in wholly ion-implanted bipolar devices, as well as in a number of specialised structures such as variable-capacitance diodes, IMPATT oscillators, etc. Since bipolar devices still account for the major part of the market, this trend is very important and will further accelerate the trend towards ion implantation. This development has come -78- about through the realisation that high doses of implanted arsenic atoms can be driven in by a subsequent diffusion to produce a desirable emitter profile in undamaged silicon. The dependence of arsenic diffusion upon concentration has been exploited, at Bell LaJos.K ' and elsewhere, to avoid the problem of penetrating tails in implantation profiles that we have shown to result most probably from scattering of ions into channels. Since the dose levels necessary for this work are 16 2 around 10 ions per cm at 50 to 100 keV, and since the numbers of wafers are large, there is clearly a need for high-current arsenic implantation. The machine shown in figure 1, developed from a Harwell isotope separator^ ' is capable of delivering 5 mA As+ beams at up to 80 keV, besides beams of a few tens of microamperes of some triply-charged species, i.e. at up to 240 keV. It is not surprising, therefore, that the demand for such machines is high. The industrial process described requires the efficient handling of batches of several dozen silicon wafers, now usually 75 nun in diameter. The vacuum engineering to meet this problem has been very successful, and wafer batches are comparable with those processed in other stages of manufacture, such as epitaxy. In some operations on MOS devices the implantation time per wafer may be as short as 1 second, and so some manufacturers are investigating the feasibility of feeding silicon wafers continuously into the vacuum chamber. Target chamber design will be an important aspect of future implanters. To a large extent, research in implanted silicon device technology has moved away from the accelerator-oriented laboratories in which it commenced into the device manufacturing organisations. This is particularly true in the USA and Japan, where it appears that ion implantation is almost synonymous with semiconductor device fabrication. A^ a result, although -79- there are still certain problems to be cleared up, there is no more pioneering physics to be done, and the main forefront of activity ia how taking place behind closed doors. This is a natural evolutionary development that marks the success of earlier research. Of course, this stage has not yet been reached in compound semiconductors, and the research being carried out at universities and other non-industrial centres is still important in removing certain obstacles which stand in the way of industrial exploitation. For this work a greater variety of ions is needed, drawn from groups II, IV and VI of the periodic table, since so far only the HI - V semiconductors show promise for device applications. Some outstanding problems in the ion implantation of semiconductors lie in the area of precise measurement and control of ion beam intensities, i.e. in the engineering aspects of the technology. Intercomparison of results achieved on different accelerators soon shows that all is not well with some of the equipment and methods in use. Calorimetry appears promising as an absolute means of calibration, while novel ways of monitoring an ion beam without intercepting it are being explored^ ' as part of a closed loop stabilization of the beam current. There is a need for highly uniform large-area beams with which to flood masked wafers. Steps towards the computer- controlled fabrication of masks, by means of focussed electron beams, are being made but it will be some time before these can replace phqtolithography and wet chemical etching. The most likely alternative appears to be the nucleation of metaj. vapour deposits on to a treated semiconductor surface as a result of localised electron bombardment in vacuo. This metal layer can then act as a mask for ion implantation, after which it may be removed by sputtering or thermal evaporation, e.g. during the annealing of the implant. This, or some equivalent process, would allow simple transistors to be produced automatically without the risks and expense of open air manual handling. -80-

ION IMPLANTATION OP METALS We have seen that the accelerator requirements of ion implantation in semiconductors arelargely met by existing designs. When we turn to other areas of materials technology this is not so true. Yet the ion implantation process offers the means of altering, controllably, the surface composition of almost any material so as to convey interesting properties. The results which have been obtained in this wider field make it appear a promising area for exploitation. The properties which can be modified by ion implantation in metals include:- corrosion behaviour, electrochemical properties, wear resistance, coefficient of friction, bonding ability. These are all surface properties, dependent upon the nature of the material within the topmost few microns. One must consider those applications for which there is a conflict between the bulk and surface material requirements, and in which the bulk composition is determined by such factors as cheapness, lightness, machinability, strength, nuclear, thermal or electrical properties, etc. A different surface composition is frequently achieved by a coating technique, 3uch as electro- plating, spray coating, application of a paint or bonded dry lubricant-, etc. These methods are sometimes inadequate, usually because of the problems that can arise at the interface. Another important way in which to view ion implantation is as a highly versatile research technique by which to investigate the effects of different additives on surface behaviour, without the difficulties of preparing alloyed specimens with similar grain structure and other parameters. It may thus serve as an exploratory study, after which some other means may be found of incorporating the desired species. Such studies can throw new light on the mechanisms of corrosion -81-

?urface lubrication, an 1,are becoming recognized as a valuable research tool. Sometimes a remarkably small addition of a certain impurity will strongly influence the corrosion behaviour. Thus yttrium has been known for some time to have thisteffect in steel, and recently Antill etal> ' have demonstrated that a shallow layer of ion-implanted yttrium can be as effective as yttrium alloyed throughout in inhibiting high temperature oxidation of a stainless ateel in C02. In this case the implanted dose was 15 2 only 3.5 x 10 'J ions per cm (figure 2). Such a surface treatment avoids the drawbacks of alloying yttrium, such as reduced tensile strength and ductility which are important in the nuclear fuel-cladding application. There are other high temperature applications of stainless steels where this process

O JO/25/Nb Q 30/25/Nb OI3»/o Y ALLOY Q 2O/25/N& O4I°/DY ALLOY • 4 V IMPLANTED 2O/25/Nb

I I O I00O 2OOO 3000 4OOO SOOO FIG 2 TIME |li| EFFECT OF YTTRIUM. EITHEB AS AM ALLOY ADDITION OR IMPLAMTED IN THE SURFACE. UPON THE OXIDATION OF 3Of25/Hb STAWLESB STEEL IN CARBOH PIOX1PE AT SOO'C -82-

could be beneficial. In a parallel study, Dearnaley et al. ' carried, out a survey of the effects of ion implantation-of..various impurities into both titanium and 18/8/1 stainless steel upon their high temperature oxidation in dry CL. Preliminary results indicate a correlation between the electronegativity of the impurity ions and the effects upon oxidation. This was a novel result, which may be understood on the basis of a model involving oxygen ion migration along grain boundaries of the metal. Equally novel was the finding that the effects were reversed in the case of the two metals: every ion that inhibited oxidation in titanium would enhance it in steel, and vice versa. This is a good example of the way in which ion implantation can be used in corrosion science. The research is now being extended to aluminium and zirconium.

% Reduction

50 i-

301

Fig. 3. The effect of calcium implantation on the oxidation of titanium in oxygen at 600°c, as a function of the ion dose (from ref. 7). -63-

In marine corrosion there are also potential applications. Grant et al; ' have studied the effect of ion implantation upon the electrochemical behaviour of an aluminium surface, and it appears that even inert gas bombardment produces strong effects. This is analogous to the behaviour of copper, in which bombardment with any ion species appears to inhibit atmospheric tarnishing. Presumably both these phenomena result from a damage process, perhaps inhibiting vacancy transport near the metal surface. Mechanical properties of ion implanted steel have been studied by Hartley et al> ' and striking changes in the coefficient of friction under dry lubrication conditions were observed. These effects are probably linked with processes of adhesion between asperities under high pressure between the bearing surfaces. Figure 4 shows the contrast between the results of implanting tin and lead into steel.

SURFACE TRACK LENGTH

SuHac* vtoek

Fig. 4- Contrasting effects of Sn+ and Pb implantation on the coefficient of friction between En 352 steel and a loaded tungsten carbide ball (from ref. 9). -B4-

It seems possible, by ion implantation, to modify the * tendency to adhere and so alter the frictional force. In other applications, such as diffusion bonding, it is desirable to enhance the grain growth across the interface at high temperature and pressure. Clearly these twc problems are related. Hartley is now extending these measurements to wear between ion implanted surfaces: wear is a complex phenomenon involving other factors than friction, and it is often related to corrosion in the sense that the high temperatures generated accelerate the production of a corrosion film, which itself wears and can produce abrasive debris. ION ACCELERATORS IN VOID STUDIES A most successful simulation of the long-term effects of fast neutron irradiation, for instance in creating voids in metals, has been achieved by utilising the high rates of atomic displacement feasible with ion beams. The objective here is to introduce damage without altering the target composition, i.e. the anti- thesis of ion implantation. This research has created a need for energetic beams of metal ions capable of penetrating well below the surface, which acts as a sink for migrating vacancies. Ion sources providing target currents of several microamperes of metal species such as Ni , Pe and Al and designed to fit conventional Van de Graaff accelerators are now commercially available from companies in Britain and the USA. Since void nucleation is enhanced by embryonic gas bubbles, e.g. of helium produced by (n,a) processes in a reactor, it is necessary to implant this helium. For the most realistic simulations it has proved desirable to irradiate with high energy metal ions and to implant low energy He+ ions simultaneously by coupling together two accelerators. The beams are adjusted to provide the correct ratio of intensities and a comparable penetration. -85-

ACCELEHATORS FOR METAL IMPLANTATION For the purposes of exploratory research into the effects of ion. implantation Sin metals and alloys one needs an ion accelerator capable; of delivering adequate target currents of many different ion species. There are many such accelerators in existence, the most efficient ones being based upon versatile isotope separator designs. These are usually limited in energy, but lend themselves to schemes such as that at the University of Surrey1* ' in which a commercial (Lintott) ion implanter is to be raised in potential by several hundred kilovolts so as to provide intense high-energy beams. Alternatively, the target specimens may be at a high potential in a suitable target chamber. In the practical application of ion implantation to metal components, economic considerations will impose a considerable restraint. For this reason, we may expect the first steps to be made with relatively small components which are either high-cost or may be crucial to the performance of a systenr '. Thus in aerospace, nuclear energy, underwater equipment, advanced motor engineering and the military field environments are found in which high reliability is demanded, and maintenance is difficult. Any extensive application of ion implantation in metals to these or other fields will almost certainly call for still higher beam intensities than are available even from the most powerful accelerators (figure 1). However, in most engineering work there is also a need for deep penetration since a composition modified to a depth of a micron, which is a deep implantation, is still shallow by the standards of conventional coating techniques. Yet if high beam intensities are combined with high ion energies the equipment becomes excessively expensive and cumbersome, and power dissipation in the target very soon becomes a problem. -86-

Research at Harwell is concentrated upon a possible way round this problem, by taking advantage of radiation enhanced diffusion of implanted material. This may be brought about as a 'result of a controlled combination of beam intensity and target temperature, the former being the more important. Substitutional impurities can be caused to migrate due to the high concentration of diffusing vacancies. Results obtained () with nickel ions in titaniunr ; provide strong evidence that enhanced diffusion, at a beam intensity of only 70 |iA per cm , carried 40 keV ions to a depth approximately 50 times their range in titanium. The percentage of implanted material retained in the target rose from 12 per cent to over 90 per sent with increased dose rate. This is another important factor in the process since, in the absence of diffusion, surface sputtering will limit the amount of material that can be 1ft introduced by ion implantation. The very high value of 2.10 2 ions per cm was achieved in this work, and there was no reason to believe that this represented a limit. Depending upon the metal and the ease of migration of vacancies through it, the temperatures necessary for such a process could be high. It should be feasible to achieve these temperatures by the appropriate combination of ion beam current, energy and target manipulation velocity. That the process would be rapid enough is suggested by the speed with which vacancies have been observed to cluster into voids in heavily irradiated metals.

The great variety of shapes and sizes of object which it may be desirable to implant will impose some problems in target chamber design. Since an ion beam is directed, it will usually be necessary to manipulate the workpiece under one or more ion beams. It may then be necessary to allow the work to cool down in a storage bay before it is exposed to the atmosphere. Each application will require a detailed study in order to -ietermine the optimum implantation conditions before the operation can be carried out efficiently on an industrial scale. -87-

ION BEAM TECHNIQUES FOR SURFACE ANALYSIS Here we are concerned not with the modification of a solid surface but with the analysis of its composition by means of an energetic beam of light penetrating ions* Measurement of 1 the energy spectrum of backscattered particles^ ^'Wf of nuclear reaction products V ?•• •'or of ion-induced characteristic X-rays ' provides an extremely useful technique of surface analysis. Charged-particle measurements, commonly made with semiconductor detectors, yield a depth resolution of better than 2005. Moreover, the results may be quantitative, with an accuracy of around 5 per cent. These features contrast with alternative methods of surface composition analysis, such as Auger speetroscopy, ESCA, or ion microprobe analysis, though these techniques can provide useful complementary information. The energy resolution of semiconductor detector systems sets a limit to the depth resolution attainable in ion backscattering, but in some laboratories magnetic analysis systems are being installed so that the depth resolution may be improved to about 108, at the expense of a lengthier bombardment time. In the semiconductor device industry, it is now widely appreciated that ion backscattering can provide valuable information on such phenomena as the interdiffusion of multi-layer metallising, the segregation of mobile impurities into "gettered" layers, diffusion proflies involving high concentrations, and out- diffusion in encapsulating films. Moreover, the channelling effect can be employed in the examination of crystal quality and the lattice location of implanted impurities. In solid state physics research ion backscattering also proves a versatile tool, and surface phenomena such as corrosion, diffusion, anodic oxidation and the composition of films and coatings can be quantitatively investigated. All this is well-established, but so far such work is carried out with existing, large-scale accelerators. The -BB- technology is well enough developed to envisage a small table- top electrostatic accelerator providing about 1 MeV and fitted with a helium ion source delivering about 0.1 (j.A on target. Modern design should provide adequate stability and; an analysing magnet should be unnecessary. Such an instrument would be an inexpensive means of surface examination that could take its place alongside the more common equipment already installed in many laboratories. During the past year a new development has taken place which promises to extend the scope of ion beam techniques for surface analysis. This is the successful production of an MeV ion microbeam, measuring only a few microns in diameter. With such beams, which may have comparable intensities to those normally employed in backscattering or characteristic X-ray methods, the spatial distribution of elements can be determined to a high resolution. The most successful work appears to have been that of Gookson.etal(17 wi) o made use of a four-element magnetic quadrupole (^18^ originally proposed by Dymnikov et al; ' and illustrated in figures 5 and 6. This equipment is attacned to a 3.5 MeV Van de Graaff accelerator at Harwell and typically provides a focused beam spot of about 5 ym in diameter, with a beam intensity of 1-2 nA of protons, deuterons or He ions. For analysis, three types of detector are available: (1) a silicon surface barrier detector for Dack-scattered particles or charged reaction products; (2) a lithium-drifted silicon X-ray detector with a resolution of 250 eV, which is adequate to resolve adjacent elements down to 2 ~ 15; (3) a sodium iodide y-ray spectrometer. These detectors generate spectra in the form of pulse height distributions and, when the beam is scanned electro- statically over an area many hundred such spectra can be -89-

Object STOp Second Proton cotiimator ; ^_ Target 1 beam T

Pig- 5. Layout of the focusing system used by Oook>;ori et al. to achieve an ion microbeam.

Pig. 6. Photograph of the ion mierobean) assembly en the Harwell 5 MV Van de Graaff accelerator.' -90- generated. A qualitative presentation has therefore been devised in which the scan waveforms are applied to a cathode ray oscilloscope and signals from the detector are used to brighten up the display whenever a selected radiation is detected. Location of a particular region of the target is achieved by an optical microscope, in which the ion beam passes through a hole drilled along the axis of the objective. By comparison with the electron-probe microanalyser, the ion beam system possesses the great advantage of an improved signal to background ratio. An ion beam produces far less continuous bremastrahlung, and thus the background is reduced by typically two orders of magnitude, a factor which is important in determining impurities present in low concentration. Since the size of the focused ion beam is comparable with that of individual elements of semiconductor microcircuits or their interconnections, there are likely to be applications in the examination of causes of failure of such components. Experiments have already been carried out on the interdiffusion of metals forming a multilayer strip only 6 nm wide. (9) Hartley et al. ' have made use of the ion microbeam to study the transfer of implanted metal, such as Pb and kg , along fine wear grooves created by a loaded ball in a steel spL. k The depth of material that may be analysed can be increased, by lapping the surface at a shallow angle and scanning the microbeam across a measured distance. Fine-scale inhomogeneities can be detected in material which may appear uniform in other tests, and the segregation of impurities to grain boundaries can be observed. The interest already shown in the possibilities of this ion microbeam makes it appear likely that similar focusing systems will be fitted to other Van de Graaff accelerators before long. Already novel uses are being made of the focused beam, e.g. as a minute source of X-rays. -91-

REFERENCES (1) DEARNALEY, G., NELSON, R.S., FREEMAN, J.H.,and STEPHEN, J.H. 'Ton Implantation" (North-Holland, 1973)^ (2) SCAVUZZO, R.J., PAYNE, R.S., OLSON, K.H.V NAGCI, J;M. and MOLINE, R.A. International Electron Devices Meeting, I.E.E.E., Washington Dec. 1972 (unpublished) (3) FREEMAN, J.H. Proc. Conf. on Ion Implantation. Reading Sept. 1970 (Peter Peregrinus Ltd., Stevenage, 1970). (4) HAINING, R.W. (private communication). (5) see Proceedings of 3rd International Conference on Ion Implantation of Semiconductors and Other Materials, Yorktown Heights, December 1972 (Plenum Press, 1973). (6) ANTILL, J.E., BENNETT, M.J., DEA.RNALEY, G., FERN, F.H., GOODE, P.D. and TURNER, J.F. (loc. cit. 1973). (7) DEARNALEY, G., GOODE, P.D., MILLER, M.S. and TURNER, J.F . (loc. cit. 1973). (8) STREET, A.D., GRANT, W.A. and CARTER, G. (loc. cit. 1973). (9) HARTLEY, N.E.W., DEARNALEY, G. and TURNER, J.F. (loo. cit. 1973). (10) CHICK, D.R. and STEPHENS, K.G., (private communication). (11) THOMPSON, M.W. Proc. Conf. on Ion ^plantation, Reading, Sept. 1970 (Peter Peregrinus Ltd., Stevenage, 1970). (12) TURNER, J.F., W. TEMPLE and DE4RNALEY, G. Proc. 3rd International Conf. en Ion Bnplantation, Yorktown Heights Dec. 1972 (to be published by Plenum Press, ''973). (13) NICOLET, M.A., MAYER, J.W. and MITCHELL, I.V. Science, VJJ_ 841 (1.97'i). (14) WOLICKI, E.A. U.S. Bav.;-1 Research Laboratory Report NRL 7477 (1972). (15) AMSEL, G., NADAI, J.P., D'ARTEMARE, E., DAVID, D., GIRARDE.E. and MOULIN, J. Nucl. Instr. & Methods, ^2, 481 (1971)- (16) POOLE, D.K. at--?. SHAV, J.L, Proc. 5th International Conf. on X-~ay Optics and Microanalysis, Tttbingen 19^8 (Springer- Verlag, 1?69). -92-

(17) COOKSON, J.A., FERGUSON, A.T.G. and PILLING, F.D. J. Radioanalytical Chem. _12, 59 (1972). (18) DYMNIKOV, A.D., FISHKOVA, T. and YAVOR, S. Sov. Phys. Tech. Phys. jfO, 540 (1965)- (19) COOKSON, J. private communication. -93-

EXPECTATIONS AND PROBLEMS OF AN ON-LINE MASS SEPARATOR PROJECT ON A HIGH INTENSITY ACCELERATOR (LAMPF)* B. J. Dropesky and B. R. Erdal Los Alamos Scientific Laboratory, University of California Los Alamos, New,Mexico 87544* U- S. A.

This presentation was originally requested to be of a general nature, including discussions of the prospects and problems with on-line separator projects at meson factories, such as SIN (Zurich), TRIUMF (Vancouver), and LAMPF (Los Alamos). Upon consulting the people at SIN and TRIUMF, we learned that there are no plans or proposals to mount an on-line mass separator project at either of these high intensity accelerators. Therefore we will restrict our discussion to the proposed facility at the Clinton P. Anderson Meson Physics Facility, commonly known as LAMPF. Most of you know that for several years we have been pursuing a proposal to mount a mass separator on-line to the one milliampere 800-MeV proton beam from the LAMPF accelerator. A stub-out from the main beam tunnel was constructed in anticipation of funding for this on-line mass separator project. However, to date this funding has not been authorized and the present LAMPF schedule calls for operating the accelerator at levels up to one milliampere during the next 12 months. Construction of the on-line facility a year or so from now would then require a shutdown of the proton beam into the entire Area A East for approximately 6 months and this is unacceptable. Also, constraints placed on the target at the on-line facility location, due to the necessity of reconstituting the beam downstream, are such that our 2 • ' -"" targets could be no more than 0.5 g/cm thick and the proton beam would be 2 - no larger than about 5 cm in area. These conditions would appear to make targeting for a versatile on-line mass separator facility extremely diffi- . cult, due to the very high power densities that would be involved. For these compelling reasons we have found it necessary to abandon the original on-line separator location. Howeverj enthusiastic support and en- couragement to develop an on-line mass separator project at LAMPF, especially by a substantial group of non-Los Alamos potential users, has been increasing with time. Therefore, with the approval of the LAMPF director and his staff, an alternative proposal is being prepared for mounting a mass separator on-line to an extension of the H beam in, so-called, Area B. * Work performed under the auspices;of the U.S. Atomic Energy Commission. -94-

The present configuration of the LAMPF experimental area is shown in Fig. 1. The location of the originally proposed on-line mass separator facility on beam line A in Area A East and the new, alternate location north of Area B on the external proton beam line are delineated. The H beam, which will be accelerated simultaneously with the milliampere H beam, is expected to be operated at 800 MeV and at a maximum intensity of 100 pA. After being magnetically separated from the main proton beam, the H beam will be deflected to the northeast. By partial stripping and magnetic deflection, up to 30 ixA can be diverted to the High Resolution Proton Spectrometer in Area C, 60 f*A can be directed through the liquid deuterium target for neutron experiments in Area B, and 10 fiA can be stripped and focussed onto the target of the proposed on-line separator facility. We have been assured that on occasion, when maximum radionuclide production in the on-line target is required, proton currents up to the full 100 ^A should be available for our project. The layout of the newly proposed LAMPF on-line mass separator facility and the heavily shielded extension of the external proton beam line, designed to handle 10 pA of proton current, are shown in Fig. 2. Steel is the principal shielding reiterial for the beam-line and water-tank beam stop, but a thick surrounding earth berm is also required. The triplet lenses will enable the proton beam to be focussed onto a target cell as small as a centimeter in diameter and than dispersed into the beam stop. Personnel access into the target vault is provided in order that major changes and repairs can be made to the target or ion source of the separator. The isotope separator, witli a 3.5 meter ion drift tube, and 55° fringe-field focussing dispersing magnet, is designed to operate up to 100 kV. It was obtained from Nucletec S.A., Geneva, and is presently set up and operating in the Radiochemistry Laboratory at Los Alamos. A beam switching system after the separator collector tank will enable up to four separated ion beams to be deflected through the final shield wall into the experimental bay. Experi- mental instrumentation representing the areas of present scientific interest in the use of this facility are illustrated on the ends of the ion beam lines. This new location and configuration for the on-line facility presents a number of advantages over the original proposal: a) It avoids the long beam shutdown period for construction; b) eliminates the possibility of contaminating the main bean line due to target failure; c) provides for control of the intensity and spot size of the proton beam a. required for the on-line ON-LINE MASS SEPARATOR FACILITY

LAMPF EXPERIMENTAL AREA

NUCLEON PHYSICS FACILITY

I O

i—ISOTOPE PRODUCTION i

FUTURE WEAPONS NEUTRON RESEARCH AREA -LOW ENFRGY STOPPED ^-RADIOBIOLOGT AND PION LINE MUON CHANNEL THERAPY RESEARCH FACILITY Fig. 1. Proposed new location of the on-line mass separator facility Fig. 2. Proposed layout of the on-line mass separator facility on the External Proton Beam -97- target; d) allows for personnel access to the target region; and e) enables radionuclide production rates to be equivalent or higher. However, it is a costly facility since theimassive amount of shielding? material for the extension of the proton beam line and for the beam stop, as well as the triplet lenses and beam stop itself, must be included in the cost package for this project. As for the scientific justification for a project of this magnitude we need only consider the significant and well-known results that have come from project ISOLDE (I) regarding the properties of many new nuclides far off the 0 stability line. And, of course, there still remain many hundreds of both neutron-deficient and neutron-rich nuclides that can be produced by high-energy proton spallation and heavy ion reactions that are yet to be studied. Important investigations of these areas can be expected to be made by the ISOLDE (II), Dubna, and LAMPF projects, as well as by the projects on the heavy ion facilities at Oak Ridge (UNISOR) and at Darmstadt. The scientific productivity of these projects will depend importantly, among other things, upon the radionuclide production rates achievable in the target and, of course, on the efficiency of getting the radioactive nuclei to the experimental apparatus for study. We have looked at this question for the proposed LAMPF facility by scaling up actual production data from ISOLDE (I) for mercury isotopes from a lead target 45.2 g/cm thick irradiated with 30 nanoamps of 600-MeV protons. The steady state activities (A) observed at CERN1 are shown in the third column of Table I. The CERN production rates (P) are shown in the fourth column. These were obtained by multiplying the (A) values by the factor:

T f = l/2(res) j Tl/Z(decay) where T, ,2r ^ is the measured residence half-time of the mercury atoms in the target-ion source system (156 seconds at 680° C) ; Ti/2(decay) is the isotope decay half-life; and e is the target-isotope separator system efficiency (2%). The fifth column shows the production rates expected at LAMPF by scaling up a factor of 330 which corresponds to a 10 \ik beam of protons impinging upon a target identical to the CERN target. It should be noted that these peak production rates are in accord with a straightforward calculation for a 45.2 g/cm lead target, 10 pA of proton beam, and reasonable cross section of 10 millibarns. These high radionuclide production rates, of course, will apply as well to the ISOLDE (II) project -98-

TABLE I MERCURY ISOTOPES dps) MASS HALF-LIFE W

1 2.3 x 10 179 3.5 sec 3 x 10' 7 x 10 4 x lO6 180 6 sec 9 1.2 x 1,8 15 1.5 x 10 181 3.6 sec 200 4.4 x 10 3 3 6 2.2 x 10 182 10 sec 8 x 10 6.5 x 10 7 10 183 9 sec 9 x 104 8 x 10 2.7 x 10 10 184 32 sec 2 x 105 6 x 10 2 x 10 : 10 185 52 sec 8 x 105 1.6 x 10 5.3 x 10 8 ,10 186 1.4 miri 9 x 10E 1.3 x 10 4.3 x 10 6 10 187 3 min 2 x 10' 1.9 x 10' 6.3 x 10 f.8 ,10 190 20 min 4 x 10e 2.3 x 10 7.7 x 10 1 ,11 193 11 hrs/4 hrs 107 5 x 10 1.7 x 10 206 8.2 min 105 6.6 x 10 2.2 x 10 and a Dubna on-line project on the up-graded 660-MeV synchrocyclotron. As a consequence of this production rate capability one can expect to be able to study the properties of nuclides several mass units farther from stability than has been possible to date. By going to smaller mass targets and smaller volume target cells, one can expect an enhancement of the shortest- lived species due to the reduction in release times for the volatile products. Through the use of highly enriched light isotopes of the target elements a further enhancement of the very neutron-deficient species is expected. In some cases,, where only a few grams of the enriched material may be available, we would then resort to operating at higher proton currents, up to 100 jiA, as the experiment requires and as the target cell and contents are capable of withstanding the higher power densities. The technical problems of targeting ir. a 10 pA high energy proton beam are not considered too severe, but going to 100 /iA will probably limit one to rather special materials and target designs, such as the Bernas type. Table II shows a coraparison of the power densities for the targeting conditions that would prevail for beam lines A and B for essentially equivalent -99-

TABLE II COMPARISON OF POWER DENSITIES FOR VARIOUS TARGETING CONDITIONS

Proton current density Target, thickness AE Power density (g/cmJ " •(MeV): ' (watts/g) Line A 200 0,5 1 400 Line B a) 10 50 100 20 b) 100 5 10 200 radionuclide production rates. The maximum dE/dx for 800-MeV protons of 2 MeV/R/cm was taken for these calculations and uniform proton current distributions were assumed. Another consequence of the high nuclide production rates and conse- quent high ion deposition rates is that more sophisticated experiments and measurements on a large number of radioactive nuclei can be carried out. This is reflected in the large number of people and variety of disciplines represented by the participants on the new LAMPF on-line mass separator proposal as listed in Table III. Three major areas of scientific studies are being proposed for this facility. Strong emphasis is being placed on the apparent feasibility of direct precision mass measurement of the short- lived radioactive nuclei, as discussed by L. Forman earlier. The measurement of nuclear spins and moments by atomic beam apparatus, optical pumping and perturbed angular correlation techniques is also an area of great interest. Detailed nuclear spectroscopy studies, especially in the deformed and transition regions, will also receive considerable effort. Important off- line experiments such as charged particle spectroscopy using radioactive targets, angular correlation measurements on low-temperature aligned radioactive nuclei, and precision mass measurements of moderate-lived nuclei are all included in the experimental proposals for the LAMPF separation facility. It should be noted that considerable expertise in the areas of theoretical physics, nuclear spectroscopy, mass spectrometry, charged-particle spectroscopy, nuclear alignment and isotope separator techniques is already represented by the Los Alamos participants on the proposal. In addition, we can call upon the advice and assistance of highly qualified metallurgists and materials scientists at the Laboratory. In conclusion, it is only fair to state that while the feasibility of the described LAMPF on-line mass separator project is not questioned and the scientific merit and support for the project is apparent, there still remains considerable uncertainty regarding the authorization of funds for the -100-

TABLE III PARTICIPANTS IN THE ON-LINE MASS SEPARATOR PROPOSAL

Planning Group ~ D. R. F;':CodTran MP-6,-Lbs"Alamos Scientific Laboratory (LASL) B. J. Dropesky CNC-11, LASL B; R^Erdal CNC-11, LASL G. M.Kelley CNC-11, LASL JvRi McConnell Iowa State University R. A. Naumann Princeton University R. F. Petsy University of Oklahoma W. L. Talbert Iowa State University Experimental Project Areas 1 - Precision Mass Measurements R. C, Barber University of Manitoba L. Formah J-16, LASL W. H. Johnson University of Minnesota •S.--G. Niisson Lund Institute of Technology J. R. Nix T-9, LASL P. L. Reeder Battelle Northwest Laboratories P. A. Seeger P-ll, LASL C. M. Stevens Argonne National Laboratory W. J. Swiatecki Lawrence Berkeley Laboratory (LBL) 2 - Nuclear Spins, Moments, and Hyperfine Interactions B. Greenebaum University of Wisconsin - Parkside J. C. Hill Texas A§M University R. A. Naumann Princeton University P. Raghavan Bell Telephone Laboratories R. Raghavan Bell Telephone Laboratories 0. Redi New York University H. A. Schuessler Texas A§M University W. A. Steyert Q-26, LASL H. H. Stroke New York University 3 - Nuclear Spectroscopy M. E. Bunker P-2, LASL W. R. Daniels CNC-11, LASL C. L. Duke Griimell College E. N. Hatch Utah State University D. C. Hoffnian CNC-11, LASL S. T. Hsue University of Northern Iowa J. D. Knight CNC-11, LASL J. L. Larson Lawrence Liverraore Laboratory fLLLI W. C. McHarris Michigan State University R. A. Meyers LLL R. A. Naumann Princeton University C. W. Reich NRTS, Idaho Falls •»• G. L. Struble LLL W. L. Talbert Iowa State University F. K. Wohn Iowa State University -101- construction and for staffing a new facility of this magnitude.

REFERENCES

1 - CERN Report 70-3 Geneva 1970, page 98 and P. Patzelt, unpublished data. 2 - ibid, page 105. -102-.

TRENDS IN THE STUDY OF NUCLEI AWAY FROM STABILITY

*) P.G. Hansen CERN, Geneva, Switzerland

and

The ISOLDE Collaboration

1. INTRODUCTION

We have often been xaminded that most particle-stable nuclei still remain undiscovered, and we can all agree that it would be valuable to have experimental methods available that would make all such nuclei accessible to experiment. This, however, does not mean that we should plan to study them all. This would require an effort that is far beyond anything that we could hope to do - or, would want to do. Therefore, in the words of the memorable Pursewarden we must "sink or skim"

The purpose of this talk is then to give examples of how experimental physicists have responded to this challenge. Most of my examples, but not all, have been selected from the work that I am most familiar with: The experiments that are being carried out by the ISOLDE collaboration at CERN. I must apologize ahead of time for having had to leave a number of interesting and important things out for lack of time. Those who want to have a more comprehensive overview of the field should consult, for example, the proceedings of the 1970 Leysin Conference ', or a number of contributions in the proceedings of the 1972 rieavy-Ion Summer Study at Oak Ridge .

It is not my role here to go into the experimental problams, but let me just remind you that in most of the work that I shall discuss the basic difficulty is to obtain sufficiently high selectivity. First of all we are interested in rare products that will often be formed with very low cross- sections. Assume that, in the worst case, we are willing to work with one count/hour, or, 10~3 dis/sec. (Those who search for superheavies still have less than that.) Now, the improved synchro-cyclotron at CERN may get a beam of 5 x 1013 protons/sec, and with a target o£ one interaction length and an assumed yield of 20% one will be trying to detect one part in 1016, or, a

*) On leave from the Institute of Physics, University of Aarhus, Denmark. -103-

cross-seetion around 10~"° cm2. This is many orders of magnitude harder than finding a needle in a haystack, which according to Enge"*^ has 105 straws. A number of papers at this conference are dealing with new techniques for studying nuclei away from stability. It is a promising sign for the field that these very difficult problems are having the attention of many physicists and chemists.

2. NUCLEAR MASSES

From the beginning it has been clear that one important task in on- line work is to map the nuclear energy surface, but it has also been evident that the problem was a hard one. It is therefore not surprising that the most important contributions are of rather new date.

At the 1970 Leysin conference Johnson pointed out that it should in principle be possible to perform on-line mass spectrometry with high precision. The first results of this type have now been reported by Klapisch and his collaborators , who have used a surface ionization mass spectrometer with moderate resolution for determining the masses of neutron-rich sodium isotopes (up to 30Na) with accuracies of 140-460 keV. In order to obtain this high precision it was necessary to determine the position of the centroid at each mass to 0.5%. This was achieved by a comparison method in which the unknown and a reference line were compared with two known masses differing by the same mass number jump. A series of new experiments by the same group have pushed this technique further, and has permitted tha determination of the binding energy of nLi.

Another method for determining nuclear masses is to measure Q values in the isobaric decay chains so that masses away from stability can be related to known ones near stability. This technique is especially difficult because of the extremely complex decays encountered for high Q-values, and it is normally necessary to work out part of the decay scheme. Results for the chains starting in the neutron-deficient mercury isotopes have been reported by Westgaard et al. , whose most complete set (the 182 isobaric chain) is shown in Fig. 1. They note that those theoretical mass formulae which are based upon the liquid-drop model with structure-related corrections succeed in following the trend of the experimental data over the whole range studied, though without reproducing the observed local structure.

In the region above tin (Z,= 50), alpha decay is a rich source of information on nuclear masses. As an example Fig. 2 compares the systematics -104-

of alpha energies for even-Z nuclei in the region Hf (Z = 72) to Pb (Z - 82) with a frequently used mass formula (9) based on the liquid-drop model. The -experimental data (10-22) are mainly from heavy-ion experiments which utilize

;;th

3. NUCLEAR SPINS, MOMENTS AND SIZES

Techniques for measuring nuclear spins and moments have been reviewed by Otten2 . A very powerful technique is atomic-beam magnetic resonance (ABMR), but intensity limitations have until now made it hard to use it on- line. Equipment has, however, been developed for extremely fast off-line 2 4 2 5^ work ' ; cen with difficult elements measurements can be made for halflives down to 2 minutes. Information on rare-earth isotopes has been given in the extensive series of papers by the Gothenburg-Uppsala group; as an example of their work one can mention the measured spins for the Tin isotopes , for which they found that the ^+(411) Nilsson orbital, which is the ground state of all odd-mass Tm isotopes in the region A = 163-173, for the two lightest Tm isotopes is replaced first by the ^2+(404) and then by the 5/2+(402) state. This finding supports the theoretical calculations of quadrupole and hexadecapole equilibrium deformations. It seems as if the intensities expected for the improved ISOLDE facility (subsection 2.2) will be high enough to permit on-line ABMR measurements23'26'. One interesting future possibility is that such measurements will reveal new odd-odd nuclei with a 0+ ground state; such cases would provide a possibility of checking the isospin mixing away from stability. An example near stability is 170Lu n y\ for which the atomic-beam measurement is consistent with the assignment x 2 fl ^ 0 implied in a determination of the isospin-violating matrix element in the Fermi-type beta decay. Measurements of spins, moments, and optical isotope shifts have been performed on-line for isotopes of mercury by Otten et al.23'29"31'. Their method, which is extremely sensitive, is based upon optical pumping with cir- cularly polarized resonance radiation. Nuclear polarization is achieved via -105-

the hyperfine interaction in the excited atomic state and it is detected through the 0°-180° asymmetry in B decay. It is of special interest that the technique used to obtain resonance (Fig.3) also provides (in an indirect way) very precise values for the optical transition energies, and thereby information about the isotope shifts (Fig. 4), which reflect the "effective" value of the nuclear radius. From Fig. 4 it is seen that 183>185Hg have almost the same volume as 196Hg. The sharp break has several possible inter- pretations , and further experiments will definitely be needed to clear up what this sudden change in the nuclear charge distribution implies. The most probable hypothesis is, of course, that one is dealing with the sudden onset of strong quadrupole deformations, whj -_h would make the case analogous to the famous N = 88-90 shape transition in the rare earths.

4. ALPHA AND PROTON RADIOACTIVITY

It is easy to detect alpha particles with good energy resolution and under favourable background conditions, and furthermore the alpha decay schemes are most often simple with one line dominating. It is therefore not surprising that studies of alpha decay have been very important tools in the study of nuclei away from stability. One example was discussed above in connection with nuclear masses. Here, we shall consider two examples oT spectroscopic applications of alpha decay in studies of nuclear structure.

Figure 5 summarizes the systematics of reduced alpha widths in the even- even isotopes of the elements with Z = 78-86. The reduced widths defined here (18, 32) have been obtained by taking out a simple barrier penetration factor evaluated for I - 0 in the WKB approximation. The data in the left part of the diagram have been obtained at ISOLDE, mainly by comparing alpha and K-X-ray counting rates for mass-separated samples. The results should be appreciably more precise than estimates based on the systematics of heavy- 21,22,33) . ion cross-sections, although the agreement for the lead isotopes is qualitatively good. The change in reduced widths across the 126-neutron closed shell is a well known phenomenon, and has been discussed theoretically by Mang3"^ and others. One notes now that a similar, sharp change seems to be taking place for the lead isotopes, where there is a change from 0.18 ± ± 0.06 for 188Pb to 1.0 ±0.3 for 192Pb 33). As i8B'189Pb decay to mercury- isotopes for which anomalous isotope shifts have been observed, it seems probable that there is a direct link between the two effects, although one has no direct clue to the interpretation. -106-

Another experiment35) related to the structure of the light mercury isotopes is illustrated in Fig. 6. In order to explain the isotope shifts for 183'185Hg one would have to assume a deformation of the order of 0.3, which would be associated with low-lying rotational excitations. One would then expect to be able to observe a, say, 60 keV 2+ level in 18"Hg as a fine- structure line in the alpha decay of 25 sec 188Pb. The resulting experimental upper limit is well below the systematics for alpha decays of nuclei with Z > 82. It would thus favour the absence of low-lying rotations in this nucleus, were it not for the somewhat, meagre systematics for nuclei with Z < 82 which indicate that in these the transitions to the rotational states are appreciably more hindered. The reason for this hindrance of the d-wave alphas relative to the s-wave alphas is not known, and it is not clear whether the effect for 1BaPb is due to the absence of rotations or to forbid- denness of the d-wave.

The question of the origin of the anomalous isotope shifts in the light mercury isotopes, is thus still an open one. One speculation has been 36) that one is dealing with a "bubble" structure , that is a structure with a reduced density at the centre of the nucleus. For such a structure, 104 3G) is expected to be a magic neutron number, and one would expect bubble configurations to appear low in the spectrum near 181(Hg, maybe even as ground states. One difficulty with this interpretation is that it fails to explain

the spins V-2) and magnetic moments (+0.50) of the light odd-mass mercury isotope(p^)s, wherea. s Ththe Nilssosimplen shelmodel lmode providel alsso a failstats e to\j& provid = 101e ,a ^~(521)suitablJe witstath e about the right properties.

For extremely neutron-deficient nuclei the proton binding energies will become negative and barrier penetration will then give rise to the phenomenon cf "Coulomb-delayed proton emission". Because of the very small cross- sections for producing nuclei far from stability, this process has not yet been observed with certainty. MacFarlane11'36^ has searched for low-energy protons in the rare earths. An extensive search based on a gas-filled spectrometer (in order to obtain a fast mass separation) has been carried out by 37) Karnaukhov et al. . It seems clear that proton-radioactive ground states will be very hard to observe unless the experimental sensitivity is improved considerably. Meanwhile, a high-spin isomer at high excitation energy (53mCo, I11 = 19/2-) provides3B) an interesting preview of the physical information to be obtained from a systematic study of proton radioactivity. -107-

The absolute width of such a very highly hindered [y 10B times according to3ti)l A19, proton transition could hardly be observed by other techniques. It seems 39) very likely ' that similar cases can be found.

5. BETA AND- GAMMA RADIOACTIVITY

Very many beta- and gairana-spectroscopic investigations have dealt with the structure of excited levels in nuclei away from the line of beta stability. Two interrelated themes have been of special importance, the transitional nuclei and the new deformed regions. We now have a good deal of evidence to show the existence of the new deformed regions (to my knowledge first proposed by Mottelson in 1961): 50 < Z,N < 82 and another with the same N range and with 28 < Z < 50. A number of problems in connection with new deformed regions and transitional nuclei were discussed in Ref. 2 and also in the proceed- h 0 ) ings of the 1971 meeting at Orsay Recently Kerek and collaborators have carried out a series of interesting investigations in the region near the new double-closed shell nucleus 132Sn, which has 50 protons and 82 neutrons. As an example of their work, Fig. 7 shows the level scheme of the odd-odd 32Sb populated in the 40 sec decay of 132Sn. The levels presumably represent very simple particle-hole excitations and should be as important to the shell-model theorist as are 208Bi, Tl. But the most exciting thing about this decay scheme is probably the beta decay to the 1+ level at U24 MeV. This transition has a log _ft value as low as 4.0, and we know immediately that it must be pure Gamow-Teller, because all Fermi decays in neutron-rich nuclei are strongly suppressed by the isospin selection rule. According to the interpretation of Kerek et al. the + 1 1 state will have to be (d3/ )~ (ds, ) , and one can work out the corres- k n /2 p ponding theoretical transition probability, which is approximately 20 times larger than the experimental one. This, of course, is a manifestation of the well-known spin-isospin polarization effect ' , but the special thing about the 132Sn case is that one can evaluate the polarization contribution experimentally without having to introduce pairing corrections, which normally add considerably to the uncertainty. Therefore a theoretical treatment of the 132Sn beta decay should be of special interest. In another investigation*"*^ the same group has investigated the decay of 0.12 sec 132In to the doubly-closed daughter 132Sn. In spite of considerable experimental difficulties they have succeeded in observing J •if>41 k?< gamma-ray, which they interpret as originating in the first excited level -108-

(presumably 3") of 132Sn. They point out that the high excitation energy shows that this nucleus is a well-developed double closed-shell nucleus. As it has not yet been possible to determine the multipolarity of the. 4 MeV transition, the interpretation as an octupole vibration can only be tentative, however, as Fig. 8 shows, the energy fits in neatly with the systematics for the single-closed nuclides. We can thus believe that the state is similar to the famous 3" state of ao8Pb, and a further investigation, if possible, would be extremely interesting.

6. BETA-DELAYED PARTICLE EMISSION

Far away from the line of beta stability the decay via delayed-particle emission becomes of importance. For a long time we have known a large number of delayed neutron-emitters produced in fission; however, it is only recently that the experimental techniques have become sufficiently advanced to allow 1,5 \ studies of the neutron spectra . For the heavier delayed-proton emitters the major part have been discovered through the technique of on-line mass separation, and the clean conditions thus obtained have given a large body of detailed information, which is summarized in Table I for the 12 presently 46-56) ,,c known cases . As an example the proton spectrum of Xe is shown in Fig. 9. The main importance of delayed-proton spectroscopy is the information that it provides about nuclear state* in an energy region that one would normally call the resonance region, and in particular about the strength function phenomena in beta decay. It has been shown that one can understand the delayed-proton spectra in terms of a compound-nucleus calculation and that we may predict absolute intensities, spectral shapes and branching ratios (see also Fig. 11). But proton detection also furnishes a very sensitive, low background method for detecting nuclides near the limit of particle stability. This property has been exploited in a series of experiments"'55' in which nuclear emulsion plates were used as detectors in a spinning-wheel set-up in order to determine half lives and proton spectra of the rare proton emitters 'l 3Xe and 116Cs, for which the ordinary counter experiments had turned out to be of too low sensitivity. The result of the former experiment is illustrated in Fig. 10. -109-

7. THE BETA STRENGTH FUNCTION AND FLUCTUATION PHENOMENA

Far away from the line of 3 stability the energy available for nuclecr $ decay is very large, and the decay proceeds to an enormous number of highly excited levels in the daughter nucleus. These levels are clearly of the same complicated nature as, for example, the resonances in slow-neutron spectroscopy, and by analogy one must be prepared to treat level spacings and the individual transition probabilities in the language of statistical physics.

The concept of a strength function serves to describe how the amplitude for a given reaction channel distributes itself over a large number of levels of a (complicated) system. Experimentally, strength is expressed as a product of the average reduced width and the level density. For 3 decay one defines the 3 strength function

S0(E) = b(E)/{f(Z, Q - E) • T^} , (1) wnere b(E) is the absolute 3 intensity per MeV of final levels at the energy E in the daughter, where f is the usual statistical rate function, and where Q is the total energy available for the 3 decay with half-life T, . Defined in this fashion the 3 strength function (in units of s"1 MeV ) incorporates both allowed and forbidden transitions. In terms of the average reduced ti transition probability B.(E) to levels at the energy E with spin and parity I.*- and with the density p.(E) one has the theoretical expression

1 Sg(E) = D" I p.(E) B7(ET , (2)

where the constant D takes the value 6280 ± 60 s.

The properties of the beta strength functions have been studied directLy through total absorption spectrometry ' for both EC and £ decays. A number of other techniques serve to give information on the beta strength function; a general survey of this field would take too far here, so we refer instead to a forthcoming review article . As an example of strength function data Fig. 11 shows the experimental results for 119Xe and Xe as compared with the shell-model-plus-pairing calculation by Martinsen and Randrup6°\ It will be seen that the renormalization of the Camow-Teller strength here amounts to approximately a factor V20, i.e. the same as lor the I32Sn case discussed above in Section 5. -110-

A number of parameters enter into a compound-nucleus description of beta decay and delayed particle emissi.on: Beta strength, proton widths, level densities. These parameters are understood to be averages over many levels (resonances) with the same spin and parity, and therefore the finite number of levels in an energy interval leads to experimentally observable fluctuations such as the fine structure in the proton spectrum shown in Fig. 9. The amplitude spectrum of these fluctuations contains information 59 ) about the spacing of the levels. In a quantitive description of this , one finds that the main source of the fluctuations is in the partial widths (beta decay and proton emission), whereas fluctuations in the level density are of little importance. The distribution laws governing the individual partial widths and also the product law, which would apply e.g. if one observes a weak proton branch from levels populated in beta decay, shown in Fig. 12c Note that these distributions are skew with an extremely large variance (for the product distribution the variance is 8 for a mean value of 1), and that even for the more familiar Porter-Thomas distribution (see inset of Fig. 12) the spectrum appears as a few large peaks on a background of small peaks. Karnaukhov et al. have recently used fluctuation analysis to derive level densities in the nuclei 1095lllSb from the analysis of the corresponding delayed-proton spectra. 8. CONCLUDING REMARKS

In this talk I have had very little time to speak about light nuclei, although there has been important developments and although we must certainly consider a nucleus such as 23A1, which is two beta-decays away from stable Na, as a nucleus far away from stability. I believe, however, that one interesting area for the coming years will be the medium-weight nuclei (say, A > 40). For these, one may hope to extend the work that is now going on in the lighter region, and one can for example hope to study the beta decay behaviour of nuclei with Z > N. The discovery62'"5 of the new isotope, 17 sec 72Kr, which to my knowledge is the heaviest N = Z nucleus yet observed, is a significant sign of experimental progress. -111-

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Delayed proton emitters with A > 50

Excited final Experimental Experimental Isotope Ref. states MeV (%) p/dis. Q - B MeV

73Kr 34.0 ± 4.0 0.862 (35) (6.8 ± 1.2 ) x io-3 4.85 ± 0.30 48 i09Te 4.2 ± 0.2 not measured not measured 7.14 + 0.10 50-52 uiTe 19.0 ±0.7 ii ii 5.07 ± 0.07 50-52 n3Xe 2.8 +. 0.2 t! IT not measured 54 Ji5Xe 18.0 ± 4.0 0.709 (58) (3.4 ± 0.6 ) x io~3 6.20 ± 0.13 46, 47 5 117Xe 65.0 ± 6.0 0.679 (14) (2.9 ± 0.6 ) x icr 4.1 ± 0.2 46, 47 116Cs ^ 3.5 not measured not measured not measured 53, 55 4 118Cs 16.0 ± 1.0 several (4.2 ± 0.6 ) x xo- 4.7 ± 0.3 53 20 8 i Cs 59.8 ± l.l not measured < 5 x 1O~ not measured 53 I 179Hg 1.09 ± 0.04 n a, 1.5 x 10"3 n 46 181Hg 3.6 + 0.3 0.158 (50%) (1.25 ± 0.30) x lcr" it 46, 47 183Hg 8.8 ± 0.5 not measured (2.7 + 0.6 ) x io~6 II 46, 47 -116-

1 1 1 I

MeV A = 182 5 M - Quadratic Term

U o o Myers -Swiatecki

x -x Zeldes a a Seeger

A A Garvey-Kelson • • Experiment '

-1

Hf Ta W Re Os Ir Pt Au Hg

Fig. 1 Naclear masses in the 182 isobaric chain given8) as excess over the liquid-drop mass. The five points far from stability (Os-Hg) define a hump which is not foreseen in the current mass formulas. Note also that the Garvey-Kelson prescription (which gives very impressive agreement near stability) leads to disas- trous predictions far from stability. -117-

E« t

Experimental alpha energies in MeV. Comparison with Myers & Swiatecki UCRL -11980

85 90 95 100 105 110 Neutron Number, N^

Fig. 2 Experimental alpha decay (kinetic) energies as a function of neutron number for even-Z isotopes in the region Hf-Pb. The dashed lines show the predictions according to the mass formula of Myers and Swiatecki ). The data have been taken from the following sources: Hf: Toth et al. Macfarlane11' W : Eastham and Grant12) Os: Borggreen and Hydel3) Toth et al.ll *»1 ) Pt: Siivola16) Graeffe17) Hg: Hansen et al.18'19) Pb: Siivola20) Gauvin et al. -118-

Fig. 3 Optical pumping of 185Hg (50 s) with light from a Z01tHg lamp. The top diagram shows the splittings due to the isotope shift (IS) and the hyperfine interaction. With the lamp placed in a magnetic field on searches for the matching points of the Zeeman components by scanning 0 asymmetry (a) as a function of the field (bottom diagram). The two hyperfine components show that the spin of 1B3Hg is %. j_From (30)] -119-

180 190 200 A |

40 |

\ "in s I y I *

1 ^

1 10

u i

• CENTRE CF GRAVIT >. rcwTDC re rcQA\/m -10 i OF 50ME RS -

-20 100 110 120

Fig. 4 Hyperfine structure and isotope shift (relative to 20Tig) for the 2537 % line of the mercury spectrum. The black dots indicate (with an error less than their diameter) the centre of gravity for this line. The dotted line indicates the volume effect on the IS; the two lightest isotopes (1B3Hg and le5Hg) are seen to have an anomalously large effective volume. 31 [From (30)]. New data ) from iei,ie9,i9»Hg further extend and confirm the systematics shown here. -120-

2.0

1.0

o 0.5 _c jQ. O -o 0) u 0.2 T3 Pb

0.1

100 110 120 130 140 Neutron number

Fig. 5 Reduced alpha widths for even-even isotopes of the elements Pt - Rn (from Refs. 18, 19, 32, 33 and references therein). The strong shell effect at neutron number 126 has been known experimentally for a long time, and has been interpreted theoretically by Mang3 '. A new effect is the strong decrease in the reduced ot-widths for the three lead isotopes of which the lightest (188Pb) decays to ""tig. -121-

SYSTEMATICS Z = 86 • 96

EXPERIMENTAL UPPER LIMIT

(38

O.i l.o EXCITATION ENERGY,MeV

Fig. 6 Experimental upper limit (heavy line) on the relative intensity of the alpha branch to an assumed ?.+ level in 181>Hg as a function of its excitation energy. The thin line represents approximately the systematics for nuclei with Z = 86-96 (there are no relevant data for Po, Z = 84). For the systematics the ordinate represents the product of the 2+/0+ ratio of reduced widths times the ratio of the barrier penetrabilities (calculated as in Ref. 32) for Z = 82, Ea = 5.975 MeV. Very little is known about the fine structure for even nuclei with Z < 82, so the ISOLDE data18) for 178Pt and 182.181fHg are given separately in the same units. (From Ref. 35). -122-

/.Osec 0' 132 50 Sn32 \

3.0MeV

Ip- = 1007.

log ft = 4.0 o _ \1,13243 " < 0.8 ns • c-- m in CN Jo cn to in ~ 2* (3)* 10777 ns

5287or 528.7or548 E

5438° 1.2(46 ) 4257 " ~ 2*.(3)- ns • in 855 IS v 0 T 14 8ns 132 51

Fig. 7 The level scheme of 132Sb as populaced in the beta decay of 40 s '"Sn according to Kerek et al.1*1). -123-

Z=50

2408 2720 2321

11flSn 120Sn 122Sn mSn 126Sn 128Sn 130Sn

N=82

4041 3277 2B31 iU 2464 f 1810

132- 134 136 138 140 142,. 144 146r 50 82

Fig. 8 The systematics of 3~ states at Z = 50 and N = 82 according to Kerek et al."*2'. -124-

Xe 115

400-

c Oi

c o o a.

20 30 35 4.0 4.5

Proton energy

Fig. 9 Delayed-proton spectrum of U5Xe measured") with a single Si counter with 15 keV energy resolution. The tail at low energies is due to positrons; it disappears when the protons are counted with a counter1 telescope. -125-

4 5 6 7 PROTON ENERGY, MeV

Fig. 10 The delayed-proton spectrum of 113Xe measured from the track lengths in a nuclear emulsion (Kef. 54). The inset shows the half-life as measured from the track density as a function ot time (actually: rotation angle of the rotating disc, which performed one full rot.it inn in 27.5 sec). -126-

-i 0

-4

Shell model with pairing corrections (u From 115" X.e delayed protons o

c 3 io5 £

From ' Xe decay scheme iR.Stippler 8. F.Munnich ) id6

Excitation energy , MeV.

Fig. 11 The beta strength function for ''''Xe as deduced from the decay scheme6"*), and the high-energy part of the 115Xe beta strength function as calcuJated from the spectrum of delayed protons (Fig. 9). The calculation assumes the empirical proton/ gamma width ratio obtained in a statistical model of delayed-proton emission56'. A change of a factor 2 in this ratio would change the lower part of the 5Xe strength function by the same factor, but would only mean about ±30% change near 7 MeV. The peak, which seems experimentally well established, is presumably the (gE, )-1 (g"<) excitation. In the shell-model calculation by Martinsen and Randrup this peak (upper histogram) comes about 1 MeV higher in energy. The renormalization of the GT beta strength thus amounts to approximately a factor 20 down from the shell model and pairing. -127-

4 5 6 7 10 11 REUTWE INTENSITY

Fig. 12 Probability density corresponding to the Porter- Thomas (PT) law and the distribution corresponding to the product of two independent PT distributed variables. Both distributions have been normalized and are shown on a scale where the mean value is 1. The inset shows as an example a sequence of PT distributed random numbers and the corresponding spectrum for a detector with the indicated (Gaussian) response function. Chapter 2: Stable isotope separation Oi-f-3 -12B-

PROCESS EFFICIENCIES IN CALUTRON SEPARATIONS* L. O. Love Oak Ridge National Laboratory Oak Ridge, Tennessee 37830

Several articles in the literature describe separators, their ion outputs, and their purity achievements; there appears to have been almost no comprehensive review of the process efficiencies achieved. This paper attempts to fill the void by summarizing calutron performance with all the elements that involved over 40,000 runs and consumed nearly 3 million hours of operation. 1 will dwell primarily with the variations experienced, both between elements and between runs with the same element. Such ranges become important in projecting the overall capability of the separator. For example, when separations are made for the accumulation of quantities of enriched isotopes, many runs are usually involved and a practical measure of the machine's performance will be an average. However, for making predictions of what ultimately can be done, two kinds of "averages" should be used - one which represents all runs over long periods, and another that involves selected runs grouped between the average and maximum achieved. The value of knowing both numbers is seen in the following example. When we were considering the collection of 5 pounds of 95% ' " W for NASA, we reviewed the previous series of tungsten collections and found that the average W* ion beam was 12 milliamperes from a single-arc unit, but there were a few runs toward the end of the collection that had average outputs as high as 18 milliamperes. We projected to the NASA representative that the runs with the higher output were more representative of what would be done in a prolonged future collection series than was the true average. When the collection of ~2800 grams of >94% 184 W was completed, the output had averaged 32 milliamperes from a two-arc ion source. Similar reasoning was applied to the process efficiency expected and the amount of charge material purchased. Had the decision been based on the average for all runs, we would have given a grossly inaccurate cost estimate and might not have made the separation. The data presented in this paper are of necessity selected subjectively, but all points are selected with the intention of representing typical performance of the calutron with each element, and including as many of the conditions as possible under which these runs were made. Since 1945. when we first started the separation of stable isotopes, there have been over 300 series of collections, a series ranging in length from less than 10 hours to more than 95,000 hours. Some elements have been processed only through a single series, while others have been processed as many as 19 times; some separations have been made for all isotopes, and some have been made for only one isotope where the impurities were measured in parts per million to parts per billion. The data are representative of runs in a series which we would try to duplicate if we were starting a new collection. A run can be thought of as being divided into three periods: start-up, smooth operation, and shutdown. !t is during the start-up that judgments are made which usually determine where the process efficiency of a particular run will fall within the range experienced for that element1. If, during this period, the ionization arc is overfed, there is a strong tendency to continue to overfeed, and this will drastically lower the process efficiency. Such overfeeding may not be at all apparent in the overall operating characteristics; therefore, attention to mi electromagnetic separation during the first part of a run cannot be overemphasized if the process efficiency is of concern.

•Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Coronration Oak Ridge, Tennessee 37830. -129-

Fig. 1. Oscilloscope trace of calcium ion beam compared with visual appearance of beam at the receiver. -130-

Figures \a b and c show pictures of oscilloscope traces of ion current vs time and of corresponding visual appearance of the receiver during the early stages of a calcium run. These serve to emphas.ze the importance of careful, yet rapid, start-up. The firs, pair of pictures shows the beam condition after the first hour of opera.ion, when the ,.m current was -25 milliamperes. Note die 20- to 30-kilocycle variation in collected current and the "blown up" condition appearing at the receiver slot. The second set was taken after the machine had been operating for about 2 hours and the output had reached 50 milliamperes. Conditions have noticeably improved. The remaining pairs are views showing the subsequent stages of the run to its full output of approximately 150 milliamperes. During this 4- to 6-hour • art-up period the amount of charge fed is constantly varied, and arc voltage a-id current are optimized around this changing feed rate. We are convinced that it is during this period more than any other that feed rates are set which ultimately determine the process efficiency for the run. That one may have an abnormally high variation in process efficiency without noticeably affecting the run performance is seen in a comparison of some run data. A few calcium runs with an average of >105 milliamperes in the collector pockets (and source drains of about 2I0 milliamperes) achieved process efficiencies up to 44.3%. !n contrast, in nearly 100,000 hours of it-: reception and 1000calcium runs, all with the same equipment and the same operating personnel, we averaged only 24%. The drop from what could be done to what was done is hard to understand, and trying to understand and explain the conditions thai resulted in only 8% process efficiency in some of the Ca runs seems hopeless. Figure 2 shows the maximum, average, and minimum process efficiencies for what we consider a typical collection series of all the elements. The deviations from maximum to minimum are some sort of an index

it il l . ...» • _ S Mq Si S K Ca Ti V Cr tFe Nki Cu Zn Ga Ge Se Br Rb Sr Zr Mo Ru Pd

MAXIMUM i JO AVERAGE { MINIMUM 10 n"H.tr,tn.il.v;1. Cd In Sn Sb Te Ba l.a Cl Nd Sm Eu Gd Dy Er Yb Lu Hf Ta W Re Os !r Pi Hg n Pb

Fig. 2. Typical ranges in calutron process efficiencies. -131- of either the process or the personnel, and, since all of us have experienced these variations, we have concluded that the se:tings required for optimum performance are more precise than our instrumentation can follow. The variation in process efficiency from element to element is easier to accept than the variation from run to run. A comparison of the process efficiency of six elements haying the best results with six of the poorer performers is shown in Table 1. The hours of separation timespent with each group are comparableVThe elements in the upper half of the table have hadabout 8000 runs and 585^700 hours of operating time, while those in the lower half have over 10,000 runs and 644,400 hours. It seems reasonable to conclude that we have had sufficient experience to find the best operating techniques if that were thei problem. The elements shown in this table are arranged according to decreasing average process efficiency,with notations indicating their first ibnizatioh potential, the ionization cross section, their relative chemical activity, and the charge material used. It is interesting that the elements which attained a process efficiency above 30% (with the exception of Cr) have more favorable ionizing properties, are high in the electromotive series, and can often be operated with metal charge. (However, because of the sparking problem, it is more practical to run some elements as compounds.) Rubidium and potassium fall in this category. Most of the elements with ionization potential above 7 electron volts have relatively poor process efficiency, but one wonders what the results would be if they could be run in the elemental form. In the early days when we were going through the periodic table for the first time, we made scans of every element to determine roughly the spectrum produced. Figure 3 shows two of these scans and points out the difference that can exist in beam composition from element to element. When there is a large number of side bands, there would certainly be a high dissipation into unwanted particles that would cause a lower process efficiency. In Professor Lawrence's earliest calutron design, he increased the collection of the singly charged positive ion beam emitted from the 10 X 150 millimeter source exit slit by using linear shims. With the aid

Table 1.

TWO GROUPS OF ELEMENTS YIELDING WIDELY DIFFERENT PROCESS EFFICIENCY1

Process efficiency (%) First lonization Elements I.P. Cross Section Charge High Average Low emf (eV)

Li 43 26 16.0 5.36 3.3 I Metal+ UC1 K 36 26 4.0 4.30 7.2 3 K2CO3 Ca 44 24 9.0 6.09 10.4 6 Metal Rb 34 19 11.0 4.15 8,4 2 RbCo3 Sr 34 15 3.0 5.60 12.9 4 Metal Cr 40 12 6.0 6.70 5.1 16 Cr2O3 +CCL, Mo 13 8 2.2 7.18 6.9 M0O3+M0 + CCI4 W 12 8 1.2 7.98 9.2 WO3+CCI4 Fe 14 7 0.9 7.89 6.3 18 FeCla Hg 9 5 1.4 10.4 6.4 36 HgS Hf 9 3 0.2 5.5 10.4 HfOj + CCU Se 6 3 2.0 9.7 5.0 17 CdSe

1 Group one: Data from 8,270 Runs, 585,700 Hr. Group two: Data from 10,652 Runs; 644,400 Hr. -132-

UCL

CfCl3

_A1JU-A-- ^' * ' JlLrJl wJL—JU.

Fig. 3. Ion beam scans made in the early days of stable-isotope separation. ill these a uranium beam of 25° divergence passed through the 90° position and arrived at the 180° position with ;i local width of ~4 millimeters. Using this arrangement, up to 30% process efficiency with outputs of 100 milliamperes per arc was attained with uranium. However, if the purity requirements are such that the locus has lo be sharper, it is necessary to limit the angles admitted to the collector, and this clipping of the beam lowers the efficiency of the separation. The separation of plutonium isotopes provides an example where the beam is restricted in the 90° position. This element usually has four adjacent masses with a one-mass-unit dispersion of just over 3 millimeters; under these conditions a beam must be well focused, and collector pockets cannot withstand erosion by even a 10- to 15-miliiampere beam for very long. These conditions cause us to limit the beam to -2°, +6°, a divergence which we think reduces the process efficiency by about 30% from what it might have been with 25° divergence. Figuu- 4 is a schematic of the baffling arrangement showing the way these restrictions lower efficiency by preventing ions fiom reaching the collector. The extent to which baffling limits the amount of beam passing through the 90° position depends on the arc conditions, which, of course, are controlled by the calutron operator. In Fig. 5 it is seen how the beam distribution at the 90° position can be changed by different modes of operation. A widely varying fa, 'or in obtaining high output and process efficiency is the pressure in the calutron vacuum system, and Fig. o is a graph showing the influence of pressure on the probability of a mercury ion getting from source to receive; when the background pressure changes from 0.01 to 0.1 micron. The two -133-

90° BAFFLE POSITION

ENERGETIC NEUTRAL PARTICLES

Fig. 4. Calutron transmission losses.

1 ; r \i 1 ! units ) a N //OUTPUT MAXIMIZED 1 o 1 1 1 J ) , V- 1

2 OVIN G PROB E c !/ \ 17 ~ 1 ^lUTP JT < MftXIMUM 1. - CURR E 1 V,. 1-12" + 12° JX 1 -12 -8 -4 REF 4 8 DISTANCE OF PROBE FROM O"-BEAM REFERENCE POSITION (in.)

Fig. S. Distribution of calutron ion beams in the 9Cf plane. -134-

curves relate to the 24- and 48-inch-radius calutrons operating at ~35 kilovolts. These data, taken with Hg* nn air as a background, fit reasonably well with the curve obtained using a cross section of 5.3 X 10"' 6 cm2. The curve obviously varies with the element and the lack of uniformity in background pressure, but Hie general effect is always one of lowering the process efficiency. Similarly, the presence of high-energy neutral particles emerging from the ion exit slit region is an added indication of losses in efficiency. In nearly all cases involving prolonged operation, protective baffle plates are severely damaged by these neutrals, as seen in Fig. 7. Intuitively, this loss must be increased during periods of overfeeding or improper use of support gas. To some extent the process efficiency is related to the chosen feed material, though the reasons still remain obscure. Our most prominent advancement in overall process efficiency is associated with internal chlorination of certain elements, particularly the rare earths and the heavy elements. Little, if any, improvement is associated with the use of internal chlorination of other elements, say iron, for example.

004 006 0.08 010 PRESSURE imicrons)

Fig. 6. Effect of background pressure on mercury beam.

Fig. 7. Baffle assembly for calutron. -135-

Listed in Table 2 are some of the process efficiencies achieved with various charge materials used by us. Some of these values are from records several years old, but they are still accurate enough for use in establishing trends rather than absolute values. While most charges are chosen to yield high output and purity, you will still note that the spread in efficiency between feeds is, in general, no larger than the spread between individual runs. Thus, we are led to the assumption that some basic limitation exists or that the combination of limiting factors sums up to be almost a constant effect. There arises then a basic question: Can sources of calutron type ever be expected to more nearly approach the efficiency-values recently achieved in Russia and Ihe U.S. with thermal ionization? Should our efforts and your own efforts he- extended toward further improving the efficiency as well as the output and purity? In routine stable-isotope separation where charge material is plentiful, efficiency for efficiency's sake is not usually of major importance. On the other hand, in second-passing and in radioisotope work, the efficiency of the process often limits the total quantity obtainable by practical approaches, and nearly always affects the economics of such work. Other situations exist where feed is limited and recovery of quantities involved is impractical; here the efficiency limits the amount of enriched material available, regardless of its monetary value. Examples of this type are encountered often enough to encourage us, as a separator group, to take a good look at the problem of improving separator efficiency. At present the variations in process efficiency appear linked to

Table 2.

COMPARISON OF PROCESS EFFICIENCIES ACHIEVED WITH VARIOUS CHARGE MATERIALS'

Charge Average Charge Average Element Element Material PE (%) Material PE(%)

Si SiCl4 8.8 Sn SnCl4 9.3 SiS2 5.3 SnS 2.2 SiS2 ± Si 6.8 SnS + HjS 4.3

K KCl 12.8 Ce CeCl3 2.6 K2CO3 26.3 CeOi + CC14 3.8 2 Ca CaCl2 7.4 Eu EuCl3 3.4 Ca 23.9 Eul3 0.8 CaO + Al 12.6 Eu2O3 + CC!4 4.1

Ti TiO2 + CC14 11.1 Gd GdCl3 1.9 TiCl4 13.4 Gd2O3+CCl4 6.9 4.9 Ge GeCl4 9.3 W WC16 4.1 GeS2 6.8 WO3+CC14 Br LiBr 1.1 Hg HgS 4.4 2.6 NiBr2 3.4 HgO 4.0 Mo MoO3 + CCU 4.8 ^. 7.6 MOO3 + Mo + CC14 Pb PbO + CC14 9.3

PbCl2 10.9

'Charge material currently preferred on the basis of over-all operation is underlined. 2 CaO + Al is used in second pass operation to avoid Ca metal-conversion chemistry in Laboratory. -136- operational procedures and charge materials more than any other parameter. Over a period of years we have improved the efficiency of plulonium separation, as shown in Table 3. The big increase was associated with internal chlorination of the oxide; subsequent improvements are associated with less easily defined operating techniques. The upper limit achieved is still relatively low, and it is this limitation which we should attempt to raise to more reasonable levels.

Table 3.

PROCESS EFFICIENCY TRENDS IN PLUTONIUM SEPARATION (90° Baffle Opening, +6°, -2°)

Percent Year Hours Low Avg. High

1959 1.9 4.3 7.0 400 1962 7.0 13.3 19.0 2700 1965 15.0 16.2 17.2 200 1967 16.0 18.3 21.0 3100 1969 19.6 20.0 23.3 350 -137-

A new method of internal metal oxide reduction in

electromagnetic isotope separation

D. ERONING, H.R. IHLE, H. LIPPERTS

Institut fur Nuklearchemie der Kernforschungsa.r.lage Juiich GmbH

Introduction

The performance of electromagnetic isotope separators with respect to the attainment of high enrichment has been considerably improved in the past chiefly by the use of inhomogeneous magnetic fields [l,2] and of multistage arrangements. [l,3,4] The overall process yield, i. e. the number of atoms collected relative to the number of atoms consumed in the ion source, however, remained low in the conventional electromagnetic isotope separation for the majority of the elements. In our work on the separation of isotopes of the rare earth elements with the laboratory separator SIDONIE II [5] overall yields were typically«5 % when anhvr"-ous chlorides were used as feed material. Some improvement was occasionail' observed by the use of the internal chlorination method with CC14 [6], when very small samples were separated. The low overall yield is chiefly caused by the low ionisation efficiency of arc discharge ion sources for the production of metal ions from a metal halide vapor. Much higher efficiencies have been measured by Bernas and Chavet for the ionisation of some elements in a conventional ion source namely 68 % for Xe and 37 % for Hg under optimum conditions. [7] • During the separation of Yb-isotopes we found that much higher yields compazed to conventional; methods are obtained when the ion source is fed with Yb-vapor produced by the reduction of the oxide with Ca-vapor.

Experimental

A double Ta-Knudsen cell as shown in Fig. 1 is used to effect the internal reduction.

Xe: total source ionisation efficiency Hg: net source ionisation efficiency (singly charged ions) -138-

AC Arc cMK>e~ C Clthodt H Heiter ovtn 1, 2 ta Tintllw TC • Tlwrmocouple KS Dadiiiton shields "« Wtll » "B°. • o«t!k of "ttil » 1C

Fig, l: Ion sourc* o.an irrtng flt for Interiil reduction of oxides

The reducing metal M - in these experiments Ca - is loaded into the left cell, A Yb 0 (MO ) is loaded into the right cell of the crucible assembly. Small 2o 3 B x samples of Yb 0 are loaded into a small inserted capsule (dotted). The temperature of the Ca charge, which is heated by oven 1 to 600 - 700 C is measured with a thermocouple. The oxide is heated by oven 2 to about 1000 C, while the arc chamber is kept at a slightly higher temperature. The Ca-vapor is led through a channel (1x5 mm) in a removable Ta plug into the reaction cell, where reduction of the oxide takes place. To provide a large surface for the gas-solid reaction the oxide is prepared by oxidation of precipated oxalate at 500 C in air. A mixture, of Ca and Yb vapor enters the arc chamber. Source operation parameters were similar to those used with chlorides except the cathode current, which could be kept lower because of the activation of the Ta cathode. The intensities of the ions formed in a typical run are shown in Table 1.

Table 1: Typical ion currents; reduction of Yb 0 with Ca-vapor

+2 +3 Ion Yb Yb' Ca CaO' ! i ! ! I [mA] I, 1,2 0,188 0,019 .. 0,720 0,0009 ! -139-

A small CaO+-ion current is observed, YbO+-ions are not registered (limit of detection: lo" amp.). The dependence of the Yb+ and Ca+ ion currents on time in an experiment,which was continued until complete volatization of the oxide, is shown in Pig. 2 (see run 2, Table 2).

23 45

ttl. it n" w* t»* ewrrwn nrtit \*mi IMfll

The dependence of the rate of reduction of Yb-0 on the Ca-pressure cannot be deduced by current measurements, since at constant Ca-vapor flow the ratio of + + the Yb /Ca ion currents was found to depend on ion source parameters. In practice, we adjust the source parameters for maximum Yb ion current at constant Ca-vapor flow. A stable beam can thus be obtained for a long time with very few high voltage sparks. Towards the end of a run, when the oxide surface diminishes drastically, the Ca-flow is increased by raising the temperature of oven 1 to maintain a reasonable Yb ion current. The reduction rate may be assumed to follow the equation

Pca

where n = the rate of Yb atom production nYb = the Ca-pressure in the reaction Pca cell = an exponent -140-

0 = the free surface of ^^2°3 f(T) = the temperature dependence of the reaction rate

and one tries to keep the product P Oyb Q constant at constant temperature of the reaction cell. For ion collection the same arrangement including a preselection diaphragm was used as described earlier in work on the separation of low abundant rare earth isotopes. [8] For direct deposition into hollow cathodes an arrangement was used similar to the one described in [9].

Results

The enhancement factors, the ion currents and the overall yields obtained with the method described are summarized in Table 2 and Fig. 3.

Table 2; Results of Yb-separations with internal reduction by Ca-vapor

Amount Amount of M M+215 isotope Run J +/I + n Purity Remarks E Yb Ca total consumed eollected [mA] [mg] [>ng]

1 13OO 1,0 0,4 15 91 0,016 90,2 2) Yb-168 2 2500 0,85 0,45 32,5 46,5 0,4 99,0 all feed material Yb-170 consumed, see Fig.2 3 0,1 0,05 21 6,5 0,29 >99,O no complete mass Yb-172 analysis was made 4 (440) 0,035 0,02 24 5,5 0,37 (99,3) hollow cathode de- Yb-174 position for hfs work ') 5 (1100) 0,022 0,01 8,5 15,2 0,24 (99,7) Yb-168 preenriched Yb-168 to 22,5 % (ORNL); hollow cathode deposition for hfs work 3) 6 2200 1,3 0,4 21,5 17,1 1,1 98,7 vi collection in Yb-170 i 7 22OO 1,4 1,1 20,5 25,5 1,45 98,7 one pocket Yb-170

1) = enhancement factor of isotope of mass M relative to isotope of mass M+2 2) 1 to 2 g were introduced in the source, in all other runs 10 to 50 mg 23 Yb-O, were used 3) Preliminary results of enhancement: factor and purity from sputtered deposits on a diaphragm of a hollow cathode collector -141-

« • 1, Vb-lU

• • 2. 16-170

• • 6, Tli-170

to1

61*11 vt Mlt d

The enhancement factors are similar to those measured, when Dy- and Er-isotopes were enriched with the same separator using the chlorides as feed material or the internal chlorination method. [8] The ratio of the Yb+- to Ca+-ion current is generally low when small amounts of YbjO are present in the source, but the chemical preparation of the oxide seems also to influence the ion current ratio. The mean value of the overall yield is 20,4 %; if one assumes a transmission factor of 0.6 to 0.8 and a mean retention probability at the collector of 0.9, the ion source efficiency would be 38 to 28 % for the production of Yb -ions from Yb-atoms. These comparatively high source efficiencies might in part be caused by charge exchange reactions in the ion source. First separations of Sm-isotopes using this method yielded Sm -Ca ion current ratios of about one and Sm ion currents similar to the Yb ion cur- E rents reported in Table 2. The reduction of u?0, with Ca-vapor is also thermo- dynamically possible under similar experimental conditions.

Conclusion

In general the method of internal reduction of a metaloxide by the vapor of another metal should be applicable if the chemical reaction is therraodyna- mically possible and proceeds with a rate, which enables the production of a constant flow of the vapor of the reduced metal with only a tolerable admixture of the reducing metal into the discharge chamber. It seems, that internal -142-

reduction of ThO cannot be achieved because of its high stability. The development of reliable high temperature ion sources is perhaps the most important supposition for a broad application of this method.

References

[l] Separation of Isotopes, H. London Eds., Chapter 15 R. Bernas, Electromagnetic Methods, George Newnes Ltd. London, 1961

[?.] L.A. Artsimovich, G.Ya Shchepin, V.V. Zukov, B.N. Makov, S.P. Maksimov, A.F. Malov, A.A. Nikulichev, B.V. Panin, B.G. Brezhnev, Atomnays Energiya 3, 483 (1957)

[3] P. Griboval, Ph.D. Thesis, University of Grenoble (1966)

Nl T. Wilson Whitehead and P.A. White, Nucl. Instr. and Meth. 103 (1972) p. 437

[5] J. Camplan, R, Meunier and J.L. Sarrouy, Nucl. Instr. and Meth. 84, p. 37 (1970)

[6] G. Sidenius and 0. Skilbreid, in M.J. Higatsberger and F.P. Viehbock, Eds.: Electromagnetic Separation of Radioactive Isotopes, Springer, Vienna, 1961, p. 243

[7] I. chavet, R. Bernas, Nucl. Instr. and Meth. 51, p. 77 (1967)

[s] H. Ihle, A. Murrenhoff, A. Neubert, U. Kurz, H. Lipperts in: W. Wagner, W. Walcher, Eds.: Proc. of the Int. Conf. on Electromagnetic Isotope Separators and the Techniques of their Applications, BMBW-FB K 70-28, 1970, p. 203

[9] A. Neubert and R. Wagner, these Proceedings -143-

PRELIMINARY RESULTS CONCERNING THE SEPARATION OF RARE GASES IN A CLOSED LOOP CIRCUIT R. Meunier, J. Camplan, M. Ligonniere, G. Moroy. Laboratoire Rene Bernas du C. S. N. S. M. 91406 ORSAY

For spectroscopic studies there is a serious need of about 30 mp 21 of Ne with a purity better than 99 %. The natural abundance of this isotope is smaller than 0. 3 %. Consequently it is impossible to obtain the wanted purity in a single pass separation. In addition even if this purity was obtainable, the separation would last about 1000 hours.

So we bought from Monsanto a pre-enriched Ne wich contains more than 50 % of Ne and is very expensive. If no recycling process is used, the amount required to load the ion source would cost about 30, 000 FF. Indeed the ion source efficiency is rather small for light masses. In case of Ne it is expected to be 5 % under the best conditions.

So we decided to recover the non ionized fraction of the neon. Instead of catching this fraction in some vessel which would be connected afterwards to the ion source, we prefer to directly send the recovered neon backwards to the ion source i. e. to operate with a closed loop circuit. Our loop differs from the one described by L. O. Love in that ours operates continuously.

When designing such a loop (cf. fig. ) various problems arise :

1 - The ion source is set at a high positive voltage while the various parts where neon is recovered are at ground potential. Hence the neon must be compressed up to atmospheric pressure in order to avoid the electrical conductivity of the gas column. This compression is achieved in four steps by the various pumps shown on the figure. In order to set the pressure in front of the needle valve, we use a constant pressure volume made by a glass vessel. If the pressure inside this vessel decreases, the liquid contained inside the shallow container is drawn into the vessel. -144-

In case of a pressure increase (this has never been observed) some liquid could be evacuated by opening V7.

As a liquid, we use glycerine wich does not dissolve the rare gas (the oils utilized in diffusion pumps do) and which does not boil when one pre-evacuates the whole loop.

The flowmeter shown close to the constant pressure volume must be used alternatively with the two following situations : i) V"4 and V^ open, V closed ; the liquid meniscus inside the glass tube moves towards the right and ii) V and V- closed, V open ; then the liquid meniscus moves towards the left.

2 - Before starting the operations, the pre-enriched gas must fill up the various volumes of the closed loop, each volume V. being at a pressure P . Hence £ P V must be minimum since it represents a gas investment, i i i With the different pumps used in our loop, HP.V. is equal to about 50 cm NTP. Note that obviously the dead volumes of the pumps are to be included in i'P V . 1 1

Once a separation is over, it is worth while to recover c* good fraction of the " invested " gas. This is done by opening V which con- i 3 nects the " recovery volume " to the loop. This volume is about 300 cm . By cooling it with liquid nitrogen, its capacity is increased to about 1 liter.

3 - Our technique for separation of high vapour pressure elements include a preselection diaphragm (shown on the " top view " part of the figure) set at the entrance of the collector box, at a place where the different beams are already separated. The unwanted isotopes are stopped on this diaphragm and are evacuated by a pump located close from this diaphragm. These isotopes must not be sent into the compressing system of the closed loop.

In our installation, this pump is a cryopump. The gas collected on it can be recovered also. This is worth while to do when the loop is loaded -145- with a pre-enriched gas. Indeed the gas caught on this pump appears to be only two times less enriched than the original gas,

4 - The gas which originates from outgasing or small leakages must be eliminated. This is done by the titanium trap. It consists of a quartz tube containing titanium sponge heated to 750°C. Thus the titanium asorbs all gases, except the rare gases. With a tube 30 cm long and 1 cm diameter, we can work for more than 12 hours.

At the time of the conference, the whole system has been tested while the valve V was closed and V open. The behaviour of the loop appeared very satisfactorily. The only trouble observed was a certain porosity of the plastic tube which is used in the peristaltic pump. This pump is to be replaced by a completly different one (membrane pump). Final results (the most important concerns the overall efficiency) are expected within a few months, as soon as the liquid helium trap which catches the separated neon will be completed.

When adjusting the separator, we let V1 closed and V open. During this period, some feed material condenses on the cryo-pump located near the collector. Since this condensed material can be easily recovered, it appears that the separator can be adjusted whithout any losses.

The same disposition (V closed, V open) could also be used to pre-enrich one isotope in order to obtain a desired purity. During the pre-enrichment period the ion source and collector slits could be enlarged so as to reduce the time consumption.

References

l - L. O. Love - Large scale ard radio, icti :s separator work at ORNL. Proceed. Marburg Conferei.ee (1970) p. 194. -146-

2 - K. Alexandre, J. Camplan, M. Ligonniere, R. Meunier, J. L. Sarrouy, H. J. Smith and B. Vassent - Sidonie, the new electromagnetic isotope separator at Orsay - Nucl. Instrum. and Meth. , 84 (1970) 45.

3 - J. Camplan, R. Meunier, J. L. Sarrouy - Contamination processes during gas collection in milligram quantities. Proceed. Marburg Conference (1970), p. 209.

ion source preselection diaphragm

oil-free I rotary pump I ; recovery peristaltic I volume pump !

constant pressure volume (glycerine)

Fig. - Sketch of the closed loop used for continuous? recycling during the gas separations. The separator is shown in a vertical plane except the region of the preselection diaphragm which is shown as a top view. The pressures in various points are expressed in torr. -147-

SIMULTANEOUS COLLECTION OF ALL MERCURY ISOTOPES R. Meunier, J. Camplan, M. Ligonniere, G. Moroy. Laboratoire Reni Bernas du C. S. N. S. M. 91406 Or say.

With a single pass separation, Sidonie.* can deliver mercury isotopes with purities as high as 99. 7 % for Hg or 4tig ^ . To obtain higher purities ( > 99. 9 %) requires a pre-enriched material for the ion source loading. Because we have a long experience in mercury separation, we thought it advisable to prepare the pre-enriched material our self.

Our collector used to collect one isotope is quite large and two of them cannot be used to collect two mercury masses simultaneously. So we decided to build a collector capable of collecting all mercury masses simultaneously. This so called multi-collector includes three parts and is shown in fig. 1. a) The dividing wings separate the different beams. At the place where the beams strike them, they are protected by a graphite slice bonded by epoxy resin which includes aluminium powder. Thus a good conductivity is obtained. The wings are water cooled and made of stainless steel. All of them are fixed together by two plates parallel to the figure plane.

b) The mercury isotopes are trapped inside the slots cut into the plate. £ ..... c) Thi« plate is cooled by the liquid nitrogen tank.

The power dissipation due to the impact of the beams on the wings ranges between 0. 5 and l.kW. That is why the wings are water cooled in order to reduce the liquid nitrogen consumption. Hence the wings must not be in thermal contact with the plate. Consequently, in order to prevent the escape of one isotope from its slot to the neighbouring one , there are lamellae fixed on the plate.

The whole collector can be moved from outside along the beam direction and, thus, can be located at the most convenient place. -148-

When a separation is over, and the vacuum broken, the collector is let inside the collector housing and kept at low temperature. The wings are demounted and, after that, the plate is unscrewed from the liquid nitrogen tank. Then the various slots of the plate can be easily attacked by niti'ic acid.

This multi-collector has been used during 71 hours and yielded 1, 6 g of the whole isotopes. The best purities obtained are as follows :

Isotope 196 198 199 200 201 202 204

Purities (%) 40 94.9 97. 1 98.2 p5. 9 99. 2 97.9

When using this multi-collector, the ion source slit is 3 x 45 mm and the extracted current about 17 mA. Nevertheless, we collect only 25 mg per hour which corresponds to about 4 mA entering the collector. We explain the difference between these two currents by the relatively low efficiency of our collector which is not, as it should be, very 11 closed ". Certainly an important part of the /apour which appears at the impact of the beam can escape from the collector.

However, the overall efficiency of a run (i. e. the ratic between the weight of the finally recovered isotopes and the weight of loaded material) reaches 12 % and the cost price of the isotopes obtained with the multi-collector is comparable to the Oak Ridge one.

References

1 - J. Camplan, R. Meunier, J. L. Sarrouy - The new electromagnetic isotope separator at Orsay, Nucl. Instrum. Meth. 84 (1970) 37. 2 - K. Alexandre, J. Camplan, M. Ligonniere, R. Meunier, J. L. Sarrouy, H. J. Smith, B. Vassent - Sidonie the new electromagnetic isotope separator at Orsay, Nucl. Instrum. Meths. , .84 (1970) 45. -149-

5 a

Fig. 1 - Assembly for simultaneous collection of all mercury isotopes. The distance between a wing and the plate is exagerated in order to make this figure more clear. The J^opjg is conecte(j insic[e a conventional pocket which is cooled by liquid nitrogen. -150-

Enrichment of 138La from natural isotopic abundance

A. NEUBERT and R. WAGNER

Institut fur Nuklearcheraie der Kernforschungsanlage Julich GmbH

intermediate type mass separators with high dispersion and currents up to the 10 mA range like the SIDONIE instruments15 are very well suited, among other tasks, for preparing some ten to a few hundred micrograms of highly enriched isotopes of very low natural abundance in single pass separations. These quantities are sufficient for optical hyperfine spectroscopic measurements and even more so for mass spectrometric measurements, e. g. neutron capture cross section determinations when the produced nucleus is stable .

1 *?R 4) La was recently asked for by a Marburg group for the first purpose and by our own laboratory for the second. Enrichments requested were 50 (or at least 30) % and >70 % respectively. From the figures of merit of SIDONIE I and II published in 1970 , enhancement factors between 1400 and 3300 and consequently final enrichments between 55 and 74 % could be expected. It was doubtful however, v.iiether this could be achieved. Therefore, to avoid acciden- tal contamination by sparking etc., a manually actuated shutter was used in both separations. Anhydrous LaCl served as ion source feed material.

It was a special requirement in the first separation that for reasons of optical performance ' , and in order to avoid difficulties in chemical re- processing, the enriched material was to be directly deposited into the hollow cathode (HC) inserts which were later used in the spectroscopic measurements. Fig. 1 (a, b) shows the arrangement. In order to prevent self sputtering at the walls of the 3 mm p x 9.5 mm HC-boring, th • was reduced to a height of 2 mm by a molybdenum diaphragm. Between this and the HC's an electron repeller (4 mm 0) of 0.5 mm stainless steel with platinum foil backing was placed. The platinum foil was intended to collect backsputtered La-mixture from the HC's for a preliminary mass analysis, as the deposited lanthanum was not accessible before the optical measurements. A linear array of 5 HC- inserts with the mentioned set of diaphragms in front was bombarded in the approximately 3O mm long line focus of our separator for 20.2 hours with a receiver current of 0.107 uA per HC average and 0.190 yA per HC maximum. Be- cause of the unfavourable geometry, only one third of the total 138La+ beam -151-

(1.5 pA average, 2.7 pA maximum) could be collected. In a preliminary experiment with x La, a retention factor 0.7-5 in the HC's was determined by activation analysis using the 1.6 MeV gamma line of the 14°La decay. So the average collected-guahtity-shbuldhavebeen-about2 8 pg'Lalper-HC? Distribution of enrichments is.shown..in'.Fig. I.e. •]^ric^ente:;^itwoLHCis-were".der'ivediby--the users ' from hyperfine structure intensities, yielding 73 % and 70 %. The latter value was controlled after the optical burn out by mass analysis yiel- 138 ding 69.0 ± O.5 % La.

o o 0 ® It ® c ® f J o o

D HC c)

Fig. 1: Collecting arrangement and results in HC separation, a) Expanded front view; b) horizontal section: S shutter, D molybdenum diaphragm, R repeller, F platinum foil, HC hollow cathode insert; c) enrichnlent results of several deposits (optically measured values in brackets).

In the second reparation a conventional receiver pocket could be used, in which the beam impinged on a wedge shaped graphite piece, and the self- sputtered La was collected on e 20 p aluminium foil during 9.4 hours with an average 138La+ current of 1.18 pA, corresponding to 57.3 pg current integral equivalent (C.I.E.). It could be seen after the separation, that the La-deposit was concentrated on a small fraction of the foil area; so it was decided to process different parts of the collector foil separately. The foil was cut -152-

into pieces A, B, C and D, as shewn in Fig. 2a, which were activated together with a standard and with the graphite piece G. 14°La activity was counted to determine the respective amounts of 139La. Piece B was again cut into 5 smaller pieces. After dissolving the deposits in nitric acid and purifying the solutions by ion exchange, mass analysis was made of the three-samples- A, G and B+C+D. Together with the 139(14O)La results, the total La amounts could be calculated, yielding the distribution shown slightly smoothed in Fig. 2b, in which also the ratios of relative deposit quantity to relative foil surface area are given. It follows that if necessary the last percents in enrichment can be gained by choosing an appropriate fraction of the collected material, as tabulated in Fig. 2c, d.

•-— -—• -c—-______^^ 1 b c) C CLa) i - \-^ \ ^ * i X i f £ < ^ £ \ A 8O.6iO.2. D B + 152ZQ.Z 3404 a) Q C c + D o. 13 ; Q 73.2 tO.Z 3066

d) Quantities A_ « o.z J J- * o.ff ea •io.y 5 A 10.J fr B*OD 11.1 • f G> 2.1.3 ' ? ? Zp 55.0 ' i C.I.E. 57.3 . b)

Fig. 2: Analysis of pocket (P) separation results, a) Unfolded collecting foil (inner pocket surface) with folding (solid) and cutting lines (dashed and dotted); b) relative deposit density distribution (denominators: relative foil piece areas, 38.4 cm = 100; numerators: relative deposit quantity, 33.1 yg La=10O); c) enrichment results; d) comparison of quantities (C.I.E. = current integral equivalent). -153-

In both separations, ion source and vacuum conditions were carefully observed to give the best possible ratio of 138La+ peak to 139La+ tail current as shown in Fig. 3. The registrations were made with a slit width of 2 nun, equal to that of thescollectorslit, • so-thatnointegration^isnecessary to find the ratio of peak to tail intensities passing through the collector slit.

Fig. 3: Peak and tail registrations: a), b) typical for HC runs; c) best ratio observed at the end of HC separation; d) , e) typical for pocket runs.

The last figure (Fig. 4) gives a comparison of the enrichment results to empirical figures of merit and to theoretical small angle scattering calcu- 8) lations . The calculated contamination factors contain only a small contribution (<10 %) from inelastic (charge exchange) scattering, the dominant phenomenon being elastic scattering (>9O %) as the contamination direction is heavy —^- light. This corresponds to the fact that in general good agreement between mass spectrometric and ionic enhan-unent factors was observed; che mass spectrometric factors often being slightly better than expected from the ionic ones. If one takes into account, that chromatism is not regarded in the calculations, but must be present as seen from the 1:2 to 1:3 asymmetry of the heavy side and light side wings of the 139La+ peak, the agreement is rather good. -154-

/ ,1

afl/ri

s id i 5 10 fa)

Fig. 4: Comparison of HC and P enhancement results to experience and theory: a) to enhancement diagrams of SIDONIE I and II adapted from refs. 1 (I: Orsay, II: Julich), o: from spectroscopic data, •, x: mass spectrometric values,- b) to final abundances vs residual gas pressure calculated after ref. 8 (dashed line: elastic scattering only, solid line: with charge exchange); solid bars: mass spectrometric values, dotted bars: optical values.

References

1) J. Camplan, R. Meunier and J.L. Sarrouy, Nucl.Instr.S Meth. J34_ (1970),37-44 K. Alexandre et al., Nucl.Instr.a Meth. E)4_ (1970), 45-54 H. Ihle et al., Proc. EMIS '70 (Marburg) = BMBW-FB-K7O-28 (Ed. H. Wagner, W. Walcher), 203-208 2 2) H. Kopfermann, Kernmomente, Frankfurt (M.) 1956 , p. 97 ff. 3) R.N. Whittem, AAEC/TM 443 R. Dobrozemsky et al.. Int.J.Mass Spect.a Ion Phys. £ (1971), 435-450 4) W. Fischer et al., Phys.Lett. 40 B (1972) 87-88 5) H.R. Ihle et al., to be published 6) R. Martin, W. Walcher, Marburger Sitzungsberichte 75. (1952), p. 5 H. Huhnermann, H. Wagner, Phys.Lett. £1_ (1966), p.~3O3 K. Mandrek, Dissertation, Marburg/Lahn 1972, p. 17 f? 7) M. Menat, Can.J.Phys. j42 (1964), 164-191 M. Menat, G. Frieder, Can.J.Phys. 43 (1965), 1525-1542 Chapter 3: Ion production -155-

THE TRIPLASMATRON : NEW POSITIVE AND NEGATIVE ION SOURCE

J. AUBERT, G. GAUTHERIN, C. LEJEUNE

Institut d'E4ectronique Fondamentale, Laboratoire associe au CNRS Universite Paris-XI, Batiment 220 - 91 405 ORSAY (France)

I - PRINCIPLE OF THE SOURCE

Previous work performed in our laboratory has shown that a plasma jet is emitted through the anode aperture of a duoplasmatron source. This 1 2 plasma jet is composed of ' : - an electron population with intensity up to several Amperes and energy in the range 20-100 eV. - an ion population with intensity up to 100 mA depending on the ion mass and energy in the range 10-50 eV. The plasma jet expands in an "expansion cup" placed behind the anode. Such a geometry allows the formation of high optimal quality and high intensity ion beam . Furthertnure, it has been demonstrated that depending on the expansion cup bias, a supplementary ionization may be obtained in the expansion cup, in relation with the ionization efficiency of the injected electron flux ; this effect is known as the "post-ionization". A gas or vapor different from the neutrals in the main discharge may be directly injected in the cup. h Based on this interesting effect, Masic and al have proposed a technological solution for a modified "mass separator duoplasmatron" as shown on Fig. 1 ; the duoplasmatron discharge is fed either Helium or Argon where as volatile compounds are fed to the expansion cup ; for metal ion production a vaporizer device is operated in a slightly modified expansion cup. Such a source presents certain advantages : there is no back diffusion towards the main discharge, thus the discharge parameters and operating conditions art- reproductible and the hot cathode is not affected : a long lifetime is obtained. Of course, these properties will be absent in a standard duoplasmatron, although a method for direct operation with metal ions has been studied by Illgen and al , especially for multiply- charged ion production : a ring shaped vaporizer oven is placed around the magnetized arc column between the "intermediate electrode" and the "anode".

A further amelioration of the performance of the "duoplasmatron expansion cup" device has been proposed in our laboratory ; the main idea is -156-

plasmo expansion cup plasma expansion cup with gas feed (Cu,Ta) with vaporizer device

extractor /

Fig: 1

Mass Separator Duoplasmatron MAS1C, SAUTTER, WARNECKE 1969

the control of the energy of che electrons entering in the expansion cup in order to adapt it to the maximum cross section of the reaction leading to the expected ions. This is obtained by a suitable bias of the expansion cup with respect to the anode of the duoplasmatron.

A positive bias gives the most interesting conditions ; a third plasma is formed in the expansion cup, which explains the name "Triplasraa- tron" given to this new operating device .

An elementary model •cup for charge production and transport mechanisms in this four electrodes structure has been elaborated from the analysis of energy spectra of the extracted particles and from plasma diagnostics with electrostatic probes

- The duoplasmatron acts as the "cathode" of the auxiliary discharge ; the variation of the electron -20 -10 current collected on the cup Electron current intensity vs cup voltage wall versus the cup voltage V is plotted on Fig 2a. -157- Bevond a critical value V , a C I mA low pressure discharge is formed. • ion The ion density increases, and consecutively, the extracted ion beam intensity (Fig. 2b) increases also : the gain can then reach one order of magnitude. - The potential of 05 third plasms located in the expansion cup is slightly more positive V and thus follows the variations of the latter. Volts This result is clearly illus- -10 10 20 trated by the variation of the -20 30 Fig. 2b energy spectrum of the extracted ions versus the cup pola- Ion beam intensity vs cup voltage rization as shown in Fig. 3 ; V V * ; the extracted ions are issued exclusively from the expansion cup plasma and their energy dispersion ( <~ 3OeV) is smaller compared to the previous conditions. - Consequently, the energy of the primary electrons issued from the duoplasmatron may be regulated continuously and independently of the pressure within 'chs expansion cup. This fact is very interesting because these electrons insure most part of the collision processes within the expansion cup plasma. It is the main supplementary advantage of the triplasmatron over the previous device. This ability of the triplasmatron will be illustrated by experimental results concerning positive and negative ion production.

II - EXPERIMENTAL RESULTS

For technological reasons, the experiments conducted on the triplasmatron source have been on the production of gaseous ions which not require the addition of an oven. We shall therefore present first the results obtained with oxygen, and then, to compare the normal post-ionization operation of the duoplasmatron with the triplasmatron, we shall present the order of magnitude of the current of "solid" ions which can be extracted, based on -15B-

VA »50v

VE| =30v B =2.7 KG

pA= 3.10 torr

= 3.103torr

Fie. 3

Variation of the energy spectrum of the extracted ions vs cup voltage

the results of Masic and al

A - OXYGEN

The extraction orifice used in these experiments is a 4mm diameter hole drilled in the bottom of the cup.

In our case, the reactions leading to formation of 0 anc1 0 ions by electron impact are : Threshold eV a~ cm max eV 19 e + 0- * 0 + 0 + e 17 ,3 4 X 10" 35 + -16 e + 0 •» O + 0 + 2e 18 10 200 + 16 e + 02 ^ 02 + 2e 12 3 X ,o" 150 -159-

Fig 4 shows the variation of extracted 0 current as a function of bias voltage on the cup. After a sharp increase, f = 0.40 which corresponds to the striking of the discharge, the ion current steadily increases with the bias voltage. The gain is about a factor of 10 with respect to "post-ionization". An increase of neutral pressure in the cup causes an increase in extracted current

for X- , 41R (radius ion neutral ** of the cup). Identical results Oxygen Positive ion beam Intensity vs are obtained for the molecular cup voltage. 0_ . An 0_ ion current greater than 10mA can be extracted under the following conditions

I = 4A 50V B = 2 Kgs arc arc Duoplasmatron — 1 Anode pressure : 10 torr Gas : Argon

Bias voltage : 45 V -2 Discharge cup Cup pressure : 10 torr Gas : Oxygen Extraction orifice : 4mm

+ 02 10mA 0 + 2mA Extracted current Ar 2mA Energy dispersion

The capacity of the duoplasmatron to produce electrons limites the extracted current. II is possible for the dmoplasmatron to operate continuously with intensities of about 50 A if thermal exchange is improved at the anode . Under these conditions, the extracted current increases in proportion to the arc current and it is possible to imagine the extinction of several tens of mA of 0_ ions.

A_-_2 Production_of_multiDly_£h^rged_ions

The possibility to produce multiply charged ior.r, with a triplasmatron can be -16D- understood as a result of the explanations in paragraph I. This is indeed ob served. The variation of the extracted beam intensity, as a function of discharge parameters (duoplastnatron and cup) follows the same law as that observed for monocharged ions. Under the same conditions as in §• A - 1 , an 0 ion current of 200 JAA can be extracted.

A - 3 Production_of_negative_ions

The three principal reactions are :

0? + e —•} 0 + 0

0 + e. —-» 0+ 0 + e o2 + o~ —» o~ + o •o- 50 Results are shown on Fig. 5 IA:4A Duoptasmatron : B = 2 kG - For a given pressure, the extracted current has a maximum as a function of f :O2flow V . This can easily be explained by regarding the cross sections for produc- f= 0.10 tion and destruction. f= 0.40 - The maximum of the f = 0.80 f = 1.20 fmnction I = f (cup pressure) can be explained in Fig 5 volts the same way (charge exchange) . Extracted current D 50 Vc is of the order of 200AJLA Oxygen Negative Ion Beam Intensity vs cup volt. for the conditions of § A-l

B - "SOLID" IONS

As already explained at the beginning of chapter II, we have not used an oven to vaporize elements, having a low vapor pressure. We have, however the results of Masic and al for a duoplasmatron with the expansion cup at the anode potential. It is therefore possible to estimate the probable results by considering the multiplying factors obtained for oxygen. This is summarized in Table I. -161-

Duoplasmatron Triplasmatron

Li+ 1 mA 10 mA

Cu+ 0,6 mA 6 mA

+ B > 0,1 mA > 1 mA

Discharge conditions as in § A - 1.

III - CONCLUSION

We have not attempted to optimized the extracted ion beams. An increase in the qualities of the duoplasmatron and also the optimization of the cup geometry and the topography of the magnetic field would permit a substantial increase of the intensity of the currents, while preserving the principal advantages of this source : no pollution of the hot cathode and the possibility to regulate the electron energy. -162-

REFERENCIES

1) ~ G. GAUTHERIN, C. LEJEUNE and A. SEPTIER - Plasma Physics 11

(1969) p. 397.

2) - C. LEJEUNE Thesis - Orsay (1971).

3) - G. GAUTHERIN - Nuclear Instruments and Methods 59_ - 1968 - p. 261

4) - R. MASIC, R.J. WARNECKE and J.M. SAUTTER - Nuclear Instruments and Methods _7J_ ~ '969 - p. 339

5) - J. SCHULTE IN DEN BAUMEN, J. ILLGEN, B.H. WOLF Proceedings of the Second International Conference on Ion Sources Vienna 1972.

6) - J. AUBERT - Thesis 3° cycle - Orsay (1972).

7) - A. ROCHE - Private communication. S -163-

A NEW ION SOURCE FOR ELECTROMAGNETIC ISOTOPE SEPARATOR R. Meunier and J. Camplan Laboratoire Rene" Bernas du C. S. N. S. M. - 91406 Orsay

1 - Introduction. Plasmar ion.sources are known as^tlie most convenient for isotope separators . In these ion sources, a magnetic field is generally used to confine the plasma and consequently to increase its density. It has been demonstrated that the positive charges can diffuse outside the plasma column, this diffusion beeing characterized by the diffusion coefficient D which is inversely proportiorinal to the magnetic induction B . This means that the plasma will more easily drift towards the region where B is low. This result can also be understood by taking into account the fact that the plasma energy decreases when it reaches regions (4) (4) where B is low . From elsewhere, in this paper by R. Pellat and G. Laval, the authors also point out that a minimum B hinders the plasma oscillations.

Consequently, we thought advisable to build an ion source having a minimum B in the region of the extraction orifice. Indeed one can expect i) a better extraction since the plasma will drift preferentially towards the extraction orifice, ii) a lower hash and iii) an increased life-time for the cathode since it will be less bombarded by the ions.

2 - Construction of the ion source. The minimum B is obtained by combining the cathode magnetic field and an external field. In a magnetron type ion source (fig. l) where the cathode is at a distance xc behind the slit, the cathode magnetic field B varies along the x axis as shown in fig. 2 (the field is plotted positive when it is oriented as the y axis and negative

in the reverse case). If one adds a homogenous external field Be parallel

to the y axis (figs. 1 and 2), the resulting field (B^ = Be + if.) can be zero

all along the slit (for x = 5 mm and a cathode current equal to 100 A, Bg must be about 40 gauss). Note that obviously the conventional magnetic field which is parallel to z is set to zero. -164-

The external field can be produced by a conventional " source magnet" located outside the vacuum tank, but this magnet should be oriented at a right angle relative to its usual orientation. Such a transformation is not very easy to do. Since B is not very high, in our experimental ion source it is obtained with a small electro-magnet incorporated in the ion source.

This ion source has been tested with Sidonie . The geometry of the front of the ion source and of the extracting electrode was a quasi flat one as frequently used in our isotope separators . The extracting orifice

of the ion source was a slit 1x20 mm . All tests have been made with

argon and an extracting voltage set at 40 kV.

3 - Preliminary results. The results presented here are very prelimi-

nary ones and must be considered as such. In particular, no theoretical

attempt to explain the observed results has been made yet.

The behaviour of this ion source is depicted by fig. 3 - This figure

needs some comments :

a) As it can be expected, the arc current depends only on IB land not of its

sign. Actually the arc current is not exactly symmetric with respect to B .

This is probably due to a slight assymetry of our ion source. The small

hump is observed for a B value (B = B , ) which corresponds to B =0 e e eh r r but also for the symmetric value of B probably because our cathode is

located in the middle of the arc chamber. The existance of these humps

is not interpreted. When B is increased beyond B , the arc abruptly starts (B =B ), c eh e es and then reaches a maximum (B = B ). Although this ion source is very e em ' different from a magnetron one, B can be compared to the cut-off field 6 S of a magnetron. Indeed in the vicinity of the cathode, the resulting field corresponding to B^ increases with the arc voltage as for a magnetron ion source. However, when operating at relatively high pressures inside the ion source, the abrupt rise of the arc current desappears and is replaced by a continuous rise.

b) - For negative values of B (i. e. when B adds to B or reinforces the -165- field in the orifice region) theextracted current is rather small. - For B = B (B =0 along the slit), few ions are extracted e eh r _ For B » B , the extracted current abruptly increases as the arc current and reaches a sharp maximum at smaller values of B than for the ' e • arc current. This could be interpreted in the following manner : the extracted current is proportional to the arc current and to D If D varies drift drift as shown in fig. 4, then the maximum extracted current appears before the maximum arc current. c) - The cathode limited mode of discharge and the pressure limited mode of discharge described by Chavet are clearly observed. d) - When working in the cathode limited mode, hash is not observed in the vicinity of the maximum extracted current. When B is larger than B 7 e B em (cf figs. 3 and 4) some hash appears.

e) - The efficiencies observed (at the maximum extracted current) are at least equal to those observed by Chavet for a Bernas type ion source. As reported in this paper, the efficiency strongly depends on the gas flow. Of course, the efficiencies of the two ion sources are compared at the same arc voltage.

The results presented here are, as we already said, very preliminary. During all the test made, the existance of a sharp maximum for the extracted current makes the adjustment of the source parameters easy to do. As can be expected from Chavet the focus quality is the same when using the Bernas-type ion source or our new ion Bource. When

operating at relatively high pressure, the current extracted at Bf = 0 (on the slit) can reach a few mA. This could be interesting since the " minimum B " ion sources are regarded as high brightness ion sources. -166-

1 - J. Camplan, Proceed. Conf. Intern. Sources d'lons, Saclay (1969). 2 - D. Bohm in A. Guthrie and R. K. Wakerling - Charact. of electrical discharges in magnetic field. Me Graw Hill (1949). 3 - W. Walcher in J. Koch - Electro magnetic isotope separators. North Hoi. Publi. Co (1958). 4 - R. Pellat and G. Laval - Les instabilites du plasma - The growth points of physics. Proceed. Florence Inaugural Conf. of the European Phys. Soc. - Riv. del Nuovo Cimento 1969 p. 214. 5 - J. Camplan, R. Meunier and J. L. Sarrouy - Nucl. Instr. and Meth. 84 (1970) 37. 6-1. Chavet and R. Bernas - Nucl. Instr. and Meth. , 5± (1967) 77. 7-1. Chavet and R. Bernas - Nucl. Instr. and Meth. , 47 (1967) 77.

Fig. 1 - Perspective view of a Fig. 2 - Cathode, external and magnetron ion source and defi- resulting magnetic fields. The nition of the coordinate system current in the cathode has the used in this paper. direction shown in fig. 1. -167-

—o- -o •

300 B 200 Br em

Fig. 3 - Arc current and extracted current versus the external field (Be) (Arc voltage - 80 V, gas flow «u. 01 cm3 s"1 NTP). The lower scale gives the values of the resulting field (Br) along the extracting slit (cathode current = 100 A, xr = 5 mm).

Fig. 4 - Variations of jr^ (hypothetical curve), arc current (experimental curve) and extracted current (deduced from the two preceeding curves) versus Bg. Both scales in arbitrary units. -168-

FORMATION OF MULTIPLY CHARGED IONS IN ARC DISCHARGE OF MAGNETRON SOURCE

W.2uk, D,Mg.czka, A.Wasiak Institute of Physics, Maria Curie - Sklodowska University, Lublin, Poland

A. Introduction A magnetron ion source was used for the purpose of ion im- plantatibn into semiconductors. Sources of this type are characterized by simplicity of constuction, good tightness of the ionization chamber, high efficiency. They also enable the production of ions of non volatile elements. Their usefulness for electromagnetic isotope separation was investigated by some authors /1,2,3/. Work parameter analyses of sources of this type can be found in literature /4»5/. In the mass spectrum obtained by means of magnetron ion sources, multiply charged ions v/ere obserwed

The purpose of this research was to find optimal work conditions for multiply charged ion production, especially for doubly charged ions of Zn, Cd, which were used for implantation in- to semiconductor compounds.

B. Apparatus The constructed ion source possesses a cylindrical discharge chamber of the capacity of 9*5 cm3 with an ion exit hole 1,2 mm in diameter and a tungsten filament drawn along the source axis. The source was located in the magnetic field, parallel to the cathode and the formed ion beam, In standard work conditions the ion source received the total electric, power of about 700 W. The current intensity of the heated cathode reached 44 A, the arc discharge current 1-2A, the anode voltage 120 V. In these work conditions it was easy to obtain currents of simply charged ions of the order of 20 ft A and -169- doubly charged ones of 0,7 A, i.e. quite sufficient for implantation purposes. The ion source was part of the electromagnetic isotope se-^ parator as described in previous papers and in recent years adjusted to implantation purposes at the maximum accelerating voltage of 70 iceV /7/. Tl' construction of the ion source is presented on fig.1.

S I Pig.1. Magnetron ion source adjusted to implantation of doubly charged ions. K-cathode, A-anode, S-ion exit hole, I-insulators, G-gas inlet.

C. Experimental data For the constructed ion source measurements of Zn and Gd ion

UQ=1OOV 40 Zn"1 30 /

a—i A 20

« 0,6 {,0 44 ifi 22 2,6 0 50 75 dOOtV] .''ig.2. Dependence of ion Zii Fig.3. Dependence of Zn ion current to the separator col- current to the separator collector as a function of the lector on anode voltage, discharge current at arc vol- Ja = 1,8 A. tage, Ua = 100 V. -170- current to xn^ r-ator -ollector as a function of the arc dia- charge current or « voltage at the remaining parameters unchanged, e.g. the arc iltage or arc current, were carrier out. Theltypical dependences obtained are presented on graph 2 and 3. The dependences of Zn ion current on the anode voltage, at the remaining parameters unchanged, are shown on fig.3.

Ztf* Zn* •MO

0 0,2 0£ 40 4,4 2,2 2.6 50 100

Pig.4. Zn ion current as a fun- Fig.5. Dependences of Zn ion ction of the discharge current current on the anode voltager. at different arc voltages.

8 On both graphs it is seen that the ion currents 6 Ua=75V increase with the arc dis- 4 charge current as well as with the arc voltage. Z However, this inci- ase is •MAI faster for multir"1"1. charged ions, especially for doubly charged ones, and in the Pig,6. Dependence of Cd ion optimal work conditions of current on the anode current the ion source the contri- at arc voltages, Ua = 50 V bution of doubly charged ion; and 75 V. in total ion current is 6-7,J. -171-

D. Disscussion of results Multiply charged ions are almost exclusively created in arc discharge conditions at low arc voltage, as a result o? stop io- nizatioh. The energies of the three first transitions from the lower to higher .ionization stage for Zn and Cd are relatively small. Suitable ionization potentials for the above mentioned metals and for standard Xe are presented on table I. Table I

Element Ionization potentials /eV/ Xo -*X+ x+-x2+ Zn 9,40 18,0 39,7 Cd 8,99 16,9 37,5 Xe 12,13 33,3 65,0

In the first phase of the discharge, during a time of the order of 10""' - 10 sec. singly charged ions are created, under the influence of the electron stream $ = n • v, where n - electron concentration. Then their concentration decreases as a result, of the successive ionization and of the increase of doubly charged ion concentration, which are further iodized into thirdly charged ions /9/. For ions of a charge Z', the concentration change nz in time can be written as :

In the formula /1/ ^z-i-*z denotes the ionization cross- section for suitable transitions, quickly decreasing^, th the multiplicity of ionization, k - the compound factor^to ion recombination oA the wall of the chamber and in the processes of interactions with atoms or electrons. In equilibrium state one can write •~TT"Z= f-'«I n some cases, as by short-lived current impulses, the last term in the formula /1/ is negligible. -172-

In this case the ion concentrations will be expressed as follows : £ & (2)

The results of the performed measurements proved that the anode voltage and discharge current increase cause an rise of the higher part of the multiply charged ions in the beam. Closer analysis indicates that the total electric power lead to the ion source plays a substatial role. The increase of the part of the multiply charged ions may be stopped as a result of cathode sputtering and reducing plasma temperature, as well as by raising gass pressure in the ion source. This effect is only poorly.marked on fig.4. The application of doubly charged ions enables a better utilization of the possibilities of the apparatus, as well as implantation of thicker layers by lower accelerating voltage. The total range R is proportional to the ion energies and can be expressed approximately as :

where : E - ion energy /keV/

Z1 - atomic number of ions

M1 - mass number of ions

M2 - mass number of atom target g - target material density. The thickness of the implanted layer is determined by the R as a projected range R in the direction of the beam, P

RP*fc-f 00 where coefficient f depends on : ion energy, ion mass, target atom mass, and is changing in the range of 0,5 to 0,8. -173-

Literature : /1/ Van Voorhis S.N., Kuper J.B.H., Harnwell G.P., Phys.Rev. 4J5, 492 /1934/ ~ /2/ Luhr 0., Phys.Rev. 49, 317 /1936/ /3/ Cobic" B., logic" D. and Perovic" B., Nucl.Instr. a.Meth. 2A, 358 /1963/ /4/ ToSi6 D..,.J.of Electroniee a. Control, Vol. JJ, 623 /I964/

/5/ 6o"bic B.f CiMn P. and To§i6 D., Proc. of the Seventh Intern. Conf. on Phenomena in Ionized Gases, Vol. I, 510 /1966/ /6/ ditto, Vol. Ill, 251 /1966/ /7/ ZukW., M^czka D. and Pomorski J., Nucl.Instr. a.Methods, 37, 249-258 /1965/ /8/ Kiser R..V., Introduction to Mass Spectrometry and Its Appli- cations, s.302, Prentiee-Iiall, INC, Englewood Cliffs, N.J. /1965/ /9/ Pigarov J.D., Morozov P.M., 2.T,]?., XXI, 476/1961/ /10/ Pasiuk A.S., Trietialcov J.P., Torhacev S.K., Preprint Joint Institute for Nuclear Research, 7-3370 /1967/ /11/ Pasiuk A.C. et al., ditto P?-4488 /1969/ /12/Mayer J.W., Eriksson L., Davies J.A., Ion Implantation in Semiconductors, s.29, Academic Press, NY - London /197O/. 5 VvJ 1M-- 'BCn^C -•' -174-

THE LENS EFFECT OF THE HOLE OR SLIT IN THE EXTRACTION ELECTRODE OF HIGH CURRENT ION SOURCES

M. Menat, I. Chavet and M. Kanter Soreq Nuclear Research Centre Yavne, Israel

1. INTRODUCTION

A reasonable knowledge of the factors determining the divergence of the ion beam in electromagnetic isotope separators is important in order to properly adjust the extraction parameters and to design accordingly the geometry of the source slit and the extraction electrodes. One of the determining factors is the behavior of the slit or hole in the extraction electrode as a diverging lens. Another factor is the influence of the space charge in the region of this slit. After finding the Gaussian magnitudes of these lens effects, it will be advisable to determine the aberrations and to devise methods to diminish or prevent them. Up to now, however, only an approximate study of the Gaussian magnitudes has been undertaken.

2. THE LENS EFFECTS OF SLIT AND SPACE CHARGE

The anode hole effect (also known as diaphragm or aperture lens effect) was calculated long ago by Davisson and Calbick for space charge free configurations. It has been treated, too, for convergent strong (2 3) electron guns ' in cases where the space charge influence cannot be neglected.

The conditions prevailing in the high ion current densities present in some electromagnetic isotope separators are not covered by the treatments mentioned. It is also not quite obvious, whether the theory of the diaphragm lens holds in the presence of considerable space-charge in a divergent beam. We therefore treated both effects together. The case of a concentric cylinder configurationC ' will be dealt with first. It is assumed that the space-charge does not widen the beam for a substantial part of the trajectory between source and extraction electrode. This can be achieved by shaping^' the various electrodes with the help of the well-known theories of Langmuir and Pierce combined. It is known(2) that the Pierce configurations -175-

of the electrodes function properly only if a grid is present in the extraction electrode. Danielson et al. ' dealt with the influence of eliminating the grid and calculated effective voltages for the various electrodes. The situation in our case is sketched in Fig. 1. Inside the extraction electrode A, on the axis, the potential is still slightly higher than the final earth potential inside the separator. Between the planes A and q, a negative voltage m prevents the electrons in the separator proper from escaping towards the source region.

Fig. 1 Schematic representation of the beam potential as a function of the distance from the exit slit

Therefore, from the plane q onwards there should be no disturbing space-charge effects. Between the source S and a certain plane p the beam is also expected to be free of space-charge disturbing effects, as has been mentioned before. The distance pq is of the order of the extraction slit width s (see Fig. 2). In the region between p and q electrostatic and space-charge forces push the ions outward. At q the ions emerge with a lateral velocity -176-

Fig. 2 Diagram of the various source and extraction electrode magnitudes

where M is the proton mass and v the iou velocity. The value of the integral may be found to a good approximation using Gauss' law, which states that the surface integral of the flux density equals the enclosed charge.

Applying Gauss' law to the region between p and q, yields

fq rq -,• -e E s h + 2h E E dz = -i-r dx, (2) o p p o J r j V(T) '

where E is the field strength at p, h the slit height, i the current density and dT a volume element. -177-

One may develop (2) and find to a good approximation q 2(1 /h)c s c9 V t 2 a dz = — ^ + r •2e V -iV 2d p 2 e a MM

where It is the total ion current. The other magnitudes follow from

Fig. 2. Both the constants c1 and c2 are of the order of unity. By inserting the result (3) into (1) one finds the sought lateral velocity u^. The situation can then be represented by a lens whose power is defined by

fTlV 2 2 l

Combination of the various relations yields

c (I /h)/~M

where the constant c, is again of the order of unity.

The term l/(2d) represents the classical Davisson arid Calbick's slit diaphragm contribution and the second term the space-charge repulsion contribution. The space-charge part strongly reminds Us of the Langmuir- Childs law of space-charge limited current density between flat electrodes

3'2

(7 8) Chavet ' has described the various links between these magnitudes and at least three other important parameters: the slit width f , the original beam opening a and the radius of curvature of the meniscus r •

By making use of his definitions, and by assuming c^ ^ 1 for the moment, one finds

l/f - |j (1 + |a+B) , (7) -178-

where 6 is defined as

6 = fs/d . (8)

This result can be rendered in at least two alternative ways:

Act « i (ct+B)[l + (4/5)ct+g] (9)

expressing the additional divergence, and

t . d SSL (10)

being the distance of the effectiveob jectpoint witl. respect to the source slit plane S.

For a round configuration, instead of a rectangular slit, the treatment is much the same. One finds

0 63 1 AaQ - i (o+B) [1+6(1+ a/6) ' ] (9 )

and

3. CONCLUSION

It is thought that the physical and optical phenomena herein discussed and the mathematical expressions describing than will be helpful in the design of the geometry of the source and extraction electrodes. -179-

4. REFERENCES (1) 0. Klfemperer, Electron Optics, Cambridge, University Press, 1971, p. 76-80. (2) P.T. Kirstein et al., Space-Charge Flow, McGraw-Hill, New York, 1967, p. 331-334; 407-419. (3) G.R. Brewer, J. Appl. Phys. 28 (1957) 7. (4) R. Meunier, Proc. Itit. Conf. "Electromagnetic Isotope Separators and the Techniques of their Applications". (Ed. H. Wagner and W. Walcher, Physikalisches Institut der llniversitat Marburg, 1970) p. 331. (5) H.Z. Sar-El et al., Nucl. Instrum. Methods, in press. (6) W.E. Danielson et al., Bell Syst. Tech. J., 15 (1956) 375. (7) I. Chavet and R. Bernas, Nucl. Instrum. Methods, 47^ (1967) 77. (8) I. Chavet, as Reference 4, p. 303. Chapter 4: Machine development sw~=f i\-3 oasi -180-

THE HIGH VOLTAGE MASS SEPARATOR AT THE CHALK RIVER NUCLEAR LABORATORIES

F. Brown Chalk River Nuclear Laboratories Atomic Energy of Canada Limited Chalk River, Ontario, Canada

INTRODUCTION The HVMS is intended to provide analysed beams of any ions up to mass 240, including radiotracer beams, with an energy of 2 MeV per charge. It is thus intended as a versatile accelerator for solid state studies, atomic collisions and atomic physics. This paper will deal very briefly with the conventional aspects and emphasise the more unusual features of the installation.

GENERAL LAYOUT The layout is fairly conventional, as shown in fig. (1) . The beam leaving the accelerator (A) is focussed by a magnetic quadrupole triplet (B) and analysed by a 90° sector homogeneous field, double focussing magnet (c) having a radius of 2 meters. The separated beams pass into a wide monitor chamber (D) behind which is the target chamber (E) . It is possible to install further target chambers, either in parallel or series.

ACCELERATOR The accelerator is a 'Pelletron' manufactured by National Electrostatics Corporation, operating in a pressure tank of

SF& at 90 psi, with a terminal voltage up to 2 MV. The general principle of the pelletron is well known, being similar to that of a van de Graaff accelerator. The principle differences are (a) the use of a chain, rather than a belt, to carry charge to the terminal, (b) provision of a rotating shaft to drive the generator in the terminal (c) an acceleration tube capable of withstanding both mechanical and thermal -181-

shock, (d) a strong column structure and (e) a large terminal capable of supporting heavy weights, and having high generating power.

Because of the terminal capacity a wide variety of ion sources can be used. At present a Danfysik model 910 source is installed, complete with all power supplies and cooling via a closed-loop liquid freon system. Gas can be supplied from one of three gas storage bottles and the source has the normal oven and heater supplies. Tha beam is extracted and f ocussed by a conventional arrangement of an independent extraction electrode and an einsel lens. Maximum acceleration between the source and terminal "ground" (i.e.. the injection voltage) is 40 keV. The power is supplied by two 400 cycle, 3 phase generators giving 3 kw total output. Terminal supplies are controlled by insulated rods and the terminal parameters are read out from meters observed through the tank base by a TV camera. The ion source is differentially pumped by a titanium sublimation pump. Finally there are electrostatic steerers in the terminal.

These details show that the terminal can provide all the features considered necessary or desireable for a conventional electromagnetic isotope separator.

The other traditional problem of pressure tank accelerators is the time required to remove the tank and to-replace it, i.e. the "turn around" time. The Pelletron operates at relatively low pressure (maximum 90 psi) and has an effective pumping and

storage system for the SFC (which is kept in gaseous form at all times). The tube and column are not sensitive to thermal shock. This particular accelerator has rapid release flanges on the tank, operated hydraulically, rather than the conven-

it tional system of bolts. Tests, so far, indicate a total turn around"time of about 40 minutes. Since the time required to -182-

cool the ion source, reload, or provide maintenance is normally in excess of this, the "turn around" penalty is small.

The Pelletron is pumped by turbo-molecular pumps located at the base. The remainder of the installation will also be pumped by turbo-pumps and is of all metal construction.

BEAM DIAGNOSTICS AND CONTROL Beam profile monitors (x-y) are provided at the tank base (before the quadrupole), at the objective slits and in position between the monitor chamber and the target chamber. The monitor chamber incorporates (a) an x-profile monitor, moveable on a track and (b) a set of slits, also on a track. Adjustment of monitor position, slit position and slit width are all external. The width of chamber and track corresponds to a mass dispersion of 4% (16 cm) either side of centre. The slits will provide the signal for external energy stabilisation of the Pelletron. In addition there are Faraday cups, beam stops and adjustable apertures at suitable locations. There are subsidiary beam steerers located after the objective slits, followed by an adjustable aperture; this arrangement can be used in conjunction with the other controls to ensure that the beam enters the magnet on a pre-determined line. Beam sweeping is achieved by a combination of magnet sweep and vertical electrostatic deflection at the magnet exit.

OPTICS Initial calculations indicated that, for a circular beam the spot size at the target should be 3.5mm and the divergence ± 1.8 mrad. Tests now show an emittance from the accelerator of roughly one half of that originally assumed. Hence there should be a corresponding improvement in beam quality at the target. The target position is slightly behind the focal point and the monitor position (slits, profile monitor) slightly -183-

ahead, so that conditions will be roughly the same in both locations. The present intention is to operate the magnet in a slightly asymmetric configuration (although the magnet is, itself, symmetrical) since this gives a lower divergence to the beam; the corresponding increase in spot size does not appear to be of importance since the dispersion of the magnet is so large. The low divergence is of importance since the HVMS will be used for channeling and scattering studies. The numbers quoted at the beginning of this section are calculated for a beam focussed initially at a point two feet beyond the true objective point of the magnet. They assume that all slits are absent, i.e. the beam is not apertured in any way.

PERFORMANCE So far the accelerator has been tested only at the factory and hence without use of the analysing magnet. Beam currents of 20 p,A were obtained using hydrogen, argon or arsenic in the sources and beam currents were very steady. _7 The pressure, measured at the tank base, was 2 x 10 torr with the source off: during operation the pressure was in the range 4 x 1O~7 to 3 x 1O~6 torr.

There was no difficulty in removing and replacing the tank two or three times during a normal working day in order to make minor changes in the ion source power supplies and then test their effects on the running characteristics- (using argon beams). This indicates that the pressure tank presents little real in- convenience to the user.

The accelerator was operating with internal energy stabilization (via generating voltmeter) and at 2 MV terminal voltage the fluctuations were less than ± 500 eV. With external stabilisation from the magnet the stability should be better than this, especially in view of the very high dispersion (1mm at -184-

the slits is equivalent to approximately 500 eV) . The stabilising circuit is designed so that if the slit signal drives the terminal away from its original setting by more than a pre-set (but adjustable) voltage, the machine automatically turns to internal stabilisation. Similar systems have been used on other accelerators for various reasons. It is especially important in the present case because the HVMS, being a multi- beam accelerator, has the standard problem common to other mass separators, i.e. ensuring that pin stabilisers do not lock on- to the wrong beam after some perturbation. The accelerator can be made to operate at low terminal voltage by shortino part of the column and. reducing the tank pressure. There was no difficulty in operating at 200 keV and it appears that with minor changes to the corona system it should be possible to operate at 100 keV.

Note that the injection voltage must be added to the terminal in order to give the actual beam energy. The external stabilising mode (slits) examines the beam energy but in the present design at least, the internal stabilising mode examines the terminal voltage. This means that an appropriate allowance must be made when the settings are made and further, the injection voltage must be reasonably constant during operation if the system is expected to switch automatically between external and internal modes. This arrangement is workable but not ideal, especially since the injection voltage can only be read out via the TV display, it appears to be an area in which one could seek for an effective but inexpensive improvement. A much more significant area in which to seek for improvement is the use of ion sources capable of producing multiply charged ions in good quant:ty. The energy range of the HVMS, as with many other accelerators, would be greatly enhanced. There is currently much interest in such sources and the HVMS terminal has the capability to use them. -185-

•iJiLHikiSgii^^^^ffitSiifls i -186-

P. A. R. I. S. THE NEW ISOTOPE SEPARATOR AT ORSAY J. Camplan, R. Meunier, C. Fatu Laboratoire Rene" Bern as du C. S. N. S. M. 91406 Orsay

The P. A. R. I. S. isotope separator built at Orsay (P. A. R. I. S. = Petit Appareil pour la Recherche, 1'Implantation et la Separation) has been initiated by the late Professor R. Bernas, in order to separate very pure stable isotopes in milligram quantities and thus to reduce the delivery time of the samples produced by our laboratory (the separations made with SIDONIE have to be scheduled two years in advance).

(2) The Paris machine uses the same ion sources as Sidonie and the same extracting system * ' i. e. a simple adjustable electrode set at a slightly negative voltage in order to obtain a neutralisation of the space charge. The high voltage supply can deliver 20 mA, The vacuum system, the power supplies etc. . . of this machine are very conventional and will not be described any more. But its magnet is used in a rather new way and this will be discussed in more detail.

The unsvmmetric arrangement of the Paris isotope separator

The magnet of an isotope separator must give a high dispersion and a sharp focus. This does not mean only a high resolving power since it is preferable to work with a machine giving 1 cm between two foci 1 mm large (for instance) than a machine giving 2 mm between two foci 0. 1 mm large. Although the first case corresponds to a smaller resolving power, it will probably gives more pure samples.

The dispersion obtained with a given magnet depends not only on the various parameters of this magnet, but also on the distance between the object and the entrance face as it is shown in fig. 1. In a sym-

metric arrangement (LT •- 1. = L ) the dispersion id D If L is smaller ' • & o O

On leave from Institute! ^ .. Stabili - Cluj - Romania -187-

than Lg) the dispersion is increased as well as the lateral magnification. However we have assumed that the increase of this lateral magnification was not important. Indeed, the image width (IW) can be expressed by :

IW = M x OW + other terms where OW is the object width and " other terms " include the various broadening of the image due to the radial aberrations of the magnet, the scattered and chromatic ions , the fluctuations of magnet and high voltage power supplies, the plasma oscillations (hash) etc.. . The preceeding equation shows that IW increases more slowly than M . In addition, (6) the studies made by Chavet suggest that, for a plasma ion source, OW is very small.

Consequently, we assumed that it would be worthwhile to use a magnet in an unsymmetric arrangement since the disadvantage of the increased M would be widely compensated by the increased dispersion. The validity of this assumption will be demonstrated latter.

Determination of the magnet parameters

It was decided to use a magnet giving a z focusing and, even more, giving " double focusing ". Such a situation has the advantage to give an image height which only depends on the slit height and not of the emitted beam direction. It was also arbitrarly decided to obtain this double focusing property by a 0. 5 index field and normal entrance and exit boundaries.

The remaining parameters to be fixed (mean radius, deflection angle, etc. . . ) have been determined by calculating with a computer a great number of situations. Various tests have been used of course in order to reduce the number of results. One of the test was the following : for each magnet, the weight of iron was calculated by a rough formula. Then the ratio of dispersion/weight was calculated and only those magnets having this ratio large enough were printed out by the computer. Finally th main characteristics of the Paris magnet are shown in fig. 2. -188-

r.nmparison between symetric (LQ = Lg) and unsymetric (LQ « Lj arrangements

Fig. 3 represents the experimental current distribution at the focus of the Paris and Sidonie separators. Both curves have been obtained v/ith a diverging beam ' ' of about 6° total aperture, and the extracting (7) geometry proposed by R. Meunier v '. As it can be seen, the discrepency between the two curves is very small. We consider these curves prove the validity of the assumption made concerning OW. Indeed the focus widthes are similar whatever the fraction of the maximum we consider. The influence of M does not appear. let t

If the Paris magnet were used in a symmetrical arrangement we can assume the current distribution would be the same since we already have the same distribution for Sidonie (symmetric arrangement) and Paris (non symmetric arrangement) in spite of the different radii of these machines. Let us imagine that fig. 3 represents the distribution of mass M and that we have to collect the mass M + 4M (not shown on fig. 3). In the hypothetical symmetric Paris, the distance d between these two o masses M and M + AM would be smaller than the distance d of the actual Paris. Consequently, for a given collector slit width (the collector slit shown in fig. 3 would admit 70 % of a beam), the collected amount of the unwanted isotope would be larger, the difference between these two amounts beeing represented by the hashed area (fig. 3). Hence the unsymmetric arrangement appears as a way to increase the purity of separated samples obtained with a given magnet..

The advantage of the unsymmetric arrangement is such that we are contemplating modification of Sidonie in order to increase its dispersion. It could be that the improvement will not be as high as expected since the increase of the exit face-collector distance will lead to a small increase of neutral particles. -189-

The Paris isotope separator is a rather new machine. No separations have been rrsde yet. A correcting profil' of the entrance face has (8) been determined and built. It corrects the radial aberration for currents of deflecting magnet lower than 40 A (maximum 100 A';. In the next step we will determine the correcting profile(s) required for working at higher currents. References 1 - J. Camplan, R. Meunier, J. L. Sarrouy - Nucl. Instr. and Meth. 84 (1970) 37. 2 - J. L. Sarrouy et al. - Nucl. Instr. and Meth. , ^i (1965) 29. 3 - H. A. Tasman - Ion optics of mass spectrometer with virtually enlarged radius - Thesis FOM 1961. 4 - H. A. Enge - Focusing of charged particles (ed. by A. Septier) Academ. Press 1967. 5 - J. Camplan, M. Van Ments and R. Bernas - J. Phys. Radium, 2J_(1961) 191 A. 6-1. Chavet and R. Bernas - Nucl. Instr. and Meth. , 47_(1967) 77. 7 - R. Meunier - Proceedings Marburg Conf. 1970 p. 331. 8 - J. Camplan and R. Meunier - Nucl. Instr. and Meth. _57 (1967) 252.

Fig. 1 - Dispersion (D), lateral magnification (M]^) and exit face-image distance (Lj) for a given magnet (paramotoro givon in thoiiguro) versus the object- entrance face distance (LQ) - D, LT and Lo are expressed in units of the mean radius. These curves are valid for = 100°, n = 0. 5, normal entrance and exit boundaries. -190-

1640

-P—-

Indei sO.S d. (mm; = 1500^ »,.,. si.5

PJL.B.I.S.

Fig. 2

ktn lource

Fig. 3 - Current distribution (experimental curves) of the images obtained -with Sidonie (symmetrical arraggement and Paris (unsymmetrical arrangement) separators. Both curves have been obtained with 40 keV Argon and a Faraday cup to mesure the current. The beam characteris tics are given in the figure.

P : PARIS 9 .SIDONIE b.a. i beam aperture crt. :current crt.d. i current density S3 -191-

DESCRIPTION AND PERFORMANCE OF THE MEIRA E.M.I. SEPARATOR

I. Chavet, M. Kanter, I. Levy and H.Z. Sar-El Soreq Nuclear ReseaYtti Centre Yavne, Israel

1. GENERAL MEIRA (Hebrew initials for high output, stable isotope separator) whose assembly has been recently completed at the Soreq Centre is undergoing its first trial runs and subsequent design improvements. This instrument has a two-fold purpose: the production of high purity isotopes in laboratory quantities, mainly for nuclear research use, and the study of the basic problems encountered in isotope separation technology. This dual aim dictates the following requirements: high output, high enrichment factor, versatility in operation procedure and convenience in use and maintenance. A high output calls for a long emission slit and wide beam divergence, a high enrichment factor for an advanced ion optical system combined with careful attention to pumping speed and other vacuum arrangements,. To satisfy the last requirement, provision must be made for different sources and source power supplies, different arc regimes and extraction modes as well as adequate mechanical design to allow easy removal of any section of the separator.

2. ION OPTICS AND BASIC DATA The ion optical system (Fig. 1) was already described at the previous Conference . The analyzer is a homogeneous magnetic sector with a relatively small deflection angle, a sloped exit boundary, magnetic shields and curved boundaries. The beam is crossed in the axial plane at the entrance boundary and the values of the optical parameters are so chosen as to eliminate aberrations to the third order. However, correction for the third order aperture 3 . . • '. aberration term (a ) will be made empirically only on a beam of average intensity in order to take into account any possible additional aberration factors arising in the extraction system of in the beam itself. The maximum length of the emission slit is 100 mm; the maximum beam divergence ±0.1 radian. ' The expected dispersion is 1190 AM/M mm, perpendicular to the beam. The calculated focal plane slope, with respect to the main trajectory, is °,1A° and the image length at half-height is 85 mm. -192-

^p^s^j"' - ' _jc

•'//ss/fss/ssss's'J/A EST! ™ •• •*

® LIST OF OPTICAL PARAMETERS R • 625 mm. f = 64° L' • 1662 mm. R' • 196I mm. e1- o L° • 1663 mm. R" • 3490mm. £"• 29° 40' 6 • 80 mm. a • O.I Rod. 7l' * 26 mm.

Fig. 1 Ion optics of MEIRA

3. ANALYZER MAGNET The analyzer is a C-type magnet weighing about 7.5 tons. The coils are wound around the pole pieces and shaped to allow free access to the entrance and exit faces of the pole pieces for shimming purposes. The main gap is 80 mm; the pole pieces are independent and separated from the yoke by two homogenizing gaps of 3 mm width. Planeity and parallelism of the pole faces are within 0.01 mm. The magnetic shields are not of the frame type; instead, each half-shield returns the magnetic flux of the fringe-field around the coils directly to the yoke. This is necessary to avoid saturation of the shields due to the very wide aperture of the beam. The magnetic field is regulated by a Hall probe and Varian Fieldial Mark II to ±10~5. The magnetic field can be reversed for demagnetization purposes:. -193-

4. VACUUM CHAMBER AND PUMPING EQUIPMENT The vacuum chamber and pumping equipment have been described at length in a previous publication( \ It is composed.of independent sections which

1 — SOURCE INSULATOR .,.16 - ENTRY AND EXIT INTERMEDIATE CHAMBERS

2 - SOURCE CHAMBER 11.14 _ MAGNETIC SHIELDS

3 - ELECTRODE FLANGE 12 - ANALYSING (DEFLECTION) MAGNET

4 - SOURCE MAGNET 13 - DEFLECTION CHAMBER

5 - VALVE ISOLATING THE SOURCE CHAMBER 19 - GATE VALVE ISOLATING THE COLLECTOR CHAMBER

6 — SECOND DIFFERENCIAL DIAPHRAGM 20 _ COLLECTOR CHAMBER

7.18 - SLEEVES 21 — DIFFUSION PUMPS AND LIQUID NITROGEN TRAP PORTS

8,10.15.17 — INTERMEDIATE FLANGES Fig. 2: General view of MEIRA are easily dismantled for cleaning or maintenance purposes. Stainless steel is used throughout except for the deflection chamber, where titanium is preferred for its nonmagnetic properties. Viton 0-rings are used and plastic components are avoided as much as possible. The pumping equipment consists of 8 oil diffusion pumps, provided with water-cooled baffles, maintaining a net pumping speed of 7000 Jl/sec. These 3 3 are backed by a Roots pump (400 m /h) and 2 small rotary pumps (25 m /h each). A liquid nitrogen trap is placed on the vacuum chamber above each diffusion pump, instead of the conventional placement, because this does not reduce the pumping speed. 5. HIGH VOLTAGE-POWER SUPPLY The high voltage limit is 50 kV and the maximum intensity available 100 mA. It was difficult to reconcile these values with the stabilization required, namely ±2xlO~°, and with the ruggedness expected from a power -194-

supply to be used on an e.m.f. separator. The power supply (Fig. 3) developed to fulfill these requirements operates as follows:

LINE

Fig. 3 Block diagram of the high voltage power supply

PS1, the bulk power supply, is capable of delivering up to 50 kV; in series with it, the smaller power supply PS2 is equipped with a regulating triode and is capable of stabilizing rapid transients (10 usec) up to ±3 kV around an average working value of 6 kV. The necessary signals are provided separately for dc and ac from a precision high-voltage resistance chain R and an additional non-precision chain R' whose essential function is to provide guard rings and corona shields to the precision chain R. The power supply PS1 is regulated through a saturable reactor SR and a magnetic amplifier MA at fair speed (1/2 sec) so as to maintain the triode in PS2 at its average working voltage. Moreover, since the working range of the saturable reactor is limited, this component is preceded by a motorized variable transformer VT which maintains the saturable reactor within its linear working range (speed: a few seconds) by comparing the output from MA. with a reference voltage REF2. Finally, since this optimum range depends on the current load, a signal proportional to the load (taken from SR) appropriately modifies the reference signal REF2.

Obviously, all these intercepted loops must be carefully adjusted for

amplification factor, feed-back, ,>iv,se-.™gle and bias in order to function -195-

properly and avoid any oscillating mood. Adequate protection devices against; H.V. surges are installed at all critical-points.

6. BEAM SHAPING 6.1. In the radial plane, the beam divergence is controlled under different operating conditions, by the distance between the acceleration electrode and the emission electrode which is also the source coyer-plate . For this purpose, the acceleration electrode is hung on two arms fixed on the diaphragm plate located between the source chamber and the body of the separator (the "differential pumpiio" diaphragm). The mechanism operating the electrode is located entirely in the thickness of this diaphragm plate around the gate valve of the beam window (Fig. 4). This mechanism commands five independent

1. Extracting electrode 2. Cylinder controlling the valve movement 3. Gate valve 4. Differential diaphragm 5. Aperture for the beam 6. Cover of the electrode mechanism A,B,C,D,E Electrical commands of the electrode (Controlled from the desk)

Fig. 4 Acceleration electrode mounted on the diaphragm plate

electrode movements indicated in the figure. Translation along x controls the beam divergence, rotation around the z and y axes orients the beam in the radial and axial planes respectively, translation along y centers the beam in the electrode slit and rotation around x corrects the parallelism between the emission and acceleration electrode slits. All these movements are remotely controlled from the desk and indicated there. -196-

Since most anticipated separations will be achieved with a divergent beam (convex meniscus), the necessary Pierce electrode .profile has been calculated for these conditions(4' and applied for the emission and extraction electrodes. No experimental results are yet available. Also the diverging lens effect of the electrode slit has been calculated for the case where the space charge of the beam cannot be neglected. This was reported in another communication of this Conference

6.2 In the axial plane, the beam crossing required by the optical system of MEISA, is obtained by appropriately curving all the extraction geometry, that is, the emission and acceleration electrodes as well as the internal wall of the emission electrode and the magnetic axis of the ion source magnet. This last curvature is obtained by sloping the faces of the pole pieces.

7. ION SOURCE The first source installed is of the Calutron type with internal cathode. It is very similar to the source utilized at Orsay except for the much longer slit and the curved wall of the cover-plate. A high- current (350 A) cathode power supply is also available for cases where a Freeman source is preferable,. The ion-source power supplies are interconnected by appropriate circuits and servo-systems in order to make the source -paration somewhat automatic. This arrangement allows rapid attainment of the required operating conditions and maintenance of these conditions without undue intervention of the operator. Three operating modes are possible: 1) Manual: All controls are manual; a resistance is inserted in the arc circuit. 2) Pressure-limited: In the first step, the arc current is automatically adjusted to the required value. The arc voltage obtained will then depend on the source pressure. In the second step, the arc voltage also is regulated to the required v^lue by controlling the gas leak or the furnace temperature. The beam current will then be constant and its value determined by the values of the mentioned parameters. 3) Cathode-limited: In the first step, the arc current is automatically adjusted to the required value by control of the cathode heating current. In the .second step, the beam current is adjusted by control of the gas leak or furnace temperature. -197-

8. PRELIMINARY RESULTS

The efficacy of the magnetic shields,was measured: with a field of 0.8 Tesla in the main gap, the field just outside the shield falls to about 10~4 Tesla. A short demagnetizing procedure was worked out lasting approximately 1 hour (1 1/2 cycle). This procedure reduces by a factor of 2 the main field inhomogeneity and improves the reproducibiJLity of the fringe-field distribution, especially at the pole piece corners where saturation effects are significant. The distance of the effective boundary to the mechanical boundary (n* in Fig. 1) was found to be 0.35 in gap units for a distance of the shields equal to one gap. The stability of the main magnetic field was tested with an NMR probe and found to be within ±10 ppm for 7 hours. The high-voltage power supply at this stage of its development gives the following performance: stability ±15 ppm for 4 hours, for 30% load change (load up to 100 mA) and ±10% line voltage variations, from 30 to 50 kV. However there is still a significant ripple which, it is hoped, will be reduced by further work. The residual pressure in the separator 24 hours after full exposure to the atmosphere is 7x10 torr without liquid nitrogen traps and 3x10 torr with the traps filled. The time needed to completely dismantle or assemble any component of the vacuum chamber may vary from 10 minutes (entrance or exit sleeves) to 20 minutes (electrode flange). The ion source has been tested successfully with an arc current up to 15 A (without high-voltage) in the pressurecor cathode limited mode but only in the first step of automatic control.

When the beam is properly neutralized, no difficulties are encountered in crossing the beam at the magnet entrance, where it passes through a diaphragm aperfture 7 mm high. However when the beam is "hashy" or when it is incompletely neutralized, the sharpness of the crossing deteriorates significantly. A fairly focussable beam of nitrogen has already been drawn from a source p-iuipr ;J v^th ar. amif-.ion slit of 100x1.5 mm, the beam current at the soi rap buing 35 mA a~d tTie total divergence 6°. However, many design details muot be completed, optimized or improved before the full performance of the machine cnn. ta achieved. -198-

REFERENCES 1. I. Chavet, Proc. Int. Conf. on Electromagnetic Isotope Separators and the Techniques of their Applications; Marburg, 1970 (BMBW-FB K 70-28) p. 366. 2. I. Levy and I. Chavet, Vacuum, 21 (1971) 325. 3. I. Chavet, Ref. 1., p. 303. 4. H.Z. Sar-El, I. Chavet and M. Kanter, Nucl. Instr. and Meth. (in press). 5. M. Menat, I. Chavet and M. Kanter, these proceedings, Communication B6. 6. J.L. Sarrouy, J. Camplan, J.S. Dionisio, J. Fournet-Fayas, G. Levy and J. Obert, Nucl. Instr. and Meth. 38 (1965) 29. -199-

An Universal Range Ion Accelerator and Separator.

URIAS

G. Sidenius and 0. Hoick

The Niels "^ohr Institute, Copenhagen, Denmark

Introduction: There are several reasons for this reDort about the old Conenharon separator. First we havi? added some rather special improvements since the last report by Nielsen and Skilbreid in 1958 ', second we want to pay homage to the late J0rgen Koch who originated 2) this unique piece of equipment and put it in operation in 1943» wherefore we are cele- brating its 5o years anniversary this year.

General Principle. The main requirement behind the last reconstruction wan to extend the usefull energy region down to very low energies (loo eV) without decreasing the beam quality and transmission. In a normal isotope separator, space charge effects will prevent operations with sufficient resolution and transmission below 5 to lo keV. The rather complicated target instrumentation in the planned measurements excluded the use of a retardation voltage on the target, wherefore the only possible solution left was to put the whole beam transport system on a potential ami in this way to obtain a retardation system. And this is in fact the most distinquished feature of URIAS, as seen in figure 1, that the beam transport from the extraction electrode through the lens system anr1 the analysing magnet to the aperture plate take place in a floating system, which will be on a potential Up, relative to ground equal to the difference between the extraction- and analysing voltage U. and the enrrfy voltage Up. Up will be able to take all values between - 5o kV anil + ?o kV. 3 in OVEN c+ ft GAS V CD -3 ro ION rt- j FLOATING POTENTIAL Ua SOURCE [U 3* 3 CD re CABINET INSTRUMENTS l-« CD X CD INS- "A en TRANS en AC. 3 o I o 1 INS- 220 § CD TRANS A.C. 13 ••i i j— P! 1 < INSULATION • LIGHT I ICHAH o s o CHAN. •d r o O o 1 1 1 O '"I 4 P | p a

cf CD I 1 a) CONTROL DESK q!

rt- eD URtAS

tf/ri/I 1 i- 4 : So.homatic anri main electrical connections. i w -201-

Fortunately the niagnet was designed with a 60 mm gap, but the beam requirement was less than 45 mm. A stainless steel pipe with a ?r> mm outside diameter and a 2 mm wall thickness: was..-successfully bend in a 9o°, 800 mm center radius segment, and flattend to a 49 mm outside thickness on the bend part, but still with cylindrical straight ends. The pipe was then covered vith two layers of a 2 mm shrinkage plastic tube, which after treatment in a hot oil bath formed a tight fitting 5 mm insulating layer on the whole analysing chamber. The stainless steel pipe was connected electrically and mechanically to the two internal chambers and the connections and vacuum sealings to the outer chambers were performed with insulating flanges. In the inner collector chamber is mounted a beam scanner anri an aperture plate with a 5 nun diameter aperture in the beam centor lino. To prevent secondary electrons escaping from the floating rry::t.em, all openings in this are covered with baffel plates. The outer chambers are each connected to a 9" diffusion pump and the normal vacuum optained is better than lo torr.

The Electrical System. An other complication of the TOIAS system is, that, excent for the map-net current supply, all the beam controlling systems have to !e floating. Thus, the systems are placed in an insulated caMnet, as shown in the unper left corner of figure 1 and are fed by insulation transformers. The controls and signals are relayed via a light channel syntem to and from the control desk. The floating system contains of the extraction- and analysing voltage U., which normally is fixed 3o kV, stability - o.o5 $, the lens voltage TJL which is varip.ble from 0 to 3° kV, the X-plate supply which is variable from 0 to 5 kV, the beam current monitor and the beam scanner supply. These are standard denies but remotely controled via the light channel system. The ion source is a standard hollow cathode ion source which is fed from rectifier supplies powered from variac controled insulation transformers. Of special design is the gas supply system . The gas flow is stabilized by apnly:ng the feed back from a constant temperature Pi rani e gauge to an electronically controled leak valve. The oven temperature -202-

is controled by placing the oven in different positions along the temperature gradient of the anode tube. Poth temperature and gas flow is controled hy insulated rods from the control desk, but a light channel system is for the moment being built. Together with the magnet supply, which is a cenventional transistor regulated supply, max. current 4o A, stability - o.o5 #, we now have all what is necessary to obtain a mass separated, well defined 3° keV ion beam at the collector aperture. This beam is floating with respect to ground potential, until we connect the main energy voltage supply Ug, which in able to go from less than loo V up to 7o kV with a load of about 1 mA and a ntahility of about - o.o5 $. The exact energy voltage is read on a o.o^ % accuracy digitalvoltmeter connected to U with a looo MOhm - o.l f metalfilm resistor chain.

i''i;rure 2. The power supply interference. -£- ion current -^-electron or electron equivalent current.

In figure 2 is shown the interference between the two power supplies d TJ which create an other principal problem. The example is for

UAA > UEE# As load on UA one have the meter resistance current i_., the ion current to the collectorplate or other parts of the floating system the secondary electron current" in the extraction gap i , the -203-

secondary olectron current from the floating system to grotind i and the leakage current iRL through especially the analysing chamber insulation and finally the target ion current I.

If one look away from the current i^, will the load on U(, i he the difference between iRS and i^A the analysing voltage ground current, which for small Ug values will be dominated hy the leak current iRL. As iR_, decreases and iRL increases with decreasing U , the load current ilg will be zero at some value of Uv, with the result that the UE supply goes out of stabilization. It is of cource possible to lower the value of Rg, hut then will the load current at high U^ values be much too high. The solution was the introduction of a constant current drain iEC of about 25o uA. The basic element of this drain is an old high voltage tricde. The constant current drain is •-ilchr'd in for II Inr.r. than IS kV.

For UR UA does the same problem exist, but as tho ratio Up/U. nuviT exceeds L1, ar.<1 II. ni'VM- g^ts below }o kV, this- problem could lie solved with a R. value of 4°o MOhm, which does not introduce any serious A load on U ,

The Retardation System: The last principal problem is, how to retard or postaccelerate the ions from the floating system to the target without destroying the spr.tial distribution too seriously due to the aperture lens effect, which any retardation or acceleration system will have on the beam. For resulting energies on the target in the vicinity of Jo keV and higher the lens effect will be so small, that it is easyly compensated by adjustment of the

accelerator lens voltage UT. Put when the ions are retarded down to low energies in a single gap, the focusing effect of the second aperture will be so strong, that some other way of compensation is necessary. As a first attempt a two gap system has been applied with a variable

voltage ¥ on the center electrode. UR is able to go from - 3o continous- ly up to + 3o kV. The principal dimensions are: gap width lo mm, aperture diameter 11 mm and lenght of center electrode loo mm. We will here refrain from a further analysis of this system, just mention that for low resulting ion onorgies there will be a beam cross-over he rare the retardation "nps where as for very low energies tho crocs-over will he inside tho center -204-

electrode. The system has shown to be able to give current densities of more than 1 mAcnf2 on the target in the energy interval from 1 keV up to 7o keV and this is quite satisfactury for the present experiments. To remove unretarded neutrals from the beam, the retardation system is followed by electrostatic deflection plates which deflect the beam about 6 decrees.

Application; URIAS is now principally used in atomic collision experiments. In a systematic investigation of the stopping in Methan of the first ten elements in the periodic system, the special feature of UEIAS has allowed measurement down to o.6 keV for protons and 3 keV for the heavier ions. In measurements of the ranges of heavy particles in noble gases, which now are being performed anr1 where it is necessary to pass quite a high current through a very small opening and keep this current stable for hours, has the high current density and stable beam condition of URIAS been of the greatest importance. It may then be concluded that even after 3o yearn service in the field of atomic and nuclear physic the old Copenhagen Separator is still going strong and performing better than ever before.

References; 1) K.O. Nielsen and 0. Skilhreid, Mucl. Instr. and Meth. 2 (1958) 15. 2) J. Koch and T. rendt-Nielsen, Mat.Fys.Medd.Dan.Vid.Selsk. 21 (1SM4) no. 8. 5) G. Sidenius, Proc. Int. Conf. Electrom. Isotope Separators, Marburg 197o 4) G. Sidenius, J. Phys. E. 4 (l97l) 771. _205_

A MINIATURE ELECTROMAGNETIC ISOTOPE SEPARATOR

P. Abrahamsen, H.Ii. J0rgensen, 0. Skilbreid Danfysik A/S, Denmark

ABSTRACT

A description is given of the design of a very compact electromagnetic mass separator covering the mass range 1 to 80 A.M.U. Curves showing ion current and resolving power as function of ion energy are shown and briefly discussed. -206-

About tlircc years ago Danfysik started the manufacture of 550 keV ion accelerators for ion implantation research. As all multielement ion sources produce ions other than those required, separation of the different ion species is a must, if a pure ion beam is wanted. This purification of the ion beam is carried out by means of some kind of ion MUSS selector. For many different reasons we chose to make tiie selection of the wanted ions before the main acceleration.

The mass separator employed.,in Danfysik 350 keV accelerators is a scaled down so-called "Scandinavian" type electromagnetic isotope separator covering the mass ranpc from 1 to 250 A.il.U.

Approximately one and a half years ago we decided to develop an ion accelerator for the industrial application of ion implantation in large volume production of solid state devices.

An instrument for this purpose should preferably be contained in a closed cabinet, be 'is simple to operate as possible, and it should be compact. The two factors which have the greatest influence on the size of an electromagnetic mass separator are the energy and the mass of the ions to be analyzed.

Up till now the elements of main interest in ion implantation for production use have been Koron, Phosphorus and Arsenic. We therefore decided to base our design on a mass analyzer covering the mass range 1 to SO A.M.U.

The: problem of obtaining satisfactorily hipli ion currents at a low energy was solved by applying space charge -207-

nei.tralization of the ion bean. Investigations '..ore made during the spring of 1972 to find the best electrode configuration for an acceleration/retardation system. Since we wanted a compact mass analyzer, and because several of our potential customers are interested in implanting at very low energies, we wished to explore the possibility of obtaining the desired ion current levels at an ion energy of 10 keV. The ion source used in these experiments is a Modified version of the hollow-cathode ion source developed by G. Sidenius and known as the M: nodel '.)11. The main modification to the source is that the original nicro-oven has been replaced with a separately heated oven of large capacity. Reasons for choice of this ion source are its high temperature capability at a modest power consumption, and its small size, which greatly reduces the memory effect when changing from one clement to another. The high temperature capability of this source also makes it possible to use most elements in pure elemental form as charge material.

The hollow-cathode ion source has a high plasma density, and the ion current which can bfc extracted from it is determined by the intensity of the extraction field.

During all the experiments carried out the ion energy was kept constant at 10 keV, while ion beam intensity and bean divergence were measured as functions of extraction voltage and electrode configurations.

An electrode configuration as shown in fig. 1 gave a maximum of beam current at an extraction voltage of 20 1;V. The divergence of the beam was measured to be approximately 0.7 % for an Argon beam of 50 to 60 uA. -20B-

Tlic extraction and retardation distance used were both ID mr- j/hile the diameter of the openings in the electrodes i.-is 2 ram.

Fig. 2 shows a cross section of the final design of the mass separator.

A double-focussing magnet with 135 deflection was chosen. Double-focussing is obtained by tapering the pole pieces so that the magnetic field J5 in radial direction is equal 1 2 to li (§ )~ / where Bo is the field strength at the O KQ middle radius of the magnet. The middle radius of the magnet is 15 en, the pole gap is 1.8 cm at that radius and the pole width is 4 cm. The maximum field Bo is 8.6 kGauss.

The 135 deflection angle was chosen in order to obtain a high dispersion and a short beam path length between tiie focal points of the magnet. On the other hand, limiting the deflection angle to 135° makes the distance between the focal points and the magnet entra?.ce and exit long enough to give ample room for convenient mounting of the ion source assembly and the exit chamber. The magnet is of C-type cons truction,

For reasons of economy, and because air cooling of the magnet was wanted, the magnet coil is placed around the return yoke. Because a magnet of this design gives very high stray fields the extraction and exit chambers are made of soft. iron. The deflection tube is made of copper.

The part of the coil which is outside the iron yoke has u:i open slot between each two layers of copper wire. The slots serve as channels for the cooling air. The riagnet power supply is an on/off or so-called "chopped" supply. -209-

The chopping frequency is 20 kHz;.

The self-inductance of the magnet coil, which is shunted by a rectifier, represents an electrical equivalent to a fly wheel.

Fig. 3 shows part of the 150 kV ion accelerator for which the mass analyzer was designed..

The wide aperture acceleration tube also serves as pumping tube for the mass separator. Not shown in this drawing is a high conductance tube which serves as a vacuum shunt between the ion source region and the exit chamber. The somewhat unusual mounting of the separator was chosen in order to obtain an upward pointing ion source and for easy access to the same.

An upward mounting of the ion source is desirable because it eliminates the danger of losing charge material into the discharge chamber or having the extraction hole clogged by loose material.

Fig. 4 gives the basic performance data for the separator in table form.

Calculations of beam cross section and resolution are based on data obtained from the beam profile monitor. We have reasons to doubt that the monitor is located exactly in the focal point of the ion beam, and future experiments will determine the correct location of it. The measured dispersion is higher than the calculated value. This can possibly be explained as a result of the fact that the real deflection angle is higher than 135°, because of ths strong stray fields from the magnet coil. -210-

mu: interesting figure in this table is the current density of the analyzed ion beam at the exit focal point.

For Argon (mass 40) the ion current density is one to two orders of magnitude liigher than it is in a high resolution Scandinavian type isotope separator, working witli ion energies of 60 to HO keV.

T'ij',. H siiows the ion current as a function of ion energy for different extraction voltap.es. During these measurements tiie ion source parameters were maintained unaltered. The measurements were made with an extraction distance and a retardation distance of 10 mm each. fig. 5 shows the same measurements made with extraction and retardation distances of 5 mn each. In the latter case the curves have shifted to the left, which means that hi flier ion current throughputs can he obtained for low energetic ions, l;or Argon ions with an energy of 1 keV, only li. IT. u,\ can be obtained in the first case while 1 uA is obtained in the last case.

l.e will iiot attempt to give a detailed explanatior for the shape of the curves. However, we have strong reasons to believe that these measurements mainly show the transmission (at thy mass separator) as function of ion energy.

As far as this can be judged from the load on the li.V. supplies, the ion current extracted from the ion source is fairly independent of the retardation field. This is also wiiat we would expect since the extraction field is practically uninfluenced by the retardation field with the electrode geometry used.

The retardation field itself will have a strong effect on -211-

focussing of the beam. i±The change in: resolution as a function of final ion energy is shorn in fig. 8. The deteoriation of the resolution-, when ion energy is decreased, coincides with a significant decrease in ion current. We believe that' this is caused by a rapidly increasingdivergenceiVbf the beam whichcan no longer be passed through the narrow deflection tube. Further reduction of ion energy will increase the divergence of the beam and the loss of ions to the walls. At a certain energy we will reach a point where the blow-up of beam in the retardation region causes the majority of .the ions to be lost to the retardation electrode. The cone of the beam, which passes through the hole in the retardation electrode, will be subject to positive focussing during passage through that hole, resulting in a low intensity but low divergence ion beam which again is well focussed at the exit.

liow much our theories have to do with reality is, of course, an open question, and rather extensive investigations are required to verify them.

CONCLUSION

The mass separator described has been highly succesful for the application for which it was designed. With a resolution of less than 200 and a mass range of one to eighty A.M.U., it can obviously not replace the high resolution laboratory type of electromagnetic isotope separator.

However, because of the high ion current density which can be obtained at low ion energies, the instrument could well find applications in atomic collision studies and high energy chemistry research. -212-

The compact size and low power consumption (less than two kW maximum) make it convenient to float the equipment electrically. In this way the useful ion energy range could be greatly increased by applying post acceleration or retardation. This technique would have the great advantage that the target area could be at ground potential. TRACTOR ELECTRODE

FIG.t

TYPICAL PliKFOKIANCli FIGUIUiS

Extraction voltage 20 tV Ion LNKRGY 10 keV Ion heaji current at oxit 300 uA

Dispersion I) . IH_t

Keaclution H - ri x M 110

Bean spot size at exi -2 r»«

Beau current density l.S B.'I/CO2

Ion bean currents for selectod elencnts

Boron (II.' ~so PA

Phosphorus ~«o MA

Arsenic ~40 MA -214-

T IMPROVEMENTS OF THE ISOTOPE SEPARATOR AT GOTEBORG FOR ION-SOLID INTERACTION STUDIES IN ULTRA HIGH VACUUM G. Holmen Department of Physics, Chalmers University of Technology, Facie, S-402 20 Gothenburg, Sweden

1 - Introduction The importance of a clean surface and a high purity material for investigations of the interaction of an ion be-"m with solid material is evident from rrany points of view. As 'n the present paper among other tilings we. discuss problems in connection with kinetic secondary electron emission from solids, the work function can be mentioned as a typical example of a physical quantity which changes during adsorbtion of gas on the surface. Both theoretically and experimentally it has been shown that a change in the work function influences the magnitude of the secondary electron emission yield. The rate at which gas molecules impinge on the surface in the equilibrium state is therefore a very important parameter in the evaluation of how to keep a clean surface. A calculation with application of the kinetic theory of gases gives for a pressure of 10 torr an adsorption rate of about 15 2 1x10 molecules/cm sec if the sticking factor is unity. This means that about one mono Layer of gas is formed on the surface in one sec. At a pressure of 10 iiorr the time to form a monolayer increases to 1000 sec. fll which is in accordance with experimental results of Hagstrum [2, 3j . These figures give an idea of how long time after a cleaning process an experiment can be performed without a great influence of adsorbed gases. The conclusion of these considerations is the necessity of ultra high vacuum in the target chamber during the time between cleaning and secondary electron emission measurement.

lrnce the bombardment at the experiment has started, the sputtering process gives a cleaning effect on the surface and if the dose rate is high enough this process keeps the surface clean. The dose rates normally used in our experiments are between 6x10 and 6x10 ions/cm2 sec. If thesp figures 12 ? are compared to 1x10 molecules/cm" sec the approximate number of impinging -9 gas molecules at 10 torr and the sputtering coefficient is between 2 and 3 atoms/ion for 40 keV Ge ions on Ge (110) ft], all the dose rates used can keep the surface clean. The sticking factor is also here assumed to be unity which is not likely,at least not at elevated temperatures. -214-

~ E

(9 8 C s

~xT -£15-

2 - Ultra Hinh Vacuum (UKV) As we have found nbove it is necessary ro have L'HV ir. the surroundings of the specimen during the experiments. To build a complete UHV isotope separator would be very expensive1 and therefore we have choosen another method to obtain UHV in the target region. As the UHV system Fig. 1 has been described earlier f5j we only give a summary of the layout. To the isotope separator [6J two chambers are connected after each other with narrow channels in between. In the chamber between the separator and the l'l!V chamber a pressure of approximately 10 torr is obtained by a 600 1/p oil diffusion pump. The UI1V chamber is pumped by a l&O 1/s ion pump mid P pressure in the low 10 torr range is obtained when no ion beam hits the target. For a few uA of e.g. Ge, Ke , Ar , Kr or Xe it is possible to maintain the pressure -o -7 below 10 "" torr. Ion currents less than 10 A. give much smaller increase of the pressure in the UHV chamber and if the chamber is thoroughly baked ;it '200 C the pressure can : 2 kept below 10 ' torr.

Fig. 1 - View of

the UHV - syslpu: -216-

3 - Cleaning method for Ge. To make ion-solid interaction studies it is not enough to have good vacuum in the target chamber, it is also necessary to have clean target conditions. As high purity materials of several semiconductors nowadays are commercially available, problems with bulk material purity should not cause any trouble in the experiments. In our experiments with Ge we have not, indeed, had any problems with the purity of the bulk material, but on the other hand impurities at the surface have influenced the results.The surface impurities on Ge consist mainly of adsorbed gases, oxides and nitrides. In order to clean the surface and restore the crystal Ge structure several processes have to be used. Before mounting in the UHV chamber the Ge crystal is etched in a chemical etch containing 50 vol. % HNO3 (100 %) j!5_ % HF (40 %) and 25 % CH., COOH (100 %). 60 ml is used for each crystal and the etching time 60 sec. When the pressure in the UHV chamber has decreased to about 10 torr, the whole target holder including the target is degassed by heating to 500-. o C for about 12 hours. During that time the pressure decreases to -Q below 10 ' torr.

In order to remove the foreign atoms in the surface layer of the target held at 500 C a sputtering treatment is made with 40 keV Ge ions to a dose of 16 7 10 ions/cm . The treatment gives the experimental conditions necessary for the secondary electron emission measurements.The possibility to introduce disorder in the single crystalline Gc for.these high sputtering doses has been investigated by electron microscopy [7] . The result showed that no defects were formed by the bombardment.

4 ~ Isotope separator. In order to make it possible to run the experiments in the UHV chamber some improvements have been made on the isotope separator. As the ion beam passes narrow channels on its way to the target in the UHV chamber, the long time stability of the ion beam position must be better than -4 52' 10 fT Therefore improvements have been made on both the stability of high tension and deflection magnet current. Changes have also been made on beam shaping and beam sweeping systems.

4.1 - High tension. The high tension from the old supply [6] has been electronically stabilized. In series with the high tension filter a tetrode has been inserted to control the output voltage (Fig. 2). The error signal is -217-

taken from a 500 kLlwirQwound potentiometer which is coupled in series with a load of high stability metal film resistors (500 M) down to ground

potential. (Veln-et 4017, 5MXI + 0,5 %, 5 ppm/°C) . The whole resistor bank

is copied with transformer oil. At the bottom of the line: a,high precision current meter is coupled for high tension reading. The error signal is ) amplified in a dc coupled amplifier and the signal is fed to the grid of \ the tetrode. The amplifier is equipped with a safety system in the last stage not to exceed 5 kV over the tetrode. The tension is measured with an instru- ; ment in a shunt over the tetrode. With the 500 kd potentiometer and the multiple switch SI it is possible to adjust the tension over the tetrode up to 5 kV for all the acceleration voltages used (15 - 55 kV). The potentiometer and the switch are handled from ground potential by insulating rods. With the described system ' is possible to control long time variations of + 2,5 kV of the output voltage. Also the ripple of the voltage is reduced. An over all long time stability of the output voltage of better than 5 x 10~5 is obtained.

85A2

.200V

75K 190K 34K 190K

':6.BV PD500

47 K

0.1 ft"

120K

«70K 28K

-200 V

Fig. 2 - Circuit diagram of the high tension stabilization system. -218-

4.2 - Magnet current supply. The old deflection magnet current supply [6] has been exchanged for two parallel coupled Oltronix power supplies B32 - 30R (32V, 30A) with external shunt. The shunt is made of a 0,12 ohm manganin band submerged in water cooled transformer oil. The over all stability of the system is better than 5 x 10

With the above discussed improvements the over all long time stability -4 of the ion beam position in the isotope separator is better than 10 m.

4.3 - Beam shaping and beam sweeping systems. To increase the ion current density at the target position a cylindrical lens system of the same type as that before the deflection magnet [6] has been included after the magnet. With the two beam compression systems and the spherical lens system [6] the beam shape can be varied from a 40 mm long and 2 mm wide line focus to a point focus with 2 mm diameter. The divergence of the ion beam is less than 0,1°.

Max 500 VPP

Fig. 3 - Circuit diagram of the beam deflection system. -219-

To enlarge the dose rate interval & new beam sweeping system has been constructed. Figure 3 shows the electronic device used for linear amplification of a 2VPP triangular signal. The max output signal with good triangular wave form is about 1000 Vpp. The frequency used is 1000 Hz. The amplified signal is fed to a pair of parallel plates located before the deflection magnet [6] . The electrical field between the plates sweeps the ion beam _J.2,5 cm at target position for 40 keV ions. With the sweeping system it is possible to vary the dose rate for most types of ions from 6 x 10 to .,,,14 . ,2 6 x 10 ions/cm sec.

Summary: With the described improvements of the isotope separator at Goteborg ion-solid interaction studies can be performed during clean target conditions -9 -IT and a pressure ranging from 1 x 10 to 5 x 10 torr for ion beams in the current interval from a few U-A to nA and dose rates from 6 x 10 to 11 2 6 x 10 ions/cm sec. Sputtering, kinetic secondary electron emission and ion implantation measurements have been performed £ A, 8, 9]

Achnowledgement: The author is indepted to professor 0, Almen for valuable discussions and to Swedish Atomic Research Council, Swedish Natural Science Research Council, Swedish Board for Technical Development, "Fonden for framjande av ograduerade forskares vetenskapliga verksamhet" and"Stiftelsen Wilhelm och Martina Lundgrens Vetenskapsfond for financial support.

References:

1. M.Kaminsky Atomic and ionic impact phenomena on metal surfaces, Struktur und Eigenschaften der Materie, Vol. XXV (1965).

2. H.D. Hagstrum, Rev. Sci. Instruments 24, 1122 (1953).

3. H.D. Hagstrum, Phys. Rev. 98, 561 (1955)

4. G. Holmen. and 0. Almen, to be pubhished.

5. G. Holmen, E. Kugler and 0. Almen, Nucl. Instr. and Meth. 105, 545 (1972).

6. 0. Almen, G. Bruce and A. Lunden, Nucl. Instr. and Meth. 2, 249 (1958).

7. The electron microscopy studieswere kindly made by Dr. S.M. Davidson, Department of Metallurgy, University of Oxford, to whom we are greatly indepted.

8. G. Holmen and P, Hogberg, Radiation Effects 12, 77 (1972).

9. G. Holmen, P. Hogberg and A. BurSn, to be published in Radiation Effects. Chapter 5: Recoil ion analysers

-220- VEL0C1. "ILTr FOR THE SEPARATION OF PROJECTILES AND REACTION PRODUCES BEHIND THE TARGET OF A HEAVY ION ACCELERATOR

H. Ewald, P. Hinckel, G. Munzenberg, F. Nickel K. GUtiner, H. Wollnik

II. Physikalisches Institut, 63 Giessen, Germany

Thin targets in the beam of the heavy ion accelerator at Darm- stadt/Wixhausen (GSI, Gesellschaft fur Schwerionenforschung) may have a thickness of about 10 atoms/cm . Behind such a target the beam will consist of 10 to 10 primary ions with relative c n velocities of about & = v/c = o.l and not more than 10 to 10 -24- 2 reaction products (production cross sections

Behind the target we have then the problem of a wanted elimination of the relatively huge intensity of the primary particles from the beam of the reaction products. If this separation can be achieved we will have the possibility to examine these products in different ways without heavy disturbances by the primaries.

This separation can be done by a focusing velocity filter. We consider different versions of such filters which principally are similar to each other and which consist of a number of static electric and magnetic fields. The first order ion optical properties of such field combinations can be described as usually in a matrix formalism by

Ky Kg K6, y Ly La L| Lfl, a° • o Zi Q 0 0 1 0 6Q | » \Pi ,0 0 0 1 i6vi -221- where the indices 0 and i relate to the radial and axial coordinates (y,z) and angles (a ,fl) of a paraxial ray at the places of the target and of the final exit slit respectively of the arrangements. 6Q and 6y are variations of the specific ionic charge numbers Q = q/m (ionic charge numbers q/ mass numbers m) and of the velocities v of the particles relative to mean values of these quantities. The two matrices in the above equations are the results of the multiplications of the radial and axial transfer matrices of all single fields and of the field free drift ways.

One version of such a field combination (Fig. 1, see also GSI- Berichte 72-3 and 73-3, Darmstadt 1972 and 1973 resp., p. 101 and 37 resp.) consists of two Wien velocity filters each having 3 symmetrically arranged separated fields (homogeneous magnetic fields with parallel bounderies and parallel plate condensers) and of an achromatic deflection system with 3 or 4 homogeneous magnetic sector fields. The first Wien filter deflects the high velocity primary particles to the side. Reaction products of a certain lower mean velocity can pass the Wien filters without deflections. This holds independently of the ionic charge number;: of the particles. That means the filters are doubly achromatic with respect to the charge numbers. Reaction products of a small range around this mean velocity can pass the Wien filters with only small deflections. The width of this velocity range ( *»+ 5 %) is defined by a diaphragm at the entrance boundary of the first magnet of the achromatic deflection system. The velocity dispersion of these selected particles which is produced by the action of the first Wien filter is compensated by the action of the second Wien filter which has a dispersion in the opposite direction. Then the achromatic system can give an optimum focusing of the selected particles at the place of the exit slit of the arrangement. The bounderies of the sector field magnets of the achromatic system are curved in such a manner that a high order focusing is achieved for broad ranges of the specific ionic charge numbers Q (+ 10 %) and the velocities v (+ 5 %) of the particles. -222-

The second Wien filter of such a combination also can be placed behind the achromatic deflection system in a symmetric position to the first one (Fig. 2). This has the advantage that smaller deflections are needed for the primary particles and that two additional quadrupoles easily can be inserted to facilitate the adjustment. The first of these weak quadrupoles can be worked as a figure-of-eight quadrupole in order to give free space for the deflected primary particles. For their good separation the first magnet of the achromatic deflection system can be a septum magnet.

Computer calculations of many trajectories of particles of the given Q- and v-ranges penetrating the target within the maximum radial and axial coordinates y = + 1.5 mm and ZQ = + 5 mm and within the corresponding maximum angles of divergence a = & = + 0.5° show that most of these particles are focused by this arrangement through an exit slit of 4 mm radial width and 30 mm axial length.

The whole field combination is doubly achromatic with respect to the ionic charge numbers and singly achromatic with respect to the velocities of the selected particles. That means that the corresponding elements of the resultant radial transfer matrix

given above are zero, namely KgQ = Lg0 = K5 =0. Moreover we

have stigmatic focusing at the exit slit, that means Ka = Kfi = 0.

Another version of the velocity filter (Fig. 3) is more flexible and simpler to a certain extent than the above one. It consists of a symmetric arrangement of two quadrupole doublets (or triplets), two electric deflection condensers and four homogeneous magnetic fields all having parallel bounderies. The dimensions and relative field strengths of this arrangement are optimized by ray tracing calculations in a similar manner as explained above. It is easier and somewhat cheaper to realize than the firstmentioned, but its focusing is less good about by a factor of three. Using in this

case the extreme values of yQ = + 1.5 mm, z = + 5 mm, 0 = + 1.5°,

fiQ = + 1.5 of the entrance coordinates and angles and the same ranges of Q and v as given above of the particles to be focused then it can be shown that most of the trajectories of these par- -223- ticles pass through an exit slit of 2 cm radial width and 10 cm axial length. Fig. 4 shows a number of radial trajectories crossing in the region of the exit slit of this arrangement. The 6 digits given at each trajectory indicate in the given sequence the yQ-, z^-, O-, &Q, 6^- and-6"-- values of the trajectories corresponding to the accompanying table. The radial width of the beam of the deflected primary ions is of the order of 10 cm in the region of the second quadrupole doublet. Its deflection can be enlarged if the fourth homogeneous magnetic field is limited in its length and acting as a septum magnet. In this manner the mean radial distance from the mean path of the arrangement is at least 35 cm in the region of the second quadrupole doublet depending on the considered nuclear reaction. By an additional small magnetic sector field these deflected primary ions can be focused again to a certain amount and shifted in a wanted direction. The deflection voltage needed in the two condensers (distances of the electrodes 14 cm) is 400 to 500 KV. The fieldstrength needed in the 4 dipole magnets is 0.7 T (distances of the pole shoes 14 cm). The openings of the first two quadrupoles are 12 cm, those of the last two quadrupoles are 18 cm. The field strenghts at the centers of the poles of these latter quadrupoles are 0.8 T.

The realization of this second arrangement (Fig. 3) which is explained here has been started now at Wixhausen. This velocity filter lateron shall be used as a first stage of a more complex recoil spectrometer. As a second stage can b.e used: a He-jet separating system, a gas-filled magnetic separator (like JOSEF at Jiilich) or a focusing parabola spectrograph (like Lohengrin at Grenoble). Additional time of flight measurements of the particles separated in such spectrographs wili be useful.

For such combinations of different stages it is of importance that we expect a rather high intensity of scattered primary particles behind the exit slit of the first stage, say 10 to 10 per sec. They may be scattered on edges of diaphragms and surfaces of electrodes and pole faces within the first stage. Therefore it could be difficult directly to combine a time of flight spectrometer with the first stage. -224-

primay particles

achromatic system nit slit Im

Fig. 1 An asymmetric velocity filter consisting of two Wien filters and an achromatic deflection system,

targrt

o e o D a DEO

Fig. 2 A symmetric velocity filter consisting of two Wien filters,two quadrupoles and an achromatic deflection system (E = electric field, D = dipole magnet, Q = quadrupole). The double arrows indicate that the first of the quadrupoles is radially defocusing while the second is radially focusing.

target

D D D D E 0 • Q

Fig. 3 A symmetric velocity filter consisting of 4 or 6 quadrupoles (Q), 2 electric fields (E) and 4 nomo- geneous magnetic fields with parallel bounderies (D) Fig. 4 A number of radial trajectories crossing in the region of the exit slit of the arrangement of Fig. 3. The 6 digits given at each trajectory indicate ^ in the given: sequence f

the yo-, zo-> ao-, V' 6Q"and'6v- values of the tra- - 1 i c» 6" 1 ?-: 6. jectories as can be S !; 1.5 -.9* C5* " :c j S derived from the i! •-- :.*• c,.- is!* 1—; ij-^— accompanying table. 1 : i! c.i c,z* ! a 1 CI. C.« ( 3 1 la I <• V C 1 C 3 ! ' « -~ - -„•" : - : -'

' 9 !1 -0.5 -J f -1S» -; i » il --* -i , -:.- j .;.- | -v ! - 1--s i -u- j -:3- • -:; -S -226-

FISSION FRAGMENTS SEPARATION ACCORDING TO MASS, ENERGY AND EFFECTIVE CHARGE IN ELECTRICAL AND MAGNETIC FIELDS by U.A.ARIFOV.A.D.BELYAEV.V.I.KOGAN.V.P.PIiOJL.A.M.USlviANDIAROV, V. A. KOSILOV, N. V. PETROV.

Passing of fission fragments with their near initial ve- losities throw electrostatic and magnetic fields gives us possibility to make space separation of fission fragments according to mass, energy and effective charge for very short time after fission. Extensive possibilities of this method in research fission process (primary nuclear charges determination /I,2/) and in interaction of fission fragments with matter /J5-7/ are successfully demonstrated by Ewald, Konecny et al on their fission frahments mass spectrograph with resolution of 150- 200/8-10/. We have made and installed in reactor the mass spectrometer that allows us to separate fission fragments according to mass, energy and effective charge for about I i/sec after fission act. Taking in account ranges of mass, energy and effective charge /IO/ and reactor neutron beam work conditions, the mass.spectrometer design was chosen with successively arranged the electrostatic field of cylindrical condenser and the sectoral homogeneous magnetic field, with opposite deflection of ions and without intermediate focal spot between both fields.From the first order angle and velocity focusing condi- -227- tions /Ij5/ and from the limiting of the values of electrical and magnetic fields, the main geometrical parameters of the mass spectrometer are /I4/: a) in electrostatic analyzer: the distance from the object to the field boundary is ^=380,0 cm, the deflection radius is ^O=274,I cm, the deflection angle is ^=I9°7f , image is in infinity ^ =00, the plate gap is d=5,000 +0,003 cm, fringing fields are screened with grounding diaphragms /I5/j t>) in magnetic analyzer: the object is in infinity u^^oo, the distance from pole boundary to image is ^-

=222,9 cm, the mean deflection radius is^0=I73,5 cm, the deflection angle is $.=400, the entrance angle of ions is g=0, the exit angle is £ =2 . Fringing field action is taken in account by means the effective field boundaries /I6/, the distances between these and boundaries of magnetic poles are determinated from the measured quantities of dissipated fields by method /I?/. Distance between both fields is chosen equal A=5I8.5 cm. Total path of ions in the instrument is II.4 m.

]L 2 3 -1OW +70UV

Fig.I. Scheme of the mass spectrometer.

Scheme of the mass spectrometer is shown in Fig.I.A part of fission fragments, prodused in the layer of 255U (260 jig/ cm2) placed in the entrance shoulder 2 in neutron flux of I.IO12 n/cm2.sec, comes out throw a split I of I mm width and 58 mm height and in the form of weak diverged beam with apper- ture limited by the diaphragm 4 with a split of the variable width (the heights of all following splits are 40 mm) passes to the electrostatic analyzer. Colliinator 3 limits the size -22P-

of neutron beam. Electrostatic analyzer /I8/ is a cylindrical condenser with the plates 13 made with high precision of bulk aluminium and fixed by means regulating device to the polysty- rene rods joined into firm framework stayed on a turning table and inserted with it into a cylindrical form chamber 12. Dia- phragms 5 and 6 screen the "fringing fields'. Through electrostatic -analyzer the ions pass having ratio of the kinetic energy E to the effectivei ct^rge ^

where V is voltage between the plates of cylindrical condenser,^ and I£are radii of curvature of work surfaces of outer and inner plates.Bipolar higb voltage power supply provides gradually controled voltage of quantity to ±70 kV stabilized to 0,01 percent to each plate that allows to pass fission fragments of any value E/2i through the analyzer with

^=272.6 cm and £2=275.6 cm. Then fission fragments pass through ion tube 8 to magnetic analyzer, that is sectoral homogeneous magnetic field, provided by electromagnet 14. Range of E/zValues is limited by the diaphragms 9 (widening) and 10 (width is 30 mm), d.E/Z )max = I percent. Ions, that have passed the electrostatic analyzer, are separated in magnetic field according to ratios

where His magnetic field strenght, and passing the chamber of the magnetic analyser 15 they come into the detector chamber II through exit shoulder 16 with vacuum valve at its end. Quantity of field in 72 mm gap of the electromagnet is controled by 0.01 per cent steps from 1000 to 10600 oersted (stability is An/H=0.0I per cent), this allow us to analyse fragments of anyM/Zt Behind the electrostatic analyzer and in the detector chamber II (in 3 cm behind focal plane), serai- conductor detectors placed, that have mask 4 mmx30 mm and 1.2 mmx40 mm, respectively, and that may be removed off the beam by bellows devices. Semiconductor detector pulse height analysis is fulfilled by 128 channel analyzer. In the detector chamber, at focal plane, a plate-holder device is situated having 40 inmx4O mm mask needed for detection of range of; -229- fission fragments spectrum by track detectors or nuclear pho- toemulsion. Pig.2 shows a photograph of the mass spectrometer. The chamber of the elect- A I rostatic analyser with a movable shield block and an electromagnet (weight is 12 t.) are arranged on two carts provided with regulating devices and hardly connected with regulating rod. Carts may be moved by means of electrical drive on the track (one of the rails is of a circle section) parallel to the axis of reactor hole. In work place the chamber of the electrostatic analyzer is surrounded by shield from reactor radiation. In process of measurements the pressure in the mass spectrometer is (1-2)-10 tor . Pumping out is ful- Fig.2.Photograph of arrangement filled by three diffusion oil pumps with liquid nitrogen traps. Adjustment of the arrangement is made by means of the parallel plane (to 18") glass plate and by means laser beam; horizontal arranging is made by means of a level. Graduation is fulfilled by o(-particles of

pxR" • : • '•' ; '• • •••••'••• '•••••••-• • ' .-.-v '• y Pu that has two intensive lines of energies 5«/t-557 MeV and

5.4-988 MeV. Coefficients CL and im of relations (I) and (2) are found by deflecting of oc-partlcles in both fields. Resolution of mass spectrometer is (Mj&AA)=700-800 FWHM on the used range of the focal plane. Fig.3 shows pulse height spectrum of fission fragments passed the electrostatic analyzer (E/^5,235 MeV/%,). The toothed character of the spec^riim is 'depended on discreteness -230- of charges. The numbers at the peaks are effective charge values definied by the electronic charge &0 .Determination of the Z values was made by comparison the spectra that were measured for a set of deflection voltages by method of /3/. Fig.4 shows spectra- of the

M/!?* values I of jt he:fission fragments,2detection;at the 2. U 22 focal plane was made by track 00 f glass detectors. Lines of both heavy and light groups, so are well separated.Measured by the semiconductor detector the pulse height spectra M/Z* so bu 100 K- lines show that the heavy fragments group M/2i lines are Fig.3«The pulse height spect- practically free from admix- rum of fission fragments taken tures of the particles that by semiconductor detector af- have near E/2* and M/Z* va- ter electrostatic analyzer. lues, but have other M,Z* and E; for light group the part of lines is found with admixtures. The Z* values of fragments, passed both analyzers, are determined from measured pulse height spectrum of each line of Fig.4 after the connection has been found between pulse heights of the both detectors for all the energy region of the fission fragments, and then one may to determine their M,Z* and E.

£°20

Fig.4. M/Z* spectra of fission fragments: a) fragments of heavy group,b) fragments-of light group. -231-

So mass spectrometer gives the possibility to separate in space in well resolution for near I^usec after- fission act the fission fragments characterized by the definite values of mass, effective charge and kinetic energy, to identify these values ;With. high precision and to: determine yields of these fragments.^This expends the range of researches for this arrangement because focal plane is taken far away of boundaries of magnetic fields.

REFERENCES I.E.Konecny,H.Ewald et al. Z.Phys.,2J5I,59(1970). 2.E.Konecny et al. Nucl.Ph.ys, ,AI00.465(1967). 3.H.0power et al. Z.Naturf. ,2baa5IfI965). 4.E.Konecny und G.Siegert. Z.Naturf.,21a,192(1966). 5.E.Konecny,K.Hetwer.Nucl.Instr. and Meth.,36.61(1965). 6.U.Hoppner et al. Nucl.Instr. and Meth. ,74.285(1965). 7.J.Albrecht und H.Ewald. Z.Naturf.,26a,1296(1971). 8.H.Ewald,E.Konecny et al. Z.Naturf, ,I9a,194(1964). 9.E.Konecny et al. Z.Naturf. ,I9a.200(1964). IO.A.A.rpeiUHJiOB H flp.npoayKTbi MrHOEeHHoro aejiHHHH,M.,lS6S. II.J.CD.Milton,J.S.Eraser.Can.J.Phys. ,40,1626(1962). I2.N.Lassen.Dan.Mat.Fys. Medd. ,26,No.5(1951). I3.J.Mattauch,R.Herzog. Z.Phys. ,82,786(1934).

I4.y.A.ApH$OB H flp. SoKJiaflH AH CCCPt204,586(IS72). I5.R.Herzog. Z.Naturf.,10a,887(1955). 16.^Hc.EapHapfl.CoBpeMeHHaH Macc-cneKTpOMeTpMs,M. ,1958.

I8.y.A.ApH$0B H flP. nT3,f 6,34(1372).

INSTITUTE OP ELECTRONICS UZBEK AKADEMY OF SCIENCES, TASHKENT. USSR. c

-232-

Computer simulation of the motion of heavy ions passing through gas filled electromagnetic separators G. Fiebig Kernforschungsanlage JUlich Institut fur Neutronenphysik

Abstract

The Monte-Carlo technique is used to study the motion of heavy ions passing through gas filled electromagnetic fields taking into account energy loss, change of the ionic charge, and deflection due to multiple scattering.

The calculations provide the line profile in the focal plane of a gas filled separator as a function of apparatus parameters (such as field geometry, gas species and gas density) and particle parameters (mass, nuclear charge, ionic charge and velocity). This study allows the deduction of optimum operational conditions for various applications.

Results of calculations are presented and compared with measurements .

1. Introduction

The aim of the present paper is to present a suitable method of deducing optimum operating conditions for various applications of a gas filled magnetic separator. To achieve this one has to evaluate the resolution as a function of various system parameters which are characteristic of the apparatus (field geometry, species and density of the gas filling) and of the ion beam (emittance, mass, nuclear charge, ionic charge, velocity).

These calculations are based on ion-optical considerations as well as on theories and experimental results available on the processes of charge exchange, elastic nuclear scattering, and energy loss which occur if fast heavy ions pass through matter. Using such information and making several simplifying assump- tions one may estimate the mass resolution from the variances of all contributing processes without considering the actual mo- -233- tion of the single particles /l-H/. However, more extensive and reliable information about the resolution is obtained if one traces the ion beam by simulating the interaction processes with the gas contents. One thus obtains the line profile in the focal plane of the separator. WithMonte-Carlo calculations it is possible in a relatively simple way to take Into'account more complicated conditions that are closer to real situations.

2. Simulation model

Only the essential features of the model will be described here. For details and references see /5/. Coordinate system and beam parameters

The particles to be investigated are characterized by their nuclear charge Z, ionic charge e = eQ(l+ 6"e), mass number m = mo(l+6m), and energy U=UQ(1+Su), where Sg, Sm, and S^ are small quantities. In a homogeneous or inhomogeneous magnetic sector field, the reference particles with ionic charge e and 1/2 ° momentum (2moUo) move in a circular main path of radius RQ (reference length), along which the constant field strength B is present. Introducing as usual the dimensionless coordinates u,v, and w, a paraxial trajectory is, relative to the main path, completely defined by its radial and axial deviations u and v, and by its radial and axial angles of inclination oc and IS. w is the azimuthal coordinate in the direction of the central trajectory. The intial conditions of the ion beam, i. e. U, e and the phase space are characterized within certain regions by probability density functions fyCU), fg(e) and fE(u,oc,v,G>) upon which the generation of a particle is based in the computer experiment, First order transfer matrices, free path length To calculate the mass resolution we need only consider the radial motion of the ions. The trajectory equations are limited to the linear case, since the influence of the higher order terms is estimated to be insignificant in comparison with the influence of the interaction processes between the ions and the gas filling the system. We specify any radial trajectory at any -234-

point w along the system by a column vector f with components u,

«*, & . The transformation of ^ from the point w = w. to w = wi+1

is given by ^. + 1 = (T)^ where (T) stands for the familiar transfer matrices of drift spaces and inhomogeneous bending magnets

including their fringing-fields. At the points wi and w^+1 collisions between an ion and the gas take place, which cause a change in ot (scattering) or in & (charge exchange).

The free path length X = wi+1 - w_L (dimensionless) is derived from the total cross section of the event "charge exchange or 2 scattering", Z = £e +JS [cm /atoms] as the random quantity

X = -U/L*Z )-ln(l - r). (1)

Here 1 = L/R , L is the target length in [cm7, L* = const* P-L is the target length in [atoms/cm ] , P the gas pressure in [Torr] , and const contains the gas temperature. refo,l] is a uniformly distributed random number.

e and e as a function of velocity

The vector y contains the relative ionic charge deviation S = (e - e )/e . The reference charge e can be related to the average equilibrium charge e of the ions in the gas as a function of the ion velocity v. On the one hand, particles with mass m , velocity v, and ionic charge e equal to e will move in the circular main path, if e = k(v/v ), where v = c/137 = 2.19 x 10 cm/sec. On the other hand, e is given by the semiempirical relationship /6/

r Io = z[l - C exp(-v/vQZ )] (v> vQ) (2)

where C and }f are empirical constants. Since e determines the effective motion of the ion beam in the magnetic field, e

should be fitted to eQ in a least squares sense, thus yielding the constant k. Simulation of charge-changing collisions

Two sets of semiempirical charge-changing cross sections allowing for single and multiple electron loss in a single collision are used to determine the ionic charge states of a particle along -235-

its path /6, II Set 1;

e_e-<: ^W-^ [^j

where eQ is given in Eq. (2) and f^ *0.404- Z. By applying these cross sections, the ionic charge states of the beam will have the initially assumed distribution fg(e) at any point along the gas filled system. Set 2:

e-1: 6Ci(ejv) - e' (%) • it*-ac

(>; v) = Q (tf~*. &^ (ej v) [t !*.»

Angert /8/ and MSller /9/ deduced Eqs. (4) from their work on

iodine ions passing through N2 with velocities 2 i. v/v £ 4.5

and charge states 6 ^ e i 13. For a , a, and bT they obtained C JJ i-i the values 0.905, 0.7 and 1. In the case of iodine ions in He we use the values a = 0.301, aT =0.47 and br =0.9 which are C i-i Li obtained by adjusting Eqs. (4) to measured cross sections.

The total charge-changing cross section is given by

It has to be realized, that the empirical formulae derived for the charge-changing cross sections are suitable only for interpolation purposes, and that extrapolations beyond the investigated ranges of Z, e, v, and the target species must be regarded as risky. -236-

Simulation of nuclear scattering The spread of the ion beam due to multiple scattering is discussed in the framework of the theories of Lindhard, Nielsen, ocharff /10/ and of Moliere /ll/. Lindhard's theory yields the 2 2 differential scattering cross section d67d%=1Ta f(^)/^ , where the universal scattering function f(^) is numerically calculated using the Thomas-Fermi screening function, a is the screening parameter, ^ = £ sin(^/2), £ the reduced ion energy, and X the scattering angle in the centre-of-mass system, By = using the total scattering cross section 1ESI£) J^6 (e/S'/e/y/e/y t we first obtain ^' by solving the integral r(*'I = E.^1'f3'((*&/#*)(/•», where r(y)e [0,1] is a given uniform random number, and then the scattering angle X - 2 arc sin(^'/f). From Moliere's theory we l 1 get Es~ Tfa /Xn f and X-^t^/d-r)' where X is a screening angle and re[0,i] a given uniform random number. The screening parameter a is in both theories used in the form a = 0.885 a /s, 9 /3 2/3 /3 1/2 where aQ = 5.29x 10~ cm and Z* £ s ^ (Z + Z^ ) . At each collision the scattering angle X has first to be trans- formed into the angle -^ in the laboratory system relative to the direction of the particle before the collision occured. v^ is then projected into the (u,w)-plane yielding Ao< , the change of direction of the particle trajectory. oimulation of energy loss

Within the free path length beween two successive collisions (charge-exchange or scattering) the mean energy loss of the particle is either calculated by applying the well known stopping power formulae (Bethe-Bloch /12/, Lindhard et al. /13/) or taken from experimental data /l'-\/.

3. Results

The following computer experiments refer to the gas filled mass separator SEMS which is in operation at Dubna /J>/. We apply our simulation method to the case of 50 MeV Xe-ions (Z = 51, m = 132) passing through helium. The experimental results of Bacho et al. /3/ are reproduced in Fig. 1 where A(BR)/BR [Sjis plotted as a function of helium pressure. Also shown are two curves resulting from computer experiments which are based on the Moliere scatte- -237- ring law with s =lfz2/3 + Z^ and on the charge-changing cross sections (3) (curve CE1) and (M) (curve CE2). The deviation of the simulated curves from the experimental one is apparently due to the application of cross sections which are correct only for special-casesF In the range of low pressure values, where charge- changing collisions are most important, it turns out.that the cross sections (3) and (4) are too large and too small, respectively. For higher pressure values the calculated resolving power is seen to be worse than the measured one. An improvement should be possible by choosing s <\Z2/^ + Z^/5 giving rise to a smaller X and thus to smaller scattering angles X . Figure 1 Measured and calculated resolu- 6- .Experiment CE 1 tion of the separator SEMS for CE 2 50 MeV Xe -ions as a function of helium pressure 2\

I ' 6 ' 8 10 PlTorrl Conclusions The present work has been done in order to develop a simulation method that is capable of predicting the resolving power of gas filled electromagnetic separators. The results of calculation deviate, to some extent, from the measurements. These discre- pancies must be attributed to the incomplete information available on charge-changing and scattering processes of heavy ions in gases. A permanent improvement of the physical input to the simulation model is necessary in order to improve the reliability of predictions. Nevertheless, it may be asserted that the present version of the simulation model is suitable for performing investigations where emphasis is placed on obtaining estimates and trends.

I am indebted to Dipl.-Phys. H. Lawin for many valuable discussions of the problem. -238-

References

111 P. Armbruster, Nukleonik 3, Heft 5 (1961) 188-19" 121 P. Armbruster, J. Eidens, E. Roeckl, Arkiv for Pysik 36 Mr. 37 (1967) 293-304 /3/ I. Bacho et al., JINR-P13-4453, Dubna 1969 /4/ a) P. Armbruster, J. Eidens, J.W. Griiter, H. Lawin, In "Recent developments of mass spectroscopy" (eds. K. Ogata and T. Hayakawa), Proc. of the Internat. Conf. on Mass Spectroscopy, Kyoto, Univ. of Tokyo Press, (1970) b) H. Lawin, J. Eidens, P. Armbruster, J.W. GrUter, K. Hubenthal, K. Sistemich Proceedings of the "International Conference on Electro- magnetic Isotope Separators and the Technique of their Applications", BMBW-PBK 70-28, p. 270 (1970) /5/ G. Piebig, Kernforschungsanlage Jttlich Report No. JU1-939-NP (1973) /6/ H.D. Betz, Massachusetts Institute of Technology, Laboratory for Nuclear Science Technical Report, Report No. COO-3O69-I, 1971 111 UNILAC - Bericht Nr. 3-67, p. 14 181 N. Angert, UNILAC-Bericht Nr. 5-68 191 A. Moller, UNILAC-Bericht Nr. 6-68 1101 J. Lindhard, V. Nielsen, M. Scharff, Mat. Pys. Medd. Dan. Vid. Selsk. 36_, No. 10 (1968) W.M. Currie, Nucl. Instr. and Meth. 7J5 (1969) 173 L. Meyer, phys. stat. sol. (b) 4_4 (1971) 253 111/ G. Moliere, Z. Naturf. 2a (1947) 133 /12/ U. Pano, Ann. Rev. Nucl. Science 13_ (1963) 1 /13/ J. Lindhard, M. Scharff, Phys. Rev. 124 (1961) 128 Ilk/ T.E. Pierce, M. Blann, Phys. Rev. 173 (1968) 390 -239-

THE JULICH GASFILLED SEPARATOR "JOSEF"

H. Lawin, J. Eidens, P. Armbruster+, J.w. Gruter, M. Rajagopalan, K. Rashid, G. Sadler, and K. Sistemich

Institut fUr Neutronenphysik, Kernforschungsanlage Jiilich,, 517 Jiiiich, Germany

Gesellschaft fUr Schwerionenforschung, 610 Darmstadt, Germany

1. Introduction

Gasfilled separators have been successfully used for investigations of recoils from fission ' * '»' or heavy ion reactions^ because of their deflecting and focussing properties. The par- tide motion depends on the deflection by the magnetic field and on the interaction processes of the fast heavy ions with the gasfilling (exchange of ionic charge, scattering, energy loss)-5' •*', The magnetic rigidity is a function of mass and nuclear charge of the ions, but is, for suitable gas fillings, approximately independent of the primary ionic charge and energy. Compared to other recoil spectrometers » '; the beam intensity achieved is relatively high,the mass resolution is however limited by multiple scattering in the gas. The separation time is given by the time-of-flight of the recoils through the separator and amounts to 10 sec.

A new gasfilled separator named JOSEF (JUlich On-line SEparator for Fission products) was installed at the FRJ-2 reactor DIDO. — • p \ JOSEF has been described in detail at the Marburg Conference The separator is in operation since autumn 1972. The results of first measurements concerning the deflecting and focussing properties of this instrument will be reported.

2. Basic design and experimental set-up Fig. 1 gives the scheme of JOSEF. Compared to the smaller separator which is in operation since 1966 for nuclear spectroscopic investigations•- an improvement in beam intensity by a factor of 300 and in mass resolution by a factor of 3 was expected. The increase in intensity is due to the use of a magnetic system with enlarged acceptance and of a high transmission electrostatic particle guide16\ which transfers the fission products -2 40-

from the source in the Readercorv high flux region near

Source changngequp'rient netic system was designed Nt-whdaw rmgoiirm. to have a high ionoptical M-wixtow (Entrance slit I Cos fling ('STorr) dispersion and short ob- M-window magazine ject and image distances. M-vwnctow (Exit Hit) Fission predict guide/' Pig. 2 shows the lower (sld'-fo / part of the 3-sector mag- Shiekimg net . Pig. 3 gives an overall view of the experimental set up. In the -hpt transport dximbtr foreground the new moving and attector syitem tape transport system named Pig .1 Scheme of JOSEF

Installation of the 92t magnet in,the reactor half -241-

; Pig. 3 view of the separator JOSEP

MARIA (Moving tape Arrangement for Isotope Analysis) nay be seen, which is used for decay investigations. The operation;!] principle and the feasibility of the identification method are 17 9 1 described in refs. ' ' V.;[i The separated fission products transferred by the second electrostatic guide are collected on a tape, which is operated in; a discontinuotis mode. After a pre- sele c t ed i rradiat i on. t ime, the tape i s move d ve ry rap idly to the counting position; located above. While the decays of the irradiated part of the tape are recorded (e. g. fi-coincident neutron or jf— radiation) a further part is being irradiated. The spatial separation of irradiation position and counting position is suitable because of lower background and better counting geometry. Compared tp the older apparatus, for the new device this distance has been enlarged (*

3. Magnetic field measurements and ion-optical properties The spatial field distribution of the magnetic system was investigated at various currents using a high-accuracy Hall probe, which was stabilized against temperature variations. The total 300-degree field region of the H-type magnet was made accessable by means of a vehicle moving precisely along the pole-shoe faces. This assures that during the azimuthal movement the radial and axial position of the probe remained constant. The homogeneous- field regions were measured with NMR probes. In the region away from the edges of the pole shoes by more than one gap width, the experimental values of the magnetic field deviate from the ideal values by less than 3*10 for medium excitations of the magnet. Even for maximum excitation the deviation remains smaller than 1*10 . The fringe-field distribution was measured at the entrance and the exit of the magnet to find out the effective field boundaries for various angular positions of the semicylinders in the pole-shoe faces.

The ion-optical properties of the magnetic system were checked by deflecting a beam of Am alpha particles. The position of the ion-optical entrance and exit axis was determined by defining the trajectories by slits installed exactly along the main path. The resulting effective deflection angle is in good agreement with the value deduced from the fringe-field distribution. This angle is 312° for the 300° pole-shoe geometry. The experimental value of the ion-optical disperion is 15 cm for 1 % ( AB9/B9) deviation, and agrees well with the calculated value. With a source and a detector diameter of 10 mm and a transmission of 5*10 a relative width of the line profile ( AB3/B9) PWHM = 7*10 has been measured. The radial and axial foci as a function of the angular position of the semicylinders were in good agreement with the calculated values.

*t» Beam density investigation

The beam density distribution has been checked in various positions in order to understand the behaviour of the beam profile along the main beam path. At the focal plane under optimum -243-

> conditions (fresh target, fun R (au) illuminated aperture of the magnet by the electrostatic guide) the expected value of the beam density for maximum fission yield is 10 cm"2 sec"1. The measured beam density for a few week long irradiated target and 20 kV particle guide voltage was 2-105cm~2sec~1. The missing intensity is due to the burn up of the fission source and the lower transmission of the guide. / Instead of the expected value only a high voltage of 20 kV could be —— V, (!• V) applied to the first electrostatic 10 20 30 particle guide because of nuclear Pig. heating of the first insulator in the high radiation field of the reactor. Investigations based on several changes of the construction are in progress, and we hope to increase the transmission to the expected value. Fig. 4 shows the fission product rate measured with a surface barrier detector at the focal plane as a function of the high voltage applied to the first particle guide. The distance between source and first insulator was 70 cm. The axial distribution of fission produce density was investigated as a function of pres- Beam density sure. For N and light fission products in q Axiol width FWHM fig. 5 beam density and ri axial linewidth as a function of pressure are given. The beam was well col]"mated at the entrance :ide of the magnet according to the Fig. 5 axial direction. Since -244-

the vertical motion of ions passing through a gasfilled magnetic field is independent of change in the ionic charge in first order, these investigations allow us to deduce the effect of multiple scattering.

5. Fission product rate as function of magnet current Fig. 6 gives the fission product rate at the focal plane as a function of the magnet current for He as gasfilling at several pressures. At 10 Torr the probability for ionic charge- changing collisions is very low. Light and heavy fission products have the same averaged ionic charge and momentum. Thus the

-•- '•-i _l

VDTon H>

H STfar a= V

! t —JIA) 250 200 60

•f\ •t- . R OB far Hi GDDTorr H*

Id1: r •V:

. — JIM 50 150 200 300 •T "^' : - J(A)

250

Fig. 6 result of the deflection of fission products in a magnetic field with high vacuum along the path length is one broad line. Even at 5.-10" Torr He light and heavy fission products are well se- 6 parated. For small currents (B5 < 0.5 10 Gauss.cm) we found -245- in addition to ^-particles accompanied by fission, light nuclei in the mass region A 1 25, which were identified by preliminary measurements of time of flight and energy. The peak observed for high currents (Bg^ 1.6.1Q Gauss.cm) seems to be caused by- = head-on collisions of fission products onihe'atoras'of'uraniuin and tantalum (covering layer on the uranium target). These contributions will be further investigated with improved identification techniques .

The width of the light and heavy fission product group indicates, that in the case of He as gasfilling the mass dispersion for light fission products is greater than for heavy ones.

With increasing pressure a shift of the peaks to smaller currents can be observed as already pointed out in ref.1^. The decrease of beam density with increasing gas pressure is due to scattering of the fission products. This effect is the greatest for the low energetic heavy fission products.

6. Resolution

For determination of the linewidth we measured the intensity of known jf-transitions of yus-isomers as a function of the magnetic rigidity. The method and technique applied here is described in ref.1 » 15). For this investigation we used the 203 keV line of 9'Y and the 197 keV line of 15 Xe. The results obtained till now are presented in fig. 7.

The relative linewidth 97 I- eg (o/o] FWHM for 'Y at 1.5 Torr N_ is |3l = 0.9 % cor- t N 2 respondind 0g9 to a mass resolution j = 1.5 %, and for 136Xe at 0.4 Tor N AI^SJ. = 1.5 % cor- 136'Xe p r responding to a mass resolution j = 2.3 %• 10'-2 10" 10°

Fig. 7 -246-

7. Calibration For fast heavy ions being deflected in a magnetic field with certain gas filling the magnetic rigidity Bg depends not only on the mass but also on the nuclear charge. Fission products characterized by different complementary values of mass and nuclear charge may be focussed at the same location of the focal plane 5' 15) . In this aspect the performance of a gasfilled separator is different from that of other recoil spectrometers, which separate nuclei according to the ratio mass/ ionic charge and energy/ionic charge. Before investigating unknown short lived isotopes the calibration of the separator has to be performed by measuring the intensity of well known g-emitters as function of the Bg-value, For JOSEF this extensive procedure is now in progress. Of special interest is the dependence of the mass and nuclear charge dispersion on different gas fillings, which may be a further help in identifying new emitters .

no 10' KB 10 99 ATorrHe 0,2TorrN2 98 i 102 t / 101 97 96 - o A 100 95 H U 99 ii 94 O=> / 98 O I / 97 "I 93 o- CD s 92 96

91 I 95

90 94

89 93

88 92

87 91

86 90 i 85 89 - 1 84 88 —"A I

85 90 95 100 105 130 135 K0 145 150

Fig. -247-

Fig. 8 gives calibration curves for the separator measured with He-gas for light fission products and N -gas for heavy fission products. For these curves only characteristic tf-lines of known longlived nucleides have been taken into account. These emitters are produced in the fission process with negligible yield; they are mainly B-decay daughters of primarily produced fission products. Thus the nuclear charge, which determines the deflection in the gasfilled magnet, is not the nuclear charge of the emitter, but the mean primary nuclear charge of the corresponding mass chain. Consequently the validity of the curves is restric- 9510' i. Torr He isscm ) ted for the identification of long £ lived secondary fission products; how- CD ever the displacement of primary fission 3 products may be deduced ' ' . 93 \ Fig. 9 shows the measured peak of the 92 B

36 37 38

Fig. 9

R. Conclusion The beam properties of JOSEF investigated till now give the en- couraging result, that the investigations of short lived isotopes under study at the smaller separator since 1966 may be continued under considerably improved conditions. The energy spectra measurements of delayed neutrons emitted by fission product precursors which could not be performed up to now at the smaller device for intensity reasons will he started again. Nuclear spectroscopy of fission products with extremely small yields seems to be Dossible. Preliminarv measurements have shown, that high energy #-lines of •' Sn reported by the OSIRIS group ' could be found at JOSEF with intensity adequate for coincidence measurements . -248-

References 1) B.L. Cohen, C.B. Fulmer, Nucl. Phys. ^ (1958) 547. 2) P. Armbruster, Nukleonik 3 (1961) 188. 3) P. Armbruster, J. Eidens, J.W. GrUter, H. Lawin, E. Roeckl, K. Sistemich, Nucl. Instr, and Meth. £1 (1971) 499. 4) I. Bacho, D.D. Bogdanpy, S. Dafroczy, V.A. Karnaukhov, L.A. Petrov, G.M. Ter-Akopyan, JINR P 13 - 4453 (1969). 5) G. Fiebig, JUl-Report 939-NP (1973) and preceding paper of this session. 6) H. Ewald, E. Konecny, H. Opower, Z. f. Naturforschg. l£a (1964) 200. 7) E. Moll, P. Armbruster, H. Ewald, G. Fiebig, H. Lawin, H. Wollnik, Proc. Int. Conf. on Electromagnetic Isotope Separators, Marburg 1970, in Bericht BMBW-FB K 70-28 (1970) 241 and following paper of this session. 8) H. Lawin, J. Eidens, P. Armbruster, J.W. Griiter, K. Hiiben- thal, K. Sistemich, Proc. Int. Conf. on Electromagnetic Isotope Separators, Marburg, 1970, in BMBW-FB K 70-28 (1970) 270. 9) P. Armbruster, J. Eidens, E. Roeckl, in Proc. Lysekil Symp. (1966) Ark. Fys. 36 (1967) 293. 10) P. Armbruster, J. Eidens, J.W. Gruter, H. Lawin, in Pr-oc. Kyoto Conf. on Mass Spectroscopy 1969, University of Tokyo Press (1970) 520. • 11) E. Roeckl, J. Eidens, P. Armbruster, Z. Phys. 220 (1969 ) 101. 12) K. Sistemich, P. Armbruster, J. Eidens, E. Roeckl, Nucl. Phys. A J139 (1969) 289. 13) J. Eidens, E. Roeckl, P. Armbruster, Nucl. Phys. A 141 (1970) 289. 14) J.W. Gruter, K. Sistemich, P. Armbruster, J. Eidens, H. Lawin, in Proc. Leysin Conf. on Nuclei far from the Sta- bility 1970, CERN 70-30 (1970) 967. 15) J.W. Gruter, JUl-Report 879-NP (1972). 16) N.S. Oakey, R.D. MacFarlane, Nucl. Instr. Meth. 49 (19t<7) 220. 17) D. Hovestadt and P. Armbruster, Nukleonik £ (1967) ^Vo. 18) A. Kerek, G.B. Holm, L.E. De Geer and S. Borg, Annual Report of the Research Institute for Physics, Stockholm, (1972) 61. -249-

OPTICAL PERFORMANCE OF THE RECOIL MASS SPECTROMETER LOHENGRIN

E. Moll? G. Siegert, M. Asghar, G. Bailleul, J.P. Bocquet, J.P. Gautheron, J. Greif, H. Hammers, H. Schrader Institut Max Von Laue - Paul Langevin B.P. 156, F 38042 - Grenoble Cedex P. Armbruster Gesel 1 schaft fur Schwerionenforschung D-6100-Darmstadt l,Postfach 541 G. Fiebig, H. Lavrin, K. Sistemich Institut fur Neutronenphysik der KFA, 0-5170-Jiilich 1, Postfach 365 H. Ewald, H. Wollnik II Physikalisches Institut der Universitat D-6300-Giessen Arndtstr. 2

Abstract : The mass spectromater for fission products "Lohengrin" presented at an earlier conference in Marburg' ', was tested with an o-particle emitting point source. The results show that the deflection field strengths and the solid angle of the spectrometer are as calculated. The foiel points of the two deflection fields were found to coincide and are separated only 5,4 cm from the calculated focal point. With full solid angle,a mass resolving power of 800 (full width av tenth maximum) was achieved. Further tests showed that the mass resolution at different places of the focal line is also satisfactory. fly narrowing the entrance slits, we arrived at a mass resolution of 8 000 (FWHM) ; this corresponds to a mass difference of about 12 MeV far fission products. If we can increase the resolution by a factor 3 to 4,the spectrometer should separate the different nuclear charges of a selected mass and give the charge distributions of fission products in a direct way.

>.. INTRODUCTiO?i Thf- mass spectrometer for unslowed fission products "Lohengrin" was presented *t an oarliar conference^ '. Now we can give the first results of the ion *Now at Salzers A.G., FL 9496 Balzers -250- optical tests of this spectrometer. It was built to separate unslowed fission fragments to get more information on fission itself, to do nuclear spectroscopy and to investigate the interactions of high energy heavy ions with matter. The variety of this work requires that the instrument should work in different modes of mass-resolution. The unslowed fission products have a broad distribution in mass, energy and ionic charge. Therefore, we obtain different mass spectra with the respective energy distribution for each ionic charge. To avoid admixtures and to obtain the highest possible intensity, the resolution has to be optimized individually for each case.

2. DEFLECTION, SOLID ANGLE AND FOCAL POINT The spectrometer is of the Thomson parabola type, it consists of electric and magnetic sector fields that are spatially separated. The total length of the main path is 23,1 m. This spectrometer was tested with a point alpha- particle source (0,1 mm in diameter) of 212Po. From the measured field strengths, we got the values of deflection radii ; these agree with the calculated values within the limits of error (2 %). (All the calculated values are given in refs.l and 2). This shows that the effective field boundaries had been correctly calculated and realized. The solid angle of the spectrometer was also verified. We had put in a slit system before the entrance to the first field to yield the central beam and four beams at the limit of acceptance. An exposure taken at 30 cm before the focal point showed all the five beams. This indicates that the solid angle of the spectrometer is as calculated. To get the focal point, the focusing properties of the electric and magnetic deflection fields had been investigated separately. The results are presented in Fig. 1. There the full width at half maximum (FWHM) is shown at various distances from the calculated focal point. The exit slit was oriented in such a way that the counting rate obtained with a surface barrier detector was sensitive either to the focusing of the magnetic field (Fig. la) or to the electric field (Fig. lb). In a third run, the exit slit was oriented parallel to the calculated mass line to get the combined effect of the magnetic and the electric fields (Fig. lc). For each case, two independent measurements are shown. It can be seen from Fig. 1, that the focal points for the electric and the magnetic fields coincide within their error limits. -251-

.FWHM Focus a) magnet

-30 -20 -10 10 20 30 kFWHM,FWTM[mm] Fig. 1 : From top to bottom : Full width at half maximum at various distances X from the theoretical focal point a) resulting from the focusing of the magnetic field ; b) resulting from the focusing of the electric field ; c) resulting from the combined focusing of both fields. Two independent measurements are shown ; they are taken by counting d-particles with a surface barrier detector ; d) exact determination of the focal point 6 7 [cm] by measuring the intensity distribution of a-particles with nuclear photographic plates. Full width at tenth maximum (FWTM), FWHM, and the ideal case, where only broadening due to geometrical effects occurs (GE), are shown. -252-

To get the focal point with a better accuracy, a nuclear photographic plate was exposed to the beam, and the distribution was evaluated by counting tracks under a microscope. The result is shown in Fig. Id. Here, in addition to the FWHM, the full width at one tenthi Bf"maximum (FWTM) arid the calculated broadening of the diverging beam, are given. The experimentally determined focus is only 5,4 cm away from the calculated one. This is a rather small deviation compared to the length of the main path (21.3 m). The result shows that the focusing properties of the fringing fields have been correctly taken into account and that the focusing strength is as calculated.

3. FOCUSING ALONG THE MASS LINE AND RESOLUTION AT REDUCED SOLID ANGLE The results of Fig. 1, were taken at the central point of the focal line. The focusing properties at different positions along the focal line (total length 72 cm) are shown in Fig. 2. The section shown represents 24 cm of the focal line. We have found a comparable resolution in a region covering 2/3 of the total length of the mass line. However, up to now the very end points of the mass line have not been tested. The results of Fig. 2 have been achieved with the maximum solid angle of acceptance, i.e. all the slits fully open. In order to test the spectrometer for higher resolution, the angle of acceptance was reduced. However, the angle of acceptance in the horizontal plane was always chosen much larger than that in the vertical plane as the focusing properties of the magnetic field are superior to those of the electric field. With this choice of the entrance slit, it is not necessary for the focal points of the electric and magnetic fields to coincide exactly ; it is sufficient to take exposures at the exact position of the focus of the magnetic field to get good results. The results obtained are shown in Fig. 3. The FWHM and FWTM are given. For comparison the calculated base width, taken (2) from v ', is also shown. For a large solid angle, the resolution is as calculated. This means that the spectrometer is well suited for separating fission products. If one reduces the solid angle, the resolution increases ; but for very small solid angles, this increase is smaller than calculated. This could be due to the fact that we did not have adequate control of the deflecting fields ; for instance, there was no nuclear magnetic resonance to survey the magnetic field. For the cases marked a and b, the corresponding intensity -253-

Fig. 2 : Intensity distributions of a-particles at different positions along the focal line. The mass resolution at FWHM and FWTM, is given. The section shown here represents 24 cm of the focal line.

10 Oorf Resolution Alnt

exp. 2a, theory 100-

5000

100

5100

2500

Fig. 3 : The mass resolution.for different solid angles (solid lines) compared with calculations from1^ (broken lines). Longer lines represent results at full horizontal aperture (2,3°) ; for the shorter lines this aperture was limited to 1° and the corresponding distributions are shown on the right hand side (a,b). The resolution at FWHM and FWTM is given. -254- distributions are also shown on the right hand side of Fig. 3. From the results obtained with a point source, the mass resolution for sources having a certain area as given in ^ '» can be evaluated. This was done in ref. 3, where it is shown, that the results extrapolated from a point source,are consistent with the calculated values. A mass resolution of 8 000 is promising and corresponds to an energy resolution of 12 MeV for fission products. If we are able to increase the resolution by a factor of 3 to 4, we can separate spatially the different nuclear charges of a selected mass chain and, thus, a direct measurement of the nuclear charge distribution will be possible.

REFERENCES : 1. Moll, E., Lwald, H., Wollnik, H., Armbruster, P., Fiebig, G., and Lawin, H., Proc. Conf. on Electromagnetic Isotope Separators and the Techni- aues of their Application, Marburg 1970, pp 241-254 (Leopoldshafen : ZAED) 2. Fiebig, G., Jul-737-FF (1971) 3. Moll , E. , GSI-Report 73-3. -255-

HIGH VOLTAGE PERFORMANCE OF AN ELECTROSTATIC SECTOR FIELD FOR UNSLOwED FISSION PRODUCTS

H. Hammers, E. Mol1x, M. Asghar, G. Baiileul, J.P. Bocquet, J.P. Gautheron, J. Greif, H. Schrader and G. Siegert Institut Max Von Laue - Paul Langevin B.P. 156, F 38042 - Grenoble Cedex

Abstract : An electrostatic sector field with a deflection radius of 5.6 m, a deflection angle of 35.35° and an electrode distance of 30 cm, has been constructed and tested. It can be operated at high-voltage values up to 700 kV after appropriate high-voltage conditioning. Sufficiently long measuring periods with low current (< 10" A), i.e. little brensstrahlung and low break-down rate, can be attained with a vacuum of 5.10 Torr.

1. INTRODUCTION The construction of the electrostatic sector field of the mass spectrometer "Lohengrin"^ ' was considered as the most difficult technological problem of the whole project. The high operation voltage values (up to 600 kV), the large dimensions of the electric field (3,5 m x 1,3 m x 0,3 m), the curvature of the electrodes (R = 5,6 m) and the high precision requirements {< 10" ) represent demands that implied a great risk for its realization. Therefore a series of preliminary experiments has been performed with a model. The sector field has been constructed on the basis of this experience. The test results show good agreement with the predictions derived from the model. 2. RESULTS OF THE PRELIMINARY EXPERIMENTS^ (Fig. 1) High-voltage conditioning in vacuum (= 10 mm Hg) is caused by field emission of electrons,if discharges are avoided. Points or whiskers on the cathodic surface, partly created by the electric field, emit a very low current up to a certain voltage beyond which it increases to very high values. Suf- ficiently high currents, however, initiate discharges and thus high-voltage breakdown. Discharges and electrode damage can be avoided by limiting the current output of the high voltage generator '. The voltage, which can be

attained with the current limited to Ir, will be called the conditioning l> Q voltage V . One can reach voltage values up to V before any conditioning. 0 r c Vc depends on pressure and the surface state ;

X Present address : Balzers AG, FL 9496 Balzers -256-

it was about 100 kV at Id"6 torr for polished aluminum and stainless steel surfaces . The current- voltage characteristic (Fowler- Nordheim equation) of the load keeps the current I- stable even for simple current limitation. The current causes a slow continuous increase

of Vc. This 1 rn conditioning according to our experiments, can be well explained.if one sup- Fig. 1 : Experimental set-up for the preliminary experi- poses that the ments : A, cathode ; B, anode ; C, iiigh-voltage feed- work function changes throughs ; 0, rotary ir-sulator (to change the gap width due to the de- between the electrodes). sorption of gases on the emitting tips. The conditioning process ends in a new equilibrium state characterized by a voltage Vp depending on Ir (table 1). Conditioning is lost, when the current is switched off, e.g. by lowering the voltage to a value Vm < vc. After some time \'c reaches Vm and the current rises to a value corresponding to V = V", if V is stabilized. Fig. 2 shows the time-behaviour of the conditioning and deconditioning process. The above -257-

mentioned explanation of these processes allows one to derive the formula^ : /, n m

where conditioning means

I i 01 / time and X depends on Ip as shown in Fig. 3. Fig. 2 : Schematic diagram of the ti me-behavi oiir of Deconditioning implies : voltage arid current during : I.jconiitroning ; II, de- conditioriihg (measuring period with low current) ; with Ic = 0. III, Less-favourable measuring situation : the vol- For deconditioning,we found values of tage remains stabilized, X * 10" h" ; they were higher at the begin- but a certain current is necessary to maintain the ning of the experiments and decreased slowly equilibrium. after many conditionings and deconditionings. For conditioning, however, we found a gradual increase of X after many experiments. To predict the behaviour of the big electric field .1 from the measured quantities of the smaller model, we assumed that the average number of electron emitting tips is proportional to the f/• area. Therefore, the current per surface area

,60 ,80 enters as the most important quantity. This Conditioning current lc(pA) assumption had some previous experimental Fig. 3 : Dependence of the evidence. conditioning parameter x on the conditioning current 3. THE DEFLECTION CONDENSOR Ic (stainless steel half- tank as cathodic surface). Fig. 4 shows a longitudinal section through the deflection condensor. All cathodic surfaces are made of aluminum (AT Mg 3), the anodic surfaces are made of aluminum (anode) or stainless steel (tank). Both elactrodes are welded structures with a very precise (10 mm) machining of the surface facing the other electrode. The other surfaces requiring less geometrical precision (tank, electrode surfaces facing the tank, fringing field diaphragms) were mechanically polished. Very rigid and symmetric box type frames surround the tank like belts at the position of the supporting insulators. They undergo a small and reproducible bending, when the tank is evacuated. -258-

Fig. 5 shows a transverse section through the deflection condensor. The high-voltage feedtr.roughs correspond to those used in the CERN- separators* '. The fringing field diaphragm at the entrance Fig. 4 : Longitudinal section through the deflection conden- side can be sor : A, anode ; B, cathode ; C,fringing field diaphragms ; moved along D, stainless steel vacuum tank (covered with an aluminum .. . .. sheet in face of the anode); E, rigid box type frames ; F, zne Deam Q1~ pumping holes. rection. Thus one can obtain a certain adjustment of the effective field boundary, i.e. of the effective field length. The diaphragm at the exit side is bent to yield a curved field boundary of R = 1.8 m, 4. OPERATION OF THE DEFLECTION CONDENSOR Fig. 6 shows a conditioning of the deflection eendensor, which starts at 0 0 V The value of Vc was 90 kV between cathode and tank, 90 kV between anode and tank, 150 kV between anode and cathode. The iccessible voltages are shown in fig. 7 as a surface in a V+ versus V" diagram, where the conditioning voltages are 3 straight lines forming a rectangular surface with a broken edge : V+ = v£ = const., V" = vl = const, and V+ + V" = V*~ = const. The first conditioning period leads from situation 1 to situation 2. The positive voltage is stabilized at 50 kV, the negative voltage is defined by the limitation of the current in the negative, high-voltags generator. Thus the conditioning voltage between the cathode and the tank is increased. The fluctuating current in the positive generator indicates a current flowing between the anode and the cathode. In our example, we reached Vr = 150 kV -259-

I Fig. 5 : Transverse section through the deflection condensor: 200 A, anode ; B, cathode ; C, cross section of the particle beam ; D, high-voltage feed throughs ; E, insulators with adjustable mountings ; F, rigid box type frame.

Fig. 6 : Conditioning of the deflection condensor with a total current of =100 uA. (much faster conditionings are possible with higher currents). between cathode and tank after a conditioning period of 55 minutes (situation 2). The new situation is characterized by : V" = 150 kV between cathode and tank,

Vr = 90 kV between anode and tank, VQ - 200 kV between anode and cathode. In our example v£~ » 750 kV, is reached after about 20 hours, i.e. we spend

about 10 hours to increase V^ from 90 kV to 400 kV at Ic = 50 pA. Conditio- ning between anode and tank (Vjfc) works in about the same way. Unfortunately we cannot reach the voltage limit V£ of aluminum cathodes with our high-voltage generators. If we suppose V^ to be about 1500 kV, we get 11 i B 0.037 h"h" for Ic = 50 PA. We found such x-values for Ic = 10 UA in the small model with 5 to 11 times smaller surfaces. The comparison with our preliminary experiments is much easier for the deconditioning, since there should be no surface dependence for I = 0. Fig. 8 shows a V+ versus V" diagram for deconditioning. This measurement with a pressure of 7 3 1 5 x 10" Torr, yields x - 2.9.10" h" between cathode and tank. These x-values correspond well to thoSe of the prelimi- Fig. 7 : Conditioning of the deflection condensor with a total nary experiments and we hope to get a current of = 100 yA. further decrease after a long operation time. Practically, a (Conditioning-at 300 yA takes about 2 hours. Conditioning is very fast compared to the measuring periods -6.4_8w:37h10 - 54-17° :21h4O L 50 which are (after conditioning to 750 kV) 5.4-8 :i3h20 -4.4 - Sh30 60 h for 600 kV, 115 h for 500 kV, 195 h for 400 kV and 310 h for 300 kV. REFERENCES : 1. Moll, E., Ewald, H., Wollnik, H., Armbruster, P., Fiebig, G. and Lawin, H., Proc. Conf. on Electromagnetic Isotope Separators and the Techniques of their Fig. 8 : Deconditioning of the Application, Marburg 1970, pp 241-254 deflection condensor after conditioning to V*" = 750 kV. (Leopoldshafen : ZAED). 2. Steib, G.F. and Holl, E..J. Phys, D : Appl. Phys., 6, 1973, pp 243-255. 3. Rohrbach, F., CERN Rep. No. 71-5, 1971. -261-

Table 1 : Voltage limits v£ [KV]for different conditioning currents, electrode materials and geometries (pressure : 2 . 10 mm Hg)

10 y.A 25 yA 40 RA 60 nA , 80 uA 100 pA

Stainless steel half-tank Stainless steel anode 240 255 260 270 270 275

Aluminium half-tank Stainless steel anode 320 345 370 380 385 390

Stainless steel half-tank Aluminium cathode 360 >400 >400 >400 >400 >400

Stainless steel anode >680 >77O >780 >785 >790 Aluminium cathode >745 -262-

BURSTS Moshe Oron* and Yehuda Paiss*, Laboratory for Laser Energetics, University of Rochester* ;Roch-, N.Y. 14627

I. INTRODUCTION: Ionsi that are blown off plasma bursts have charge, mass, and energy distributions that are related to the plasma parameters and composition. The determination of these distributions is essential when plasma parameters and/or composition are studied. Earlier works measuring those distributions used a combination of two static analyzers in series, e.g., an electrostatic analyzer that singles out an ion beam of small energy range followed by a magnetic analyzer that determines the charge to mass ratio, or a time-of-flight analyzer followed by an electrostatic or electromagnetic device. Many plasma bursts are required to generate a distribution function since each shot gives information related to a small energy range. Some works ' are using the parabola spectrograph in which eneray and yield distributions are determined simultaneously. In this approach it is difficult to analyze the nuclear properties of the collected species since the collection takes Dlace on a parabolic trace and not on a single spatial collection spot, when collecting over a small region of the parabola the collection efficiency for a single q/m is reduced. In the following sections, a dynamic spectrometer is described in which the yield and energy distributions are determined simultaneously for each plasma burst. The spectrometer described has a single spatial collection point for each (q/m) enabling to collect the separated species or make further use of a desired ion beam.

2. THE DYNAMIC MASS SPECTROMETER: An electrostatic energy analyzer or magnetic momentum analyzer can be represented in their simplest case, for small deflection angles, by the angle of deflection in the field:

* on leave from Soreq Research Center, Yavne, Israel. -263-

and 9 ,2! e - a m m v e m where Q&, 6m are angles of deflection in electric and'magnetic fields, (q/m) -charge to mass ratio,v-velocity7of\;thel:io|> and E,B electric arid magnetic field strength.

BY OSin an electric fifeld E 2 °?° H := Ebv or magnetic field

B = BQv, where-Eo,:Bo are constants; the angles of deflection 6e' 6m are only :(?#n) dependent^ and y independent- Fields of this kind can be generated for pulsating ion sources in the following way: a laser pulse of short duration hits the analyzed surface, and a burst of ions is emitted. The ions travel a distance L and then enter the E or B field. When a time dependent field E = -$ or B = 5& is applied to the analyzer, where t is elapsed time from burst, the. 8e, 9 are only (q/m) dependent. Using the described principle, it was shown^6* that for a capacitor-like electrodynamic set up (Fig. 1) , the ion tra-. jectory is described by

_+ x y _ cj . , ccso i x ) + xtana mo L Lcosa

A(X,,Y.)

Fig. 1 Schematic diagram of an electrodynamic analyzer. Here for each (q/m) there is a spatial collection point X . which is independent of. v. ,7» For the magnetic case it. can be shown that the trajectory is described by (Fig. 2):

* = SL Boln v.'here is the angle deflection and S is the distance measured along the trajectory of the ion, measured from point c (Fig. 2). -264-

Here, as for the previous version, there is single spatial collection point for each (q/m).

Fig. 2 Schematic diagram of a magnetodynamic analyzer.

The energy distribution of the ions can be determined by the time-of-flight method, using the fact that the x component of the initial velocity v is not changed by the E-field in the electrodynamic version. In the magnetodynamic version, the time-of-flight calculation should take the B field into consideration since the Lorenz force is normal to the trajectory, Resolution calculations for energy and (a/n) are presented in,(6)

3. EXPERIMENTAL: Based en the electro-dynamic version (since fast time varying magnetic fields are more difficult to generate) a spectrometer was built, for the on-line diagnostics of laser produced plasma. In the study of laser induced thermonuclear fusion, LiD pellets are irradiated with pulsed high power high energy Nd:glass laser (=20-50 joules in pulses of 10 sec. duration at 1.06um wavelength), and the analyzer is designed to separate H,D,He,Li, and Oxygen ions emitted from the plasma (0 is an impurity). The analyzer operates over a charge range from +1 to complete stripping and energy range 1-10 Kev. Since the highest expected energy is =10Kev, the potential E = =0- is KV 2 t applied only at t = 2ys and the value of E - 8—(ys) giving KV cm E(t=2|u.s) ~ 2— . The ions are collected in faraday CUDS, and currents of few PA are typical for the system. The system operates at 1G Torr. -265-

Fig. 4 presents the schematics of the laser-fusion experiments and Fig. 5 the analyzer design;

SOUO STATE DETECTOR

X-RAY SHIELD

\ CALORIMETER

BEAM-SPLITER

I METER CZEHNY-TURNER SPECTROGRAPH LEADSHIELDEO -SCINTILATOR-PMTUBE I POLARIZING PLATE NEUTRON DETECTORS PHOTOOOOE*

Fig. 4 Schematic diagram of target chamber for laser induced fusion experiments. (8)

Highvoltagsin V

Separated current probes,(Mognetic suppresion of Secondary electrons)

To amplifier and scope Fig. 5 Electrodynamic analyzer assembly. -266-

4. TYPICAL RESULTS: The typical experimental results are the time history of the charge collected at each charge collector. In the example shown below, a pellet of lOOym diameter of LiD was irradiated by a neodvmium glass laser (1.06vi) palse +? of duration =100ps and energy =25 joule and ions of Li_ ", Li_ + 3 were separated and collected simultaneously/. Fig. 6 shows the oscilloscope trace obtained for the two. From the two curves shown the relative and absolute particle yield can be obtained by integration of the curve and knowing the solid angle assumed by the entrance slit of analyzer. Velocities and energy distributions are derived from this curve by changing variables. 5. DISCUSSION: The describee1, technique can be applied to other nuclear reaction studies, e.g., electron, proton or neutron pulse induced nuclear reactions, e.g., fission and spallation, the qen- eration of ions in the- Mev energy region calls for very raoid changes in electric or magnetic fields which are difficult to generate in the present state of the art, but nevertheless deserve a serious evaluation. Another wav to realize a r>ul-

.+3 y \ /

I sJ

Fig. 6 Typical time history of charge, collected simultaneously for Li-+3 and Li-"1"2 emitted from a LiD pellet when irradiated with a 25 joule Nd:glass laser pulse of lOOps duration. The traces shows two velocity groups in the Li_ 3 ions. -267-

sating ion source is to use the pulse that generates the nuclear reaction, e.g., electron or proton beamas, the ionizing element, or to use a pulsed laser beam as the 'ion source1 , in this way only particles of Kev energy are produced, for which analysis is feasible. The last suggestion has the disadvantage of diluting the reaction products in ion3 of target material hence increasing space charge problems.

6 • ACKNOWLEDGMENTS: The authors wish to thank Drs. M. Lubin, L. Goldman, and J. Soures for the helpful discussion and direct assistance. The authors are grateful to Mr. F. Willis for building the electronic parts of the analyzer. This work was supported by the Laser Fusion Feasibility Project at the University of Rochester. 7. REFERENCES: 1. N. I. Alinovskii, et al., p.25 in Recent Advances in Diagnostics, Vol. 3. Edited by V.T. Tolok, trans- lated by Consultants Bureau, 1971. 2. N. I. Alinovskii, et al., p. 30, ibid. 3. Yu. A. Bikovskii, et al., Sov. Phys., Tech. Phys. 13, 986 (1969). 4. W. Kaufmann Phys. Z. 4, 55, 1903, known also as J. J. Thomson spectrometer. 5. S. Neumann and H. Ewald, Z. F. Physik 169_ (1962) 224. 6. M. Oron and Y. Paiss, submitted for publication in Rev. Sci. Instr. 7. M. Oron and Y. Paiss, to be published. 8. Laser Fusion Feasibility Project Annual Report (1972), Laboratory for Laser Energetics, University of Rochester, Rochester, New York. -268-

Velocity Separation of Heavy Ions by High Frequency Deflection *

by P. R. Krueger, B. I. Persson, E. Kankeleit Author's adress: Institut fiir Kernphysik der Tech- nischen Hochschule Darmstadt, D-6l Darmstadt, SchloBgartenstr. 9

To produce superheavy elements at the UNILAC (Heavy ion accel erator under construction) of the GSI in Darmstadt, one can use e tthh e followinfg g reaction typesyp : „„ l^l^O 1^44 28284 1 The compound nucleus formation, e.g. ' Te + J Sm -* 114 r>Qp 114 + 2n could be used. One would then search for the de- 25& 2? 76 Thexcitee fusion-fission-reactiond compound nucleus in, the.ge forwar. U d+ direction U->( .lS4) -*• b ° 112 + other reaction products, coul& d be used combined with a search for the heaviest reaction product which is emitted with^large probability in the forward direction and at an angle determined by the kinematics.

Due to the broad charge distribution of the ions, pure magnetic or pure electrostatic separation of the nuclei of interest from projectiles and background is not feasible. However, in both reaction types the velocities are well defined by the kinematic, practically to - 1$. As the production cross sections will be very small (^lnb) for both reaction types and as only thin targets (less than 1 mg/cm ) can be used due to the energy loss, the beam of wanted particles has to be separated very well from the projectiles. While a static Wien- filter will separate the particles only according to their velocities, the HP-Separator will in addition separate according to the travel time of the particles. As the consequence one can remove beam particles, which have been slowed down at places outside the target (especially in the edges of beam defining slits) and thereby obtained a velocity near that of the compound nuclei.

*(Work supported by the "Gesellschaf t f?tr Schwerionenforschung" (GSI), Darmstadt, West Germany.) -269-

The primary beam is bunched by the accelerator. The repetition frequency is 27.12 MHz; each bunch is 0.3 nsec wide. This bear.i will penetrate the target and it is hoped that it there will produce long living compound nuclei or very heavy reaction products. By a quadrupol dublett the'beam will be focused into an high-frequency electric field which is synchronized with the accelerator. The flight path between target and IIF-field, and the phase of the deflector field are adjusted so that pri nary particles will get the maximal opposite deflection to compound or other wanted particles. After the main part of the projectile beam has been deflected away, the wanted particles will enter a second HF-deflector, where the first deflection will be cancelled. 1 ) The q/m-divergence of the beam is cancelled, however a smal i parallel displacement remains. 2) The projectiles which entered the first deflector at the same time as the compound nuclei will now be rejected with high probability. 3) Those reaction products of low velocity which are able to pass the 1st deflector in the same phase as the compound nuclei but some periods later, can be separated in the 2nd deflector unit, thereby preventing measurements to be influenced by them. 0 The velocity acceptance is ± 1.5 % if two deflector units are used.

The particles fulfilling the separation conditions will be focused on to a certain plane. Because the highly chromatic beam one cannot produce a sharp image of the 6 mm wide beam -270- spot on the target. A waist of 6 cm in axial and 1 cm in radial direction will be obtained. The Luminosity will be about lo"-5 cm2, and the accepted aperture about o,2 msr. The separation efficiency, i.e. the ratio of the intensity of the incident pro- n jectiles in the beam of wanted particles, will be about lo^, as obtained by Monte-Carlo-calculations. This figure applies to measurements in the direction of the accelerator beam and depends somewhat on the type of reaction. The transmission of compound nuclei will be about 5 %•

If one does not need very high separation efficiency and good ion optics, e. g. in using the HP-separator as a pre-separator for a He-Jet or chemical separation arrangements, one deflector stage with additional focusing is sufficient. The transmission of compound nuclei will then be increased to 3o % and the velocity acceptance to 5 %.

It should be mentioned that a 6-dimensional phase space (e.g. as used in the computer program Transport) is not sufficient for our beam transport calculation. The beam has to be described by a 7-dim.phase space using the velocity deviation as an additional dimension.

Peak field strengths up to loo kV/cm are needed, since the deflection plates may not be longer than 3o cm to obtain a reasonable transit-time-factor. To provide the mentioned apertures a distance of 6 cm between the deflection plates is necessary. A deflection peak voltage of 6oo kV is then required.

Some models operating at higher frequency have been built to measure the power needed to meet these requirements. A capacitively shorted A/4 - coaxial - line needs 9o kW power for continuous operation. We have found a new structure fulfilling beam transport and other requirements that needs only 5o kW. We hope that by optimization of this structure this value can be still improved. Chapter 6: Ion physics techniques -271-

PROGRESS REPORT ON THE ION IMPLANTATION FACILITY AT ORSAY J. Chaumont, F. Lalu, M. Salome, C. Seide. Laboratoire Rene" Bernas du C. S. N. S. M. - 91406 Orsay.

The Orsay ion implantor has been used for the last two years in a wide range of experiments, including solar wind simulation , implan- (2) tation enhancement of EPR in metals , lattice location of heavy impuri- (3) ties in metals , position sensitive semi-conductor detectors and implantation effects in superconductors. The purpose of this paper is to indicate (4) the main features of this machine (previously described in ) and to describe its operation for various applications. 1 - Ion source. We use the same Bernas-type ion source as on the isotope separators at Orsay. This ion source has been previously described , The only difference is the reduced size. The extracted current can be larger than 2 mA (instead of 20 mA) but is usually about 1 mA. This ion source can ionize all the elemnts except thofje of the platinum group. About 40 elements have already been implanted ; the maximum currents through the definition slit range from 40 to 400 uA according to the element. Ion extraction, beam formation as well as space charge neutralization are obtained by a simple electrode at a potential slightly negative relative to ground

One advantage of this ion source is that it can deliver significant currents of multi charged ions. As an average we can easily obtain 10 % of doubly charged ions and 1 % of triply charged ions for most of the elements. This ability combined with the relatively high extracted current is such that for instance a total current at the collector of 30 \iA of P or 120 uA of Yb or 70 uA of AM .. . etc may be obtained.

2 - Loading materials . Feed materials for an isotope separator ion source have been already studied * " '. These loading materials are usually convenients for long runs. In our ion implantation facility we -272- frequently have to change the implanted element, i. e. we need very flexible feed materials. This led us to develop new methods for some elements.

2-1. Phosphorus . Red phosphorus evaporates at 150cC. At this low temperature the ion source would have to be especially adapted in order to maintain easy control of the phosphorus flow. A conventional ion source can be loaded with the copper phosphide CU5P2 w^c^ *s decomposed under vacuum at about 600°C producing phosphorus vapor. With a 1 mA extracted current, the phosphorus current in the collector reaches 300 uA.

2-2. Silicon. SiS is a very convenient loading material but is unfortunately rapidly destroyed by air. Hence we have tried to synthetise this compound in situ. Two methods have been developped : a) Silicon is sulfurin ated by a CS2 flow exactly as many elements are chlorinated by CCl,. b) A powder containing FeS + Si is heated at 700°C. We prefer this second method because the mixture can be prepared in advance and does not require special precautions for tis conservation and use.

2-3. Boron. BC^-J *s commonly used as a feed material, but this compound beeing very corrosive is not very easy to use.

Very good results have been obtained with a mixture of B O and Al F When heated at about 400cC, volatile fluoride and oxyfluoride compounds are formed, and we have obtained 50 |iA of boron ions with a good reproducibility. The only trouble is that the mixture must be protected against humidity.

2-4. Aluminium. The metal is not convenient because it combines with the graphite of the discharge chamber. A good feed material is a mixture cf Al F3 and Al powder.At 700 °C a volatil aluminium monofluoride Al F is probably formed. With this mixture a 300 uA Al+ beam is easily obtained. -273-

3 - Magnetic analyzer. The magnet is quite small (400 Kg). Its radius is 30 cm and the field index is 0. 3. When used in a non symmetric arrangement it : a relatively large dispersion (5. 5 mm between masses 100 and 101 perpendicular to the beam) and a good resolution ^' as is shown on fig. 1.

At the maximum magnetic field of 1 Tesla and under 50 Kv acceleration voltage ions car. be analyzed up to mass 90 (singly charged ions) or 180 (doubly charged ions).

4 - Implantation chamber. Our ion implantor includes a separator proper followed by an implantation chamber. The only connection between these two parts consists in a slit which is usually 1. 7 x 10 mm . While the separator is evacuated by oil diffusion pumps the implantation chamber is evacuated by a turbo-molecular pump. Thus a very clean vacuum is obtained inside the implantation chamber.

Behind the slit are two sets of plates which are used to sweep the selected beam. Thus inside the implantation chamber we have a uniform 2 dose over an area as large as 10 x 10 cm .

5 - Target holders. Inside the target chamber we have three possible target holders.

The first one is set at ground potential and can support 6 samples 2 as large as 100 x 50 mm . The second one, which may also support 6 samples is insulated relative to the target chamber which remains at ground potential. When set at a negative voltage (maximum 100 kV) the ions energy can reach 150, 300 or 450 keV for singly, doubly and triply charged ions respectively. When set at a positive voltage and associated with a simple optical device, the ion energy can be divided by 4. The third one contains a sample holder mounted on a goniometer. It can also be set at voltages up to 100 Kv. Thus on monocrystalline targets chanelling can be achieved (or avoided if necessary). Crystal orientation .74- is obtained via 50 KeV proton bombardment inside the ion implantor itself. Fig. 2 shows a channeling pattern on an iron crystal.

Doses are measured accurately by a current integrator set up in an insulated box and directly connected to the target holder.

Summary. In spite of its small size, basic simplicity and ease of operation, the Orsay ion implantor has proven to be quite a versatile machine. Ion energies between 5 and 400 KeV have been obtained on fourty elements. Implanted doses range from 10 to 10 ions/cm . It is presently applied to studies in nuclear physics, solid state physics, lunar science, metallurgy and chemistry. Strong interactions with ion beam users has led to many of the reported technical developments. This is an essential and stimulating feature of our work.

References.

1 - J. P. Bibring, J. Chaumont, G. Comstock, M. Maurette, R. Meunier, and R. Hernandez - Lunar Science IV eds. J. W. Chamberlain and C. Watkins, 1973, p. 72. 2 - P. Monod, H. Hurdequint, A. Janossy, J. Obert, J. Chaumont - Phys. Rev. Letters, vol. 29, Nb 19 (1972). 3 - F. Abel, M. Bruneaux, C. Cohen, H. Bernas, J. Chaumont, L. Thome. Solid State - Communication - To be published. 4 - J. Chaumont, M. Baran-Marzak, J. Camplan, R. Meunier, J. L. Sarrouy - A. V. I. S. E. M. Versailles Sept. 71 - Le Vide : ° 152. 5 - J. L. Sarrouy et al. - Nuclear Inst. and Meth. , 3Q (196b', Z, \ 6 - J. Camplan, R. Meunier, J. L. Sarrouy - Nucl. Inst. and Meth. 84 (1970) 37. 7 - J. H. Freeman and G. Sidenius - Proceedings 2n Int. Conf. Ion Sources Vienna, 1972, p. 724. 8 - C. W. Sheridan, H. R. Gwin, L. O. Love - ORNL 3301. 9 - J. Camplan, R. Meunier, C. Fatu - P. A.R.I. S. - The new isotope separator at Orsay - These proceedings. -275-

SPECTRE Kr Fig. 1 - Mass spectrum of Kr. The I amplitude at the minimum between masses 83 and 84 is 10"3 of the mass 84 peak.

Faraday cup silt Hl-

Fig. 2 - Channeling pattern on an iron crystal, obtained from measurement of current on the electron repeller. 78 80 82 ' 84 86

Fe crystal alignment in isotope separator

3.0

-o-

2.0 50keV protons

1.0

i • I i I i I i l i » i I i I i l i I i I I I I I i I l I I I I I I. J. I 10 14 -14 -10 -6 -2 0 2 Tilt (?) -276-

A Versatile Isotope Separator System for Ion Implantation and Ion-Surface Interaction Studies

J. S. Colligan , J. H. Freeman*, W. A. Grant , M. J. Nobes , and G. W. Lewis

TJniversity of Salford, Salford 5, Lancashire, U.K. •A.EJl.E., Harwell, Didcot, Berkshire, U.K. -277-

Introduction

In the Atomic Collisions;in Solids Group of the University of Salford a wide variety of the research topics involve: the useLjof: ion beams, A large proportion of the programme is supported by a specially modified isotope separator^with various on-line facilities. The general versatility of the ion source ' and in particular tte ease with which intense sharply focussed beams of most elements can be obtained over a wide range of operating conditions has proved to be particularly convenient for a broadly bassd University ion implantation research programme. In this paper we will discuss the overall layout of the modified separator system and its relevance to the experimental programme before returning to cover some particular aspects of the machine performance in greater detail. These include measurements of the random energy spread of the ion beam and of the low energy extraction behaviour of the ion source. Isotope Separator System

A schematic diagram of the principal components of the isotope separator system is shown in Fig. 1. The basic machine is a variable—geometry isotope separator which uses a standard Harwell ion source with a 4 cm extraction slit and has a 60°, 40 cm radius homogeneous field analysing magnet fitted with wide pole tips and rotatable inserts. The ion source provides intense beams of a wide variety of ion species at energies up to 40 keV. Useful intensity beams (~^A) can often be obtained by employing doubly or triply charged ions and this raises the energy range to 120 keV. The variable analyser geometry allows the focal length, the dispersion and divergence of the ion beam to be controlled over wide limits and this versatility has been exploited in the on-line facilities that have been added to the basic machine. The variable focal length of the separator * has proved extremely valuable since it can be U3ed to optimise the position of the focal plane for particular experimental requirements and to move it over distances of several metres which allows the use of multiple target chamber assemblies. The first experimental stage after the analysing magnet i3 a general purpose earthed target chamber. This is used for the implantation of samples at energies up to a maximum of 120 keV (using triply charged ions). The stage is equipped with a Faraday cage and magnetic secondary electron suppression and a heater for implantations up to 500°C. Precise geometrical -27B- alignment also allows ion implantations to be carried out into channelling directions in single crystals. This primary earthed target stage can be readily retracted from the beam line and the ion beam re-focussed into a secondary high voltage stage. To minimise the problems of space-charge "blow-up" of the intensity beams, and to simplify the construction, the 150 keV acceleration chamber is designed with a single gap acceleration structure as shown schematically in Fig. 1• Since it is desirable for implantation experiments to maintain the beam shape as far as possible, the lens has also been designed to minimise focussing effects. Fig. 2 shows the change in ion beam shape of a well collimated ion beam as a function of the potential of the post-acceleration lens. At voltages up to 100 kev the angular convergence produced by the focussing effect of the lens is less than + 1a'in both the horizontal and vertical planes. In order to increase the uniform implanted sample area this acceleration stage is fitted with a reciprocating target arm and a linear drive motor. This allow'* the ion beam to be kept stationary on the accelerating lens axis. In contrast in the primary earthed target stage single axis sweeping of the tall beam is sometimes employed to increase the doping area. This is achieved by superimposing a small ac signal on the stabilised dc accelerating voltage; this results in a horizontal scanning of the beam on emer- gence from the magnet.

Immediately following the HP target stage is a quadrupole triplet assembly. The ion beam can be focussed through the previous target stages into this ion-optical arrangement which is used for beam handling and shaping. The normal beam from the separator has a rectangular section which is useful for conventional implantation experiments. However a circular beam may be preferable for ion-surface interaction or charge transfer studies. Furthermore to undertake clean-surface experiments an ultra-high vacuum is required so that differentially pumped aperturels must be introduced into the beam line. It is «cTantageous therefore to design an ion-optical system which will allow maximum transmission of ion current through such apertures and, for this reason, an astigmatic focussing system comprising an electrostatic quadrupole triplet was constructed. Preliminary results on the performance of the quadrupoles when handling 20 keV Ar+ beams are presented later.

Immediately following the quadrupole system (Fig. 1) the ion beam can be manipulated into one of three beam lines by an electrostatic deflector -279- stage. This consists of a set of three pairs of deflector plates mounted vertically above one another and movable in a vertical direction. The pair of curved deflector plates appropriate for beam deflection into the desired beam line can be simply selected by the vertical adjustment provided. Of the three extended beam lines, two are for permanent attachment to experimental apparatus, whilst the third is available for attachment to various simple, easily removable, target chambers. One permanent line leads, via differential pumping and further beam manipulation stages, into a 20" diameter stainless steel uhv chamber whilst the other is intended to transport the beam, via similar stages, into the target holder section of a 100 keV EM6& electron microscope. This latter instrument has a modified column stage that will allow an ion beam to be directed onto a sample already mounted for imaging within the microscope.

Experimental Programme

The principal research programmes based on the isotope separator system are listed in Table 1 below.

TABLE 1 Principal Experimental Programmes based on the Isotope Separator System

1. Radiation damage in semiconductors. 2. Atom location in semiconductors. 3. Range measurements. 4. Ion collection/saturation effects in semiconductors. 5- Ion implantation/gas release studies in metals. 6. Low energy ion deposition for epitaxial growth. 7. Charge transfer cross-section measurements for heavy ions. 8. Surface analysis by sputtering/secondary ion emission. 9. Catalytic reactivity of implanted metals. 10. Corrosion studies of implanted metals. 11. Simulation study of lunar carbon chemistry. 12. Implantation alloy effects in metals. 13. Threshold sputtering under u.h.v. conditions. 14. The effect of ion implantation on magnetic bubbles. 15. Simultaneous evaporation and ion implantation of thin films. 16. On-line electron microscope studies of radiation damage. -280-

A large percentage of the work of the group was originally concerned with ion implantation into semiconductors but interest has recently widened to include many other topics of relevance to ion-oolid interactions as illustrated by the table.

The techniquelofIRutherfordbackscattering of'lightions in conjunction with the channelling phenomenon is extensively employed for measuring both the damage resulting from ion implantation in semiconductors and for loca« ting the atomic positions occupied by the implanted species . Targets which have to be doped for radiation damage and atom location studies are usually implanted in the earthed target stage or in the HT 3tage if higher energies (and greater penetration depths) are required and are then transferred to a Van der Graaff accelerator for analysis. The heater provided in the primary target stage is extensively used for investigating the effect of elevated temperatures or for reducing radiation induced damage.

A topic of particular interest in ion implantation is the study of the maximum attainable dopant concentrations. The maxima arise at high implant doses by virtue of the competition between ion injection into the solid and target erosion by sputtering. The intense beams produced by the ion source 7 8 in the present system enable this problem to be investigated for a wide variety of ions within an acceptable implantation time scale.

In the field of implantation into metals the research is centred around the chemistry and electro-chemistry of implanted samples. It is possible, for example, to alter the corrosion behaviour of the surface layers of metals by forming a suitable ion-implanted alloy . Similarly the catalytic behaviour at metal surfaces should be amenable to investigation and altera- tion by ion implanted impurities whilst the formation of nitrides and carbide r, etc. within surface layers is also a possibility with ion implantation. The versatility of the ion source in producing intense beams of a wide range of ion species i3 of particular advantage in these studies since the very high doses required are readily achieved in moderate implantation times.

The i3otopic resolution of the system enables research topics involving implantation of radioactive ions to be carried out. In particular the measurement of ion ranges at extremely low doses is possible in semiconductor single crystals. The channelling of heavy ions in such single crystals is easily masked by the radiation damage induced loss of -281- crystallinity. Isotopic separation allows investigation of channelled ion ranges using the necessary low doses combined with the high count rate required during subsequent target sectioning. Ejection of radiotracers is also of advantage in thermal desorption experiments which form a large proportion of the research group.'.s activities11'12. The release of implanted species during thermal cycling can be used as a measure both of radiation damage and the location of the species within the target crystal; Low ion doses combined with high counting rates are also usually required in this field.

A number of projects are scheduled using the uhv chamber on one of the three beam lines. These projects are designed so that the experimental apparatus required in each case is mounted off en identical flange which can be fitted as required to the main chamber. The research topics include sputtering, surface analysis by secondary ion emission and the measurement of charge transfer collision cross«-sections for heavy ions. The electron microscope attached to the second permanent line is for investigation of ion induced radiation damage and the introduction of the ion beam directly into the microscope target stage provides the means of measuring the dynamics of events immediately following ion injection into solids.

A number of the programmes listed in Table 1 have employed target stages positioned in the third beam line. For example the simultaneous evaporation of thin films and ion bombardment using an inert gas has been employed in an effort to produce more adherent films, and a simulation study of lunar carbon chemistry has also been carried out . In this latter project C* and D?* ions were injected into lunar fines to simulate the solar wind and to provide information .z carbon chemistry at the lunar surface: in particular the reaction of carbon and hydrogen to form CH^ has been investigated.

One final research programme to be mentioned here is the epitaxial growth of silicon layers by low energy ion deposition. The apparatus used in this project can be easily substituted for the quadrupole assembly shown in Pig. 1 and is represented schematically in Pig. 3« As discussed later, low energy ion beams of adequately high intensity are difficult to obtain and the alternative technique of extracting beams from the ion source at high energies followed by deceleration of the beam at the target is preferred. As indicated in Fig. 3 the same power unit is used to supply both '.ie ion source acceleration voltage and the retarding volt&ge on the -282- target, with a simple battery providing the nett energy of the ion beam. The retardation system is also designed to steer the low energy beam away from any accompanying flux of directed fast neutrals produced by charge- exchange in the main beam transport 3ystem.

Ion beam quality

In this final section we discuss a few properties of the ion beams produced in the separator system, with particular reference to the ion beam quality, and al30 present preliminary data illustrating the performance of the quadrupoie lens system. As mentioned earlier for high current low energy requirements it is preferable to extract intense beams from the ion source when operating under norma1 voltages and then to subsequently retard these beams, after analysis. However, in order to fully exploit the :i.30« tope separator for those applications requiring much lower intensity beams at low energies it has proved desirable to study the behaviour of the ion source at extraction voltages considerably below those normally used. In addition estimates have been made of the random energy spread, or emittance, of the extracted beams. Some preliminary results, obtained principally on an identical separator at Harwell, are given below. A more detailed account will be published elsewhere.

(i) Low Energy Extraction

Figure 4 shows typical current - voltage extraction curves for ion beams of boron, antimony and argon. In each case, a quite standard ion source was used and the beam current at the collector stage of the separator was measured as a function of the accelerating potential of the ion source. For all three elements it can be seen that the beam current falls gradually as the voltage is reduced from 40 kV down to about 10 kV and then drops off much more rapidly. Nevertheless, it is interesting to note that even for voltages as low as 5 kV, the focussed beam current is considerably in excess of that obtainable on most purpose-touilt, low-energy ion accelerators and that currents which are quite adequate for a wide range of ion-beam interaction studies can be obtained at much lower voltages. Although there is a perceptible deterioration in the quality of the focussing of the separated beams as the energy is reduced, it can be seen from Figure 5, which shows an antimony beam at 5 keV, that the i3otopic resolving power remains adequate for most requirements.

An examination of the ion beam behaviour clo3e to the ion 3ource -283- reveals that the total extracted current at low voltage is considerably in excess of the beam measured after analysis at the collector. At such low energies, the bean exhibits an abnormally large angular divergence in both the vertical and horizontal planes. It is thus apparent that the drop in the resolved beam current as the extraction voltage is reduced, is due, in part at least, to a reduction in the beam transmission through the instrument. It was thus rather surprising to find that in further experiments, both at Salford University arid at Harwell using a variety of modified ion source electrode geometries with much smaller extraction gaps, there was no significant improvement: in^ the transmitted current. As we shall see below, this anomalous:beam divergence does not appear to be due to a random energy spread of the extracted ions nor is it evident that it arises entirely from space-charge repulsive forces. It should thu3 be amenable to some measure of control by a suitable design of the ion beam extraction and focussing lenses and further studies of this particular topic could possibly lead to significant and worthwhile improvements in the very low energy separator performance.

(ii) Ion Beam Emittance

The emittance, or more precisely, the random energy spread of the ion beams, is of particular importance in considerations of the use of the separator for (a) precise ion channelling studies, (b) low energy experiments, (c) ion microprobe studies.

It has been apparent for a number of years, from the excellent quality of focussing of such high current separators, that the ion beam emittance must be very small. This is confirmed also by the quality of beam parallelity, and hence the high transmission, which can be obtained from thn slit extraction ion sources used in such machines. In spite of these indications, no precise measurements of emittance appear to have been made. It is important to note that in respect of the machine applications listed above, even quite low random energy spreads could be important. Thus a lateral energy of only + 1 eV in an accelerated beam with a directed energy of 40 keV would correspond to a total random angular spread of over 0.5°. This would militate against the use of simple single aperture colli- mation for precise channelling studies and it would limit the fine focussing capability of the beam. In particular at much lower energies, it would result in an unacceptably large divergence of the extracted ion beam (e.g. ~ 4° at 1 keV). -284-

Pigure 6 shows a diagrammatic representation of a simple experiment designed to measure the quality of the ion beam at a short distance from the ion source. The beam was intercepted by a defining plate with a precisely positioned array of 0.5 mm diameter holes, The pattern of holes corresponded to a beam divergence of > 2 in the horizontal plane and a beam height of 2 cm in -the vertical plane. The;;size and: position of the transmitted bundle of fine rays was then determined by allowing them to strike a plate covered with cardboard or with a white plastic sheet. Under normal operating conditions (i.e. at 40 keV and with milliampere intensity beams) the cardboard or plastic is very rapidly carbonised and the resultant mark forms a useful permanent record of the beam shape and position. This procedure has been compared with a variety of other beam measuring techniques and has been found to provide a useful and simple qualitative measure.

Figure 7 shows a typical burn pattern obtained with a 30 kV argon beam. By comparing such patterns obtained with varying exposure times, it is also possible in principle to obtain a rather more quantitative measure of the spatial intensity distribution of the discrete rays. In practice it was found that the beam spots were quite sharply defined and that there wa3 little evidence of a significant low intensity extended spread. The beam patterns obtained in this way provide valuable information on three aspects of the ion source extraction behaviour. Thus, by measuring the precise position of the horizontal row of spots in the median plane, it is possible to graph the paths of the individual rays as shown in Figure 8. These confirm the sharp focussing behaviour of the separator by indicating the formation of a virtual source whose width is very much smaller than that of the ion extraction slit. The position of this virtual object on the beam axis can also be precisely determined. This is of importance in considerations of the effects of space-charge and plasma boundary geometry on the ion beam formation.

In the vertical plane the size and the position of the central row of spots provides information on the parallelity of the beam and also on its random energy spread. Since the ion beam is extracted from a k cm long slit, any significant random velocity component of motion should result in a readily measurable increase in the vertical spot dimensions. As can be seen from Figure 8, the extent of the broadening of a 30 keV Ar+ -285- beam is at most a fraction of a millimetre over the 15 cm gap between the defining plate and the target.

In a series of similar measurements which extended down to extraction voltages of about 2 kV, it proved to be difficult to detect any consistent ht°a^^ ^fectBM: lareer magnitude.:^However, at the lower energies, the degree,of blackening contrast produced by the ion beam is significantly recced and is thus more difficult to measure. A more refined technique for the measurement of the spatial distribution of the rays would be desirable.

Nevertheless, these preliminary measurements indicate that the random energy spread does not exceed + 0.1 eV. This value which is comparable with the temperature of the walls of the ionisation chamber of the source appears surprisingly low and justifies further study.

(iii) Quadrupole lens system

The electrostatic quadrupole triplet was constructed by mounting segments of solid copper bar of radius 2.5 cm with their curved faces opposite each other as indicated in Pig. 1. Yfhen appropriate potentials are applied to these electrodes the separated ion beam passes through three approximately hyperbolic fields in series and experiences refraction in the vertical (x - z) and horizontal (y - z) planes of differing amounts depending on the polarity and magnitude of the voltages applied. To determine the spatial distribution of the ion beam after focussing two 2.4 mm diameter probe rods were mounted on movable arms so that they could be swept through the ion beam whilst the current was being recorded; one rod was mounted horizontally and could be moved vertically in the y-direction and the other was mounted vertically for movement in the x«»direction. Preliminary results for the transmission of a 20 keV, 3*5 cm x 3 """ rectangular section ion beam are shown in Pigs. 9 and 10. In Pig. 9 the image has been rotated by 90 , that is, the image is rectangular with its long axis in the horizontal plane, whilst, in Pig. 10, an approximately circular spot image is obtained. It is clear therefore that the quadrupole 3ystem can drastically modify the beam profile and its performance is, in this respect quite satisfactory. However, the preliminary results indicate a rather poor transmission efficiency of only about ten percent. Further investigations are currently being carried out to improve these factors and also to study the effect of •space-charge' on the focussing behaviour of the lens assembly as a function of the intensity, mass and energy of the transmitted beam. -286-

References

1. J. H. Freeman, Proc. Int. Ion Source Conf. (i.N.S.T.N. - Saclays France) 1969 al30 AERE Report 6138.

2. J. H. Freeman, G. A. Gard and W. Temple, AERE Report 6758, 1971, Techniques for the production of heavy ion beams.

3. J. H. Freeman, proc. Int. Mass Spec. Conf. (Kyoto, Japan) 1969, also AERE Report R6254, 1970. 4. J. H. Freeman, Proc. Int. Conf. on Electromagnetic Isotope Sep. ed. Wagner and Walcher (Marburg, Germany. BMBW-FB K70-28) 1970, also AERE Report, R6497. 5- A. B. Campbell, W. A. Grant and G. A. Stephens. Radiation Effects 17, pp. 19-24 (1973). 6. G. Carter, W« A. Grant, J. D. Haskell and G. A. Stephens. Radiation Effects 6, pp. 277-284 (1970). 7- J. L. Whitton, G. Carter, J. N. Baruah and ff. A. Grant. Radiation Effects i£, pp. 101-105 (1972). 8. G. Carter, J. N. Baruah and W. A. Grant. Radiation Effects 1j6, pp. 107-114 (1972). 9. V. Ashworth, G. Carter, W. A. Grant, P. D. Jones, R. P. M. Proctor, N. N. Sayegh and A. D. Street. Paper presented at International Conference on Ion Implantation into semiconductors and other materials, New York, 1972.

10. W. A. Grant and J. N. Baruah. Radiation Effects 1J_, pp. 1>17 (1973). 11. A. Cavaleru, D. G. Armour and B. Navinsek. Paper presented at 6th Yugoslavian Symposium on Physics of ionised gases. Split, 1971.

12. A. L. M. Davies and G. Carter. Radiation Effects 1£, pp. 227-233 (1971). 13- C. T. Pillinger, P. H. Cadogan, G. Eglinton, J. R. Maxwell, B. J. Mays, W. A. Grant and M. J. Nobes. Nature 235, No. 58, pp. 108-109 (1972). 6O variable geometry magnet

CO o if n

on source I UHV target 2, chamber p r 4OW m

W aux. I O target chamber t+ O •a

i o 1 Schematic diagram of Isotope Separator System - UNIVERSITY OF SALFOBD -2BM-

E o o o

c o O) --is u

Figure 2i Focussing effect of post-«.cceleration stage I

CD BEAM DEFLECTOR PLATES »# BEAM DEFINING CO PLATE

6OV et O NEUTRAL BEAM

I ro CD

ION / KT ; UNIT SOURCE TARGET 4OkV

1 SEAM DEFLECTOR T I H.T. UNIT o CO s lOcm s scale rndication

SCHEMATIC DIAGRAM OF LOW ENERGY DEPOSITION SYSTEM -290-

lOmA

OlmAl

u E a v

O-J/uA

MAXIMISED BEAM CURRENTS AS A FUNCTION OF ACCELERATING POTENTIAL

Figure 4: Current/voltage curves for low energy extraction -291 -

Figure 5t Sb at 5 keV, effect of low energy on isotopic resolution -292-

Pigure 6: S Schematic representation of experiment to measure ion beam quality -293-

Figure 7* T|ypical ion bean burn pattern (30 keV Ar ) -294-

E u i> |s. IA n - — ro in N o> m - in f> 666666 6666

HO.

\ \ \ 1 1 III 5~ \ \ \ 11/ Q:1 \ \ \ 1 I I S \ \ \ fin i

F Sli t M M if Illl = /f/l \v\ 'o i///l

Figure 8: Extrapolation of burn pattern to indicate the poaition and width of the virtual source. -295-

2mm. dia. probe

•••••:?•••<<•;'•• ;•-,:••• 3\V-v ;. --i*,.. probe displacement (cm)

Horizontal line focus (u scan)

Figure 9: Horizontal line foous produced by quadrupole triplet -296-

2mm. did probes y scan

2 3 4 probe displacement (cm)

Spot focus (x and y scan)

Figure 10s Spot focus produced by quadrupole triplet -297-

10N liAMC'-y IN KVAPOIfATSD TARO •yiTj

A.Arnesen and T.Noreland Institute of Physics, University of Uppsala, Uppsala

Introduction When making ion implantations it is sometimes necessary to know the depth distribution of the implanted ions in the target material. Determinations of such distributions have been made for many combinations of ion and target with the anodic oxidation-, back scattering-, etching- and polishing methods, which all have limitations in applicability and accuracy. The lack of detailed data for several ion-target systems to be used in electron spectroscopy measurements thus led to attempts to determine depth distributions with evaporation techniques.

Method description The method is simple: With the now commersially available crystals it is possible to determine the thickness of evaporated layers with great accuracy. Thus layers of different thicknes:; of the material to be investigated are evaporated on a suitable backing. Then, in analogy with the anodic oxidation-, etching- and polishing techniques, a radioactive isotope is implanted. The activity is measured, the evaporated layer dissolved and finally the residual activity is determined. Using a standard evaporation (or sputtering) chamber it ic possible to treat a large number of materials in this way. One may however get problems with "island formation" in very thin layers and varying structure due to the evaporation conditions (1).

Results The method has been used to determine depth distributions for Co in Al and Au. Layers of Al and Au of different thicknesses were evaporated on glass plates and measured with a commercial crystal meter. The meter was calibrated to an accuracy of ±2.5 $>• The use of glass backing was motivated by its high surface finish and its good resistance against the solvents used. As ion accelerator we used the isotope separator at Uppsala. Since only trace amounts were implanted no saturation effects can be expected. After having measured the total implanted activity, the layers of M and Au were dissolved in HC1 (with CUSO4 as catalvst) Md in a(lua regia respectively. Finally the _ -298- renidual activety in the glass was measured. The results are shovm in fig. 1 and 2.

Wesiduai activity "fa 100 •30 keV 57Co in Au 25 keV 57Co in Au

10.,

1..

100 Penetration depth nr,/cn •'if;. 1. Integral ran^e distributions for "'Co in Al and Au.

/ 2 Penetration probability % per ug/cm

•?l) keV JlCo in Al 10..

in Au

20 30 40 50 60 70 Penetration depth ug/cm? Fip. 2. Differential ranrn dintributions for ^7Co in Al and Au. -299-

Kach curve represents an average of three different measurements. Tho reproducibility showed, to be fairly the same for both targets and both energies with a spread in the measured residual activity figures ranging from 1 unit deep in the target to about "5 units closer to the surface. We have not yet made any experimental comparison between our results and those which can be obtained with the anodic oxidation method for bulk materials. However, an interpolation between the median range values for K and Kr in aluminium reported by Uhler (2) reives values falling within our experimental errors. In this report we have not taken into account the influence of th« glass backings on the obtained distributions. For the pro.jectile-target combinations used, nuclear stopping is dominant at these energies, and thus the ideal backing should be made of a material with nuclear charpp and mass number close to those of the evaporated layer. If this is not the case, one will not get the "right" fraction of projectile atomr; reflected from the backing into the target layer. Since glass in a mixture of comparatively light elements, this effect will be ^r for gold. The differential range distribution curves for Co in should therefore probably be shifted somewhat towards shorter range values.

References 1. C.P. Powell, J.H. Oxley and J.M. Blocher, .jr., '-M., Vapor Deposition, •lohn Wiley and sons, Inc., New York, 1966.

?. ,). Ilhler, Arkiv F.ysik, 2±, S/J9 (19^5). B 0 -3Q0-

A STUDY OF DOSE EFFECTS ON THE RETENTION OF IONS IMPLANTED AT KEV ENERGIES

D.C. Santry Chalk River Nuclear Laboratories Atomic Energy of Canada Limited Chalk River, Ontario, Canada

INTRODUCTION Ion implantation has become an established technique for ion range measurements, damage and diffusion studies, as well as for modifying properties of solids by ion doping.

In the case of semiconductor materials, it is realized that ion implantation is a rather violent process, consequently ion damage and dose effects are recognized as important factors. Metals, however, are generally considered more toler- ant of ion dose.

It is the purpose of this paper to show that such may not be the case, and that any study of metals which involves ion implantation, should include the effect of ion dose on such measurements.

EXPERIMENTAL The study consisted of implanting a radioactive tracer (dose 12 2 < 10 ions/cm ) into various target materials and measuring the retention of the radioactivity as a function of stable ion dose. 2 A circular targat with an area of 1.0 cm was mounted in a Faraday cup for ion implantation. The sweep feature of the magnet power supply*" was used to move a 2mm by 3 cm mass separated beam of 40 keV ions across a 1 cm by 1 cm aperture

Alpha Scientific solid state supply with digital field control, model 40 type 330. -301-

in front of the Faraday cup. The implant covered an area of 2 1.0 cm , consequently the integrated dose, as measured with a digital current integrator* was given directly as ions/era2. Beam currents used were in the range of 1 to 5 namp for stable beams. All targets were removed from the Faraday cup after each implant for radioactivity measurements, in addition, targets were weighed initially and after each implanted dose. Any observeable weight loss gave a measure of the amount of target sputtering which occurred.

RESULTS 85, The retention of Kr activity in various target materials, as 84 a function of Krdose, is shown in Fig. 1. Although the range of Kr ions is the same in Au and in W, we see that implanted ions are more easily lost from Au than from w. Thus the

I015 behaviour of ma- DOSE (iONS/cm2) Fig. terials towards retaining the implanted species within the solid is influenced by a combination of at least two factors; the depth distribution of ions in solids, and the sputtering effect of ions on target materials.

The loss of implanted radioactivity from Cu, Zn, W and Au at lower doses, indicates that these materials will have range distributions and dopant retentions which are dose dependent. Studies on C, Al and Si targets indicate that ion retention

Tomlinson Research Instruments model 2000AEC -302-

is less sensitive to implanted doses. The effect of dose on range distributions in these materials has yet to be determined.

A correlation of weight loss and range measurements indicated that another effect was occurring which influenced ion retention. Although the effect was exhibited by most materials, it can best be illustrated by the behaviour of Cu targets. Range p c profile measurements of 40 keV Kr in Cu indicated that the 2 median range of the implanted ions is 45 ng/cm and that 88% 2 of the ions are contained within the outer 130 p.g/cm of a Cu target. However, the retention curve for Cu in Fig. 1 shows that 88% of the implanted activity was lost at a dose of 2 x 10 ions/cm where the measured weight loss was only 21 M-g/cm . Thus, more activity is lost than can be accounted for by a direct sputte.ring of the target. The effect observed is referred to as radiation enhanced diffusion in which ther^ is a generation and diffusion of vacancies created during heavy ion irradiations. There is, therefore, a migration of Kr to the surface of Cu where it can leave the solid.

For a given target element there can be variation. i*i the rfc- tention of an implanted species which depends on the crystalline structure of the material and its surface composition. For example in Fig. 2 the higher retention of an oriented tool- « o, single crystal of W SINGLE CRYSTAL (Ill) is related to the 80- enhanced range of 60 ions due to the 40 channe1ing e f fect.

20 We see also that the retention of 10" DOSE uoNS/cm*) pig. 2 Kr ^s greater in the oxide of the metal. Consequently the presence of an oxide -303-

or carbon deposit on the surface of a metal can alter the amount of ion retention.

Comparisons were made on the retention of ions in W and Au, two high Z materials frequently used as targets or target backings. At the Marburg Conference I had shown that the range profile of a 40 keV ion was nearly identical in W and Au(l). Any differences observed in the retention of ions in W and Au would be due to effects other than ion ranges. 85 In Fig. 3 are shown release curves for Kr implanted into W and Au when bombarded with ions of masses 7 , 20, 40, 84, 132 and 202. The numbers in brackets are the measured median ranges of the ions in the target DOSE (lONS/cm2) materials. As ex- IQOr- (150) pected , increas ing the mass of ion increases the amount of Kr released for a given dose. A mass

Ar(43) saturation effect is Kr (23) 7 10 id observed since Kr, Xe DOSE (lONS/cm*)

Fi 3 and Hg gave identical release curves in W, while Xe and Hg gave identical curves in Au. This may '.n r.rt n?e dn~. te differences in ion penetration r^pths. rrrrxant d Kr is influenced by the entire dose of ui, Ne c. Ar, but o.ilv .s. fraction of the Xe or Hg dose. Notice that Tcr hot:* u^ivjet materials there is a normal sequence, of .'ncif-tising release with increasing ion mass. -304-

Release curves for radioactive tracer implants of one isotope bombarded with a stable isotope of the same element are shown in Fig. 4. Again the numbers in brackets are the measured median ranges of 100 No (70) the implanted ions. eo Under these experimental conditions

40 the median range of 40 kaV IONS IN TUNGSTEN the radioactive tra- 20 cer implant was S iols 10" .d identical to that of DOSE (IONS/cm*) the stable bombard- 100- ing ions. The

80- Na(7O) curves show qualitatively the extent to which the initial portion of an im-

Kr(23) plant is affected by In (18) a subsequent dose.

DOSE ( IONS/cm2) The positions of the Fig. 4 Kr and Xe release curves are not in the normal sequence of increasing ion mass, even though the Kr data shown are the same as in Fig. 3. It appeared as if the inert gases, in the presence of radiation damage, might more readily diffuse to the surface of a target and be released. However this was shown to be unlikely since Kr and Rb, when bombarded with Xe, gave identical release curves (see Fig. 5). Included in Fig. 5 are values for the weight losses as measured for several ion doses, if release was due to sputtering only, retention values of 50% would re- 2 quire the removal of 23 M,g/cm of Au or W. The observed weight losses for 50% retention were < 13 M-g/cm2 for Au and ~ 6 \ig/cm2 for W. -305-

So far I have discussed dose effects on the retention of ions during the implan- 100 tation process. I

a ^Vc__4qk*x« 80 would like to also briefly mention H60 z o WkeV^Kr o D that ion dose can ~40k«V i- also affect subse- 20 quent measurements on implanted tar- 10 Itf" icT DOSE (IONS/cm2) gets. For example, Fig. 5 in Table 1 are listed the results of implanting 40 kev ions of Xe into Al and Kr into W, then growing anodic oxides on the surface of the TABLE 1 metals. At doses Dose Effect of Implanted Ions on Anodic Oxide Formation >10 ions/cm the con- Retention of Implanted Activity (%) version from metal to Dose Aluminum (ions/cm ) Tungsten metal oxide (anodized) anodized stripped anodxzed stripped lo14 100 j.4.4 1.00 16.2 permits a fractional 15 10 20.3 8.6 39.6 ia.8 release of the implant- 16 10 19.0 9-7 26-7 12-8 ed ions. Thus Kr and Xe are not necessarily "inert markers" for ion implantation purposes. Weight loss measurements indicated that differences in retention values obtained after stripping off the oxide were due to heavy ion damage in the metals which enhanced the rate of oxide formation.

The final point I wish to make is that doses used in ion im- 14 . ,2 plantation are frequently in the region above 10 ions/cm where dose effects may be important, implants of stable nuclides for nuclear reaction studies, Rutherford scattering or 14 charge particle induced X-ray studies, require at least 10 to in15 ions/cm2 for analysis. Note that a monolayer of ions implanted into Al represents ~ 1.6 x 10 ions/cm . -306-

13 2 Radiotracer implants are assumed to be < 10 ions/cm but the actual dose and implanted area are seldom measured.

At Chalk River, implants are routinely performed with targets positioned inside a Faraday cup, so that doses are measured even for radiotracer implants. Experience has shown that: (1) Cross contamination under actual operating conditions can be significant. (2) Extraneous interfering beams are frequently present and contribute to the dose. (3) Implanting into a very small target area requires a large dose to provide sufficient radioactivity for precise measurements. (4) The use of swept beams to minimize ion damage and to produce homogeneous implants increases the cross contamination factor (see Table 2 for Xe implants under typical operating conditions). These

TABLE 2 factors combine to cross contamination of Mass Separated Beams 13 (Collected Through a 0.5cm wide x 1.Ocm high aperture) produce doses of 10

Hasa 131 collected <*) to 10 ions/cm for Mass position fine focus boam swept beam supposedly tracer (2 mm x 2 cm) (1 cm x 2 cm) implants . 130 0.48 1..25

131 98.86 97..56

132 0.66 1..19 CONCLUSIONS Studies on the release of implanted radioactivity as a function of ion dose showed that the release was greater than could be accounted for by target sputtering. The technique described is useful for establishing dose limits where diffusion and sputtering of implanted ions become significant, it is recommended that the effect of ion dose on subsequent measurements be made for all studies involving ion implanted samples. REFERENCE 1. SANTRY, D.C. and SITTER, C.W. 1970. Proc. Int. Conf. on EMIS (Marburg) Rept. BMBW-FB K70-28 p. 505. -307-

High Energy Chemistry with Low Enerqy Accelerators

G.K. Wolf, Lehrstuhl fiir Radiochemie Universitat Heidelberg**

The implantation of energetic ions is a technique extensively used in the field of solid state physics. In contrast applications of this method to chemical problems are onlv little known ~ , despite of the fact that accelerated ions may induce unusual chemical reactions and effects.

Our group at the University of Heidelberg and the Nuclear Research Center in Karlsruhe is working for some time in this field and I want to give a survey on our activities. Because of the limited extent of the contributions I have to restrict myself to some selected examples.

Before mentioning them a brief description of our experimental installations will be given.

We are using a 100 keV ion accelerator designed by Danfysik which is equipped with a 30° magnet for cleaning the beam. On the ion source side we have a variable gas inlet system, which enables us to let in simultaneously different gases, either feeding the ion source or transforming non volatile solids to volatile ones. On the target side we use different arrangements, consisting typically of a metal wheel with up to 16 targets and installations for defining the beam size, measuring the beam current and repelling electrons x*The reported experiment were performed by M. Becker, H.G. Burkhardt, T. Fritsch, W. Froschen, U. Heinstein, E. Mohs -308-

(Figure 1). The targets may be irradiated between -180 and +25O°C. As far as chemical compounds are concerned we install tablets, produced with an IR press.

The analysis of the irradiated targets was in most cases done by chemical methods, sometimes also by means of MoBbauer spectroscopy and E8CA (Electron Spectroscopy for Chemical Analysis).

Mostly radioactive isotopes were implanted in order to be able to distinguish between the atoms originating from the beam and from the target.

4) 1) Radiation damage in chemical compounds

Chemical compounds very often are suitable targets for the production of radionuclides, especially the corresponding elements being to volatile or in connection with on line mass separation the radioactive products from a metal target being not volatile. In these cases the radiation damage to the target is of major importance specially in heavy ion accelerators. The irradiation with low energy accelerators allows to simulate the experimental conditions and to extrapolate the results.

We studied for example the behaviour of Ga isotopes im-

planted into SrCl2 since there is some interest in chlorides as targets. Recoil Gallium from nuclear reaction and Gallium implanted with low doses f 10 atoms/cm^) turn easily into Gallium chloride and eva^. \rate carrierfree from the target. Applying higher doses the yield of volatile Gallium chloride decreases slowly from 70 % at 1013 atoms/ -309-

1: target arrangement for implantations in chemical compounds

(electron repeller) ZwischenblendeyO V Blende -310-

? 1 fi ? cm to 15 % at 10 atoms/cm - This shows the damage of the target latti.1* hindering the formation of product compounds and restricting the irradiation time for every target.

Looking from a different point of view to the problem one may use also the formation of a product compound as an indication of the progress of the damage. Figure 2 shows for example the percentage of implanted Cr turning into

Cr (CO),. in a matrix of Mo (CO) c as a function of irradiation A 5) 6 5 dose

Fig. 2: Formation of Cr (CO)c in Mo (CO), as function of the —• + 6 6 bombarding Cr dose

Retention

44-

36-

28-

20-

4H TJ3 TTT ^> 1u 1Cf Dose [Ions] -311-

As long as every incoming 1Cr-ion causes an isolated 5l damaged zone in the lattice, the Cr(CO)c formation is D high (50 %) . As soon as the damaged zones begin to overlap the carbonyl yield begirsto drop rapidly. Thus, the drop in yield give.s an good indication of the overlapping of damaged zones. Besides this such dose against yield functions may say also something about the mechanism of the chemical reaction of the implanted ion with the matrix. Without going into any detail one example will be mentioned:

After bombard ement of a Co compound CoRen)-C1,~|NO, with 15 2 57 •!- 59 + ions/cm of Co" + Co the targets were examined with a Mossbauer spectrometer. Despite of the fact the lattice being nearly destroyed under these conditions about one half of the Co was found as a complex compound. This was an indication that the implanted Co does not only displace the Co of the target - a mechanism occur ing at lower doses - but also reacts with fragments of the destroyed molecules forming new or rearranged complexes.

2) Chemical syntheses in microguantities

A normal chemical reaction occurs all reaction partners having about the same temperature. The reactions of implanted ions on the other hand take place under conditions where one reaction partner (the matrix) has low temperature the other one (the implanted ion) a very high one. Thus unconventional reactions may proceed leading to products being yet unknown or difficult to synthesize otherwise. Unfortunately the quantities which one can make in that way are small. -312-

Depending on the sensitivity of the matrix against radiation damage they range from 101 - 10 molecules. Two examples for the lower and higher limit are: a) The generation of mixed carbonyl compounds: The implantation of Mn+ or Co+ into Rhenium or Rhodium carbonyl leads to the formation of volatile Mn and Co compounds , which very probable are binuclear carbonyls with different central atoms. Very little is known until now about such compounds from 'normal" experiments.

b) The synthesis of graphite compounds: While only 101 4 atoms/cm 2 can be implanted into the

carbonyls without destroying the matrix; there exists "* 7 18 2 the possibility to incorporate up to 10~ - 10 atoms/cm into graphite in order to generate compounds. This is a region where X-ray analysis is practicable.

As a preliminary experiment in this field we performed implantations of K ions. The potassium compound is well known and its formation should proof the chanca to synthesize also other hitherto unknown compounds. Comparing a normal 17 1 fl graphite surface with one containing 10 - 10 implanted K ions one realizes immediately a blue colour vanishing at high temperatures and after chemical treatment. This properties correspond to a compound containing potassium in every third graphite layer. In the moment we are trying to incorporate also other elements into graphite and to analyze the resulting products with X-rays. -313-

3) Surface reactions

Implantations of ions change the surface properties of the targets, as well by turning a crystalline surface into an amorphous one as by incorporating the bombarding particles into the material. Since katalytic processes take place directly on the surface they should be affected by energetic ions.

We studied this problem taking the hydrogenation of ethylene (C2H4) on metal katalysts as example. Without going into details two results are given: a) With Ni foils bombarded with 1015Kr+ ions/cm2 the reaction rate was MOO times higher than with untreated Ni. b) With Cobalt foils bombarded with 1015 Kr+ ions/cm2 the increase came even to a factor of ^10 . Thus Cobalt foils showing normally no katalytic activity turned into katalysts.

4) Reactions of energetic ions with gases

For the study of gas phase reactions ve are building and testing two types of arrangements. The main purpose of these studies is firstly to investigate the radiation damage in gaseous compounds caused by energetic ions, and secondly the chemical reaction of the energetic ions themselves with gases. These investigations are of general interest as well as for the separation of short lived nuclear reaction products in the gas phase. -314-

In contrary to the installations normally used in ion-molecule reaction studies where only the charged reaction products are analysed we want to analyse also the neutral products with a quadrupol mass spectrometer. This is only possible with an intense beam of bombarding ions. In order to achieve this beam one has to drop the demand for good accuracy in energy.

Our first development (fig. 3 a) consists of a retardation lense, a reaction chamber, a baffle for the separation of reaction gas and reaction products and the quadrupol mass spectrometer. It delivers 10 ions of 100 eV - 20 keV and will be used mainly for the damage studies.

The second elaboration (fig. 3 b) consist of a sputtering target, mounted in the reaction chamber and the mass spectrometer. It may also be equipped with a separating baffle. It delivers 10 atoms/sec of 1 eV - 100 eV depending on the combination beam/target and is considered for the study of chemical reactions between atoms of 1 - 100 eV and the reaction gas.

Both concepts are in the test phase and we hope that we will be able to perform the first "serious" experiments during the second half of this year. Reactiongas- supply a.

Accelerator

1Cf"torr

Retardation^ lonsource 20 keV-lons -2 yy |10 - 10 tOlT ^^ Faradaycup and Reactionchamber

Turbomolecularpump

Ouaclrupole mass spektrometer Tjrboniolecuiarpump

Fig. 3 a: Arrangement for the study of reactions of energetic ions anc atoms with gases Concept with retardation lense b. Quadrupole -1 mass spektrometer

Accelerator 10 torr Penning

10 torr Gasinlet Beam Target

sputtered atoms!

Turbomolekularpump

Fig. 3:b Arrangement for the study of reactions of energetic ions and atoms with gases

Concept with sputtering target -317-

References

1) T. Andersen, G. Sorensen, Nucl.Instr.Meth. 38.' 2O4 (1965)

2) G.K. Wolf, T. Fritsch, Radiochim.Acta JU, 194 (1969)

3) G.M. Jenkins, D.R. Wiles, Conference Chem.Inst. of Canada,

Quebec 7.6.1972

4) G.K. Wolf, to be published, GSI-Report 1973

5) U. Heinstein, Diplomarbeit, University of Heidelberq 197 3

6) F. BaumgSrtner, H.G. Burkhardt, priv.Com. 1973 -31 8-

The UNISOR Project

E. H. Spejewski, R. L. Mlekodaj, H. K. Carter, W.-D. Schmidt-Ott, E. L. Robinson, R. W. Fink, J. M. Palms, W. H. Brantley, B. D. Kern, K. J. Hofstetter, E. F. Zganjar, A. R. Quinton, F. T. Avignone, W. M. Bugg, C. R. Bingham, F. Culp, J. Lin, J. H. Hamilton, A. V. Ramayya, M. A. Ijaz, J. A. Jacobs, J. L. Duggan, W. G. Pollard, R. S. Livingston, C. E. Bemis, E. Eichler, N. R. Johnson, R. L. Robinson, and K. S. Toth

UNISOR,* Oak Ridge, Tennessee 37830, U. S. A.

1. Introduction The UNISOR consortium was formed for the primary purpose of studying nuclei lying far from the line of beta stability by means of an isotope separator placed on-line the Oak Ridge Isochronous Cyclotron (ORIC) at the Oak Ridge National Laboratory. This consortium is unique in that the member institutions have provided the majority of the initial capital equipment and continue to provide a substantial portion of the operating costs from their own internal funds. The U. S. Atomic Energy Commission provides the remainder of the operating costs as well as additional capital funds. The availability of ORIC for this project is fortunate in several respects. It is centrally located with respect to the member institutions, providing convenient access for most of them, and it is capable of providing moderate intensities of heavy-ion beams. The beams that are presently available and useful for this project are shown in Table I. The ORIC also provides intense light-ion beams, of course, but these are not used because the shielding has not been designed for them.

_ , UNISOR is a consortium of fourteen institutions, and is supported by th :n and by the U. S. Atomic Energy Commission. The member institutions (authors' initials) are: University of Alabama in Birmingham (ELR), Georgia Institute of Technology (RWF), Emory University (JMP), Furman University (WHB) , University of Kentucky (BDK, KJH), Louisiana State University (EFZ) , University of Massachusetts (ARQ), University of South Carolina (FTA), University of Tennessee (WMB, CRB), Tennessee Technological University (FC, JL), Vanderbilt University (JHH, AVR), Virginia Polytechnic Institute and State University (JAJ, MAI), Oak Ridge Associated Universities (JLD, WGP), and Oak Ridge National Laboratory (RSL, CEB, EE, NRJ, RLR, KST) . Chapter 7: General descriptions of

ISOL facilities -319-

The facility is also intended to Particle Energy (.'urrent be used off-line for separations of (MeV) (euA) 12C3+ -1 stable and radioactive materials, for 45-67 12C4+ implantation, and for atomic physics 80-120 •1 -•1 studies employing the separator as a 69-103 14N5+ low-energy heavy-ion accelerator. 107-161 1 16O4+ 60-90 •1 II. Layout 16O5+ 93-140 •L The UNISOR facility is housed in 20Ne5+ 75-112 ML 5 an addition to the ORIC building, 20 6+ Ne 108-162 •*•(). 2 Fig. 1. This addition also contains Table I. ORIC beams used several other experimental facilities, for UNISOR with UNISOR occupying approximately half of the total space, as shown. The facility is designed so that the isotope separator can be used either for on-line studies, or off-line as a conventional isotope separator. The drift tube, which is shown connecting the two lens boxes, connects to the housing of the off-line ion source, permitting fast and convenient switching between the two modes. The separator control console is located adjacent to the collector chamber in the experimental area, convenient to the experimental equipment. This area is arranged so that it is possible to perform on-line experiments using two different mass beams. In addition, a lead cave will be installed to permit studies of longer-lived species which are simultaneously produced.

III. Isotope Separator The isotope separator is a commercial version of the Scandinavian type. It is a 90°, 150-cm radius-of-curvature device, having a resolution Am/m < 1/2000. It is capable of providing well-focussed beams at the focal plane in the mass range of approximately ±8% of the central beam, providing either a line or a spot focus. The ion sources provided are the standard Nielsen-type oscillating electron and the Sidenius hollow-cathode sources with appropriate housings and extraction electrodes. In addition, a split-magnet housing is provided for the Nielsen-type source to allow the cyclotron beam to enter the ion source. The collection chamber of the isotope separator is provided with an airlock for insertion and removal of collection foils. This is particularly useful in performing focal-plane collections of the side masses while an -320-

ORNL-DWG 73-5036

TRANSURANIUM FACIUTY"

' .* NEUTRON FACILITY

POLARIZED DEUTERONS

m gg^-ON-LINE 'iM ION SOURCE

m$~ OFF-LINE COLLECTOR §f ION SOURCE

BEAM SWITCH

Figure 1. Addition to the ORIC building housing UNISOR. -321-

on-line experiment is occurring on the central mass because this arrangement permits only a short interruption of the on-line experiment while the collector foil is raised to the position of the airlock. The back end of the collection chamber is removable, allowing the installation of a beam switchyard. This permits the extraction of a separated beam at 0° and at ± 30°, as shown in Fig. 1. The port at any of these beam lines can be connected to a 2.5-meter long beam-extension tube which is provided with two focussing elements. A single-mass beam may thus be removed to a distance relatively far from the other activity simultaneously being separated.

IV. Tape Transport A tape transport unit, Fig. 2, has been constructed for the collection of a single-mass beam and the transportation of this activity to detector stations. This instrument is modular in design, permitting the installation of modules which can be designed to fit the requirements of a specific experiment. The modules initially constructed are a collection station, two "standard" detector stations, a right-angle detector station, and two units containing the tape reels and drive mechanisms. The collection and detector stations are constructed with portholes for the insertion of detectors. This design permits the use of a wide variety of

different detectors at any port, Wu „„,,..,.

requiring at most the constru- SEPARATED BEAM ction of a special "flange" for v ALUMINUM-COATED \ MVLflR TAPE a given detector. With one DETECTOR PORTS reception, the ports are sufficiently large to accommodate detectors as large as 8.8 cm in diameter. The collection station is provided with five ports. Two of these are primarily designed for particle detectors, being smaller in size than all the

others and viewing the deposi- TAKE-UP REEL"' tion side of the tape (the "front") at 45° angles. The other three ports view the Figure 2. Tape transport system. -322-

back and edges of the tape. Each of the "standard" detector stations contain four ports, viewing the front, back, and edges of the tape. The right- angle station contains two ports viewing the front of the tape at a 90° angle to each other, as well as ports viewing the tape edges. The tape is aluminum-coated Mylar, the activity being deposited on the aluminum side. It is driven by a stepping motor which can operate either in a pulsed or in a continuous mode. In the pulsed mode, the minimum transfer time between adjoining stations is approximately 0.9 sec. The control unit for tnis instrument is the same type as that used at TRISTAN, and was constructed at the Ames Laboratory through the kindness of Prof. W. L. Talbert and Mr. J. R. McConnell. Because of space considerations, the tape transport is presently connected to the beam-extension tube on the 0° beam line.

V. Target/Ion Sources During the past eight months, we have been developing two types of target/ion source combinations. Since heavy ions are used for production, both use relatively thin targets and both take advantage of the large linear momentum transferred to the product atoms. One of these systems uses a separate bombardment chamber and a halium-jet system to transfer the products to the ion source. The advantage of this system is that almost every element can be used as a target in it, although the efficiency does not appear to be high as the other target/ion source. The helium-jet system and its operation are discussed more fully in another contribution to this conference. The other target/ion source under development is the "Pingis type," so called because it is based on a concept used in the Pingis project at the Research Institute for Physics, Stockholm. The initial configuration of this type is shown in Fig. 3. A standard Nielsen-type ion source with a hole cut through the anode cylinder and heat shield, is used. A thin foil target covers the hole in the anode cylinder, held in place between a shoulder on the anode cylinder and a carbon ring. The target is open to the cyclotron beam which-enters through the center section of the split-magnet housing mentioned above. Those products which recoil out of the target are stopped by a catcher foil which is mounted within the plasma region. Since the catcher is at a high temperature, the product atoms very quickly diffuse out of it into the plasma. Those products which do not recoil out of the target can diffuse -323-

out since the target is at a temperature of approximately 1000°C. CARBON CATHODE The advantage of this WITH EXTRACTION HOLE target/ion source is that it QUARTZ INSULATOR appears to have a high efficiency, and promises some possibility of performing "chemistry" within CARBON ANODE CYLINDER the ion source. Its disadvan- JARGET FOIL tage is that only refractory CARBON RING WITH fHHt AD materials can b~ used as targets HEAVY ION BLUM because of the ..ligh operating QUARTZ INSULATOR temperature. Several different elements have been used as targets in CARBON CATHODE this arrangement. Only two, Mo and Nb, have operated entirely satisfactorily. Targets of Ni, Rh, and Pd have either ruptured or developed holes within a short Figure 3. The Pingis-type time of operation (several min- target/ion source. utes to several hours). Targets of Zr and Ta have not been used sufficiently long to determine their behavior, and other refractory materials have not been tried. We have tried several modifications in attempting to solve the target breakage problem, but none have yet been completely satisfactory. The most successful modification is to place the target at a larger distance from the filament by means of an extension, and to rotate the catcher foil 180° so that it acts as a heat shield for the target. In this configuration, the targets maintain their integrity for long periods (approximatley 8 hours) with a moderate-intensity cyclotron beam, but have much shorter life- times with beams of about 1 uA. When the target foils remain whole, the operation of this target/ion source is very satisfactory. Assuming theoretical values for the production cross-sections, we have estimated efficiencies of a few percent for products from Ag to I, and 30-50% for Xe. The dwell time within the ion source also appears to be good, since we have seen 3-sec 116I in our tape transport system with no obvious reduction in the amount of activity expected. -324-

Plans for an On-Line Mass Separator at the Heavy Ion Accelerator UNILAC at Darmstadt

E. Roeckl, W. Dumanski Gesellschaft fiir Schwerionenforschung mbH, Darmstadt, Germany R.Kirchner and W. Lauppe Institut fiir Kernchemie, Universitat Mainz, Germany

1. Introduction. Due to the promising features of a heavy ion beam for the production of isotopes far away from stability, several proposals for experiments along this line have been worked out for the Heavy Ion Accelerator project at Darmstadt. Two types of experiments may be of special interest within the scope of this conference:

12 3) 1) Mass and/or time-of-flight separation ' ' of recoil products with their primary energy, momentum and ionic charge as defined by the reaction.

2) On-line separation of isotopes after slowing-down and reionization (ISOL systems).

We want to report on an approach of the second type, combining more conventional target ion-source systems with a standard mass separator (The project of coupling a He-jet with mass separation as followed by the Giessen-Mar- burg collaboration will be discussed in Session I of this conference).

2. The Heavy Ion Accelerator UNILAC. The heavy ion accelerator UNILAC ' , which is presently under construction at the Gesellschaft fur Schwerionenforschung mbH Darmstadt, is planned to accelerate ions up to uranium to energies up to 10 MeV/amu. Design aims 14 13 for the beam intensity are 10 particles/s for mass 70, 3-10 particles/s 12 for mass 184 and 2-10 particles/s for mass 238. According to the present schedule, the first beam of an energy of 5.5 MeV/amu will be extracted end of 1974. This beam energy should be available for experiments beginning in 1975, whereas the maximum output energy will not be reached before mid 1975.

supported by Gesellschaft fur Schwerionenforschung rnbH, Darmstadt, Germany. -325-

3. The Target Problem.

:\olid targets, exposed to a very intense heavy ion beam, have to stand the-. extreme strain caused in a layer of a few mg/ci/' by heating, sputtering and production of lattice defects. As these effects restrict very much the choice of the target material and configuration, investigations7'8^ are underway at GS1 with the aim of developing targets of reasonable life-times.

Even with additional "anti-sputtering" layers, the expected sputtering rates of a few atoms per incoming particle will limit the life-time of a thin target to a day or so. The base target temperature should perhaps be raised in order to increase the annealing of defects and to reduce the temperature fluctuations occuring due to the beam structure (5 ms beam pulse, 25% duty cycle). The thermal strain introduced by the beam (for in- 13 stance 2.3 kW in the case of 10 " uranium ions/s of 6 MeV/amu) can probably be kept within acceptable limits (i.e. below the melting point of the target material) by rotating the target, by blowing up the beam spot or by splitting the target into a sandwich of thinner foils. Going to the other extreme, a relatively thick (about 1 mm) target might be feasible, too, as it is less sensitive to sputtering and as the cooling by heat conduction is then more effective than by radiation.

All in all, it seems feasible to meet the target requirements by modifying existing target ion-source systems. Moreover, the operation of an ISOL target in the beam of reaction products behind a velocity filter is being considered.

M-. The On-Line Mass Separator Facility.

4.1. Installation at the TRIGA Reactor at Mainz University. To study target-ion source systems and to get a mass separator and related equipment into reliable operation by the time the UNILAC beam is available, a separator was installed at the TRIGA reactor at Mainz University.

Because of easy access, the target is placed in the external neutron beam. 7 At 100 kW power the thermal neutron intensity amour. _s to 4-10 neutrons/s over a target area of 30 cm2. Even after a considerable intensity gain by cooling the present Bi filter to liquid nitrogen temperature, the neutron -326-

intensity will be far lower than for similar facilities in other laboratories (for a review see Ref. 9). In the pulsed mode, neutron bursts of 25 ms FWHM and 12 MWsec integrated power can be produced every 15 m.

4.2 The Mass Separator. The separator consists of a system of electrostatic lenses and of a 55 magnet, and is essentially of the ISOLDE

9) tyPe .

The present status of the collector area is shown in Fig. 1. The stable beams are analyzed by a standard scanner (Fig. 2). Long- lived samples can be collected on an aluminum strip and Fig. 1. Magnet and collector area of the separator. L = vacuum lock, T = tape station, D - detector removed via a va- array. cuum lock for off- line counting. Short-lived isotopes are stopped in a 6 mm wide copperized computer tape and transported on-line into a beta-gamma detector set-up by continuous or discontinuous transport. This tape station, with its collector carriage movable along the focal plane, will later be used for a control of the separator during on-line operation. A second moving tape collector is under Fig. 2. Scan of the stable xenon isotopes. construction to be set up in a beam-line downstream from the focal plane. -3?7-

5 . The liranyl Stearate Target. First measurements have been carried out with a cold powder target o" the emanation type, which has been applied successfully for the release o' 9) noble gases :oi many separator facilities . 2.^ g of uranyl stearate c^- - 235 ' ta'-ing 0.61 g of ' U, are kept between two sheets of paper filters. The gas for stabilizing the plasma ion source sweeps through the target and enters the ion source via a 1 cm 0 x 50 cm transfer tube. 87-91 lSV-lHO The longer-lived isotopes Kr and Xe have been identified. The 3 89 saturation decay rates amounted to 1.6x10 dps for Kr and to 3 139 (2.7-8.3)>'"0 dps for Xe, varying somewhat from run to run. As the fission yield, the neutron flux and the amount of fissionable material are known, an overall efficiency could be determined to be (3-12)%. This number accounts for all losses (chemistry, transfer, ionization, separator transmission, collection). 99 93 The shorter-lived nuclides such as 3.0 s "Kr and 1.2 s Kr have been identified by means of the moving tape collector in the pulsed reactor mode. This mode also allows the investigation of the time characteristics of the release of activity. The tape transport is running continuously at a low speed of 10 cm/s, while the rate of a 5 cm 0 x 0.3 mm NE 102A beta detector ir. multiscaled. Fig. 3 shows the delay curve 90 for the release of Vr. The spike is due to the background from the neutron pulse. Following the gap, corresponding to the tape transport time, the release profile can be seen. If the half-life of the isotope is known, the time characteristic of

30 40 -•- CHANNEL NUMBER the "competing" diffusion process can be determined. Assuming a single decay compo- Fig- 3. Time dependence of nent (see Fig. 3) and neglecting cumulative the release of 9°Kr from a effects, a half-time of 6.8 s was found for uranyl stearate target. s for 139Xe. 90 Kr and

6. The Uranium Tetrafluoride Target. So far most of the ISOL targets relied upon the volatilization of the isotopes in the elemental form. Only in a few attempts11'12'13* have volatile compounds been used. -328-

Weber et >. ' ' " studied the release of fission products from solid UF and found a rapid and effective release of Se, Kr, Nb, Mo, Tc, Ru, Sn,

100 r- Sb, Te, I, Xe. The temperature dependences of the yield (Fig. 4) can be interpreted in terms of volatilization of fluoride and oxyfluoride compounds and seem to be promising with respect to elemental discrimination.

Studies of a UF target for the mass separator are under- 400 600 800 0 200 400 600 800 •>- Temperature !°C) way. The target chamber is Fig. 4. Volatilization curves of Zr, Mo, shown in Fig. 5. No on-line Sb, Te, I from a UT^ target (from Ref. 14). The arrows indicate boiling or sublimation tests have been performed so temperatures. far, but off-line tests have revealed promising festures concerning vacuum,

Ion source high temperature (<800° C) and ion discharge chamber Transport source operation. tube heater Copper transport ub 8. Integrated Target Ion-Source Systems. Integrated target ion source systems to be used with a heavy ion beam have been developed at Dubna and at 18) Stockholm . Furthermore, a modification of the Bernas-type surface 19) lomzation source is planned

Before starting work .long this line, we want to look in.- ,• -ssible modifications of the ion source with the aim Fig. 5. UF target chamber. of increasing the source temperature and the ionization efficiency. Here both a reduction of the source dimensions and the operation as a unoplas- matron are under investigation. For this purpose, an ion-source test set-up has been put into operation which allows off-line tests of ion sources with an accelerating voltage of up to 30 kV. -329- We wish to tnank K.H.Burkard, P.F. Dittner, 'J. Ssckert and M. Weher for their help in the experiment. We are grateful to many colleagues from t!.f If.OLDE collaboration for valuable comments, and especially for making the tape station available to us.

References:

1) GSI-Bericht 73-3 (1973). 2) P. Armbruster, GSI-Bericht 73-2 (1973). 3) H. Ewald et al., contribution Dl to this conference. 4) GSI-Bericht 73-7 (1973), to be published. 5) K. Blasche, D. Bohne, Ch. Schmelzer and B. Stadler, in Proc. of the Int. Conf. on Nuclear Reactions induced by Heavy Ions, Heidelberg (1969), p. 5 18. 6) D. Bohne, in Proceedings of the 1972 Proton Linear Accelerator Conference, Los Alamos, (1972), Report LA-5115, p. 25. 7) H. Prange, in GSI-Bericht 72-10 (1972), p.l. 8) F. Nickel, in Ref. 3. 9) W.L. Talbert, in CERN 70-30 (1970), p. 109. 10) A. Kjellberg and G. Rudstam, eds. , CERN 70-3 (1970). 11) J. Alstad, B. Bergersen, T. Jahnsen, A.C. Pappas and T. Tunaal, in CERN 70-3 (1970), p. 111. 12) G. Wolf, contribution H8 to this conference. 13) Private communication from the ISOLDL collaboration (1973). It) M. Weber, N. Trautmann and G. Herrmann, Radiochem. Radioanal. Letters b_ (1971), 73. 15) M. Weber, N. Trautmann, H. Menke, G. Herrmann and N. Kaffrell, Inorg. Chem,Letters 9_ (1973), 519. 16) M. Weber, N. Trautmann, H. Menke, G, Herrmann and N. Kaffrell, to be published in Radioehimica Acta (1973). 17) A.P. Kabachenko, I.V. Kuznetsov, K. Sivek-Vil'chinska, E.A. E.kakun and N.I. Tarantin, Dubna Report JINR-D7-5769 (1971), p. 204. 18) I. Bergstrom, K. Fransson and M. af Ugglass, in Proc. of the Int. Conf. on Electromagnetic Isotope Separators and the Techniques cf their Appli- cation, Marburg (1970), Report K7O-28, p. 44. 19) R. Klapisch, private communication (1973). -330-

PINGIS - AN ISOL-SYSTEM AT A CYCLOTRON FOR HEAVY IONS

K. Fransson, M. af Ugglae and A. Engstrom Research Institute for Physios, S-104 05 Stockholm, Sweden

1. INSTRUMENTATION An electromagnetic isotope separator for on-line operation has been constructed and tested for studies of nucleides far from the line of beta stability. This ISOL-system, named PINGIS, is attached to the 225-cm cyclotron at the Research Institute for Physics, Stockholm. In spring 1971, the 225-cm cyclotron was shut down for the final phase of an extensive improvement programme. According to the present schedule, the machine will be in full operation again for research in the autumn of 1973. The improvements are expected to result in a better performance, in particular regarding the acceleration of heavy ions such as 12C1}+ (120 MeV), lH5+ (lUC MeV) and 1605+ (l60 MeV). Before the shut-down, the 225-cm cyclotron was able to accelerate protons, deuterons and alpha particles, the latter to a maximum energy of U3 MeV and a maximum external beam current of 2 yA. The reaction 238U(a,f) was then used to test the PINGIS-system and will be the only experiment described in this report. Figure 1 gives a general view of the principal parts of this particular ISOL-system, which works in the following way: The cyclotron beam hits a target inside the ion source of the isotope separator. The target is heated so that the reaction products emerge from the target. The ion source is of the well-known Nielsen type with modifications for heavy-ion on-line use and for increased temperatures (* 1500°C). A cylindrical container is placed in the anode cylinder wall on the plasma side of the entrance window for the cyclotron beam (see fig. 2). The windows consist of commercial niobium foils with a thickness of 9.1 mg/cm2. The target inside this container consists of a solid but porous disc of natural uranium dioxide sintered with pure carbon fibre (diameter • 5 ym) wool. It is believed that most of the material is converted to a uranium carbide which can resist very high temperatures and that the diffusion of fission products in the voids between the fibres is rapid. As can be seen in figure 1, the isotope separator is mounted vertically<• This was the way to get a reasonable concrete shielding (1.2 m) between the cyclotron beam target and the collector area of the isotope separator where the detectors require a low background radiation. The isotope separator is of the Scandinavian type with a magnet of a 2-m radius, 90° deflection angle ^TRANSPORT TAPE

HIGH VOLTAGE CAGE HEAVY ION BEAM -^»-

WINDCW INb! WALL TARGET

ANODE CYLINDER

CONTROL DE5K FILAMENT HEAT SHIELDS MAONE! COIL Um CONCRETE SHIELD INSULATOR (BN1

LENS SYSTEM TARGET-ION-SOURCE

OASLINE(Al,O,l • CONTAINING > THERMOCOUPLE -

CYCLOTRON BEAM TUBE

Figure 1 Figure 2 A general view of the PINGIS-system The PIKGIS target-ion source used in with the external beam from the the 238U(a,f) experiments. cyclotron entering at the lover left corner.

and a homogeneous field with 90° entrance and exit angles. The ions are extracted from the ion source with an extraction electrode which is made of aluminium and water-cooled. The distance between the outlet and the electrode is adjustable during operation by a telescope arrangement. The ion beam is focused by a lens system with four equidiameter cylinders. To get the desired height of the line-shaped beam at the collector the beam can be squeezed by an electrostatic quadrupole lens at the magnet entrance. The design and the vertical construction ensures proper alignment at the components of the lens system and the electrostatic quadrupole. Their common optical axis can be moved during operation in order to align it with the outlet of the ion source. The alignment problem is solved in this way as the target-ion source in turn has to be well aligned with the cyclotron beam. The adjustment mentioned above is less than 1 mm and the parallax introduced between the electrostatic lens systems and the magnet is unimportant since tha pole gap of the magnet is 1+0 mm. The construction details of the acceleration system can be seen in figure 3. The central beam leaves the -332-

DEFLECTION PLATES

MIRROR rC ALIGNMENT

E'.ECTRIC QUADRUPOLE LENS

>.EN5 5Y5TEM 4 ELEC fRODES

RADIAL ADJUSTMENT SYSTEM

HEJWV ION BEAM

Figure 3

A cross-section of the electrostatic configuration in the acceleration chamber• -333-

magnet in a direction parallel with the floor and 80 cm above it. The magnet and the collector permit collection of a relative mass range of ± 7 %, One mass-separated beam is admitted through a slit in the focal plane to a tape transport system behind the collector. The tape station is of a preliminary but very useful design which will be modified and improved to fit forthcoming experiments.

2. PERFORMANCE In on-line systems of this kind the production rate is an important factor. In the reaction 238U(a,f) with E = 1+3 MeV and I = 2 yh the production rate in the uranium target is about 108 atoms/sec for the mass region 92-lUT. The most probable charge for a given mass is about the same for a-induced fission of 238U and thermal neutron-induced fission of 235U except around the double-magic nucleus 132Sn where the reaction 235U(n ,f) produces nucleides further out from the 3stability line than 238U(a,f). Which elements are preliminarily separated through the system is a question that is difficult to answer because the decay schemes of the short- lived isotopes produced are not known. The following elements, however, are definitely processed by the system: As, Br, Kr, Rb, Sr, Ag, Cd, In, Sn, Sb, Te, I, Xe and Cs. Other elements are uncertain such as Ba and La. The transition elements between Z=39 and Z=U6 do not come through. The delay time between production in the target-ion source and collection in front of the detector has not been studied in a systematic way. It depends of course on the element in question, the temperature of the target- ion source and many other things. The most short-lived isotopes so far ob-

12 96 served and studied are °mAg (\/2 = 0.32 s) and Rb (Tl/2 = 0.21 s) but

9l the decay of *Kr (T ,? = 0.25 s) has not been seen. The overall efficiency of the whole system has been calculated for different elements and different runs. The values are between lCr1* - 10-2 and, so far, it has been impossible to find a relation between efficiency, element, operation parameters of the ion source and target temperature. The overall efficiency of the isotope separator itself when running on xenon gas and under favourable conditions is around 8 %. The admixture from adjacent masses in the collected sample are under running conditions less than 10-3 and on some occasions less than 10- .

A more detailed report of the PIHGIS-system has been accepted for publi-

cation in Wucl. Instr. and Meth. -334-

3. PRELIMINARY EXPERIMENTS In order to gain some experience of the capabilities of the entire system a comprehensive scan of the decay of fission products has been undertaken. Nearly all the isobaric chains from mass numbers Qh to 98 and from 116 to ikk were scanned. A few mass chains (9^-98 and 120-128) were selected for further and more thorough studies. In particular the decays of odd indium isotopes with mass numbers 121-127 have been investigated. From this introductory examination of products from cs-particle-induced fission it is evident that the PINGIS-system is well suited for nuclear spectroscopy. The interest will now be focused on nucleides heavier than ^09g^ produced in reactions with heavy ions from the reconstructed cyclotron. As fission will compete strongly here, the investigated nuclei are expected to be hidden in the background if separation is not carried out. In this region it is thus of special value to use the type of product separation offered by the PINGIS- system. -335-

THE RECONSTRUCTED ISOLDE FACILITY AT CERN

S. Sundell, P.G. Hansen, B. Jonson, E. Kugler, H.L. Ravn and L. Westgaard

CERN, Geneva, Switzerland

1. INTRODUCTION

The ISOLDE on-line reparator system at the CERN Synchro-cyclotron (SC) first started its operation in October 1967. Almost 3900 hours of on-line experiments were conducted at the facility until its temporary shut-down in May 1973 in conjunction with the SC reconstruction work.

It is the aim of the SC improvement programme to increase the intensity of the extracted beam of 600 MeV protons by as much as a factor of 100 or to approximately 10 yA. Starting with the shut-down of the SC on the 7th June, a period of one year is foreseen for this work, during which time the ISOLDE facility will also be entirely reconstructed. The changes planned for ISOLDE have several purposes: (i) to incorporate further shielding between the target area, the collector chamber, and the experimental area, in order to allow for greatly increased radiation levels; (ii) to improve the separator according to the experience gained during the years oc operation; (iii) to rearrange the experimental room, thus obtaining more space, and to make provisions for the new beam-handling arrangement ana target systems

2. ISOLDE-2 LAYOUT

The main differences between the old layout and the one foreseen for ISOLDE-2 are shown in Figs. 1 and 2. It involves a displacement of the whole separator to another branch of the extracted proton beam.

The solution adopted to cope with the problem of the high radiation levels consists of a double shielding wall with two moving doors. The analysing magnet is now built into the shield nearest to the target, so that the present long drift tube is avoided. This, together with the new positioning of the target, has gained about 8 m of space in the experimental room. The second shield is placed after the collector chamber and the new switchyard of the separator so that the high amounts of radioactivity accumulated at this point will not intefere with the experiments. These -336- will, in the future, be performed in the four secondary beam lines planned in the experimental area. In one of the beam lines it is foreseen to install at a later date a second magnetic analysis stage with relatively low dispersion. If required from the point of view of space or background considerations, pure beams can then be bent up by means of electrostatic deflection to a laboratory on the floor above.

The control desk for the separator will also be located in this laboratory (Fig. 2).

The racks on high tension will be placed in the area between the two shields in order to decrease the length and hence the capacitance of the cables leading to the target and ion-source room.

3. SEPARATOR IMPROVEMENTS

Apart frcm the analysing magnet and the dispersion and collector chamber described earlier , all other parts of the separator will be reconstructed. Together with the long drift tube the second electrostatic lens can be omitted, so that a simplified and more stable operation of the separator is obtained. The new first lens chamber and the differential pump stage is shown in Fig. 3. The main features of the design are the following: the differential-pumping chamber with the extraction electrode can, due to its separate support, easily be removed from the lens chamber. This allows the whole lens unit to be demounted as one unit, so as to facilitate cleaning and realignment of the radioactive lens. On the insulator between the differential stage and the lens chamber, a series of plugs and quick connectors containing all the supplies for the target and ion source are mounted, which allows the remote connection and disconnection of the new target and ion-source systems.

Another important problem in the target and ion-source region is the ionization of the air due to the proton beam and secondary radiations. The resulting load on the accelerating HT supply (about 200 uA in the present system) has until now caused separator instabilities only under special circumstances.

However, with the future beam intensities, special precautions will have to be taken in order to avoid excessive leakage of current from the target and ion-source unit. The solution adopted here is to enclose it in a metallic shield connected to a separate unregulated HT power supply -337-

approximately at the same tension as the acceleration voltage. The strong leak current will then be drawn only from this supply.

The vacuum system of the facility will be equipped entirely with turbomolecular pumps. The oil-free vacuum obtained with these pumps is a requirement of some of the new target and ion-source systems. But also the ion beam instabilities experienced previously because of oil film on lenses and chamber walls should be avoided in this way.

These improvements, together with numerous small modifications of the electrical system of the separator, should simplify the operation and ensure high stability.

REFERENCES

1) E. Kugler, these Proceedings.

2) H.L. Ravn, S. Sundell and L. Westgaard, these Proceedings.

3) The ISOLDE isotope separator on-line facility at CERN (Eds. A. Kjelberg and G. Rudstam), CERN 70-3 (1970). -338-

ISOLDE OLD LAYOUT

1 Target-ion source assembly 2 Lens 3 Magnet U Collector chamber 5 Switch yard 6 2nd. analyzing 7 Collection and counting equipment

ISOLDE

Fig. 1 Layout of ISOLDE-1 and ISOLDE-2 1. Bending magnet 6. Analysing magnet 2. Proton beam lenses 7.' Switchyard 3. Target and ion 8. High-tension source equipment A, Electrostatic lens 9. Electrostatic quadrupole lenses 5. Fast beam shutter 10-15. Experimental equipment 16. Control desk 17. Lift and stair- case 18. Proton beam dump 19. Target dump

Fig. 2 Layout of 1DOLDE-2 -4 "k-

b i s

Fig. 3 Lens system and differential pumping stage -341-

The ORSAY on-line separator

For the ISOCELE collaboration : R. FOUCHER, P. PARIS*, J.L. SARROUY INSTITUT DE PHYSIQUE NUCLEAIRE. 91406 - ORSAY-FRANCE • CENTRE DE SPECTROMETRIE NUCLEAIRE ET DE SPECTROMETRIE DE MASSE 91406 - ORSAY-FRANCE

A separator, called ISOCELE, on-line with a synchro-cyclotron and principally intended for nuclear spectroscopy, is under construction at ORSAY. Numerous technicians and physicists of two laboratories are concerned in this project which extrapolates the various techniques which have been under development for several years. The assembly of the constituent parts has not yet been completed, so this paper only describes the principal features of the system.

I - General arrangement.

The synchro-cyclotron at present produces an external beam of 0.2 ^A, 157 MeV protons. 79 MeV deuterons, 157 HeV a and 236 MeV 3He particles can also be accelerated. The accelerator will have to be modified in 1975 and its expected beam is 7 (iA of 200 MeV protons. The geometrical characteristics of the separator were imposed by the necessity to use the place presently available near the accelerator and to solve the problem of protection against neutrons. It was also decided to minimize the target-ion source distance in order to increase the system efficiency and to use a medium current type of separator, able to work in association with various ion sources. It was not possible to provide sufficient protection before the collector. The latter has to be included in an intermediate unsafe area and most of the mechanical devices are to be remotely controlled. The general layout is shown in the figure.

II - The ion source.

The distance between target and source can be adjusted from 30 cm to zero by changing the final orientation of the proton beam. Owing to its

-343-

considerable versatility and the experience attained with it at ORSAY the BERNAS ion source (1) will be used at first, without excluding any other type. The maximum height of the extracting slit will be 15 mm and the maximum ion beam current 10 mA. In fact, the current intensity extracted from the source will in practice usually be small. But the possibility of extracting higher currents can be very useful, for instance to handle sources with strong gaseous carriers or with important parasitic stable beams. So, no limitation is expected on this point.

Ill - The magnet.

A magnet proposed by J. CAHPLAN and proceeding from the general ideas described by him at this Conference, was adopted. Main parameters are as follows :

source-magnet distance 1.80 m deflection angle 75° mean radius 0.80 m index 0.5 mean gap height 66 mm magnet-focus distance 2.97 m dispersion 1990 mm (1 cm between masses 199 and 200, perpendicular to the beam) maximum mass range at the collector : +_ 10% of the central mass maximum radial aperture of the emitted beam : 4° (± 2°) angular magnification 0.67 (maximum aperture of the beam at the collector : 2.7°)

Two kinds of corrections have to be made on the magnet : first, as already applied ro several magnets (2), the entrance boundary will be corrected to minimize radial aberrations. Secondly, the exit profile will be curved so as to obtain a 30° angle between the main path and the focal plane. Without correction, this angle is expected to be 19°. Any small deviation of the n = 0.5 index value can change the focal plane position but will eventually be corrected by acting on the exit boundary angle. It seems to be also necessary to control any focal plane displacement corresponding to changes in the object location due to modifications of the ion source or extraction parameters. Such a small -344-

correction will be made by acting on windings consisting of printed circuits fixed on the two polefaces. Pumping at the target and source levels will be achieved by oil pumps. Apart from this, the vacuum will be maintained in the whole separator by three, or perhaps four, cryogenic helium pumps, the helium autonomy of which is more than one week.

IV - The collector.

A first protective wall will be built between magnet and collector, the target-source-magnet room being almost completely surrounded by walls of concrete and iron. This protection will not be sufficient to allow access to the collector during the runs, but will give complete independence to the other users of the synchro-cyclotron. From the collector, two masses selected by slits will be deflected by electrostatic cylindrical condensers, acting also as energy and charge filters, and transforming both divergent beams into parallel beams. The first will be in a fixed position and will permit the extraction of one of the different masses for off-line spectroscopic work or for chemical studies. The second deflector will render the beam parallel to the focal plane and will be removable along this same direction. So the exit beam orientation will be rather independent of the exact selected mass. However, this beam will undergo a small translation due to the variation of the angle of incidence, along the focal plane. The transport line which follows will be suitable for the amplitude of this translation and with the small possible vertical divergence of the beam beyond the focal plane. Its exact realization will depend on the experimental characteristics of the separator.

V - switch magnet.

The two ion beams will be carried beyond the last protective wall, 1 m to 1.5 m thickness of concrete. The second ion beam will be once more analyzed by a second magnet, available at ORSAY, which can switch between three directions, "wo of them being effectively in use at the beginning. Due to the number of -adial crossings produced by the quadrupole electrostatic lenses calculated for the transport line, the second stage -345-

dispersion will add to the first one. Before the second magnet, an intermediate focus will be convenient for the use of a p spectrograph.

VI - Present state.

The present state of the ISOCELE construction is the following (May 73) : the proton beam transport line has been built and works correctly. The spot surface is about 0.5 cm2 on the target. The first stage magnet has been delivered and measurements and corrections of the magnetic field are beginning. All the constituent parts of the vacuum tank have been ordered and are now being received. A mechanical system allowing remote controls of the four movements of the extracting electrode has been built. A 60 kV - 10 mA supply has been built and works satisfactorily, associated or not with a device to stabilize the beam. Thanks to collaboration of the AMES Laboratory, * transport tape system is under construction. A second system will be used for angular correlation measurements. We have now to construct the ion lines beyond the collector.

VII - Source experiments.

Beside the ISOCELE proton line, another one is used for on-line studies with various target-source systems. The first one collects volatile products on cooled metal plates which are periodically transferred outside the irradiation room to a Ge(Li) detector. This target can work at high temperature and melt some refractory metals like platinum. Modifications to extract and implant the ion beam with a few kilovolts and to place targets inside the ion source are being made. Many tests are foreseen using these facilities. It is planned to make the first trials of the separator in September and first experiments at the end of this year.

References

(1) J.L. SARROUY et al., Nucl. Instr. 38 - 1965 - 29. (2) J. CAMPLAN, R. MEUNIER, Nucl. Instr. 57 - 1967 - 252. -346-

ZBIK; h PROJECT OF All ISOL Z'iK'^la ON 'IE- :,..ACTOR A. Piotrowski, P. Klep&cki, A. Sulik, J. Ludziejewski and J. Jsstrzebski,Institute for Nuclear Research, Swierk, Poland

1. Introduction An isotope separator is now under contruction at the Institute for Nuclear Research in Ewierk for on line experiments in the reactor. This note is to provide information on the present status of the isotope separator construction, some characteristics of the new reactor and the adopted fission product transport system from the reactor core to the separator ion source.

2. Isotope separator The isotope separator is based on the ISOLDE principle '(L) 5° magnet, K=1.5m £ =35-5)• The magnet is composed of six 40 mm thick Brmco plates. The maximum coil current of 150A produces a 0.38T field in the gap of the iru.gnet yoke. The mtgnet power supply will be a commercial device (Danfysik) The 50kV high-tension supply used gives a long term stability better than 10""^. The installation of the magnet is scheduled for September of this year. All mechanical parts of the design have been constructed at the Institute workshop.

3. Ion sources Two types of the ion sources are in preparation: the plasma type ion source and a source based on the surface ionization principle. Both are modified versions of the previously described sources '-' . The plasma type ion source was tested together with the isotope separator ion optics. The efficiency measured for Ar •111 was 35%. The radioactive J Cs source, introduced to the ion source in the form of CsCl, was also prepared. In this case the ion source efficiency was determined as 38%. The detailed description of the ion sources will be published later . -347-

4. Reactor

For the projected on-line experiments the uranium t:.r,\:u v.ill be placed near the core of the new reactor (KARIA) in L'viierk. The reactor will be in operation at the begininf; of 1v?5, v.ith a thermal neutron flux of t-bout 5x101^ near the core and about 1CK n/cm s at the output of the allocated channel. The 120 mci diameter of this channel makes the instalation of the ion source and focusing optics near the reactor core rather difficult.

'j>. Transport ^ystem

A He-jet system is studied to provide transport of the slowed down radioactive fission fragments from the uranium target, placed near the reactor core, to the EM separator ion source (distance about 10m). The 14 MeV neutron induced fission is now used to investigate the different transport conditions. Although this part of the design is only in a very early stage, we hope, basing on available literature (o.f;. rsfs.-"'"J ) that the system applied will be useful for extracting fission products from the reactor with a good efficiency. A much more difficult problem is the introduction of the He-jet to the isotope separator ion source. In the first stage we plan to use for this purpose the surface ionization source.

6. Concluding comments Although the construction of the isotope separator is well advanced in our laboratory, the main problems of putting it on-line must still be solved. It has been decided that the installation of the separator at the reactor will be realized only if the He-jet collaboration with the separator ion source gives satisfactory results. -348-

Refe:rences 1 The Isolde Collaboration, A. Kjelberg and G. Rudstam ed., CiiRN Rep. 70-3 /197O/ 2 A. Piotrowski, V. J. Raiko and H. Tyrroff, Pribory i Technika Eksperimenta 21 /1972/23 3 G.J. Beyer, E. Herrmann, A. Piotrowski, V.J. Raiko and H. Tyrroff, Nuol. Inst. and Meth. ^6 /1971A37 4 A. Piotrowski and Z. Kozlowski, to be publ. 5 K. Wien, Y, Fares and R.D. Macfarlane, Nucl. Inst. and Meth. J1O3_ /1972/181 6 H. Dautet, S. Gujrathi, VK.J. Vrfiesehahn, J.M. D'Auria and B.D. Pate, Nucl. Inst, and Meth. 1_02 /1973A9 -349-

NUCLEAR SPECTROSCOPY USING ON-LINE MASS-SEPARATOR IN LENINGRAD SYNCHROCYCLOTRON

E.Ye.Berlovich, E.I.Ignatenko, Yu.N.Novikov

Leningrad Nuclear Physics Institute Academy of Sciences of the USSR

A short description of the IRIS system using on-line mass - separator is presented.

Nuclei far off the beta-stability line are attracted wide - spread attention of investigators'1'2'. This fact is explained by the possibility to observe new physical phenomena'7^ such as proton and two-proton radioactivity' ', correlated neutron pair emission'5{ delayed emission of different particiest6/, delayed fission''', and also other characteristic processes^ '*" . The study of nuclei far off the beta-stability line is slso interesting for the investigation of nuclear structure. It gives the possibility to research the nuclear property evolution with the change of nucleon number in the wide range of mass numbers. Clearing up^the mechanism of the nuclear shape transition from spherical.to nonspherical one in so-called transition regions^ ' '11» 12/is important. Short-lived nuclei far off the beta-stability line can be studied using particle beams from accelerators and reactors only in the immediate vicinity of an irradiated target. The extraction and identification of short-lived nuclides are carried out by special rapid radiochemical methods using mass-separators on-line with the source of particles generating nuclear reactions. Similar installations are created by more than ten scientific centres. Leningrad Nuclear Physics Institute, Academy of Sciences of the USSR is planning to realize the system IRIS (Investigation of Radioactive Isotopes at the Synchrocyclotron). The IRIS system is based on the 1 GeV proton synchrocyclotron, it is placed in a special annex closest to the experimental -350-

hall of the accelerator. Fig.1 represents the schematic plan of different system assemblies in the annex. The building consists of the bunker for the targets and for the input part of the mass-separator and experimental hall,divided by 6 m iron shielding wall. The proton beam will be transported to the target by means of a vacuum tract. Beam focusing will be carried out "by quadrupole lenses placed in the bunker. At the output of the accele- 12 rator the proton intensity and average density are 1.0.10 p/sec and 5.8.io'1'1 p/cm ggc respectively. In the experimental 1 *1 hall of.the accelerator the same parameters are 7.8*10 p/sec and 5.6.10'1'' p/cm2sec'1^'. Fig.2 illustrates the beam cross profile. After the target the beam will be damped in the concrete wall with the iron trap. Target devices will have specific construction features depending on physical and chemical properties of radioactive products extracted. It is planned to use "cold" targets under usual temperatures, "hot" ones heated up to 800 - 1000° G and targets combined with an ion source. The radioactive products being formed in the target device will be transported through the warming tube to the ion source. Storage for changeable targets and special jib for their transportations are provided in the bunker. The system described will use a mass-separator of Scandina- vian type made by the firm "Nucletec". Its main technical pro - perties are given in Table I. Similar type mass-separator is described in different publications' * *>' and operates now in CEEH and Dubna, TABLE I

The average trajectory radius R = 1.5 m The deflection angle of the average trajectory 0 = 55° The output arm length L =2.1 m The inclination angle of the focal plane y = 30° The dispersion measured at the right angle to the beam for one percent of a relative

mass difference ij ._ ^ rnm 1'ho range of mass analysed simultaneously at the collector ^ l\ = + 15 i The magnet weight T - 1 ton The magnetic field strength in 50 mm gap H - 4700 OJ •The magnet supply current 1=19\ a The ion source of Nilaen type with efi'ici - ency 5 + 10 % The vacuum system gives the possibility to afford the residual gas pressure in the muca-separator chambers no more than 10"b mn Hr;

7ig.1 shows that a part of the deflation system and cul - lector chamber of the nass-separator are iiiount»d into the :ihi- ciciinfj; v/2li. It givffs the poaaibility to rule out in".jnvenii:nt :.;y.:;ly[;i v:ith a l;>ng ion channel conaecliug t'-•;;•.• i:n sourcj v.'iLh Lht; '.ci'le-jtion chamber realized in the i.ioat on-line :iC^j<.<:i:.i ! / a;, "//1 in our system ions v/ill bti tr.-inGportad !;•.•• -iit; cc,'l..c- ciun chamber by means of the lens uyutem and ari-jrt ion ciiunncl. ly long-lived isotopes y/ill be extracted fr.iiu tiic chamber by means of aluminium foil through vacuum sluice. Short-lived isotopes will be transported in different di - rections to the tapes of the tape winders with the help of tn« deflection systems and ion channels ant! further to tiit- uctec - tors for invLjti.vjtions. :ai detect in;; devices are placed in .-y+0 m"- e;-:p<..rim'.ntal hall. -352-

Fig.1. The scheme of the complex IRIS. 1 - muss-sep.iratur, 2 - target, 3 - isotope channels, 4 - detectors, 5 - hi©h. voltage supply, 6 - magnet supply, 7 - desk, 8 - electronics, 9 - physical devices, 10 - proton beam, 11 - lenses, 12 - TV- ar - ranp;ement, 1J - jib, 14 - target storages, 15 - vacuum pumps, 16 - proton's trap, 17 - shielding door. lCM

Fig.2. The cross profile of a proton beam, a) - at the beam, output from the accelerator chamber, j) - in the experimental hall of the accelerator.

1. Proc.Conf.Lysekil. ArfC.fys., 36 (19b?). 2. Proc.Gonf.CERN.CERN preprint 70-30, vol.1,2 (1;70). •j. ri.Ye.Berlovich.CERN preprint 70-30, vol.1, 497(1v7O). ^. V.I.Goldanskii. Ann.Rev.Nucl.Gci., _16, 1 (1966). 5. lil.Ye.Berlovich,O.I.i.Golubev,Yu.N.Novikov. Pis'inu JiTF, 12, . 289 (1970). 6. V.A.KarnaukhoVjG.M.Ter-Akop'yan.Yadernaya Fizika,2»6(19c>L)). . H.D.Macfarlane.A.Siivola.Phys.Rev.Lett., 14,11^- (1965). . 7* E.Ye.Berlovich, Yu.N.Noviliov. Hrys.Lett., 29.E, 155 (^9o9). 8. E,Ye.Eerlovich,Yu.N,Novikov.DAN SSSR, 188, 1023 (1969). 9. E.Ye.Berlovich, O.M.Golubev, Yu.N.Novikov. Izv.Alcad. Nauk, . 3SSR, ser.fiz., ^4, 675.(1970). 10. IS.Ye.Berlovich.Izv.Akad.Nauk,SSSR,ser.fiz.,29,2177 (1965); Acta Phys.Polon., A^8, 645 (1970). 11. E.Ye.Berlovich.Yu.N.Novikov.Pis'ma ^TETF, 2, 281 (1965). 12. il.A.Gorensen. CB3N report 70-30, vol.1, 1 (1970). 13. N.A.Abrosimov.A.A.VoroMev.Vestnik mead, ilauk,;.:;';::*,M11 (1 ;'/?')• 14. a.r;udstam,3.i.u.Tuell,G./uidC:rsoon.Nucl.InGtr.;,lcth. ,28, (1964). -354-

15. G.Kusiol, V.I.Raiko, H.Tyrroff. Preprint JOE, F6-4487, Dubna (1969). 16. Proc. Intern. Conf. on Electromagnetic Isotope Separators, Llarburg (1970). 17. Nucl.Inrjtr.Meth., 58 (1965). Chapter 8: !SOL beam handling and collection techniques -356-

A beam switching principle for ISOL facilities

G. Andersson Department of Physics, Chalmers University of Technology Goteborg, Sweden

The problem of bringing on-line separated isotopes to the source positions of measuring instruments becomes a non-trivial one, when several mass numbers are to be studied simultaneously and, preferably, independently. Two basically different approaches have been practiced at ISOLDE: mechanical transfer from the focal region of the separator by means of collector devices movable across the mass spectrum, kept in constant position, and the more direct method of extracting the desired beams and directing them into the instruments, changing mass numbers by displacing the mass spectrum.

The first-mentioned alternative, represented by a magnetic-tape system 2) and the so called T-meter , has the advantage that the settings of the separator can be left unchanged during a run, which is favourable iroia the point of view of machine operation and facilitates the planning of experiments. But any arrangements of thic typ_ tend to imply either severe shielding problems or unacceptable transfer times. For most investigations extraction of the beams gives far more satisfactory conditions. At ISOLDE a switchyard has been in successful use for some years, where one electrostatic deflector lens en each side of the undeflected central beam make possible the simultaneous study, well away from the separator, of three mass numbers. A disadvantage of this system is the interdependence of the channels; only one of the mass numbers can be chosen freely.

An arrangement that combines the advantages of the two alternatives, avoiding the drawbacks, can be obtained simply by making deflector Lenses instead of collector devices movable in the focal region and i.- such a way that their output directions are unchanged. This is exactly realized in the idealized case,shown in the figure (next side),that the separated ion beams are parallel. -357-

SEPARATED BEAMS

\ \ \ , FOCAL\ • PLANE \ \B'

Two deflectors in the form of sectors of cylindrical capacitors are indicated with output directions A-A' and B-B', respectively. The minimum distance between beams picked up with a pair of lenses is seen from positions A -B to be equal to half the thickness of a lens, neglecting the beam width! The combination A^ demonstrates the inevitable cccurance of shadow effects in some situations.

Obviously the relation between dimensions and voltages is crucial for the

feasibility of the arrangement. The elementary formula to apply for singly

charged ions is

2 Ua r = V where r is the mean radius of the cylindrical capacitor, a the distance between the plates, U the acceleration voltage of the separator and V the lens voltage. Suppose, for instance, that r = 50 cm, U - 75 kV and V = 3 kV. We then obtain a - 1 cm. Allowing for some shielding outside the lens

.lates ^his 6ti:.l -- Aes it possible to pick up neighbouring masses up ,,-,„,.rds ma.s numoer- 200 in an ISOD3E type installation.

in r^-v- f. oeam pattern in the focal region is not parallel but so-e-

-,hat tf Wt. Thus the condition for a beam to follow the proper path -358-

through the lens, that the entrance edge is perpendicular to the "beam, is not generally fulfilled. Small deviations, however, can be compensated for by changing the lens voltage, causing the radius of curvature of the beam to differ from the mean radius of the deflector. The resulting error in the output direction is taken care of by the refocusing lenses supposed to be incorporated in all extracted-beam systems. In cases of larger deviations from parallelism, direction-correcting ion optical device? may have to be included at the; entrance side. For practical reasons, however, it is suitable to limit the relative mass difference, and thus the angular spread, covered by one ejector lens and to work with a series of lenses slightly overlapping each other as to mass range. It may be remarked that operation close to the focal plane is not a strict requirement in types of separators where the opening angles of the focused beams are small.

The general ideas outlined here can, of course, be applied in a variety of fashions, depending on the actual demands. In the detailed design of such a beam handling system, the focusing properties of electrostatic sector fields may, for instance, be used to advantage.

The plans for a switchyard based on this principle at ISOLDE are described in the contribution by E. Kugler

References

1) G. Astner et al., unpublished. Cf "The ISOLDE isotope separator on-line facility at CERN" (Eds. A. Kjellberg and G. Rudstam), CERN 70-3 (1970) p. 9.

2) M. Alpsten, G. Andersson, A. Appelqyist, B. Bengtsson, E. Borgentun, and B. Jonson, CERN 70-3 (1970) p. 39.

3) S. Sundell, Proceedings of the International Conference on electromagnetic isotope separators and the techniques of their applications, Marburg 1970, p. U73.

h) E. Kugler, these Proceedings. -351-

DEVELOPMENTS IN AREAS OF ON-LINE FISSION-YIELD AND DIRI-T.T MASS MEASUREMENTS AT THE LOS ALAMOS SCIENTIFIC LABORATORYt

S. J. Balestrini and L. Forman University of California, Los Alamos Scientific Laboratory Los Alamos, New Mexico, USA

ABSTRACT 0 T Q On-line studies of U fission yields with fission spectrum neutrons were made with the Bernas surface-ionization technique at the Godiva IV no o burst reactor facility. The target was 300 mg of U in a porous graphite mixture. Ion emission rates were measured by Z-direction motion of the collector. The isotope collection time was varied from 0.1 to 4.0 seconds with respect to the reactor burst. Six sets of data gave the relative yields of Cs in the mass region 138 to 146. Th\. absolute independent fission yields were obtained by normalization using radiochemical data and chain yield estimates. Work has also progressed toward encapsulating the Bernas source in a thin graphite container to reduce the hazards of working with highly radio- 235 active targets. Ion emission rate measurements on encapsulated U indicated 146 that the ^0.2 second Cs is an accessible nuclide. One of the authors (Leon Forman) has been charged with organizing the direct mass measurement section of the LAMPF on-line isotope separator proposal. At the Skovde meeting, we should like to report the scope of this work and the participants and their institutions.

INTRODUCTION At the 1970 EMIS conference at Marburg, Germany, we described a method developed at Los Alamos for determining independent yields of alkali nuclides from neutron induced fission. This manuscript describes changes in that technique for studies of fission induced by fission spectrum neutrons and for working with highly radioactive fissile targets.

'Work performed under the auspices of the U. S. Atomic Energy Commission and in part sponsored by the Advanced Research Projects Agency of tne Department of Defense (Order No. 1836). -360-

In addition, a program for the direct mass measurement of radioactive nuclides produced at the proposed LAMPF on-line isotope separator is briefly described. So far, this endeavor has resulted in a collaboration representing virtually all active mass measurement groups on the North American Continent and two on-line mass spectroscopy groups.

FISSION YIELD MEASUREMENTS Briefly, the on-line fission yield measurement method at the Los Alamos Scientific Laboratory employs an intense burst of neutrons from the Godiva IV reactor to produce fissions in a target material suitably located in the source (12) optics of the spectrograph. ' The ion source design employs the Bernas surface-ionizacion technique. " Ionized alkali fission products are accelerated into a 51-cm magnetic lens and collected on the focal plane of the spectrograph for a preset time. Each mass position is later assayed by beta counting. Th3 resulting isotopic abundance analysis can then be normalized using available radiochemical data. The collector is designed so that it can move at a steady rate in a direction transverse to the beam spectrum. The burst signal initiates the motion, and the resulting beam deposition trace on the collector describes the ion emission from the source as a function of time. The standard method of fabricating a target is first to extrude a mixture of uranium oxide and graphite with furfuryl alcohol as a binder, through a die and curing it at a high temperature in an inert atmosphere. The result is a stock of target material in the form of a hard, thin rod of a mixture of uranium carbide and graphite. The target is mide by breaking off a suitable length from this rod. The relative abundances of 138Cs, 139Cs, 141Cs, U3Cs, 145Cs, and 146Cs on o formed by fissioning U with fission spectrum neutrons have been measured recently by this method. The Cs nuclides of mass 140 and 144 were not observed because: the beta activity in these mass chains is too small to yield information. The target was a 3.6 mm diameter cylinder 9.5 mm long containing 1OQ tyo C 300 mg of high purity U (less than 0.025% U) in the graphite matrix. The target was enveloped in Ta to form the source oven and maintained at a steady 1880°C during each irradiation. For this work, the center of the reactor was moved to about 35 cm from the target with all possible moderating matter removed. A typical irradiation burst lasts for about 30 microseconds and produces a total of about 10 neutrons. - 5 L, 1 -

The emission rate for Cs from this source oven, when maintained at 1880°C, was determined a3 a function of time using the moving collector technique in order to correct the observed Cs abundances for decay and ingrowth from precursors during the finite collection for the 238U yield times. Collection times were varied from 0.1 to 4.0 sec in the course of six irradiations. The weighted averages of the corrected relative abundances are normalized to 141 Cs as unity and listed in Table I. The estimates for the independent fission fission yields in Table I were obtained from the relative abundances by normalizing as follows: 1) The fractional cumulative yields for 139139X u and 14114X1 e have been recently measured. (4) The fractional independent yield of Cs for each mass chain was estimated by subtracting the cumulative yield of Xe from unity and correcting for the small contribution from Ba that can be inferred from systematics.^ The

+ 0050 + 027 fractional cesium yields thus obtained are 0.052 °- and Q.395 °' - 0.0063 ' - 0.075 respectively. 2) These values were multiplied by the respective mass chain yields a. listed by Meek and Rider*- to provide estimates for the independent 139 141 yields of Cs and Cs. The relative abundances are normalized to these values and given in Table I. The independent fission yields are plotted in Ki't,. 1 and compared to theoretical values. The solid curve is obtained by multiplying the chain yields from Ref- 6 by the fractional independent yields of Cs from Ref. 5 and the dotted curves • ire ranges of probable values. The normalized yields tend to be lower. The Cs (Z - 55) yields are also lower relative to those of adjacent even Z nuclides in thermal fission of 235U.^

TABLE I RELATIVE AND ABSOLUTE YIELDS OF Cs FROM OO O FISSION OF U WITH FISSION SPECTRUM NEUTRONS Absolute Independent Mass Relative Abundance Fission Yield. % 138 0.033 ± 0.024 0.057 ± 0.042 + 0.053 139 0.198 ± 0.034 0.333 - 0.059 + 0.27 141 1.00 ± 0.11 1.69 - 0.30 + 0.33 1.86 142 1.10 ± 0.14 - 0.36 + 0.36 143 1.06 I 0.17 1.79 - 0.39 + 0.087 • 0.502 '45 0.298 ± 0.034 - 0.095 146 0.065 ± 0.014 0.109 ± 0.025 -362-

10 1 1 1 r

2 I0" ! 1 i i i 140 145 MASS NUMBER

Fig. 1. Comparison of the independent fission yields of Table I (plotted as o's) compared to theoretical estimates obtained by multiplying chain yields •* by fractional fission yields (solid curve). The dashed curves indicate ranges of probable values.

ENCAPSULATION OF HIGHLY RADIOACTIVE TARGETS A preliminary experiment has been conducted to see whether Cs atoms produced by fissioning 235U can diffuse readily through the wall of a graphite capsule. The purpose of the work was to determine whether one could reduce the hazard of working with a highly radioactive target nuclide by encapsulating it and still give a satisfactory on-line source. A capsule to contain a - JU target was machined as a cylindrical graphite sleeve 0.51 mm thick, closed at one end and large enough to accommodate a 6.35 mm long and 1.8 mm diameter target. After inserting the target, the capsule was sealed with a graphite stopper by braising with nickel. -3C3-

Results with and without the capsule, from an oven temperature of 1350°C indicate that the capsule reduced the ion emission only by roughly 30% fen- 5 sec collection times. Further study of the time dependence of the ion emission rate from the source with the capsulate target, using the moving collector technique, revealed that about 35% of the ions available are released quickly with a mean time of about 0.15 sec. In the course of calibration and testing, the source oven and capsulate target were heated repeatedly and extensively to temperatures as high as 1900°C. But on examination after the experiment, no alpha activity was detectable outside the capsule. This evidence justifies confidence for encapsulation as a means of containment.

DIRECT NliCLIDIC MASS MEASUREMENTS AT LAMPF It is anticipated that the Clinton P. Anderson Meson Physics Facility (LAMPF) will be one of the most prolific sources of radioactive nuclides. B. J. Dropesky is presenting at this conference the status of the proposed on-line isotope separator project for LAMPF which would isolate many of these nuclides. A collaboration has been formed for the purpose of determining the nuclidic masses of these radioactive species. Coauthorship in the on-line isotope separator proposal section ' describing these mass measurements includes: W. H. Johnson (University of Minnesota), R. C. Barber (University of Manitoba) and C. M. Stevens (Argonne National Laboratory) representing the direct mass measurement groups and L. Forman (Los Alamos Scientific Laboratory spokesman) and P. L. Reeder (Eattelle Pacific Northwest Laboratories) representing on-line mass spectroscopy efforts. The broad basis for scientific interest in these mass measurements was documented by S. G. Nilsson, J. R. Nix, P. A. Seeger and W. J. Swiatecki. Three methods are considered for the direct mass measurements. They are off-line determinations of longer-lived species, on-line studies using the isotope separator as an integral part of the mass measurement instrument, and introduction of the isotope separator beam into a high resolution double focusing instrument. All three methods have their respective advantages.

REFERENCES (1) S. J. Balestrini, L. Forman, K-. Wolfsbarg, T. R. Jeter, Proceedings of the International Conference on Electromagnetic Isotope Separators and the Techniques of Their Applications, Marburg, Germany, 1970, H. Wagner and W. Walcher, eds. , (Physicalisches Institut der Universitat Marburg, 1970), pp. 68-77. -364-

(2) L. Forman, S. J. Balestrini, K. Wolfsberg, and T. R. Jeter, in International Conference on the Properties of Nuclei Far From the Region of Beta-Stability, Proc. Intern. Conf., Leysin, Switzerland, Aug. 31-Sept. 4, 1970, Vol. I, p. 589 (CERN Report No. 70-30, Geneva).

(3) R. Bernas, "Advances in Mass Spectrometry", Vol. 4, Institute . of Petroleum, (1967).

(4) K. Wolfsberg, private communication, 1973, to be published.

(5) K. Wolfsberg, private communication, 1972, of updated calculations by a combination of the methods outlined by A. C. Wahl et al., Phys. Rev. 126, 1112 (1962) and by C. D. Coryell et al., Can. J. Phys. 39, 646 (1961).

(6) M. E. Meek and B. F. Rider, Compilation of Fission Product Yields, Vallecitos Nuclear Center, January 1972, NEDO-12154.

(7) A. C. Wahl, A. E. Norris, R. A. Rouse, and J, C. Williams, in Proceedings of the Second International Atomic Energy Symposium on Physics and Chemistry of Fission, Vienna, Austria, 1969 (International Atomic Energy Agency, Vienna, Austria, 1969), p. 813.

(8) "Proposal for an On-line Mass Separator Facility at LAMPF to Study Products of Reactions Induced by an Intense 800 MeV Proton Beam", B. J. Dropesky et al., in preparation. This proposal will contain a section entitled "A Nuclidic Mass Determination Program for Radioactive Nuclides Produced at LAMPF" by R. C. Barber, L. Forman, W, H. Johnson, S. G. Nilson, J. R. Nix, P. L. Reeder, P. A. Seeger, C. M. Stevens, and W. J. Swiatecki. -365-

ON THE COUPLING OF AN E.S. QUADRUPOLE DOUBLET WITH POST-ACCELERATION OR RETARDATION COLLECTOR SYSTEMS AT THE FIRST ORSAY SEPARATOR

CM. Truong, J. Fournet-Fayas, G. Levy, J. Obert. Institut de Physique Nucleaire, Universite de Paris-Sud, 91 406 ORSAY.

1 - INTRODUCTION

In many nuclear physics experiments, the required size and shape of isotope targets given by an E.M. Separator can be of great importance. In 3-spectrometry sharp line or quasi-punctual radioactive sources are demanded, while for Van de Graaf accelerator, isotopic targets of desired area with both required thickness and homogeneity must be reached. Besides, it is obvious that, for these purposes ion beam deceleration techniques should be used in order to obtain either suitable R-sources for which a shallow penetration is required, or in the latter case, thick stable isotope targets with thin backings. On the other hand, many problems occured, especially in separators working with emission slit ion-sources as for our two-stage Orsay senara'.or when the beam, with its usual characteristics (curve shaped imap.e divergence, z-length...) is directly used into the retardation lens. (2) P. Bautz et al. pointed out the important farts that a) a parallel beam aperture, b) an inclination towards the lens axis, c) a beam convergence or divergence... greatly influence the general feature of the !:ocus by increasing aberrations or reflected ion losses. In order to reduce these difficulties, an electrostatic quadrupolr- doublet was added between the focal plane and high voltage collector devices.

2 - DESCRIPTION OF THE ARRANGEMENT, (see figure 1)

2.1. Focal slit unit: including electron repeller and current measu- rer, movable in x and y directions, slit size 30 mm long x 0-10 mm wide.

2.2.3. Quadrupole doublet, located close after the focal slit (sec for example ^3'4'5'))

- aperture = 90 mm diameter - pole = 110 ram diameter (&1.2 x aperture) -366-

- pole length = 100 mm - gap between the two Q = 0-150 mm - high voltages : +_ 0-15 KV

2.4. Diaphragm unit (with electron repellers) in order to avoid electron flow in either direction and possible high voltage breakdowns from the retardation zone to the doublet.

2.5. High voltage collectors a - Ion-implantation set-up. The testing system, performed in our laboratory is described elsewhere b - Ion-retardation devices, mainly : 1 - Uhler's type retardation lens - Details are shown in figure 2. - The top of H.V. plates are located 15 mn after the diaphragm unit. - Earth and H.V. shields are mounted around the retardation plates and target support. - The target is movable along the beam axis. - the plates : 120 mm long, 50 mm wide are separately movable with respect to median plane. - A heart-shaped cam provides a linear sweeping motion of the target : 2 2 area = 10 x 10 mm and 15 x 15 mm , 6c/min. - A cooling device and thermocouples are included in the target support. 1 - Plane geometry retarding lens (figure 3) The so-called "A7 configuration" / p \ as theoretically studied and constructed by J.S. Dionisio with z -length = 70 mm, y-electrode gap = 11 mm distance between electron repeller and electrodes = 35 mm The target support hold by a nylon rod can be easily get out through a vacuum lock.

3 - WORKING SPECIFICATIONS

3.1. In "classical" separations (40 KeV ion energv) since the Q-doublet easily provides a straight line of desired length instead of a curved one, the collection conditions are more convenient.

3.2. Preliminary attempts indicated that, for post-acceleration and ion-implantation technique, the Q-doublet can readilv be used in order to -367- obtain either a wide area impact or an accurately concentrated beam (for instance, in "located ion-implantation" semi-conductor experiments').

3.3. The retardation devices operated quite satisfactorily. By acting on the different parameters - position of the target along the beam axis, quadrupole and retarding lens voltages, the best focusing conditions for the application in view are readily achieved. In the case of stable isotope collection, a rather wide area focus is used while the target is mechanically swept. On the other hand, the retardation plates are located in such a manner that the resulting electrostatic field gives a slight deviation to the decelerated beam, hence the backing foils are not reached by the neutral atoms.

3.4. When replacing the retarding plates by an inner cvlinder of appropriate size and working with suitable parameters of the doublet, we (9) easily obtained a quasi punctual ion impact . However, with respect to the normalized ion source emittance, generally a more accurate position of the target is necessarv.

4 - PRELIMINARY RESULTS.

4.1. Stable isotope targets of 8 mm diameter with different isotopes (Ca, Si, Mg, Fe, Ti, W...) using various carbon backing densities were nrenn- red. 2 As an example, we obtained a few targets of Ca with about 40 Ug/cm on carbon backings of the same approximate density (at residual energy from 100 eV up to 1 keV) . However, many others problems occured about decelerated stable isotope thick targets (saturation values, electron bombardment and heating of the target, neutral atoms, differential dilatation between fragile backing and isotope layer, precise measurements of homogeneity and reached thicknesses, etc...)(9'10). It still remains that our deceleration arrangement provides a more versatile method in stable isotope target investigations.

4.2. A few radioactive samples of neutron deficient mercury isotopes were prepared with this experimental set-up for low energy electron spectroscopy studies (E^ 10 keV). -368-

Preliminary leasurements ' using one of these samples ( ' Hg and l93mHg + ' 93Hg) (dimensions : 0.5 x 15 nan ; total activity^ 0. 2 mCi ; kinetic energy of the collected ions : 600 eV) with a semi-circular spectrograph and pre-acceleration showed an improved resolution over the same spectra obtained with full ion energy collected samples (E = 40 keV). This performance illustrates the possibilities of this method of ion-collection in a medium intensity research isotope separator. Two kinds of developments are now in project on this subject : A - The replacement of the mechanism of the present experimental arrangement by a more suitable one in order to investigate the optimum focusing conditions of the set of quadrupoles and retarding lens. B - The use of a TTV2 double focusing g-spectrometer to investigate systematically the influence of the source thickness oa the low energy line shape. Indeed it is very difficult to make an accurate line shape analysis of the low energy electrons detected with a photographic plate.

5 - CONCLUSION

The use of an E.S. quadrupole doublet close after the focal plane and working in connection with either a post-acceleration system or a retardation collector as outlined above, provides a very convenient and more practical means in beam-handling and focusing related to collection problems. In the future, this set-up will be extensively used in all post- accelerated and decelerated collections and systematic investigations in building-up of stable retarded isotope thick targets will be performed.

REFERENCES

1 - R. Bernas, J.L. Sarrouy, J. Camplan, J. Phys. Radium, 2J_, 101 A (1960) 2 - P. Bautz, B.A. Brandt, H. Wagner, Proc. Int. Conf. EMIS, Marburg (I97O)p.337 3 - H.A. Enge, Rev. Sci. Inst., W, 248 (1959). 4 - B. de Raad et al., CERN. 66-21. 23 June 1966. 5 - K.G. Steffen, High Energy Beam Optics. Interscience XVII - John Wiley and Sons, New-York. 6 - H. Bernas and J. Obert, Nucl. Inst. Meth., U)]_, 423 (1973). 7 - J. Uhler, Arkiv f. Fysik, 24_, 349 (1963). 8 - J.S. Dionisio et D.X. de Lima, Nucl. Inst. Meth., 61, 260 (1968). 9 - A. Fontell and E. Arminen, Ann. Acad. Sci. FENNICAE, A. VI, 244 (1967) -369-

10 - A. Fontell and E. Arninen, Can. J. Physics, 47, 2405 (1969). 11 - W. Berg, P. Kilcher and P. Paris - unpublished work. 12 - J.S. Dionisio, A. Marion, Ch. Vieu in Annuaire 1968-69 - the two latter ref. from "Centre de Spectrometrie Nuc.lSaire et dc Spectromc'trie de Masse - CNRS - Orsay, France".

figure 1

Figure 1 : 1. focal slit - 2/3. Quadrupole doublet - 4. diaphragm unit 5. retardation system - 6. motor: The overall lenght is about 55 cm -370- figure 2

figure 2 : 1. earth shield 2. H.V. shield 3. retardation nlates 4. tarqet holder and rai1 5. heart-shaned cam 6. insulatinq rod

figure 3

Figure 3 : 1. slit - 2. electron repeller - 3. retardation electrodes - 4. target holder - 5. nylon rod - 6. valve - 7. vacuum lock -371- RAPID CHEMICAL SEPARATION TECHNIQUES COMSINED WITH AN ISOL-SYSTEM

B. Grapengiesser and G. Rudstam \ The Swedish Research Councils' Laboratory Studsvik, Fack S-611 01 Nykoping 1

1. INTRODUCTION

In considering various methods of fast chemical separation we have found thermochromatography to be a very promising one. Earlier measurements at this laboratory have demonstrated it to be well suited for a combination with on-line isotope separators. We have chosen to carry out the chemical separation after the mass-separation although the other alternative should also be possible.

2. DESCRIPTION OF THE EXPERIMENTAL EQUIPMENT

The basis for the separation method is that there exists a fairly well-defined temperature, the deposition temperature, at which a gaseous chemical compound begins to adhere to a surface. This deposition temperature depends both on the nature of the chemical compound and on the material of the surface. If the wall is changed, for instance by building up a layer of adsorbed molecules, the deposition temperature might change. Thus micro and macro amounts of the same chemical compound might have different deposition temperatures, and one would always expect the depo- <. sition temperature of micro amounts (in this context amounts too small to build up a monomolecular layer on the surface) to be higher or equal to , that of macro amounts.

t J$ The separation apparatus ("thermoseparator") is shown in Fig. 1 together with a typical temperature distribution. It consists of two con- *V1 centric quartz tubes (length 800 mm, dia 20 ram and 10 mm, respectively), , the outer one surrounded by heating coils. The heat from these coils is ^ smoothed by a layer of copper foil between the quartz and the coils. A V copper heat shield and a steel tube surround the coils. The steel tube * can be moved axially through a hole in the collector chamber wall of the ^ isotope separator, vacuum sealing being achieved by means of two 0-rings. -372-

j^ _y j

50 ICO ." 7u0 730 EO0 mm

600

O—O—O o .o 400 -•

UJ Icc uj 200

J I I I 200 600 800 LENGTH mm

Fig. 1. Thermoseparator with temperature distribution

One end of the apparatus is provided with a narrow entrance tub?:. It is positioned so as to allow a selected ion beam to enter and to hit a collector placed in an extension of the concentric tubes. In order not to disturbe other experiments a geometrical arrangement is chosen so that most other mass beams pass freely. The entrance tube can be blocked by means of an electromechanical shutter.

The choice of collector material depends on wu-ther or not a chemical reaction between the material and the incoming beam is desired. In the simplest case the collector is inert and is kept hot so that the ions, after being stopped and neutralized, rapidly diffuse to the surface and evaporate. Suitable materials are graphite, molybdenum, or tungsten. Foils -373- or filaments of thes- materials ara than, mounted between t*:o electrodes and heated by direct current. This method is suitable only for few elements, however, because the deposition temperature of the ator-.s evaporated is usually high. As the incoming beams are carrier-free, there is no possibility of forming more volatile di- or polyatomic molecules. Thus, the method can with the present apparatus only be used if the element of interest, in tha carrisr-free elemental fora, has a deposition temperature less than about 400 C. Unless the collector is kept very hot, the diffusion to the surface will be the process determining the rapidity of the chemical separation procedure.

In order to increase th^ number of accessible elements one may use a collector material in whicli thi inconing atoms form volatile compounds by hot atora reactions. We have ir.vestigated sodium chloride for the production of volatile chlorides, but i v:.d» variety of other materials are also possible. Unfortunately, tha diffusion rate of large halida molecules is expected to be low at temperatures below the nelting point of the collector materir.l, and this neans a slow separation. Therefore, one should not rely upon diffusion in this case. A better approach is to adjust the temperature so that the collector caterial sublimates at a suitable rate liberating the compounds trapped. The rapidity of the separation method is then determined mainly by the tima for sublimating a layer of the collector material corresponding to the penetration depth of the incoming ions. Obviously, the sublimation rate should not be too high as the collectors might then hava to be replaced at too frequent intervals.

The other end of the separation apparatus contains a section with a catcher whose temperature can be adjusted and thermostated in the range from -100°C to 350°C. Tha material of the catcher has been platinum in the present experiments. A steep temperature drop between the main oven temperature and the catcher temperature is en advantage so that the compound of interest is deposited on the catcher ar.d not on parts of the quartz tube.

Measuring equipment is placed behind the catcher and as close to it as possible. The window in between is thin so that efficient beta detection can be performed. For Tray spectroscopy Nal- or Ge(Li)-detectors have been used. -374-

The temperature of the collector surface is measured using an optical pyrometer. With the hulp af a mirror arrangement the collector is viewed through the entrance tube in such a way that the measurement can be done during the run. The temperature of the other parts of the system is measured by means of thermocouples.

3. DEPOSITION TEMPERATURES

Obviously, essential input data for the use of the separation apparatus are the deposition temperatures and, in the on-line application described here, notably those of micro amounts of species in the elemental state or of molecules containing only one atom of the element of interest. An extensive study of deposition temperatures has been carried out using the thermochromatograph described in ref. ' provided with an external oven which allows heating the sample to above 2000°C . Some relevant deposition temperatures are extracted from ref.2) and given in Table 1.

Table 1

Deposition temperatures of carrier-free elemental germanium, arsenic, bromine, antimony, tellurium, and iodine on quartz and platinum

Element Tracer DepositioDeposi n temperature, °C nuclide quartz platinum

Germanium ™Ge 570+30 > Arsenic 78Aa 700

Bromine 8"Br * 200 z 600

Antimony 130Sb > 800

Tellurium 133Te 570+30

Iodine 135l 150+50 > 525

Xenon 135Xe < - 50 -375-

4. EXPERIMENTAL RESULTS FOR BROMINE AND IODINE ISOTOPES

A series of experiments have been carried out with bromine and iodine isotopes produced at OSIRIS. For iodine Table 1 shows that with the catcher kept at 140°C and the main oven at 400°C a pure sample should be formed. This proved to be the case. At mass number 135 no traces of antimony or tellurium activities were seen in the catcher position, and in the mass range 137 - 140 neither xenon, cesium, nor barium activities were found. This also proves that the decay product xenon is continuously and efficiently removed from the catcher position (in spite of possible beta- recoil catching effects).

Using a molybdenum foil as collector the saturation activities of the isotopes 136 I, 137I, and138 I have been measured as a function of the collector temperature under otherwise identical running conditions. The results are presented in Fig. 2. Evidently, a plateau must be reached at high temperature. Such a plateau was found for138 I. From these measurenents the diffusion constant can be evaluated (cf. ^

i£oo zooo TEMPERATURE t

Fig. 2 Saturation activity of three iodine isotopes as a function of the foil temperature -376- With the diffusion constant3 known the activity at saturation can be calculated as a function of the half-life of the isotope studied. This has been done for iodine ions impinging into molybdenum held at three typical temperatures. The results are given in Fig. 3,

Bromine isotopes were also studied in a series of measurements using the same temperatures of catcher and oven as in the iodine case. The efficiency of the isotope separator is very low for selenium, and this element is not expected to interfere in the measurements. According to Table 1, no other parent elements are expected to interfere either, and the temperature settings chosen should therefore ascertain pure bromine samples. This proved to be the case. No activity of the parents (germanium, arsenic, and selenium),nor of the daughter (krypton) was found.

0.01

01 12 5 10 100 HALF-LIFE seconds

Fig. 3. The ratio between the disintegration rate at the catcher (An.) and the rate of atoms hitting the collector (M) divided by the effi-

ciency (n) is plotted versus the half-life for three values of D (in m /a). In the case of iodine diffusing in molybdenum these values are typical for 1600°C, 1800°C and 2000°C respectively. -377-

Figure 3 shows th.nt the separation apparatus should be useful also for quite short-lived apecies. In experiments designed to test its performance in this respect the iodine isotopes (half-life within brackets) of mass number 134 (52 nin), 136 (83 sec and 48 sec), 137 (2b sec), 138 (6.6 sec), and 139 (2.5 sec) were easily detected, and a ..«ak activity at mass number 140 (0.86 sec) was also found. Similarly, the bromine isotopes of mass number 86 (56 sec), 87 (56 sec), 88 (17 sac), and 89 (4.6 sec) were easily seen, and a weak activity was found at mas* number 90 (1.6 sec).

The efficiency(n)of the thermoseparator was determined experimentally yielding results in the range 5 - 20 %. A calculation taking into account the geometrical dimensions of the apparatus has indicated a value of 7.5 %

5. DISCUSSION AND FUTURE PLANS

The separation method as described in the present article has proved to be well suited for use in an ISOL-system. So far, only bromine and iodine have been studied with the apparatus, but work is going on to apply the method to a larger range of elements. In most cases the deposition temperature for the elemental state will probably be inconveniently high. Then hot atom reactions in the collector material will have to be used to produce more volatile compounds of the elements of interest. Thus the sublimation method is likely to become very important. In this connection it might be mentioned that a rapid evaporation from solid halides such as uranium tetrachloride or uranium tetra- 4 5) fluoride has been found to take place ' . Such a collector material might therefore be of great interest in a therraoseparator. REFERENCES

1) L. Westgaard, G. Rudstam, and O.C. Jonsson, J. Inorg. Nucl. Chem. Jtt, 3747 (1969).

2) G. Rudstam and B. Grapengiesser, The Swedish Research Councils' Laboratory Report LF-44 (19^3).

3) B. Graper.giesser and G. Rudstanu The Swedish Research Councils' Laboratory Report LF-43 (1973).

4) M. Weber, N. Trautmann, and G. Herrmann, Radioehem. Radioanal. Lett. 6^ 73 (1971).

5) B. Neidhart, K. Bachmann, S. Kramer, and I. Link, Radiochem. Radio- anal. Lett. 12, 59 (1972). -3 79-

ON-LINE ELEMENT SEPARATION AT THE COLLECTOR END

P. Hornshtfj

Institute of Physicss University of Aarhus, Denmark

B. Jonson, H.L. Ravn and L. Westgaard CERN, Geneva, Switzerland

The number of elements available from on-line mass separation has increased rapidly in recent years. At the ISOLDE facility1^ at CERN more than 30 elements can be obtained as primary products ' , and another 10 can be obtained as decay products of those. In this contribution we consider the situation when it becomes desirable to perform an additional element separation at the collector end of the separator. This will often become necessary when several elements are separated simultaneously, especially in order to observe isotopes far from stability, which will have much smaller yields than those nearer stability. One may also consider the situation when unfavourable half-life relations make chemical separation of a daughter radioactivity necessary.

On-line chemical separation methods based on thermochromatography are 1,) being developed by the Studsvik group . In this paper we describe a simple apparatus for on-line separation of mercury and platinum. The method, which exploits the difference in vapour pressures of the two elements, may evidently be adapted to other combinations of elements (or maybe compounds) which show a sufficiently great difference in their evaporation rates from the collector foil.

The separation apparatus is shown in the figure. The ion beam from the isotope separator is collected on a thin (0.02 mm) gold foil, four of which are mounted in a windmill-like arrangement at right angles to the beam plane. After the end of the collection period the windmill is turned 90° by means o'" a stepping motor, so as to bring the foil in front of a small heating lamp (and at the same time the next foil is shifted to the collecting position). The lamp (OSRAM 64635 15 V/1.50 W) gives a hot spot about 5 mm in diameter. Due to reflection losses the desired temperature (700-800°C) could only be reached after the deposition of a layer of carbon on the side of the gold foil facing the lamp. A thin sheet of molybdenum -5H0-

STEPPING / MOTOR

H EAT ING LAMP COLLECTOR FOIL

DETECTOR

On-line element separation apparatus is used as support for the collector. Such a mounting gives a small heat capacity which is essential in order to avoid that the detector is over- heated when the collector, after a short cooling interval, is stepped to the measuring position.

A series of test experiments with gold foils on which mercury, gold (daughter), and platinum (grand-daughter) activity had been collected, have been performed off-line. Using a heating period of ^ 4 sec and a temperature of 800°C, less than 0.1% of the mercury activity and more than 90% of the platinum were left on the foil. Only one test has been made on-line: for 183Hg we used a time sequence of 10 sec collection, 5 sec heating, and 5 sec cooling. About 10% of the mercury activity remained after heating. A small misalignment between the heated area and the collected activity may be the reason for the large retention. A more precise definition of the beam spot, for example by the introduction of a narrow collimator in front of the collector, will presumably lead to a separation factor closer to what has been observed in the off-line measurements. -381-

REFERENCES

1) The ISOLDE isotope separator on-line facility at CERN (Eds. A. Kjelberg and G. P.udstam), CERN 70-3 (1970).

2) H.L. Ravn, S. Sundell and L. Westgaard, these Proceedings.

3) F. Hansen, A. Lindahl, O.B. Nielsen and G. Sldenius, these Proceedings.

4) B. Grapengiesser and G. Rudstam, these Proceedings. -382-

THE BEAM HANDLING SYSTEM FOR ISOLDE 2

E. Kugler

CERN, Geneva, Switzerland

1. INTRODUCTION

In the new ISOLDE facility1 the collector chamber of the isotope separator will be enclosed between two shielding walls and will not be accessible during runs. Experiments in the collector chamber will therefore largely be restricted to collections of long-lived isotopes. Thus all other experiments will depend on the extraction of a number i,f ion beams and an efficient transport of these beams to the various collector positions in the experimental area.

Each one of the four beam lines will at a given time accept only one isotopic beam, but the new switchyard described below gives an alrcost free choice over a wide range of isotopes without any change of the mass spectrum in the separator itself.

In the new long secondary beam tubes, however, the space-charge effect is negligible because of the extremely low currents of the radioactive beams and should still be small even with stable beams of up to 0.1 ]iA usnd for testing and calibration.

2. THE SWITCHYARD

For the new switchyard2 (Fig. 1) pairs of electrostatic deflector plates with narrow spacing will be used. This type of deflector has been successfully used in the old ISOLDE facility. The cylindrical plates of radius 400 mm and height 100 nun have only 10 mm spacing, just sufficient to accept one ion beam. The angles of deflection are 13°, 26°, 36° and 56°. The voltages which have to be applied to the deflector plates are of the order of ±1.5 kV for 60 keV ions. Grounded shields are used on all deflectors to eliminate interference with neighbouring beams.

The deflector plates can each be moved independently along the direction of the deflected beam in order to intercept different isobaric beams. This movement guarantees constant alignment with the lenses in the beam lines, and allows the experimenter to cover a range of 200 mm across the direction o£ the ion beam in the collector chamber. (This corresponds to 20 Hg isotopes being accessible without change of the mass spectrum.) Owing to the fact that the beams of different isotopes are not parallel (the variation is approximately .j' over the whole width of the collector chamber), small variations of the deflecting voltage will be necessary when changing from one isotope to the; next one.

Thus the switchyard deflector plates represent an optical element which varies in position an^ in power. Therefore the lenses which follow the switchyard have to be very flexible.

3. THE ELECTROSTATIC QUADRUPOIE LENSES

A symmetric triplet configuration (Fig. 2) was chosen in which the two outer quadrupoles have half the length of the central element and are kept at the same potentials. The physical dimensions of the lens are: aperture 40 mm, electrode diameter 46 mm, length of the outer elements 80 mm, length of the central element 160 mm. Guard rings (length 30 mm) are inserted between the quadrupoles with the aim of defining the fringing field in a better way and in order to improve the mechanical stability of the lens. The vacuum chamber is easily demounted to facilitate alignment and cleaning of the lens.

4. THE OPTICAL LAYOUT

Numerous beam calculations3 resulted in the optical layout shown in Fig. 3. The objects for all calculations were always assumed to be waists in the beam of diameter 2 mm, situated in the focal plane of the isotope separator. (The system of beam scanner and sniffer will in the future be moved along the focal plane in the collector chamber.) The initial beam divergences were estimated from the geometrical dimensions of the separator and were of the order of approximately 2 mrad.

The first quadrupole lenses will be adjusted to give almost parallel beams. The voltages of these lenses are expected to vary between 1 and 2.5 kV, depending on the setting of the switchyard. Another set of lenses will then be used for refocusing the beam at the end of each beam line (focal point 0.4 in behind the lens). - 3 B 4 -

In beam line 2 as well as in beam line 3, intermediate lenses L2B and L3B are foreseen to focus the beam on collectors for off-line work. The plans foresee the installation of a second analysing magnet in beam line 2; it may then be necessary to move the lens L2B to the position indicated on Fig. 3 in order to obtain (together with the action of magnet M2) a line focus in the slit position.

It is expected that the beams in the system will nowhere exceed 20 mm in diameter. In the collector positions the beams should theoretically be focused to waists of less than 1 mm diameter.

5. FUTURE DEVELOPMENT

It is hoped thac the second analysing magnet M2, with even comparatively small dispersion, should clean the beam from contamination of isotopes nearer stability, which are produced in much higher yield. Improvements in the purity are essential for work on isotopes very far away from the line of stability, A preliminary calculation with a homogeneous sector magnet (1 m radius, 30° bending angle) showed that in combination with lens L2B a separation of mercury isotopes in the order of the line width could be reached. The beam can at a later stage be transported to the next floor, where the background is lower.

6. BEAM TESTS

A test line was built up consisting of a pair of deflector plates with angle 35 , a quadrupole lens, and 4.80 m of beam tube, which is followed by another quadrupole lens. The beam could then be observed with the help of an x-y scanner. The performance of the lenses was excellent. The best focusing with a spot diameter of 1-2 mm was achieved with lens voltages which fell within 3% of the calculated values. ivi.ij rt

COLLECTOR TANK SWITCH YARD

03 I

-200 + 250

Fig. 1 Electrostatic switchyard - 3 a 6 -

S886S

o 3 U

60 -387-

ra u

D- O - 3 b H -

REFERENCES AND FOOTNOTES

1) S. Sundell, P.G. Hansen, B. Jonson, E. Kugler, H.L. Ravn and L. Westgaard, The reconstructed ISOLDE facility at CERN, Con- tribution to this conference.

2) I am indebted to Dr. G. Andersson for valuable discussions on the subject.

3) B. Hedin, Computer program BEAMOP, Internal Report CERN-MSC-71-6 (1971). :c:.: OF u .,:,j A l-'O.t

WITH L UP TO ~i>o ?.Y CO».JI;;AJIC;. c/ J^ ...

AIU-OA'JID A;JD Tiki aE,ji ...Ji'iiCU

,, .J.-.rtukh, v. /...vaeic.iikov, .;.^rb*, C-. „•'.(.:- v' .L.i..ikheev , 7. /. vrolkov, J.'..ilczynski

Laboratory of Kuclear Reactions, Joint Institute

for Nuclear Research, Jubna, ,: ,ufl.

i. Introduction

'i'he use of on-line mass-separators at accelerators and in reactors offers a variety of possibilities for trie identification of new nuclei and the study of their properties, r.owever tne ion sources of these mass-separators are, as a rule, suitable for tne study of only certain chemical elements, while versatile ion sources maice it difficult to estaolisn z of tne nuclei under investigation. ;i'hese difficulties are especi'.liy significant in the case of the isotopes very remote from the beta st'oility line because their production cross sections are small. Juerefore, special interest is taken in the methods facilitating the simultaneous determination of Z ana •> of nuclei, irrespective ol their cneraical properties.

,i method of charged particle identification is described nere which is Based on tne combination of the magnetic analysis and tfteAii,r: methoa/:L'2/. ;, telescope of thin and tnick semiconductor detectors is put into the focal piano of tho. mrpolic analyzer.

Z. apparatus and the method of t.ieasuremei.es rhe magnetic analyzer with a homogeneous field and the aouole focusing of the second order was a version of tne -590- analyser developed earlier for analyzing 40U iv.e.'/c electrons It has the following oasic parameters:

-;adius of central trajectory 1.26 m je'.'lection angle 70

B IB kli max Jolid angle 3xlO~J sr "omentum dispersion 12.7 tnm/lCj i.oinentum resolving power with a source 10 mm in diameter 0.3-/J i'he analyzer magnet is of armour type, it weighs 40 tons. Jhe magnetic field is stabilized within 10 \o and measurea oy tne nuclear resonance method. Jhe silicon surface-barrier detectors && are 26-2UU pm tnick, plane-parallelism is ~ 0.1 Mm and tne sensitive region is 10-^5 rnr.- in diameter. lJulses from the^B and JS detectors after amplification are fed into a converter for two-parameter pulse- hei£ht analysis, i'he chosen region of the two-dimensional Ckti, £U-Aii spectrum is recorded, in the 4096-cnannel analyzer, operating in the 64x64 channel mode. Trie two-dimensional amplitude spectrum obtained in bonfeardmeri of . the 232Th target 'J mg/cm2 thick, with 290 i,:e7 40^r ions accelerated in the Uuona 310 cm cyclotron is snown in /i^. 1. Jhe detection angle of the nuclear reaction products is 40°. itie ^^ detector is 36 p thick, •i'n.e magnetic field is 7.27 k3. Jhe identification of tne maxima was carried out as follows, xhe energy of all particles, which passed through tne magnetic analyzer and hitted the telescope, had fixel values determined oy equation

where jf; is magnetic rigidity, ^i is the ion charge, j-.nowing - 3 9 1 -

tnese energies and using the range-energy curves one can determine the energies released by a deiinite isotope in theAi and H detectors and, consequently, establish its position on tne Aii,i) plane, x'he energy calibration of tne *i and ± detectors was carried out according to elastically scattered ions. The initial energies of all particles were also aeterminea according to relation (1) with respect to tne energy o± tne elasticr.ily scattered ions, This made it possible to reduce considerably the influence of errors in the determination of ion initial energy ana the &E, detector thielcness on tne accuracy of identification. i'he discrepancy between the calculated position of t;.e ma:;ima and the experimental one (.b'ig. i; is witain one channel in both A-Eand ~& . l''ig. 2 snows the data aedcribing the JJ separation of the Ar-Lg isotopes, iiach point has been ootained by summing up the

counts in the AE channels, corresponding to a deiinite element in the fixed charge state in tne two-dimensional spectrum shown in i''ig. 1.

3. t..ass reparation

On the basis of the Beth.e-j.loch formula at&J « i.Q we get o A E * E^- const A-Z . C <-; /or our method, from formulas (l) and (2) for isotopes in a given charge state and with fixed magnetic rigidity, we ootnin

.. comparison of (3) and &) snows tnat tne difference in

the specific ionization of the neignoouring isotopes of one

element is increased twofold when our metnoct is used, compared

to tne conventional ^E,E metnod. -392-

±ne relative variation of E for tne isotopes of a given element depends on the oE detector thickness, ass

E = E0~AE= COnst/A-ConstA according to relations (1) ana O) one can easily show that

dE _ __ (E0/lE)+2 adA

E " (E0/AE)-1 "A

vhe expressions for ^E and dii/E can also oe ootained by using the empirical range-energy relation -173 R=a€" ^;

_he obtained expressions are clumsisr than relations (3) and ^4), out provide close results, rhis makes it possiDle to use relations (3) and (4) also .Cor rather thick AE detectors.

The peak width in our spactra is determined by the statistical spread of pulse amplitudes in the AE and £ detectors and by the target and detector dimensions, oince the coefficient of

Horizontal magnification of our magnetic analyzer is u.36 and momentum dispersion is l

10 mm in uiameter, provide an amplitude spread less than 2yi in tne 6ii and E detectors.

/ig. 3 shows the spread of energy losses in the ,-\ j£ detectors as a function of the initial energy lost oy the particle in the detector. The measurements have been performed at the magnetic analyzer exit with a 3 mm slit.using Ar and P ions accelerated in the 310-cm heavy ion cyclotron, i'he spread of

energy losses in the *E detectori has been determined from measurements of the full width of peaks nt half-maximum

in the total-absorption detector E placed behind the ^E

detector. -393- following relation has been used

4u •ine experiments yielded j^ =255 ^e/ and 0^IJO>2.D i..e/ for ..r

31 and Eo=178 i.e/ and £"(£<,) =1.9 ...e-f for P.

1 i he optimal energy resolution of the A£, detector is ootained when its thickness corresponds to the absorption of ~4O.-' of the initial energy, 'x'he dotted line of fig. 3 snows tne 40 theoretical energy resolution that can oe obtained using j.r

ions on the basis of C .'fsehalar' s work/4>5'// without talcing into

account the changes in the Initial particle charge, me contri-

bution to the experimental energy resolution from inhomogeneities

in detector thickness in the region ot&2/$o £, 0.4 does not exceed 0.5^. 'i'his value has been determined from the experiments

on the irradiation of the various points on the sensitive surface

with alpha particles, .he calculated contrioution to tne energy

resolution from the multiple scattering leauing to an increase

in tne *£ detector effective thicKness aoes net exceed a few

tenth percent. 1-he main contribution to tue energy resolution

is apparently made by fluctuations in the effective uuarge of

particles to be detected. ..hen silicon detectors are used, the channeling effects play an important part especially in the case of tae .i-ulta- neous ueteotion of isotopes with sharply different yielas.

,iS. 4 shows the effect ox cnanging the angle oetween t««A.i aetector plane and the main particle trajectory in tae exit ox t*e magnetic analyzer. *ae «* detector plane turn, around an axis ,hich is perpendicular to the median plane of the analyser •n a ^"e ions with an ana goes througH the deteotor centre. -,.e -= ot 161 ,e, have "oeen «t..t... , * *

thick. -394- 4. Application of the Liethod Cur technique has oeen usea to identify neutron-rich isotopes of light elements in experiments with laeavyions accelerate: in the uubna 310-cm cyclotron. In these experiments the new 18 20,21. 22-24 23-25,, 25,26.. 29,30., 31-33., CP , ' .., 0n , i, ' Ue, » i,,g, AI, 33~363i, 35~38P, 39)403, 41'42C1 isotopes have been obtained in bombardment of 232Th with 180. 22Ne,'40x..r ions7'1'6'77'. Our

method was also used for determination of the ground state •i-i po /R/ masses of ' 0 from Q-value measurements' '.

Keferences

1. _••.. C-.Artukh, G.x''.(jridnev, V.L.^ikheev, v.7.7olkov. .jucl. i'hys.,

^137, 348 (1969).

2. A.ir.Artukh, \T.V.Avdeichikov, J.Ero, (i.^'.Oridnev, /.L.j.iikheev, /./.Volkov. i'lucl. Instr. and 1,-Ieth., 83, 72 (197U). J. .i.u.Afanasiev, Y.A.Goldstein, G.A.,.,avitski, z., •/ .ijtepula,

8. i^.G.Artukh, •J.i'.G-ridnev, v.L.r.akheev, '/. V. volkov , ! J ..•ilczynaki. imcl. Phys., ^192, 170 (1972). [ -395-

iVig.l. iWo-dimensionalAiijE -aifi spectrum obtained in bombard ? TP ' 40 raent of a J Th target with 290 iv.eV ^u.ir ions.

i'ig. 2. Yields of i,g, Al, ,i, i',o, 01 and ,r proauced in tne 232!'h + 40Ar reaction at E=290 me/ and 0 -40°. _ ••; n, .

Fig. 3. The energy resolution of & £ silicon detectors as a function of the initial energy lost by the particle in the detector, xhe dotted line shows the theoretical energy • resolution for ^ ions on the basis of C.Tschalar's work'

r'ig.4. The cnanneling effects as a function of the angle between the AE silicon detector plane and the

9=-5° • main particle trajec-

CHANNELS S W7lf P, -397-

!'NOVEL ION BEAM HANDLING AND COLLECTION TECHNIQUES AT THE TRISTAN FACILITY"

W. L. Talbert, Jr., J. R. McConnell, J. K. Halbig, and G. A. Sleege Ames Laboratory, USAEC and Department of Physics Iowa State University, Ames, Iowa 50010

As experimental conditions become more demanding at the TRISTAN facility, it has been necessary to continue development of techniques to accommodate the studies under way. This report will briefly describe

four new facility configurations which have become available in the past

year.

As the studies at TRISTAN have progressed to the point of pressing

the limit of low yield and short half-life! the use of stable masses to judge focusing conditions in the separator has become isss possible. This

is so since, in many studies, no stable mass beams appear at the collector

within the mass range of the separator. A beam position stabilizer has

been in use for some time which has proved to be successful for beams

down to a few pA. In order to improve ion beam scanning sensitivity, an

amplifier (output/input=l V/nA) has been used with mechanical scanners

with reasonable profiles for nA beams. This amplifier has now been in-

stalled at a non-mechanical (electrostatic deflection) scanner which

intercepts a beam other than the one being used further in on-line

studies, to obtain beam profiles down to the 100 pA region.

The scanner consists of two plates, 18 cm by 5 cm, with 1.2 cm

separation and formed of double-sided printed circuit board, with the

outside surfaces serving as grounded shields. A simple Faraday cup be-

hind a 0.5 mm slit is situated 5-5 cm beyond the deflection electrodes. The construction is such that all but the aperture is covered by a grounded

shield to avoid effects of adjacent beams. A satisfactory scan is achieved

with about 200 V peak-to-peak symmetric driving of the deflection plates.

In practice, the ion beams are first focused using the visual profiles of

stable mass beams, then positionstabilizedto the inass value of interest,

and finally an adjacent beam scanned for fine adjustment of beam quality.

A "beam skimmer'1 has been developed to control the separator vertical

deflection in order that constant activity may be obtained for experiments

in which a continuously moving tape is utilized. The basic principle of

the skimmer is to compare the actual activity digitally with a preselected

O AU ro

-I5V -±r

BEAM SKIMMER

Fig. 1. Block d?agram of beam skimmer electronics. -399-

% rate, and to adjust the separator vertical deflection to change the de- i? posited beam intensity. This procedure simply automates the traditional

\i practice of lowering the viewing screen in the collector box of the sep-

•y[ arator to adjust the beam quantity deposited at the moving tape collector.

•1 A block diagram of the skimmer is shown in Fig. 1. The count input

|; from a discriminator is scaled and compared to 1% of the selected count

f rate. Deviations with the selected count rate exceeding \% are converted

;: into analog signals which are used to change the deflection plate voltage

s (the multiplier scales down the analog voltage to about 1% of that cor-

:=; responding to the existing deflection plate voltage).

S More significant to the overall TRISTAN facility has been the devcl-

$ opment of two new moving tape collectors, one of which was adapted spec-

:\ ifically for use on a high-resolution n^/2 (3-ray spectrometer. Both units

;J use essentially the same transport configuration, in which large reels for

V; tape supply and take-up (35.6 cm in diameter and with a capacity of about

;i 2000 m of tape) are mounted in a separate vacuum chamber. Special fi-aturc

:; of the tape collector for the 3-ray spectrometer will be discussed later,

s and the essential features of that used in normal spectroscopic studies

Vft are outlined below.

•S One of the characteristics desired for this tape collector was that fit of flexibility. For this reason, a modular concept was usec in its de- lf sign. The tape transport, tape supply, and tape Vake-up system was built

H as one unit, to be connected to the experimental ports by connecting vacuum lubes. The tape transport is a capstan driven by a stepping motor to make possible accurate control of tape speed and position. The capstan is driven through a gear box with three gear changes, and is capable of moving the tape up to 60 cm/sec. The loop of tape being driven is maintained at a constant tension of 170 g by tension arms which also actuate the step motor drives for the motions of the supply and take-up reels. The capstan and tape reels can be driven in reverse, allowing the tape to be rewound in about 70 min. A 260 1/s turbomoI ecular pump is attached to the tape vacuum chamber and the large vacuum connection pipes to the detector modules provide good pumping conduction to detectors located within the vacuum system.

In the design of the detector stations, it was decided to provide for as many detector ports as possible, with interchangeabi1ity between

the first two stations. The first station has four ports in addition to the beam input line, the second station has five ports, and the third station is equipped with four ports. While the ports are not unusual in de-

sign, they can be fitted with re-entrant cups having thin windows, or

they can be blanked off or used to mount special detectors. While no

shielding is included in the design of the detector stations, a total of

100 cm shielding can be placed between the detector stations and the tape

take-up reel, and 28 cm shielding space exists between the first two de-

tector stations. An external stand serves to hold the desired experi-

mental shielding arrangements. The third detector station is regarded

as adjustable in distanc from the second station, as well as being

adaptable for special purpose detection, such as kn$ counting or an angular

correlat ion array. -401-

The new moving tape collector is shown in Fig. 2, which is a photograph taken before it was mounted on TRISTAN. As mounted, the tape vacuum chamber is fastened to the floor.

Using essentially the same transport system, a tape collector has been installed in the high-resolution TV'2 P-ray spectrometer to enable for the first time, on-line 0-ray spectrometry of mass-separated activities.

The two modes of the moving tape collector are indicated in Fig. 3, in which it is assumed that the tape is used with discontinuous motion only.

For the parent enhancement arrangement, the spectrometer is programmed to take data as the ion beam is collected. The beam is deflected and tape moved before daughter activities build up at the deposition point. In the daughter enhancement mode, the collected activity is delayed to allow the daughter to build up, and moved to the spectrometer source position.

In order to maintain a constant source position in the spectrometer,

regardless of spectrometer field setting, the ion beam is position

stabilized during collection. This is made possible by a groove made

down the middle for the length of the tape, so that each half of the tape

functions as a stabilizer pin current pick-off. Thus, the tape, which is

0.025 mm mylar bonded to 0.009 mm aluminum, has a groove 0.3 mm wide and

0.0I mm deep to separate the two halves electrically. Graphite current

pickoffs are usea for the stabilizer inputs, and stabilization is achieved

by control of the switching magnet deflection (and therefore the ion beam

entry into the spectrometer). A picture of the tape collector is shown Fig. 2. Moving tape collector ION ION BEAM BEAM BETA-SPECTROMETER BETA-SPECTROMETER^

Fig. 3. S-ray spectrometer tape collector modes. To left, parent enhancement; to right, daughter enhancement. in Fig. k, which features the tape transport box (at the top) and the "snout-1 lo direct the tape into the spectrometer source position.

With the four improvements described above, the TRISTAN facility prov'des capabilities to perform a variety of "mportant measurements in the study of short-lived radioactivities.

Fig. k. (3-ray spectrometer tape col lector. -40 5-

IMAM3 i''OR 011-LINE ATOMIC-BEAM EXPERIMENT- AT TIE; 1G0UX: FAO1 !,J :<^'

C. Ekstrom and I. Lindgren Department of Physics. Chalmers University of Technology, Goteborg Sweden M. Olsmats Institute of Physics, University of Uppsala, Uppsala, Sweden

1. Introduction

The atomic-beam magnetic resonance (ABMR) method has been used for measurements of spins and moments of radioactive nuclei for about two decades, and during that time more than 300 nuclei have been subject to 1 °} investigation ' . Obviously, the results obtained have had a great impact on our present knowledge of the nuclear structure. This is especially the case in regions where more systematic investigations have been performed. So far, practically all atomic-beam experiments have been of conventional off-line type. A natural extension of this kind of research would be experiments of ou-line type with an atomic-beam machine connected directly to the isotope producing facility. It would then be possible to go further down in half-life, hence reaching nuclei farther away from the region of beta-stability. Two production methods are generally used to reach such nuclei, spallation and fission. In both cases a mixture of a large number of nuclei is normally obtained, and an intermediate isotope sepevratiou step is required in order to make an atomic-beam investigation meaningful„ The atomic-beam apparatus at Uppsala is presently being reconstructed for on-line experiments on isotope-separated spallation-productE in connection with the ISOLDE facility at CERU, Our time schedule follow closely that of ISOLDS II j reconstruction of the system during 1973, installation during the spring 1974, and tests of the entire system during the fall 1974. The minimum production rate required for a spin determination with the ABMR method is in a typical case of the order 10 -10° atoms/sec ' . The higher beam intensity expected after the reconstruction of the CERM synchro-cyclotron, together with further developments of the present target system, will make accessible a large number of isotopes for ABMR investigations. -406-

2. The atomio-beaia magnetic resonance method

The principal mechanical design of a focusing atomic-beam apparatus is shown in Pig. 1 (top). The bottom part of the figure shO7/s the beam trajectories for atoms at resonance (with an exaggerated radial scale).

COLLECTOR DISK

!URNA3l£ PLATE

MAIN OVEN CALIBRATION CNEM

Fig. 1

In a conventional experiment a beam of free atoms, formed by evaporation of the sample material in the main oven, traverses the main chamber of the atomic-beam apparatus. The strong inhomogeneous fields of the focusing A- and B-magnets, acting on the magnetic moment of the atoms, deflect the beam in such a way that it will not reach the collector unless a change in sign of the magnetic moment is induced by a radio-frequency field superimposed on the static homogeneous C-field. Information on the hyperfine structure of the atoms is obtained from the experimentally determined resonance frequencies. In weak magnetic fields (Zeeman effect for the hyperfine structure) the experimental resonance frequencies give directly information on the nuclear spin since the electronic quantities involved are known in advance. The magnetic dipole and electric quadrupole hyperfine constants, and from them the corresponding nuclear moments, can be determined by observing the higher order contributions to the resonance frequency in successively stronger fields. -407-

5. On-line application ofABl.ffi at ISOLDE

The planned location of the experimental equipments in the hall after the reconstruction is indicated in ?ig. 2.

The two set-ups intended for measurements of nuclear spins and moments, optical pumping (OP) and atomic-beam magnetic resonance ('JIE:), will share one beam tube on the heavy mass side. This is a natural position oince the experiments will be performed mainly on nuclei not too Car from the beta-stability line (as far as concerns the Mfli experiments). The most difficult problem in the on-line application of atomic- beam experiments at ISOLDE will be the focussing of the 60 keV ion beam from the isotope separator on a small spot at the oven position of the atomic-beam apparatus, and the subsequent evaporation of the collected activity in the form of neutral thermal atoms. According to calculations , the ion beam may be focused by an electrostatic quadrupole lens (triplet, total length 17 cm, radius 10 mm, voltages 1.2, 2.2 and 1.2 kv), after a 90° electrostatic deflection (r = 40 cm), to a spot of the size 0.6x0.4 mm on a collector at the oven position. The activity will be evaporated from the collector bo- local heating. It is supposed that a satisfactory atom to ion ratio will be obtained by choosing a suitable collector material. Vest runs using the isotope separator at Uppsala are being prepared. -408-

There are two classes of elements which cannot be studied with conventional atomic-beam techniques, namely: a) elements with no thermally accessible electronic state with J ^ 0, i.e. noble gases, elements of the second group of the periodic system (Be, Mg, Ca, 3r, Ba, Ra, Zn, Cd, Hg) and Pd, Yb and Pb. b) elements with very high evaporation temperature, like the refractory metals. Generally, atomic-beam work is simplified if the element to be investigated has: a) low evaporation temperature b) small electronic J value (5/ 0) c) large electronic g value d) only one electronic level thermally populated Easiest to study from this point of view are the alkali metals, the halogens, elements like Cu, Ag, In, 3n, Gb, Te, Tl, Bi, Po and some of the elements in the iron and rare-earth groups. However, also many other elements have been successfully studied by the ABMH method.

The production rate required for a spin measurement with the atomic-beam technique depends on a number of factors. Below we list the more important ones and give for each of them a typical value: a) fraction of hyperfine states contributing to the signal (O.i) b) oven efficiency (O.1) c) transmission (10 ) d) transition probability (0.5) e) detector efficiency (0.3) With a production rate of 10 atoms/ sec one would in this typical case yet a counting rate of about 10 counts/min at the detector. The general detection method for neutron-deficient isotopes is by the X-rays following the K-capture, using a thin scintillation crystal. Measurements in the ISOLDE hall have shown that the counting background with such a device is at present about 10 counts/min during beam on. This background is not expected to be appreciably higher after increasing the proton intensity, since the shielding will at the same time be improved. A signal of 10 counts/min will then be easily detected and a production rate of 10 atoms/sec can therefore be expected to be sufficient in a typical case. In favourable cases, particularly when oc counting can be used, a production rate of 10 atoms/sec might suffice. -409-

The detection of the resonance signals .vill be performed on-line; at the collector end of the atomic-beam apparatus. A collector disc system has been constructed in which the discs after collection automatically are brought to the counting positions. In order to increase statistics, eight detector positions are prepared. As mentioned above, c*. counting is the most efficient and will be used when the probability for a-decay is appreciable. In other cases X-ray detection will normally be used. A minicomputer will handle the data accumulation raid analysis. Several other functions in the experimental routine will also be controlled by the computer. Several target systems have been developed at ISOLDE, and among the spallation products released, all elements except those having a ,T -.- 0 electronic ground state may be studied by the A3T.TK method. The nost promising target systems for ABLE experiments seem to be the high temperature targets with oxides of Th, Ce and Zr releasing isotopes of a large number of elements Tl - a.J?r, .-.Sb - Cs and .... - ,J:b, respectively. The highest yields of the alkali metals, Kb, Cs and 7r, however, have been obtained from the molten metal targets (Y, La and Th) with surface ionization. Several isotopes obtained as daughter activities from high yield products may also be studiedj the halogens Br, I and At obtained from the noble gases Kr, Xe and Rn, respectively, produced by the metal hydrozide targets. Similarly, the noble metals Gu, \g and Au from Sn, Cd and Hg, produced by the molten metal targets with Ge, Sn and Pb, respectively. It would also be of great interest to extend the systematic study in the rare-earth region by investigating short-lived isotopes of Eu, 3m and Tm which are produced with the rare-earth targets Gd-La and Lu-La. Atomic-beam, experiments for measurements of nuclear spins and moments will constitute a natural complement to the optical pumpin,- experiments at the ISOLDE facility; the latter technique being particularly suitable for elements having a J = 0 electronic ground state. inferences

1) G.H. Fuller and V.W. Cohen, Nucl. Data Tables j>( 1969)43?. 2) C. Ekstrom, Dissertation, Goteborg, 1972. 3) I. Lindgreu and M. Olsmats, Ark, Fys. _36(1967)155. 4) 3. V/annberg and B. Kugler, Private communication. ,W?

; ,:i;.3:i:. .•,,;u:-."i':-:--'^L'.v"TE;'; .YJIET,: F,: ^•—i. ..:•. wow: i ke KbgluiM ' •.•partmoat o.t' Physics, Uni.'frsity oi' -"• tri iki.c'lni

Abstract. A collecting system is described which can easily be fully auto- matized. The system is designed for source collection in an mass-separated ion beam. When not in use, the system aan be removed from the beam rapidly, with preservation of the vacuum.

Sources for off-lins measurements were earlier collected manually. However, off-line work with short-lived isotopes is substantially facilita- ted if the sources are collected automatically. In the following a collecting system designed for use at the ISOLDE II mass separator in GERM is described. In the construction of this system, care was taken to minimize the number of steps needed in a collecting cycle, thus reducing the requirements on the electronic control circuits. The numbers in parentheses refer- to the drawings on next page.

The sources are collected on foils, mounted at the end of cylindrical holders (i) of diameter 9 mm and length 26 mm (rabbits). The rabbits are stacked in a vertical magazine (2) with a capacity of 120. From the magazine the rabbits are fed into a 7 mm deep slit in a wheel (3). They are then in collecting position.

After end of the irradiation, the wheel makes half a turn, the irradia-

ted rabbit is released, and drops down into an air lock5 and a new rabbit enters collecting position.

A. motor, placed outside the vacuum chamber, drives the source wheel via a telescope axis. The 180 rotation is controlled by a micro-switch. The source wheel is given a profile which allows the bulk OL rabbits above to slide down smoothly when the wheel is turned. -411-

The air lock, constructed by M. J Obert at CEil!, consists of two sliding valves (4) and between those a small space (5) connected to u pre- vacuum pump. When a rabb:Lt is to be transmitted out of vacuum, the space between the two sliding valves is evacuated and the upper sliding valve is opened. Then the rabbit is dropped down into the space between the sliding valves, the upper sliding valve is closed, air is let into the air lock, the lower sliding valve is opened, the rabbit drops into a pipe below, and the lower sliding valve is closed. Then the rabbit is transi'erred to the therminal for off-line measurements by application of compressed air to the pipe. The procedure from end o.C irradiation uvfcil the rabbit is brought out of vacuum will consume a time of 8 seeondes, An additional time of 7-10 secondes is required for the transmission to the off-line equipment. The magazine is hanging in a fix axis (6) at the top. A rotation a few degrees around this axis will remove the magazine and collecting mechanism from the beam and thus give free way for on-line experiments, N V/A Schematic, cut viewd of the system

beam Chapter 9: ISOL target - ion source

techniques - 41 2 -

A VERSATILE INTEGRATED TARGET-SURFACE IONIZATION SOURCE FOR ON-LINE ISOTOPE SEPARATIONS

S. Amiel, Y. Nir-El, M. Shmid, A. Venezia and I. Wismontsky Nuclear Chemistry Department Soreq Nuclear Research Centre Yavne, Israel

Elements with low ionization potentials or high electron affinities may be separated rapidly and selectively at high efficiencies from neighboring elements in a mixture of nuclear reaction products using surface ionization.

Positive surface ionization was demonstrated mainly by the Orsay group where alkali isotopes down to extremely short halflives were identified and studied. Preliminary attempts were made in our laboratory to apply negative surface ionization for the study of halogen isotopes formed in fission

An integrated target-ion source designed to function both for negative and positive surface ionization was constructed (Fig,l). A large area target consisting of graphite foils coated with uranium is placed inside an heated chamber and exposed Tantalum loniztr to a thermal neutron beam. The fission products diffuse out of the target chamber and pass through an heated tantalum guide tube, where they undeifcO ionization and are extracted into the mass separator (SOLIS). In the

Graphit* a «•• case of negative surface ionization, where halogens Boron Nitris TantalvH teal* II are separated, the intense electron emission is deflected Fig. 1: Layout of ion. source -413-

by a small magnet placed near the exit of the ionization tube. The temperatures of the target and ionization chamber are controlled separately to optimize speed of separation, selectivity and ionization efficiencies. Diffusion time, efficiency and specificity as functions of the temperatures and dimensions have been scudied.

Diffusion halflives Td were calculated from the experimental results using the expression

R Nt(coll.) = eAd VW

Nt(coll.) = number of atoms x collected at time t . e = overall efficiency R = direct formation rate (atoms/sec) , In2

This expression assumes a single exponential component in tlif diffusion and neglects precursor contribution. A more elaborate formula is used for accurate computer calculations. Examples of diffusion halflives for Rb and Cs at different temperatures of the diffusion chamber are given in Fig. 2.

Examples of collection and decay of activities obtained for masses 92, 142 with positive surface ionization, and for masses 136-138 with negative surface ionization, are shown in Figs. 3 to 8. The dependence of the activity collected on the diffusion halflife and the radioactivity halflife of a separated nuclide is given in Fig. 9 .

Calculated surface ionization efficiencies of fission produced alkalis and halogens are given in Figs. 10 and 11. The experimental results were found in general agreement within a factor of 10 with the predicted values (e.g. 0.03% for I at VL400°C and >1% for Rb).

Ihe specificity of separation was studied in detail for iodine. The contamination of iodine isotopes by the corresponding isobars was found to be < 1:1000. The decontamination is due to the long diffusion time of tellurium and its low ionization efficiency. The diffusion

half-life of tellurium was found to be Td = 80 min at 14oo°C. -414-

I0O—| Td (sec) DIFFUSION HALFLIFE FOR Rb AND CS 2 gr URANIUM on~IOOcm2 GRAPHITE FOILS (-03mm)-6cc DIFF CHAMBER

IO—)

i nso 1250 I35O Temp CC1 Fig. 2: Diffusion halflife for Rb and Cs

Using this source independent counts per channel fission yields of iodine isotopes 92Rb growth of masses 135-139 were studied (Fig. 12). Further experiments dealing with nuclear spectroscopy and fission yield studies of alkalis and halogens from fission are in progress.

REFERENCES 1. A. Venezia and S. Amiel, Nucl. Instr. & Meth. J57, 307 (1970)

n. beam on

K*. 0 K> 20 30 4O 50 6O 70 80 90 channel

Fig. 3a: A = 92 growth -415-

counts per channel n beam off A = 92 (pos. SI) 2gr.235U n flux 1,5 x I08 \ P def. eff.~3% 5 cycles 2- 0,95 sec. per cncmnel

I03- i 1

5- Fig. 3b: BACKGROUND A = 92 decav

o2- \

\ T = 57,4 sec. 5- LT^" 4.85 sec W \ (4^sec ^Rb+^sec^b) *Rb

I 1 i i i I i i i 150 channel

couns per chonnel 500-] n. boom off

A= 142 (pas SI)

• * 8 \'\ n flux 1,5 x I0 .Yk" \ ° Td- 10 sec 200— Poet eff-3% *. * 1 cycle Fig. 4: • m i A = 142 growth -••* 100- * and decay.

\"'s* -

\ 50-

\ 24 sec

S "^ s 20- YI beam on 1" —i r~ 1 T—r~ 1 1 1 1 (3 10 20 30 40 so JJO^SO 100 HO120 130 WO CO 60 TO{ counts :.:T 5 sec. A = I36( neq SI)

(7 cycles) 23S n beam off 2qr. U / flux 15x10" IO«- Td - 20 sec P det eff ~ 3%

\ •» %

Fig. 5:

^ A = 136 growth and decay.

I---.2- OC 20C 100 "OC 50C

counre per sec A=l37(neg SI) !20 cy: tb 2qr. 235U

,n beam cm n. bean off n flux l^xlO8 r * Td —an «M> i 0 det. eff.-3% i i s 10 - i ! \ • Fig. 6:

900- i y A = 137 growth i / \x. and decay. i

j l37 .i \ 3,9 m Xe i i i 50- I m i \H5S l I i i \ i i I i . > • | i i i i 1 Ci 00 time (sec) 2°° 3t» -417-

counts per sec. I37I Activity Growth W3- ~6 cc diff. chamber

600-

Fig. 7:

A = 137 growth.

son Calculated act. for Td=24sec

•

1 1 1 1 t oo 200 ! time (secj

counts per ysec. (350 cycles) A = I38( neq SI) o3- n beam on n beam off

n flux l,5xlO8 Td -40 sec.

500- det. eff. - 3%

Fig. 8: A = 138 growth and decay.

50-

10 -4 18-

MAXIMUM SATURATION ACTIVITY SATURATION ACTIVITY AS A RJNC'h-vii

OF THE RATIO Td/T /T|/2

OOI 50 00

Fig . 9: Saturation activity as a function of the ratio T./T , .

Positive Strfoce lomzaUon (calculated ) 01—1

W-l l+aex

taoi- I I W-I e (ev) (ev) I5OO°K To 0 0,33 b 4,2 37" W 0,3 032 Ta 0,3 0,82 3,9 = l500°K W 0,6 0,99 = 2000 °K 6 Ta -13 6.OXIO" 5,7 5 =2500°K 38Sr W .-1,2 5,6xlO' = 3000°K Ta -1,0 2^x10"* B 5,2 56 ° W -0,7 2^xlO"3 LAl -3 -2 0 W-l (ev!

Fig. 10: Calculated positive surface ionization. -419-

O.l-i

Negative Surface Ionization Efficiency (calculated) A-W a exp- ilO *- KT €•- A-V,' e*p- I- kT

a = ^(Halogens)

Fig. 11: A A-W Calculated lev) ev ) 'I5O0*K|20OO"K'| negative To -C.7 35Br 3.5 1,2 « IC " surface W(Th) 0,8 0.72 0,60 ', ionization 4 To -1,0 I.2II0"4 ?,51 rcr 0 1 o = 1500 K 53 0,92 0,82 10-- W(Th)05 6 b=2000°K To.-1,8 3,O«!O"7 aoxio" Se (2.4) 2 c=25OO°K 34 W(TW-O,3 2.5xlO"z 4slO"

d=3000°K 6,6KI0"S 2,5 jlO"6 2,2 To -2,0 52Te W(Th!O,5

o A-W

Yield

056 FRACTIONAL INDEPENDENT YIELDS OF IODINE 2M FROM U*nfh

(normnli7ed to n

Fig. 12:

"NORMAI" VAUJFS Independent D32- (Wnhl I9f,9) fission yields of iodine.

I3S 136 137 138 139 Mass re -420-

ON-LINE STUDY OF GASEOUS FISSION PRODUCTS; RAPID TRANSFER TECHNIQUES

H. Feldstein and S. Amiel Nuclear Chemistry Department Soreq Nuclear Research Centre Yavne, Israel

Short transfer time and adequate knowledge of the transfer time function are required for independent fission yield determinations in on-line mass separation studies.

The apparent yield of the separated fissinn product counted at the collector must be corrected for the decay occurring during the transfer from the irradiated target (especially in the case of short-lived isotopes) and for the decay of precursors in the target during irradiation and transfer.

The time required for a fission product to reach the collector may be considered to be composed of the time required to leave the fission source (by diffusion, emanation or recoil) then the transfer time and the mean residence time in the ion source. Since it is difficult to analyze separately, each of the components mentioned above overall transfer functions have been derived for different configurations of target, transmission line and ion source, at different gas flow conditions,, and the contribution of each stage to the overall delay was evaluated. Molecular flow

The targets used for the separation of fission-produced noble gases were based on emanation from either UO -stearats or a mixture of U_0_ and Z j o Ba-stearate; the release of halogen precursors from these targets was less than 1%. The emptying krypton and xenon isotopes were transferred by helium carrier (with M.5S Kr or Xe) at molecular flow conditions through a 1" diam. tube into a plasmatron ion source. The separated isotopes were beta-counted and the resulting curve of activity versus irradiation time was analyzed according to the method proposed by Winsberg to obtain the transfer time distribution. Two target configurations were studied: 1) a 20 cc single compartment circular cell, in which the fission source was sandwiched between stainless steel gauze and filter paper and 2) the same cell as above but divided into several small compartments, to minimize possible unevacuated pockets in the target. The resulting curves present the probability P(T) of an atom reaching the collector within time T (Fig. 1) and the transfer time distribution (Fig. 2). Some improvement in the transfer time distribution was achieved by using the second target configuration. A well-defined peak was obtained at 1.8 sec (Fig. 2), but the overall transfer time remained within 3 sec, also obtained for the single compartment target.

The effect of the length of the connecting tube on the transfer was measured in two different connecting tubes: the mean delay time between irradiation and the rise in activity at the collector was 1.6 sec. for the 30 cm tube and 1 sec. for the 9 cm tubev (Fig. 3). Since the flow time in the tube should be proportional to its length, 0.8 sec. minimum residence time may be attributed to the evacuation of the target chamber. This indicates that for the determination of fission yields using molecular flow one must consider both the delay due to the length of the connecting tube, as well as that due to target chamber evacuation.

Laminar flow

Fast transport through long distances may be obtained by laminar flow, but since the ion source operates at reduced pressures, the removal of the carrier gas is a prerequisite for the use of laminar flow for on-line isotope separation. This can be achieved by employing tha recent development in the helium jet technique . It has oeen found that the nuclear reaction products recoiling from the target into helium are incorporated in high molecular weight clusters which are formed in the helium medium by impurities like water vapour or organic substances introduced in it . The formation of the clusters is significantly enhanced by a high energy radiation field o like UV of wavelength shorter than 2000 A or that due to intense nuclear radiation^ }. The clusters, containing the fission products, may be transported by laminar flow through plastic or stainless steel capillaries several meters long. Upon emerging into an evacuated space they form a narrow conical jet (dispersion angle < 4°) which may be collected quantitatively at a substantial distance from the end of the capillary or introduced directly into the ion source. Since velocities of up to sonic speed may be obtained -422-

in the capillaries, a line several meters long may permit transport within 0.1 sec™.

The overall transfer time in that case is a function of the pumping capacity and the volume of the target chamber. Assuming a complete mixing in the target chamber one obtains a simple exponential relation between P(T), the probability of reaching the ion source within time T (sec), v, the pumping capacity (cc/sec) at the pressure prevailing in the target and V, the target volume (cc)

r VT 1 - exp - ^

In this case T . , the time required for 50% of the recoils to leave the target may be defined as ftn 2 V/v (sec) and by choosing a suitable V/v ratio it may be made as short as is dictated by the experimental conditions.

The helium jet technique has already been employed by various groups in the study of very short-lived hafnium isotopes formed in nuclear reactions , californium fission products ' , U-238 fission products produced by 14 MeV neutrons , and other products of nuclear reactions '

Experiments to couple a differentially pumped helium-jet skimmer with an isotope separator ion source are be^'ng carried out.

Discussion

A comparison between the two experimental assemblies possible, s.g. emanation target - molecular flow - ion source and recoil target - laminar flow - ion source, with respect to trasfer time, is given in Table 1. The emanation process is highly selective for the noble gases, while no such selectivity in the formation of high molecular weight clusters has been observed . Studies are in progress to overcome this by using nuclear spectroscopy and introducing a degree of selectivity in the ionization process. -423-

TABLE 1 Evaluation of the transfer time components in two target-ion source assemblies

Transfer time (sec) Molecular flow Laminar flow tube: 9 cm long tube: 5 m long 1" diam. 0.5 mm diam.

Release from target: (10) emanation recoil

Evacuation of 20 cc 0.8 minimum for v/V=l 2 sec target chamber M..5 mean for v/V=10 0.2 sec

Transfer through •vO.2 connecting line

'a. in ion source

Ov . _ mean VL.9 0.4 - 2.2 transfer time

v = pumping capacity (cc STP/sec) V = target chamber volume (cc) REFERENCES 1. L. Winsberg, Nucl. Instr. and Meth. j>5 (1971) 19-22. 2. M. Oron, S. Amiel, Proc. Intern. Conf. Electromagnetic Isotope Separators and the Techniques of their Applications, Publication BMBW-FBK70-28., (Eds. H. Wagner and W. Walcher; Bundesministerium fur Bildung und Wissenschaft, Marburg 1970) p.87. 3. R.D. MacFarlane, R.A. Cough, N.S. Oakey and D.F. Torgerson, Nucl. Instr. and Meth. 73 (1969) 285. 4. K. Wien, Y. Fares and R.D. MacFarlane, Nucl. Instr. and Meth. 103. (1972) 181. 5. H. Dantet, S. Gujrathi, W.J. Weisehahn, J.M. D'AURIA and B. Pate, Nucl. Instr. and Meth. JL07 (1973) 43. 6. H. Jungclas, R.D. MacFarlane and Y. Fares, Radiochim. Acta 16 (1971) 141. 7. H. Feldstein and S. Amiel, Nucl. Instr. and Meth. (in print). 8. Wblf-D. Sctaddt-Ott, R.L. Mlekodaj. C.R. Binghaia. Nucl. Instr. and Meth. (in print). 9. D.F. Torgerson, R.A. Gouch and R.D. MacFarlane, Fhys. Rev. 174 (1968) 1494. 10. A.C. Wahl and W.R. Daniels, J. Inorg. Nucl. Chem.6 (1958) 278. 1 1— 1 1

1.0 / o H U W 0.9 ,J Target with O compartments/// \ 0.8 w S 33 en / H en M 0.7 / Single O [14 f compartment 33 W 0.6 target

0.5

0.4 // M H 0.3 - -

O H 0.2 - - Pi H / 0.1 - 0.0 J, 0 10 20 3.0 4.0 Time elapsed after fission (sec) JIG .1: Probabilities of reaching the collector within time T after fission, for two target configurations.

to 30 w Target with comportments _ 70

< 60 oc? oi Hi 50 oi w I fa S« 40 compartment target E-i 30 In O 20 Z O H 10 PQ S H co 1.0 2.0 3.0 M Time after fission (sec) a p(T) - Distribution of transfer probabilities as a FIG.2: function of time, for two target configurations. .-425-

1 I I T- i i r —T—

5 Neutrons •( 1 •< •• - - 4h Activity atjr collector^* t 3 1.6 se o — ? 2h A

/ i i i i 0 2 4• 6i 8i 10 12 14 16 Time (sec)

V) 6 'c Neutrons 3 5

••/» Activity at *' collector k. O r 3

•*= 2 B I* •

5 10 15 Time (sec) 91, FIG:3: TIME DISTRIBUTION OF COLLECTED ACTIVITY OF Kr(T]/2=8.4 sec) AFTER IRRADIATION BY A SQUARE NEUTRON PULSE. A) 30 cm CONNECTING TUBE. B) 9 cm CONNECTING TUBE. -426-

THE APPLICATION OF CERAMIC OXIDES AT HIGH TEMPERATURES AS ISOL-TARGETS

F.IIansen, A.Lindahl, O.B.Nielsen and G.Sidenius

Niels Bohr Institutet, Copenhagen, Denmark

1. Introduction. By the application of on-line isotope separation of the products formed in high energy proton reactions, it is for intensity reasons of decisive importance to utilize the ability of the proton beam to penetrate targets of great thickness v/ith moderate energy loss. Accordingly, the key problems of target technology are related to the methods for obtaining a continuous liberation of carrier free reaction products from large amounts of the target element. In liquid targets a migration of reaction products to the surface may take place by the combined effect of diffusion and convection, whereas only the diffusion is effective in targets in solid state, but in this case the distances to be transversed can be kept short by the use of a powdered material. Practically the speed of the liberation process seems mainly to be determined by the evaporation from the target surface (ref. 1) . It has been found that mean delay times of less than about one minute can only be obtained if the target system is kept at a temperature, which corresponds to vapour pressures of the order of one atmosphere for the product element in question.

The ISOL production of less volatile elements is thus determined by the possibility of keeping suitable target elements at elevated temperatures. Usually the temperature range is limited due to reactions of chemical nature between target and container, and liquid targets have scarcely been operated above 15oo° C for longer periods of time. One of the few promising possibilities for extending the temperature region is offered by the use of ceramic oxides as targets. Many of these have excellent chemical stability at temperatures above 15oo°C, and are easily prepared in a powdered or porous state.

The target problems are further accentuated when attempting ISOL production of elements heavier than mercury based upon the spallation reaction, as thorium and uranium are the only possible target elements. Our decision to test the possibility of using thorium oxide as target is inspired by its exceedingly good stability at the highest temperatures.

2. The Ion-Source Target System. The targets were tested in a modified version of the multiple integrateel target ion source (MITIS) system, which has been I

Fig 1

described previously (ref. 2,4) (Fig 1). It is based upon the use of ion sources of the hollow cathode type (ref. 2) , which operates at a high temperature with a modest power consumption by virtue of its small dimensions. The usual life time of an ion source under on-line conditions is above 20 hours, but in order to secure sufficient operational reliability, provisions are made for automatic exchange of up to five ion sources.

WE URGE I AND ION SOURCE ARANGEMENT IN MITIS 73

Fig 2

The present arrangement of target and ion source is shov/n in detail in Fig 2. The target oven has been significantly modified relative to the earlier design (ref. 2,4). Originally the body of the oven was made of boron nitride, and heating was by

"US - A ? B - means of a wire of tungsten or tantalum. Although we have had excellent experiences with boron nitride at high temperatures when supplied as rods of small dimensions, it turned out to be difficult to obtain larger pieces, which were chemically stable above lloo°C. An oven made of graphite and heated by a tantalum wire could be operated for more than 24 hours at 16oo C. This was still considered unsatisfactory, the main weakness of the design being that the oven obtains a considerably lower temperature than the heating element. The present oven therefore consists of a directly heated tantalum tube (Fig 2). The tube is 45 mm long, 19 mm in diameter and has a thickness cf o.5 mm. Both ends are fitted into holes in supports made from 2 mm thick tantalum plate, and it is closed by two conical plugs 2.5 mm thick. The connection from the target to the ion source is through a 4 mm dia tantalum tube, which is welded onto the larger tube. Carrier gas is supplied to the ion source by a thin tantalum tube passing through one of the end plugs of the oven. The heating current is conducted by the tantalum supports from water cooled feed throughs. With a shielding around the oven as sketched on Fig 2, 2ooo C is reached at a current of 7oo amps, correspondincj to 975 watts. The temperature is controlled by a thermocouple placed inside the oven in the volume occupied by the target material.

3. The Target Material. The oxides tested were prepared by neating of the nitrates in the atmosphere at about 800 C. The resulting granular compound was ground in a mortar. Some further outgassing took place during the heating up in vacuum.

Initial tests were performed off-line with the oxide contained in the target oven. Thorium oxide (ThO~) could be kept at 22oo°C for about 3o hours without essential deterioration of the system. At 23oo C oxide and tantalum started reacting and the tube was destroyed after some hours.

In similar tests, lanthanum oxide (La2O3) seemed to disintegra- te at about 14oo C, whereas cerium oxide (CeO2) was stable until 155o C. An on-line run aiming at the production of the elements antimony, tellurium and iodine was therefore carried out with cerium oxide.

The powders as used as targets had a density less than half of that of condensed material. During exposure to high temperatures, some sintering took place, but with little change of volume, leaving the material in highly porous state.

4. Operational Experiences,

a. The Thorium Oxide Targets.

In a series of on-line test runs in the 600 MeV proton beam at -429- the ISOLDE facility, all the elements from no.79 gold to no 8 7 francium were produced in a stable and reproducible manner The efficiency of the target-ion source stage seemed relatively independent of the element, the differences in yield given in the next section being mainly attributable to the variation of the nuclear cross sections.

Investigations of yields as a function of temperature revealed a picture not quite simple. We observed good yields of radon at a temperature as lowQas 5oo C on fresh target material, of astatine at about looo C, and of bismuth and lead at 14oo°C, whereas francium was not released below 15oo°C However, the target properties depended strongly on the thermal prehistory in the sense that a target, which had been at a higher temperature for some time showed inferior behavior when the temperature was lowered to the initial value. This phenomenon did not cause severe troubles, but it was necessary to keep the target on a stable, or better on a slowly increasing temperature in the running periods. A target which had been used at temperatures from 16oo C to 21oo C for more than 48 hours showed indeed the best performance observed. Only for francium, the high yield could not be maintained for long periods.

counts / ID sec '°*Rn from 21 g ThO..-largit

30 i

-T-, tot "*Rn

proton beam off -p •

,00 200 300 J9° __JS?_ i£ ™ *J

Delay times were measured in a few cases by the method of switching the proton beam on and off. Fig 3 shows the result for radon with a target temperature of 18oo C; a quantitatively similar result was found for astatine at 2ooo C. About 5o pet. of the activity was released with a delay definitely less than lo sec. This is the shortest period, which could be estimated with the lo sec interval between the measured points. A similar fraction had delays of about 75 sec in both cases, whereas a few pet. showed periods essentially equal to the half life of the isotope used for the test. We shall not attempt a detailed interpretation of the above phenomena, but only summarize that for each element the delay curve must be considered a complicated function of temperature and tt.er- -h iO- mal prehistory of the target, and probably also of diffusion and adsorbtion phenomena in other parts of the system. For each isotope in question the efficiency is determined by the resulting delay curve in relation to its life time. One test run was devoted to the study of the release of fission products from thorium oxide. Silver, cadmium, antimony, tellurium, iodine, xenon and caesium were observed, with efficiencies which seem to be similar to those obtained for the heavy elements. Elements of the group of light fission fragments were also seen in about the same intensity, but were not isotopically analyzed.

b. The Cerium Oxide Target. The tests on this target are still preliminary. Antimony, tellurium, iodine, xenon and caesium were observed with about equal intensity. The yields were, normalized with respect to target amount and proton beam strength, about 10 pet of those previously found for xenon from a target of cerium hydroxide. Delay times were not measured, but I (T^ 3 sec) was observed with good intensity. 2

5. Yields. The radioactive products were measured partly on-line in one of the ISOLDE beam tubes, partly off-line on samples collected on "end strips" in the collector tank of the isotope separator. Because of the large number of masses to be surveyed, the yield curves are based on measurements of only a representative set of isotopes. Each result was reproducible within a factor of two. A consistent picture could be obtained with the only assumption that the distributions have a reasonably smooth behavior.

-15 -10 -5 T dtccys/SK 21g ThO, ron deficiency neu 5 • 10" protons /sec 1 10s 0"' yiel d stability at mass no 1 y o • Hg from Pt 200 10' relativ e r / •• -— Tl from fh 203 o' • - - Rn from 7b 217 / I 10' Fig 5 / 10' 0' Fig 4 * • 82 U 86 Mass no

Au Hg Tl Pb Bi Po At Rn Fr The results are summarized in figs. 4 and 5. Fig 4 shows the shape of the yield curves for radon and thallium from a thorium target, and in addition the most recent shape for the yield of mercury from lead (ref. 5). The yields are given as a function of "neutron deficiency" relative to the stability line. Unfor- tunately the concept of a stability line contains an ambiguity of at least one mass number in this region due to the distorti- on caused by the closed shells. -431-

The distribution curve for mercury from lead is on the neutron deficient wing mainly determined by the competition between neutron and proton evaporation. The radon curve, which extends less to the low mass side, is presumably influenced by an additional competition with fission, whereas the low relative intensity of the light thallium isotopes can be explained by the larger distance from the target nucleus, as reactions with protons of 600 MeV only rarely leaves a product with an excitation energy sufficient for the emission of more than 40 particles. The lightest spallation product observed with sizeable in- 1 or tensity was in fact as light as Hg, corresponding to the reaction "TTh(pf lip 37n) . Fig 5 shows the maximum yields observed for each element. The points for gold and mercury are based on extrapolations from points measured outside the region of high yields and are tentative. The production of radon corresponds to 60 pet of that found earlier for a target of thorium hydroxide at room temperature when normalized to the same amount of target material and proton current. The yield of mercury is orders of magnitude lower than obtainable from a lead target. This is due to competition with the fission of thorium, and especially to the larger "distance" from target to product nuclei.

REFERENCES

1) E. Hageb0, Proc. Int. Conf. On Electromagnetic Isotope Separators And The Techniques Of Their Application, Marburg Sept. 7-10 1970 p. 146

2) G. Sidenius, ibid p. 423

3) P. Patzelt, ibid p 158 4) A. Lindahl, O.B. Nielsen and G. Sidenius, A Multiple Integrated Target Ion Source System, Proc. Int. Work- shop Meeting On Techniques And Problems Of On-Line Se- parators, Ginosar and Soreq Dec. 14-18 1970 5) H. Ravn and L. Westgaard, internal ISOLDE report 27-7-1971 o

NEW MOLTEN-METAL TARGETS FOR ISOLDE

H.L. Ravn, S. Sundell and L. Westgaard

The ISOLDE Collaboration, CERN, Geneva, Switzerland

1. INTRODUCTION

The ISOLDE on-line isotope separator facility ^ at the CERN 600 MeV Synchro-cyclotron has now been in operation for 5\ years. In parallel with the running experimental programme, considerable effort has gone into the development of new target and ion-source systems. An early acrount of this work was given by Hagebo ' at the foregoing Conference in this series.

The ISOLDE targets that have been in current use for experiments can be divided into three classes:

i) Hydrous oxides of Zr, Ce, and Th at room temperature for the production of Kr, Xe, and Rn, respectively. Due to the high emanating power of the target materials, the noble gases are given off with short delay times, and the over-all yields are good . For long-term uses, however, operational difficulties occur, owing to the release of bulk gases (mainly water vapour) from the target. Such problems are likely to become still more serious with the strongly increased proton beam inten- i,) sity anticipated for the improved ISOLDE facility .

ii) Molten-metal targets. The first generation of these systems consisted of molten Pb for the production of Hg, molten Sn for Cd, and molten Ge for Zn. In all of these cases chemical selectivity is ensured, since the neighbouring elements to the product of interest have too low vapour pressures to be released at noticeable rates. A particularly favourable property of the molten-metal targets is their high stability in vacuum during prolonged heating and irradiation periods. The new systems described in the present report constitute an extension to other groups of target and product elements, characterized by their higher chemical reactivity and by the higher temperatures needed for the melting and evaporation processes. In general, the vapour pressure conditions are such that several elements are released simultaneously from the melt. The properties of the whole range of molten-metal target systems are summarized in Table 1, to which detailed references will be given in the main text of this contribution. - 4 3 3 -

Table 1: Survey of ISOLDE molten-metal targets

i-t lolal I'luuiiel i-lenient Mix. ••atill.lt l.m iJ Weight clement -".nil ^ >:u 11-1 weight ."'/,'.. " ,l:;r.v' ' K ' (HI (°C> l-..'-ei 1

be-La alloy 50 18 1300 L'a Sc-U alloy 5H 18 1300 K Sc-l.a alloy 5U 18 1300 Ar Y-La al loy till 50 1300 Sr

Y-La alloy HO 50 1300 Rli 1 \ ill', ":KI. "a, (.:: " S'JL" 1 - Y-I.a alloy (id 5(1 131)1) Kr

l.i 70 7(1 MOD Ha 10', '-'Ha

11 l.i 7(1 70 1400 Cs I"", '-"tV 'l - ( S -ei ! - ' "-H • 1 1-1 ••-. I

L.i 70 7(1 1.1(111 Xe J • 1(1' , ' - ' Xt- '".Vr I:,:. •-ei l -

1 lh-1,.1 alloy 98 30 1400 Ra (i • in' , • • 'i;.i - "' 'li:i (J.II mini

; lc 31 Th-La alloy 98 30 1400 1'r - lli",-"- - ir • 1 r I o sec 1 - ' "''I i 1 i'li -•.-. t ill-La alloy 98 30 1400 Kn ll-Cr alloy 190 182 1300 Ra ll-Cr alloy 190 182 1300 l-r U-Ll- alloy 190 182 1300 Hn Cd-La al loy 9b 90 140U bu 1 '", 11-7Lu M'Lu c: seel - Gd-La alloy 9b 90 1400 Sin S - !0s, '"Sm 1J7Sm (51 S?C) - l.u-La alloy 90 59 1400 Yb I.u- La alloy 90 59 1400 Tm s li- 134 134 1100 Zn « - 10', ' In "T-li ( 1.1. mm 1 k 6 D6 ? Sll 203 203 1100 Cd 3 10 . * ' '' t;d "'CA ( 1 1 mini- '"U! ( f..K sL, I I'h 270 27(1 70P Hg 5 » 10', '"llg '"llg I II .47 sec|- '"llg ( :.'} mini

SURFACE IONI5ATI0N SOURCE

TARGET MATERIAL

Ta CONTAINER / N/ Fig. 1 Early version of second-generation target and ion-source arrangement iii) Hot oxide targets. A group from the Niels Bohr Institute has developed a system in which a powdered (or sintered) stable oxide is kept at temperatures in the neighbourhood of 20Q0°C, and where the products being released are brought by diffusion into a hollow-cathode ion source . So far the elements from Rn to Au have been produced from ThO2, and likewise the elements Xe to Sb from CeO2.

2. NEW TARGET SYSTEMS

2.1 Alkali element targets

The first to become operational in the series of second-generation molten-metal targets was the system to produce Cs from liquid La (Ref. 6 and Table 1). An early version of this arrangement is shown in Fig. 1. The target material, which is heated to above its melting point by ohmic losses, is kept in a tantalum cylinder placed axially in the proton beam. Among the spallation products formed in high yields in the bulk of the target, the elements Ba, Cs, Xe, and I are released from the surface. (Te and Sb, which form stable compounds with La, are retained.) The volatile products are brought to the ion source by thermal diffusion through the heated tantalum transfer tube. In the case of Cs, the ion source is constituted simply by the constriction at the end of the Ta tube heated to ^ 1000°C by ohmic losses. Due to its low ionization potential (3.9 V) the efficiency for thermal ionization of Cs on Ta is high at that temperature, while the other species are essentially pumped off as neutrals. Thus, the chemical selectivity is restored.

Figure 2, taken from Ref. 6, shows the observed saturation yields of Cs isotopes after mass separation. An over-all efficiency of around 30% has been measured for this system. It may be of interest to note that with the anticipated 100-fold increase in proton beam intensity, radioactive ion beam currents up to a few nanoamperes will be available at ISOLDE-2.

With target and ion-source arrangements analogous to the one described for Cs, the alkali elements Rb and Fr have been produced from La-containing alloys of Y and Th, respectively . Similarly, although this has not yet been tested on-line, it seems certain from off-line experiments that K can be made From a Sc-La alloy (see Table 1). The purpose of alloying the target metal is to lower its melting point to a manageable level. In this respect, two important conditions must be met: the vapour pressures should be low enough to avoid migration of any component of the target into the -435-

120 125 130 135 140 MASS NUMBER

Fig. 2 Saturation activity of Cs isotopes from molten-lanthanum target transfer tube and ion source, and the rate of chemical attack on the container walls should be kept low.

Because of their high chemical reactivities, the liquid metals and alloys discussed here act as getters for residual gases as oxygen, nitrogen, and hydrocarbons. In general, however, the compounds thus formed dissolve readily in the melt, so as to leave no contaminating layer on the surface. (In case of excessive absorption of impurities the melting point may rise to too high values, and it is therefore essential to maintain a good vacuum.)

For the production in good yields of very neutron-rich Fr isotopes (as well as Ra and Rn isotopes, see below), the use of a molten U-Cr alloy as target material is considered (see Table 1). Here the problem of chemical corrosion of the container is particularly serious. Preliminary laboratory experiments indicate that the problem might be solved by the transformation in situ of the inner surfaces of the usual Ta container into TaC. The use of crucibles of yttrium metal might offer an alternative solution. -436-

2.2 Alkaline-earth targets

In order to obtain a sufficient surface-ionization efficiency for the alkaline-earth elements, for which the first ionization potentials are about 1.5 V higher than for the corresponding alkalis, it is necessary to replace the ionizing surface with a material having a higher electron work function than Ta. This has been achieved by lining the inner surface of the constriction at the end of the Ta transfer line (see Fig. 1) with a piece of Re foil, shaped into a tube. At the same time the temperature of the tip was raised to 1700°C. As indicated in Table 1, good yields have already been obtained for the elements Ra and Ba , while the on-line production of Sr and Ca so far has not been tried.

A difficulty in the use of the p^es^nt alkaline earth targets, in particular for experiments with vt;ry neutron-deficient isotopes, is that they do not discriminate against the accompanying alkali elements. Two different methods have been considered to restore the chemical selectivity.

It was at first attempted to make use of the fact that while the alkaline-earth atoms are ionized only at the Re surface in the end of the transfer line, the alkalis are effectively ionized already in their first collision with the wall of the heated Ta tube. By coupling the terminals for the d.c. heating current in such a way that a retarding electric field is set up along the transfer line (except for the constriction), the alkali ions should be trapped, provided the field is strong enough (and no neutralization from collisions with impurities occur). Onrline experiments with such an arrangement gave a reduction of the Ba and R3 ion beams of a factor of only 3 to 4. Thus, even though the principle is operative, a sufficiently high separation factor is not so easily achieved.

Another approach represents a deliberate use of the tendency of the alkaline-earth elements to form molecular ions of the type MeF , while the alkalis do not form such ions by surface ionization. An alkaline-earth side band may therefore be obtained, shifted by 19 mass units with respect to the alkali spectrum. An enhancement factor BaF /Ba of about 10 has been observed in on-line experiments, where the fluorine was supplied only from impurities in the commercial tantalum construction materials. Off-line tests with controlled addition of fluoride show results that are promising for the future application of this type of element separation. -437-

2.3 Rare-earth targets

Among the rare earths, the elements Sm, Eu, Tm, and Yb have sufficiently high vapour pressures to he released efficiently from a target melt at 14-1500°C. The first ionization potential- e around 6 V, and in order to attain a satisfactory efficiency in a thermal ion source it is necessary to go to quite high temperatures. The rare-earth source developed at ISOLDE, shown in the inset of Fig. 3, is a modified version of the ion sources de- 8) 9^ scribed by Beyer et al. and by Johnson et al. ' for off-line use. The thermal source itself, which constitutes a prolong' ^.on of the tantalum transfer line, is made up of a Re tube of diameter 3 mm, heated to 2800°C by means of electron bombardment from a surrounding filament. With this system good yields of Eu and Sm have been obtained from a molten Gd-La 1 o) alloy . A similar system to produce Yb and Tm from a Lu-La alloy is envisaged, and promising results have been obtained in off-line experiments. For the rare-earth targets there do not seem to be any simple ways to achieve chemical selectivity other than what is provided by the differences in vapour pressures. 2.4 Other systems

As indicated in the Introduction, it is not clear whether the emanating hydrous oxide targets for the rare gases can be used if the high proton beam intensities of ISOLDE-2 are to be fully utilized. A possible alternative is to combine the molten-metal targets described here with a plasma ion source. By keeping a low temperature along the line between target and ion source» only the noble gases are transmitted. On-line experiments have shown that

the yield of Xe from molten La is comparable to that from the old CeO2-xH2O target (see Table 1).

For reasons that are not completely understood, only very low yields of iodine have been observed from a molten lanthanum target with a hot transfer line and a plasma ion source as shown in Fig. 1, and this kind of system therefore does not seem promising as halogen targets. (These elements can instead be obtained from hot ceramic oxides or molten salts, sse the Introduction and below.)

Early studies at ISOLDE showed that occasionally indium was produced together with cadmium from a molten tin target, an observation that could only be explained as being due to chemical effects1. Later work has shown that small amounts of chlorine present in the system, lead to a rapid release of indium in the form of InCl, which is sufficiently stable to resist 1. Target container; 2. Target to ion-source transfer line; 3. Ion-source; 4. Extraction electrode; 5. Water cooling; 6. Heat screens; 7. Cathode for electron bombardment; 8. Ion-source ground terminal; 9. Gas inlet;

SURFACE IONISATION SOURCE ELECTION SOMIAKKMENT 10. Low-current feed- through; 11. Water-cooled high- current feed- through; 12. Ion-source magnet; 13. Electrical plug-in connectors; 14. Water-cooling connectors; 15. Separator HV insu- TARGET ASS EMILY WITH PtASMA ION 5OURCE lator; SURFACE IONISATION SOURCE DIRECTLY HEATED 16. Target vacuum chamber; 17. Electrical cables. Fig. 3 Target and ion-source assembly with insats showing the two different thermal ion-sources. -4 39-

breaking up in the transfer line. Although not yet tested on-line, specific r.urface ionization of indium as In is expected to take place in a hot ion source of the rare-earth type. Similar kinds of chemical transport reactions may also in other cases offer a useful complement to simple evaporation and diffusion for bringing a desired product rapidly out of the target and into the ion source.

3. DELAY TIMES

The different processes responsible for the hold-up of the products between the moment of their creation in the target volume and thsir extraction as ions from the ion source cannot at present be accounted for in detail. However, as discussed in Ref. 11, one can determine a composite "delay curve" by observing how the yield of a certain isotope falls off with time after the proton beam is switched off. The probability distribution of delay times is in principle found by differentiation of the measured delay curve.

3.1 Alkali-element systems

Figure 4 shows the delay curve for 130Cs (T, = 30 min) from a molten o "5 La target at 1240 C. (The figure also shows the increase of activity when the proton beam is switched on again.) The curve is very close to be a single exponential over two decades. Consequently, the probability distribution must also be nearly exponential, and one may define an average delay time t = 1/X, where X is the exponential time constant. The value obtained in the present example is t =48 sec. (Evidently the distribution must have a physical cut-off towards very short transfer times — the ionic flight times are of the order of 50 ysec.) A striking illustration of the occurrence of delay times much shorter than the average is the observed "reduction factor" (defined in Ref. 11) of about 0.003 for the isotope 219Fr, T, =20 maec. (This means that the observed yield of 219Fr is about 300 times lower than would have been obtained if the delay were infinitely short.) The delay curve for Fr from molten Th-La has

not been measured, but a single-exponential distribution with tflv = 44 sec, as found for Cs from La at 1400°C, would theoretically predict a reduction factor for 219Fr of 0.00065 (the ratio of the nuclear average lifetime to the average delay). Since Fr has a higher vapour pressure, the delay is expected to be shorter than for Cs. The measured reduction factor of 0.003, corresponding to t *v» 10 sec, is therefore at least in qualitative agreement with the simplified concept of an exponential delay-time distribution down to quite short delay times. 10 -

100 150 200 250 Seconds Fig. A Delay curve for 13'Cs from molten lanthanum at 1240°C. Collection and counting times 5 s

The average delay of Cs from molten La goes up from 44 sec to 101 sec when the target temperature is decreased from 1400°C to 1100°C. The trend it. simi1jr to what was found for the old Hg and Cd systems .

With the main purpose of studying the influence of the total volume and the aren of the extraction hole, a test was performed with a small La target containing about 1/10 of the usual amount of La, while keeping the surface- to-volume ratio approximately the same. The transfer line was shortened to ^ 5 cm and the extraction hole widened to 8 mm in diameter. The delay curve obtained for 130Cs could hardly be distinguished from the one measured for the usual large volume target, only the yields being a factor of 10 lower. This is taken to indicate that the evaporation from the surface is the time- limiting step (i.e. the same conclusion as drawn in Ref. 11 for the Hg and Cd systems), and furthermore that re-absorption on the surface of atoms once released is not an important factor in determining the over-all delay. Maybe the out-gasing from the target and container materials is still strong enough to bring the total pressure above the region where the conditions for molecular flow are met. — t - 4 1 —

._ Improved Hg target. Effects of shaking

About two years ago it was found that vip,arnu< stirring of the molten Pb gave a reduction of tiiu tit delay times. In a test experiment with a target of dimensions 2.7 x 10 cm (half-cylinder), mechanical shaking of the molten lead at 700 °G brought the average delay corresponding to the main component of the delay curve down from 120 sec to about 40 sec. . in the present version of the Hg target the shaking is accomplished by placing the target cylinder, heated by ohmic losses from an intense a.c. current, between the pole pieces of a strong permanent magnet. The vibration of the melt is sufficiently violent to break up the surface into droplets.

The yields of Hg isotopes from the improved target are given in Fig. 5. Also shown is the yi.eld curve for the first-generation target. Besides the shaking the new system incorporates also other improvements, such as a heated transfer line and a wider extraction hole (diameter 3 mm).

180 184 188 192 196 200 204 208 A

184 188 192 196 200 204 208 A

Fig. 5 Saturation activity of Hg isotopes, first and second generation molten-lead targets - 4 n; -

The influence of stirring has been tested as well for the systems Zn from molten Ge, Cd from molten Sn, and Cs from molten La. In no case has a positive effect been found. (For the first two targets the tests were not very conclusive, however, because of technical difficulties.) It is believed that the effect for Hg is due to the breaking up of a contaminating (oxide) layer, which is always observed to be present on the surface of molten lead, even under good vacuum conditions.

A. MECHANICAL DESIGN

The present version of the ISOLDE target assembly is shown in detail in Fig 3. A major difference from the first-generation systems is the suppression of the long drift tube between the target and ion source. The target and source, connected through a short transfer line, are now forming a unit which is enclosed in its common vacuum container. This container,which is made out of aluminium, carries on its front all the necessary supplies in form of quick connectors. The corresponding plugs on the separator are placed on the HV insulator of the differential-pumping chamber. By means of a mechanism not shown, the whole assembly can be connected or disconnected by remote control, For easy access the lid of the vacuum chamber is shaped into the form of a "hat" which can be removed without touching any of the connectors.

The target container and transfer line are as a rule made out of cotr- mercial tantalum. (An exception will be a future molten U-Cr target, as discussed above.) The target cylinder is suspended on two water-cooled high-current terminals which carry the heating current (< 1000 A a.c.) The all-tantalum high-temperature plasma ion source is shown mounted in the assembly, while the two different surface-ionization sources are drawn as insets.

Not shown are the heat screens surrounding the target cylinder. As the solubility of Ta in the target melt is in general not negligible, it is important to keep temperature gradients low in order to avoid excessive transport of dissolved Ta, leading to seiious corrosion of the container.

Also not shown is the permanent magnet used for stirring of the molten lead in the Hg target. -443-

3. CONCLUSION. OTHER LINES FOR FURTHER WORK

Molten-metal targets have proved to represent a fruitful line of development for ISOLDE. As shown in Table 1, about 20 elements are either available as primary products from such systems, or are believed to be within reach with present techniques. In addition several other elements are formed as decay products in sufficient yields to be useful for experiments.

However, novel target types are also considered, and to the list given in the Introduction, a fourth class might perhaps already be added: molten- salt systems. In a preliminary test run, mass-separated beams of short- lived At and Po isotopes, as well as (long-lived) fission-product I and Te, have been obtained from a target of molten ThF^ -LiF. Time, so far, has not permitted the development for practical use of this particular "chemical" target system, or of other analogous systems that can be envisaged.

Acknowledgements The authors acknowledge with pleasure the. support from and interest shown by many members of the ISOLDE Collaboration, and in particular the help from our colleagues in the CERN-ISOLDE Group, P. Gregers Hansen, Erich Kugler and Bjorn Jonson.

mi -444-

REFERENCES

1) The ISOLDE isotope separator on-line facility at CERN (Eds. A. Kjelberg and G. Rudstam), CERN Report 70-3 (1970).

2) E. Hagebo, Proc. Int. Conf. on Electromagnetic Isotope. Separators and the Techniques of their Applications, Marburg, 1970, Report BMBW-FB K 70-28 (1970), p. 146.

3) P. Patzelt, ibid., p. 158.

4) S. Sundell, P.G. Hansen, B. Jonson, E. Kugler, H.L. Kavn and

L. Westgaard, these Proceedings.

5) F. Hansen, A. Lindahl, O.B. Nielsen and G. Sidenius, these Proceedings.

6) H.L. Ravn, S. Sundell and L. Westgaard, Phys. Letters 3_£ B, 337 (1972). 7) L. Westgaard and the ISOLDE Collaboration, Proc. Eur. Conf. on Nuclear Physics, Aix-en-Provence 1972, Vol. II, p. 170, and Bull. Am. Phys. Soc. JL7_, No. 10, 907 (1972). P. Kornsh^j, P.G. Hansen, B. Jonson, H.L. Ravn, E. Roeckl, S. Sundell and L. Westgaard, to be published.

8) G.J. Beyer, E. Herrmann, A. Pitrowski, V.J. Raiko and H. Tyrroff,

Nuclear Instrum. Methods 96^ 437 (1971).

9) P.G. Johnson, A. Bolson and CM. Henderson, Report UCRL-73982 (1972).

10) P.G. Hansen, B. Jonson, H.L. Ravn, S. Sundell and L. Westgaard, to be published. 11) E. Hagebo, A. Kjelberg, P. Patzelt, G. Rudstam and S. Sundell, Ref. 1, p. 93. W ? H

lie-Jet Un-Line Ion f UNLSOR Mass Separator

Wolf-D Scimildt-Otl md ". i. 'n.4-.Hi,ii UNISOR, Oak Ridge, Tennessee J783O, U.S.A.

Abstract In a recent publication transport efficiencies for recoils from heavy ion reactions with the use of He-jet systems and •".•asibil i ry studios of such transport systems for the on-line operation oi the UNISOR mass separator were reported. Subsequently, new capillary-skimmer systems have been constructed and connected on-line with the separator. Gamma and alpha-ray spectra have been measured on-line and the total efficiency imp.-oved to the point that it is now 0.1% using a hollow cathode ion source.

I. Introduction At UNISOR, two on-line ion source systems are under development : (a) He-jet on-line ion source systems using cold targets, a He-jet transport system and either a hollow cathode or an oscillating electron ion source, and (b) a modified oscillating electron ion source using target foils of high melting point inserted directly into the anode cylinder of the ion 2 source. The feasibility of various He-jet transport systems for the on-line operation of the UNISOR mass separator has been reported in a recent publication. At present, modifications of the transport system were performed and the transport system was connected on-line with the UNISOR mass separator.

II. Target Chamber A new target station 35 x 25 x 25 cm has been installed at the UNISOR beam line of the Oak Ridge Isochronous Cyclotron (ORIC) . The 300 cm3 target chamber which has been used in previous experiments was inserted into the vacuum system (Fig. 1). Degrading foils can be used for a rapid

On leave of absence from University of Goettingen. *UN1SOR is a consortium of University of Alabama, Emory University, Furman University, Georgia Institute of Technology, University of Kentucky, Louisiana State University, University of Massachusetts, Oak Ridge National Laboratory, Oak Ridge Associated Universities, University of South Carolina, University of Tennessee, Tennessee Technological University, Vanderbilt University, and Virginia Polytechnic Institute. It is supported by these institutions and by the U. ... Atomic Energy Commission. change of the energy of the heavy 0R""--0*G 7i-" ion beam. The beam current is monitored with a Faraday cup assembly. Entrance and exit windows are mounted on water cooled frames and can easily be exchanged. The reaction recoils which are knocked out of the targets are stopped in a He-gas atmosphere. Thin foils of Ag, Ni, and Rh and rare earth oxides (141Pr,142Nd) evaporated on 25pm Be foils have

ENERGY DEGRADING been used as targets. In the FOILS experiments with Ag, two target foils were used (see Fig. 1).

III. He-Jet Transport System HEAVY ION The reaction recoils are swept BEAM out of the target chamber with the He flow through a 5.5m teflon capillary (0.86mm i.d.). Transport Fig. 1. Target station with efficiencies of 60% have been meas- He gas chamber. ured using 2 booster pumps at the 30£ collector chamber. The recoils leaving the capillary are emitted into a cone with a 3° opening angle. To overcome the pressure step between the target chamber where the reaction recoils are thermalized and the volume where the ion source of the separator is operated, differential pumping at cone shaped skimmers was applied.

IV. Construction of a Skimmer-Hollow Cathode-Ion Source System The collection chamber (Ref. 1) has been connected with a hollow cathode ion source."^ Already in the first experiment, i;^Dy activity (comp. Sect. 8) was detected in the focal plane of the separator. In order to reduce the distance between skimmers and ion source and to improve the alignment of the system, a new arrangement has been constructed (Fig. 2). Capillary and skimmers are mounted at the ion source front plate. The capillary is aligned with a 12 cm brass tube. The distance between capillary end and stainless steel skimmer no. 1 can be varied. Skimmer rDANFYSIK, Jyllinge, Denmark, model 911. - ••! .'I 7 -

ORNL-DWG 73-

ANODE CYLIMOER

HOLLOW CATHODE SUPPORT GAS- !0N S0URCE

TEFLON CAPILLARY

CURRENT

SKIWMER NO. 1- MAGNET ION SOURCE SKIMMER NO. 2 HOUSING

Fig. 2. Skimmer-hollow cathode-ion source system.

no. 1 is a pointed cone with 40° opening angle for the outer and 26° for the inner cone. A second skimmer can be inserted into the anode cylinder of the ion source. The opening is large enough so that all recoils which pass through skimmer no. 1 also pass through skimmer no. 2. The collection chamber is evacuated by the 2 booster pumps (pumping speed 800 £/sec each), the volume between the skimmers by the diffusion pump located under the separator ion source. The pressure in the ion source can be adjusted by the variation of 4 parameters, the He flow, the distance between capillary and skimmer no. 1, the opening of skimmer no. 1, and the amount of added support gas (a mixture of Kr and Xe has been used as mass marker). Typical parameters are given in Table 1.

V. Construction of a Skimmer-Oscillating Electron-Ion Source System Using the experience with the hollow cathode on-line system a new construction was carried out. The transport system was connected with an oscillating electron ion sourcet+ by fitting a stainless steel cylinder in between 4 the collector chamber and the ion source base plate (Fig. 3). Only one skimmer (cone opening angles 45° and 25°) with 0.5 mm diameter hole is used. In comparison with the data of Table 1, the pressure in the ion source is reduced by a factor of ?. for otherwise the same experimental parameters.

^DANFYSIK, Jyllinge, Denmark, model 910. -440-

Table 1. Experimental Parameters of He-Jet On-Line System Using the Hollow Cathode Ion Source

Pressure in target chamber 0.7 atm He-flow rate 5 cm3/sec STP Pressure in.collection chamber 10"3 Torr Capillary-skimmer no. 1 distance 4 mm Opening of skimmer no, 1 0.7 mm Opening of skimmer no. 2 1.5 mm Pressure reading at ion source diffusion pump 6 Using the He-jet only 5-10- Torr With support gas 7-KT5 Torr

VI. Impurities in the Transport

System ORNL-OWG 73-522?

An increase of the absolute - CURRENT FEED THROUGH transport efficiency has been S-- MAGNET observed when "small" amounts of CCIL, were introduced. In more recent experiments these results are confirmed. A similar effect is observed with the He-jet ion source system. The He gas was bubbled 30 sec through a bottle with CCli,. The cransport system SKIMMER h OSCILLATING was subset, ;ently evacuated. Sub- ELECTRON ION SOURCE " sequently, pure He was introduced and the cyclotron beam was Fig. 3. Skimmer-oscillating turned on. The level of short- electron-ion source system. lived activity in the focal plane

of the separator increased and then gradually decreased during a 2 h run. After reintroducing CCl^, the activity level went up again. Using a comparatively large amount of CCl^, less activity was observed and deposits of soot have been discerned at the skimmers after the experiment.

VII. Time Performance of the He-Jet On-Line Systems He-flow rates 5 to 10 cm3/sec STP were presently used. An upper limit for the hold-up time of the reaction recoils in the target chamber can be estimated to be the tima necessary to sweep out once the target chamber. - 4 '4 1 -

This yields 42 sec for the 300 cm3 chamber at 0.7 atm and 5 cm':/sec STF flow. The actual hold-up time Is shorter since the recoils are th'jrmalized near the capillary entrance. The sweep out time was calculated by assuming laminar flow. For the flow through a 5.5 m capillary with 0.86 mm i.d., the calculated time is 35 sec in accordance with the measurement. For future experiments smaller target chambers are under construction. The transport time of the recoils through the capillary is 5.5-10"* sec assuming sonic velocity. The hold-up time in the ion source can be estimated for the equilibrium between the infusion of the recoils into the ion source and the effusion through the exit hole using the expression t=ln2-4V/(c-S). The parameters fcr both ion sources are given in Table 2.

Table 2. Calculation of Hold-up Time in the Hollow Cathode Ion Source I and Oscillating Electron Ion Source II

Ion Source I Ion Source II

Ion source volume V 0.1 30 cm3 Exit area S 1.3-1O-3 1.3 •10"2cm2 Average Velocity c for mass 115 610 530 m/sec Hold-up time t 3.5-10-3 1.2 •10-1 sec

Experimental Results Several test experiments have been performed in order to investigate the influence of the various parameters of the system (Sect. 4) on the on- line performance. During these runs, the total efficiency has been gradually increased by (a) reducing the distance between skimmer and ion source, (b) using a small entrance opening to the ion source, (c) reducing the pressure in the ion source and (d) adding a "small" amount of CClt, to the He transport gas. A list of the nuclear reactions used and of the identified recoils which were collected in the focal plane of the separator is given in Table 3. As an example, in Fig. 4a and 4b the gamma spectra of mass A = 114 and A = 115 are given which were measured following a 10 tnin collection. In this experiment the total efficiency of the on-line system was estimated using theoretical cross section values of 200 and 100 mb for the compound nuclear reactions which lead to u'lSb and 115Te, respectively. The efficiency presently achieved is M).l% for the hollow cathode ion source. One experiment performed with the oscillating electron ion source had an efficiency vhich was 4 times smaller which may have been due to excessive amounts of CCl^ (camp. Sect. 6).

Table 3. Test Experiments Performed

Target Heavy Ion Beam Energy Detected Activity

141 2 15O Pr2O3(239)jg/cm ) 97 MeV Dy(7.2 min) 12C 76 MeV 15ODy(7.2 min) 97 MeV e5Ga(15 min) 103Rh(3Omg/cm2) 4He 80 MeV 102Ag(14 min) Ag(2.6mg/cm2) 12C 76 MeV 111|inSb(8 min) 115Te(6 min) 115Sb(31 min)

OBNL-DWG 75-15226

i 3 MeV ]

Y OF GAMMA QUANTA ENERGY OF GflMMa QUANTA

Fig. 4a. Gamma spectrum Fig. 4b. Gamma spectrum 114 of msb (8 min). of 115Te (6 min), 499 keV y-ray of 115Sb.

References 1- W.-D. Schmidt-Ott, R. L. Mlekodaj and C. R. Bingham, Nucl. Instr. and Methods 108, 13 (1973).

2. E. H. Spejewski, Proc. Conf. EMIS1 Billingehus, Skoevde, June 12-15,1973. 3. R. A. Naumann, Proc. Heavy Ion Summer Study, Oak Ridge, June 12-July 1, 1972, p. 233. -451-

PRODUCTION OF PURE SHORT-LIVED Te AND Sb SOURCES WITH AN ISOTOPE SEPARATOR SEMI-ON-LINE SYSTEM.

M.E.J. Wigmans, B.O. ten Brink, R.J. Heynis, P.M.A. van der Kam, L.A. Paanakker and H. Verheul

Natuurkundig Laboratorium der Vrije Universiteit, Amsterdam., The Netherlands

Decaystudies have been performed on several neutron-poor short-lived £b and Te nuclei, using mass separated sources. Natural tin foils were irradiated with protons (15 - 28 MeV)and He (20 - 42 MeV) from the AVF cyclotron of the Free University. The activated foils were transported with a rabbit system to the ion source of the isotope separator. This system has been 1 2) developed by Dulfer and has been described in more detail elsewhere ' .

To investigate the optimizing of the separation procedure of Sb and Te isotopes produced in Sn, we simulated the irradiated targets with Sn foils containing 0.1% stable Sb or Te as impurity. The beam of one of the stable isotopes of Sb or Te was catched in a Faraday cup at the source collection position, and the current was measured as a function of the time after the arrival of the foil in the ion source. By current integration one can obtain the separation efficiency. In figure 1 this separation course is given for the separation of Sb from a Sn foil, using two different filament currents. At the same time scale the temperature change of the foil is given. This change is measured by determining the time, necessary for melting foils of different materials in the ion source. The Sn foil is taken out of the ion source after 50 seconds. Chemical analysis and experiments with long-lived Sb showed that at that time 99% of the Sb has evaporated out of the foil. Meanwhile the temperature of the foil is so high, that the Sn itself has a high evaporation velocity.

When the foil is not removed out of the ion source it turns out that the separation efficiency decreases very rapidly each next separation, due to -452-

Sn accumulating on the walls of the ion source and dominating increasingly the ion source pressure.

However after taking the foil out of the ion source still Sb is separated, the beam intensity decaying exponentially with time. An explanation of this can be, that the Sb which evaporates out of the foil precipitates on the relative cold walls of the ion source and reevaporates with a velocity determined by the filament current. This also explains the great difference in slope for the two filament currents of fig. 1.

>•

< ..... on '•••• Ifj|=38A CD cr

x m !fi|s40A "«*»xx»x Til T(°C) 4 3 -*• TIME(min) H00 B •- 40A — 38A 1C00

600

200 TIME(min) 1 2 3 fig. 1. A. Separation of stable Sb from a Sn foil at two different filament currents. At t = 0 the foil arrives in the ion source. After 50 seconds (time marked with an arrow) it is taken out of the ion source again. B. Temperature of the Sn foil at the same time scale. -453-

Following this procedure, the separation efficiency for Sb from Sn foils remains rather constant during a great number of separations and amounts about 0.5% for Ifil = 40A. This number is calculated neglecting the neutral beam and escape of secondary electrons, but it is confirmed by the separation results for e.g. Sb, which was produced by irradiating the same foils with 25 MeV protons.

When If., = 38A it takes more time to get the same efficiency as for

Ij... = 40A. For studying short-lived Sb activities as ' Sb (Tn = 3.t min) 113 s and Sb (Ti = 6.7 min), the Wk current is thus preferred. During these radioactive separations the Faraday cup is placed in the collector room of the separator and used as a monitor by catching the 121 Sb beam in it.

The separation of Te from Sn foils has very different properties, the evaporation velocity at a given temperature being much higer than for Sb. The separation course for Te from Sn foils can be seen in fig. 2, in which for comparison, the separation of Sb from Sn foils is also shown. I 125 Te

if)

CC < CC CO CC < l21Sb r 1 "*" TlME(min) 1 2 3 fig. 2. Separation of stable Te and Sb from Sn foils. I = At t = 0 the foil arrives in the ion source. For producing a pure Te source the foil is taken out of the ion source after 25 sec. This moment is marked by an arrow. -454-

As can be seen from this figure it is possible to separate only Te from Sn by taking the foil out of the ion source 25 seconds after arrival. This has been tested with Sn foils in which long-lived Sb and Te activities had been produced. When such a foil had been in the ion source for 25 seconds, it turned out, that afterwards all the Te activities had disappeared, while the Sb activities had remained for ^ 100% in the foil. In this way a separation not only to A, but also to Z is performed.

This Te-Sb separation is very good as can be seen from fig. 3. In this figure the decay curve of the 497.3 keV y ray of Sb is shown. Sb is the daughter of Te, which is produced by separating a Sn foil, after irradiation with HO MeV He, in the way described above. This decay curve has a significant ingrow of 6 minutes ( Te). Analysis of the curve showed, that at t = 0 (end of the separation) > 99% of the source consists of Te, this in spite of the fact, that, compared to Te, a large amount of Sb has been produced directly in the foil

TIME(min) 0 fig- 3. Decay of the U97.3 keV y ray of 115Sb, using a 115Te source.

This separation possibility has great advantages for our investigations of the decay of Te and Sb :sotopes.

Firstly the contamination in the Te sources of Sb and also Sn and In -455-

activities with the same massnumber is suppressed far enough to make the recognition of weak Te y rays possible.

Secondly, it was possible to assign two newly found activities to the isotopes Te and Te uniquely. The possibility that these activities belong to isomeric states of the Sb daughter nuclei could thus be excluded,

Thirdly, it becomes possible to study the decay of short-lived Sb nuclei fed by the decay of the longer lived Te mother nuclei, as in the case of 112 Sb (T, = 53 sec). This half-life is too short to use Sb-separation with an acceptable efficiency (see fig. 1), but using the mother activity has the advantages of a longer half-life and a shorter separation time. Identification of the y rays can be done with the 53 sec ingrow in the 1.8 min decay ( Te), thanks to the pureness of the Te source.

Some results of the decaystudies performed on sources produced in this way will be presented at the 1973 Munich conference on Nuclear Physics.

References:

B.O. ten Brink, G.H. Dulfer, H. Verheul, Proc. of the Marburg Conf. (1970) p. 2) G.H. Dulfer, thesis, Vrije Universiteit, Amsterdam, 1971 -456-

Experience v/ith chemical compounds as tarqets for the production and isotope separation of carrier free nuclei

G.K. Wolf, T. Fritsch, J. Dreyer, Lehrstuhl fur Radiochemie der Universitat Heidelberg, 75 Karlsruhe, Postfach 3640 Germany

1) Introduction

Until now very little experience has been qained with chemical compounds as targets for the production and mass separation of short lived nuclei. Nearly for all ISOL systems in operation elements are used as targets.

They have the advantage of leading to the evaporation of the nuclear reaction products as elements being easier to ionize than compounds. Besides this the radiation damage in the target is less serious. On the other hand a number of elements (Zr, Hf, Nb, Ta, Mo, W, noble metals) cannot be evaporated and ionized as elements. For them the evaporation as compounds is inevitable.

We started therefore systematically to investigate the behaviour of nuclear reaction products evaporating from chlorides and oxides in a plasma ion source. Special attention was paid to the role of sweeping or reaction gases transporting products to the ion source.

2) Experimental set up

The experiments were performed with a Danfysik ion accelerator equipped with a 30° magnet and a gas discharge -457-

(Nielson-type) ion source. The tarqets irradiated with different kinds of particles were heated above or a little below the melting point and the nuclear reaction products swept together with various qases into the ion source. In all cases the over all yield of the evaporation and mass separation step was determined.

We designed a gas inlet system (fiq. 1) which provides the ion source with Cl, F, Xe, Kr, and other qases or gas mixtures under defined pressure conditions. These gases may either pass over the heated target or are fed to the apparatus behind the oven or directly into the ion source (fig. 2) . Besides these solid compounds may evaporate and enter the ion source via a second oven.

3) Evaporation and mass separation of nuclear reaction products from SrCl- as target

It is well known ' that a number of elements evaporate with

high yields from SrCl2- We used therefore SrCl? bombarded with oC-particles in order to produce Zr and Y isotopes and

with 600 MeV protons in order to get Ga, Get As and Se isotopes as a test system.

The irradiated targets were heated in the oven of the ion source to 800 - 900°C and the volatile nuclear reaction products ionized under various experimental conditions. The percentage of the different nuclei reachinq as separated masses the collector plates was determined relative to their total amount in the target. H- Hi Mi • nH> I—" ID Beschleunigcr 3 accelerator ft vQ en 01 01 H- (0

«rtluorkohl«n\ sserstoff J fluorine compounds

-8517- Mass separator yield of nuclear reaction products from

irradiated SrCl2

oveni gasinlet 2 ion source gas inlet 1

gasinlet 3

Nuclear Gasinlet Gasinlet Oven 2 Yield C'AJ Reaction 1 3

Sr C12+ Krypton —— Mass- —(Decompo- oc-Particles marker sition) ii ecu — Mm Zr 0,5-1 ,Y —- SrCl2 + Krypton Mm GaO,6 ; 600 MeV p As,Se,G6<0,01

II Krypton — As,Se Ga 0,5 As 1,0 Se 0,2 Ge <1,0 II —_ Ga 0,4 As 0,4 CClA Mm Set 0,03 GeO,6 II — ecu Mm Similar but no Ge

Fjg. 2; Mass separator yields of nuclear reaction products

from irradiated SrCl2 -460-

Fiqure 2 shows the most important results: The chlorides of Zr and Y evaporatina from the taraet bombarded with Ot-particles decompose with Krypton as sweeping gas, CCl^ fed directly into the ion source does not work, too. But with CCl. passed over the taraet one gets reasonable results. A Zr/Y separation takes place if the transfer line between oven and ion source is kept rather cold.

From the proton irradiated SrCl- only Ga is carried with Krypton to the discharqe chamber and ioni2ed. CCl. works as good as in the case of Zr + Y. In contrary to the Zr/Y case also CCl. in the ion source is affecting the yield of the most products. This behaviour proofs the decomposition of Zr and Y between oven and discharge chamber and the reaction of As and Se with the material of the discharge chamber being responsible for the loss of products. This interpretation is supported by the fact that stable As and Se from the second oven act similar to CCl. suppressing the reactions of carrier free As and Se with the source material.

The results show that it is possible to use chemical compounds as targets for mass separations taking into account the special chemical behaviour of microquantities. Applying in a controlled way inert or reactive gases and carrier compounds one is able to influence the selectivity of the procedure.

4) Evaporation of fission products from compounds as targets

With the experience from the above mentioned experiments we studied the evaporation and ionization of fission products, produced in salt mixtures. U^/py^ and u n /RrCl were irradiated with thermal neutrons and heated in the oven of the ion source to 700 - 800°C. The yields of different separated fission products were determined under various experimental conditions, as described under 3). To avoid evaporation of uranium chlorides the CC1. was always fed into the system behind the tarqet (inlet 2 of fiqure 2). Table 1 gives a survey of the results.

Since we collected only the mass ranqes with the hiqhest fission yields and only isotopes of at least some hours half live the list of possible products is incomplete.

Remarkable is the high iodine yield from the oxide target with Krypton and the possibility to evaporate and ionize Zr and Mo (and probably also Nb) .

Table 1: Mass separator yields of fission products from different targets

Tarqet Experimental Yield Conditions /%/ + U3Og/B2O3 Krypton J + 10 Gas inlet 1 J 2 700°C

+ U-.OQ/SrClo CC1. J 1 - 2 ^ 4 Tf=> 0 5 Gas inlet 2 *r n l ° gi

U/graphite CCI. J+. 1 % Gas inlet 2 ™+ °; -462-

Wanting to avoid molten salts in order to achieve faster diffusion times or to meet the conditions of a heavy ion accelerator one may use also targets consisting of uranium/ graphite sandwiches. The evaporation of Zr (and probably Mo and Nb) is also possible from such targets (table 1), if one applies CC1, without letting it have contact with the uranium.

All procedures mentioned work satisfactory and reproducable under off line conditions. The next steps are now the on line tests which we hope to perform in the following months. For this purpose we developed in collaboration with the GSI-group a suitable heated target with variable gas inlet for the separator on line with the reactor in Mainz.

References

1) G.K. Wolf, T. Fritsch, KFK-Report 1257 (1970)

2) K. Bachmann, GSI-Renort 71-1, 115 (1971)

3) G. Herrmann, GSI-Report 71-1, 138 (1971)

4) A. Kjelberg, R. Rudstam, CERN-Report 7O-3 (1970)

5) W.L. Talbert, jr., J.R. McConnel, H. Skank, BMBW-FB K 70-28, (1970)

6) G. Wolf., T. Fritsch, BMBW-FB K 70-28, 168 (1970)

7) G.K. Wolf, GSI-Report 71-1, 123 (1971) B oOcl

NEW TARGET ARRA^GZICNT FOR THE OSIRIS FACILITY

Ch.Andersson, B. Crapengiesser, and G.Rud:;tara The Swedish Research Councils' Laboratory, Studsvik, Nykoping, Sweden

1. ORIGINAL TARGET SYSTEM

A special feature of the OSIRIS on-line isotope separator is the combined target - ion source arrangement. The target material consisting of uranium oxide is plated onto the inner mantle surface of the graphite cylinder used as discharge chamber in the ion source, thus avoiding the connecting line between a separate target and the source. Since the diffusion is believed to be more rapid in graphite than in uranium oxide ' a second cylinder made of graphite cloth is placed inside the discharge chamber. Fission products recoiling out of the target material get caught in this cylinder, diffuse to the surface, evaporate, and get ionized. This kind of target - ion source combination has worked well. Eighteen fission elements pass primarily through the separator, and the delay between production and measurement is known to be less than a second for a large part of the activity formed. There are limitations with the system, however. Apparently, only a target thickness corresponding to the recoil range in the material is effective - an increase of the thickness above this limit does not increase the amount of fission products reaching the graphite cloth. This means that the effective amount of target material is around 300 mg for the actual size of discharge chamber. An increase of the amount of target material will only give increased amounts of more long-lived products which survive the slow diffusion through the uranium oxide.

2. PRINCIPLE OF A NEW TARGET ARRANGEMENT

For certain kinds of experiments high activities are mandatory. A way of increasing the intensity of the samples has therefore been looked for. As it is highly desirable to keep the delay in the system short the idea of placing the target material inside the discharge chamber has been retained. -464-

rapid diffusion in graphite is also a feature to be retained. The investigation has resulted in the development of a cylinder of graphite cloth stabilized by means of glue and then impregnated with a solution of uranyl nitrate which is subsequently converted to uranium oxide by vacuum annealing. The target, thus obtained, consists of a layer of uranium oxide(possibly converted to uranium carbide at the high temperature in the ion source) on a framework of graphite. The cylinder, which is mechanically quite stable, fits into the discharge chamber of the ion source. No adjustments of the parts of the ion source are necessary.

3. PREPARATION OF THE GRAPHITE CYLINDER

The first step in the construction of a target is to prepare a cylinder of graphite cloth. This is done by means of the pressure tool sketched in Fig 1. The graphite cloth is wound onto a perforated roller, and some glue is added. The roller is then pressed between the two half- cylinders and .dried at 120°C for about two hours and ther. at 400°C in nitrogen atmosphere for 1.5 hours. After this, the graphite cylinder is removed from the roller and annealed at 900 - 1000°C in a vacuum oven for 6-8 hours (to constant weight). By this procedure a stable cylinder is formed of dimensions (outer and inner diameters) determined by the dimensions of the pressure tool. screws

press ^1 ' I perforated roller end plate

( / / handle handle press

nuts

Fig. 1. Pressure tool (for preparation of graphite cloth cylinders) IMPPJ3GNATI0N OF TH^ C'LIN'D?"

Preliminary impregnation experiments vher? pieces of graphite cloth were immersed into an aqueous solution of uranyl nitrate and then dried and ignited, showed that the amount of uraniua taken up was rouglily proportional to the strength of the uranyl nitrate solution. Sous results are given in Fig 2.

D- gi 40 5i 5

>i 30

JO le d craphite cloth pieces immersed n) into hot aqueous solution 'c C 20 d> c O cylinder after one pressure impregnation U n A

UO2(NO3)2 « 6 H2O in water Fig. 2. The capacity of graphite cloth to take up uranium from uranyl nitrate solutions.

The following impregnation procedure was adopted for the graphite cylinder. The cylinder was put into a container (see Fig 3), and the uranyl nitrate solution was sucked through. After the impregnation, the cylinder was rotated for 3 - A hours under a heating lamp and then placed in a furnace at 120°C over night. Next, it was ignited at 900°C in a vacuum oven for 4-6 hours in order to convert the nitrate to oxide. This procedure gave a quite homogeneous deposition of uranium oxide in the different layers of the cylinder. If desired, the uranium content can be increased by re- peating the impregnation procedure.

The amount of uranium on the cylinder was measured absolutely by means of weighing or activity measurements taking advantage of the 135 keV y-ray in the z35U-decay.

5. RESULTS OBTAINED AT OSIRIS WITH DIFFERENT TARGETS

Target cylinders containing 3.3 g graphite cloth and 1.5 g uranium-

235 (I) or 2.2 g graphite cloth and 2.7 g uranium-235 (II) were mounted in

the ion source, and the yields of various elements were measured. , -solution

to vacuum pump FT

inner plug

graphite cloth cylinder

container

Fig. 3. Apparatus for pressure impregnation

The flux of thermal neutrons was estimated to be 4.0xlCT n/cm -sec in tha target position at the reactor power 1 MW . and the fission yields were estimated from Wahl's work . The activity of samples of certain r,'^s numbers chosen so as to correspond to definite fission products was measured by means of a ATT beta detector and an intermittently working tape transport system. The over all efficiencies of the separator for various elements were calculated.

The accuracy is very good for immediately repeated measurements (mean i viation <± 2 % in cases with good statistics). Long-time variations of about + 20 % in intensity are common, however. It is also found that two different ion sources, judged to be identical, produce the same intensities within ± 30 %. Here the main reason for the deviation is unknown (possibly differences in ion source temperatures) . The neutron flux is reproducible with a much higher accuracy. The results are given in Table 1. - 4 6'/ -

Table 1 Efficiency for various elements obtained with old and new target arrangements

Element Mass number Half-life Efficiency °/ measured sec 00 Old system New sy5tm U (0.3 g) I u (1.5 g) II u (2.7 g) Zn 76 5.7 25 Ga 76 30 27 Ge 79 19 2.9 As 81 28 0.16 Br 87 56 0-2 o.4O Kr 91 9.1 0.25 Rb 91 58 3 2.5 Sr 94 72 0.080 Ag 117 73 16 15 Cd 119 200 24 26 In 124 3.2 30 27 22 Sn 127 247 3-4 3.0 0.76 Sb 133 148 4.5 4.0 4.3 Te 135 19 0.6 0.60 I 137 24 0.4 0.90 0.71 I 138 6.6 0.30 Xe 139 39 0.08 0.35 Cs V-l 25 0.4 0.45 Ba 143 12 0.014 0.020

Taking these uncertainties into considerav-cn it seems that most elements are processed with practically the same efficiency by the three targets. This means a considerable increase in sample strength as the new target (II) contains about ten times more uranium as the old one. An increase in efficiency is noted for bromine and iodine (about a factor of two) and for xenon. In the latter case the comparison is not rJequate, however, since tbe eari'er low results may have been caused by escape of xenon from the r-ignetic tape used for collecting the samples.

Target cylinder II * is -rily •.esto.d tor four elements. The low result (4 times It"6 than ear'.ier) fci. ..in cannc. be explained. One attempt to ,-;se how the decay rate inf iaence ' t>.t vie'a was done in tf.t latter test.

n8 It was found that the yiei;l uf T C^.., *> 6-6 sec) was 40 % of the yield of 137I (t . = 7k sec). According to ov.iiet types of experiments this - 4 6 8 - proportion is about the. HST2 for earlier tarpitG.

Table 1 reveals large differences in separator elficior.cicc fron element to element. The efficiency ranges from 0.002 % for barium to 2.7 % for indium. Selenium, the transition elements yttrium to palladium and the rare earths do not seem to pass through the separator at all. The high values for silver, cadmium and indium make v.p for the low fission yields, and strong samples are obtained even in the valley of tha yield-mass curve for fission.

It may be mentioned that very strong sources can be obtained, for instance about 3 mCi for 133Sb at 1 MW reactor power.

6. CONCLUSIONS

Our experiments have shown that it is possible to imprsgnate a cylinder from graphite cloth with considerable amounts of 235U~target material without unduly delaying the emission of the fission products from the target surface. This is evident from the fact that the 3 sec 124In fission product is processed with equally hish efficiency from the new target cylinder as from the old one containing much less uranium.

Obviously, the method of impregnating graphite cylinders is not restricted to uranium but should be useful also for other target materials.

REFERENCES

1) S. Borg, I. Bergstrom, G.B. Holm, B. Rydberg, L.-E. Da Geer, G. Rudctam, B. Grapengiesser, E. Lund, and L. Westgaard, Nucl. Instr. & Meth. 9^, 109 (1971).

2) I. Amarel, R. Bernas, J. Chaumont, R. Foucher, J. Jastrzebski, A. Johnson, R. Klapisch, and J. Teillac, Arkiv for Fysik 36_, 77 (1967).

3) Private communication from reactor staff.

^' A.C. Wahl, A.E. Norris, R.A. Rouse and J.C. Williams, Proc. 2nd symp on phys. and chem. of fission, IAEA Vienna, 813 (1969) and private cotraiuni- cation (1969). Saoci5.-

ON LINE SEPA:?ATIC;; O? r7;'.rTo;r F:;A. AT TIIS oTii^oiiuir:: . :A>JTU.: J.P.ZIRN1I3LD and L.HCHUTZ University Louis Pasteur 3TE A3 BOURG- FRAI I •: 7.

I - INTRODUCTION The main purpose of this experiment consists of isolating short period fission products (of the order of one second) especially in the rare earth area. The principal characteristics of the various subsystems constituting, the on line equipment, which include the fission product transfer by means of a capillary tube, the molecular jet apparatus and the hollow cathode ion source, have been investigated. Further on we shall present the properties of the assembly as a whole. II - FISSION PRODUCT TRANSMISSION The principle of the transmission measurement is to produce aerosols combined with gold particles, to pass them through a capillary tube and to put them on a collector. A 3-counting method has enabled the determination of the proportion of clusters deposited on the collector and on the various surfaces (catcher foil and walls). We definite the transmission as the ratio of these two quantities. A certain number of parameters has been varied : - the capillary tube length : 10 - 20 - 40 - 100 - 200 - 400 cm - its diameter : 0.3 - 0.5 - 1 mm - its nature : teflon, stainless steel - the feeding pressure of the gas - the nature of the carrier gas The results show that the absorption is proportional to the square root of the ratio of the capillary tube length to the gas flow. This latter quantity has been systematically measured because no analytical formula is available in the case of turbu- lent flow.