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Experimental Programme at ISOLDE

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Experimental Programme at ISOLDE Experimental programme at ISOLDE For the past ten years, the Isotope in addition, background problems are targets is characterized by a typical Separator On-line (ISOLDE) has been becoming severe in this region, not time constant of 30s. in action at the CERN 600 MeV because of inadequate shielding from Recently a new target concept using synchro-cyclotron. It is the finest the target but because several refractory metal in powder form or facility of its kind in the world providing experiments are using the full available fibrous carbides has proved to release a wide range of intense secondary ion beam intensity of 6nA (a very high a variety of elements on a very short beams of short-lived nuclei for ex­ figure for a radioactive particle beam time-scale. Due to the combined effect perimental investigation. Its ex­ corresponding to a Curie at saturation of short solid diffusion paths and perimental programme involves some for an incident proton beam of 1 uA). in temperatures above 2000°C, the delay 100 scientists from 15 research the near future, some of these distributions have components in the centres. problems should be solved by install­ region of 1 s. The physics interests and ex­ ing a vertical ion beam tube to the two For example, high yields of rubidium perimental techniques were described rooms above. By using a second nuclides are obtained by irradiating a in the February issue 1967 when the analyzing magnet as the vertical 50g/cm2 niobium powder target at separator began operation. It is now in deflector it should be possible to 2200°C with a 1 uA 600 MeV proton its second generation', and is some­ improve the isotopic purity of the beam beam. Usable yields extend down to times called ISOLDE-2 since a series of and on-line operation in these low 74Rb with a half life of 65 ms which is improvements were introduced at the background regions should, in the heaviest nuclide produced to date time when the SC was being rebuilt to favourable cases, bring the detection in which the number of protons equals provide higher beam intensities (see limit for rare decay modes down to a the number of neutrons. 73Rb is not February 1973). reaction cross section of 10"40cm2. observed and the cross section for its ISOLDE-2 can provide unstable production must be at least a thousand nuclei with half-lives down to a few times lower than 74Rb. This probably milliseconds, to as many as ten ex­ Target and ion-source development indicates that the limit of stability has periments in a typical run. This is now been reached. achieved by applying several new ex­ A key feature in an on-line isotope Like most other target materials the perimental techniques. New target and separator is the target and ion-source. niobium powder liberates several ion-source units make it possible to The target should assure a fast, con­ product elements simultaneously (Sr, use proton beam intensities of up to tinuous liberation of the radioactive Rb, Kr, Br and Se). A variety of techni­ 10uA and they have increased the nuclei produced in large amounts of ques have been developed to assure range of elements available for study target material. Together with the ion chemical selectivity. Often this is ob­ to about forty. A remote handling source, the unit selectively forms an tained by ionizing only one element out system allows a quick change from ion beam which preferably should con­ of the mixture. In the case of the alkali one element to another. Beams of six tain only the isotopes of the chemical metal rubidium, isotopically pure elements only have been used during element under study. beams are formed by means of positive the first two years of operation of The development of this experimen­ surface ionizing effect on the inside ISOLDE-2. tal technique is a field of radio- wall of a tantalum tube heated to There is a versatile beam handling chemistry which also involves metal­ 1200°C. system that supplies ion beams of lurgy, high temperature chemistry and Likewise the halogens fluorine, different nuclei to four measuring surface physics. At ISOLDE, pro­ chlorine, bromine, iodine and astatine stations simultaneously. The heart of duction methods for the isotopes of can be selectively ionized by means of this system is a newly developed about forty elements have been the negative surface ionization effect. electrostatic switchyard with four sets developed through* a systematic in­ Such systems are under development of deflector plates which move along vestigation of possible target and ion and a recent test showed promising the axis of the outgoing beams, so that source configurations. Most of the yields of bromine ions. each of the four beam tubes has a free targets consisted of molten metals choice of a wide range of isotopic held at temperatures between 700 The experimental programme beams without displacing the mass and 1400°C, problems of containment spectrum in the separator. Six ex­ and/or volatility of the target material The following experiments are at perimental set-ups are installed in the preventing operation at higher present under way: lower experimental room. temperatures. The delay in the release 1. In 1972, a sudden change in the This does not meet the demand and. of the reaction products from the liquid shape of the mercury nucleus was 15 Perspective view of the isotope on-line (6) Nuclear spectroscopy (a, (3, Y) separator, ISOLDE. The 600 MeV proton beam (7) High resolution mass spectrometer (1) from the CERN synchro-cyclotron is focused (8) Optical pumping and laser spectroscopy on the target ion source unit (2). A beam of (9) Atomic beam magnetic resonance radioactive ions is formed by a 60kV (10) Collection of radioactive sources for acceleration stage and is mass analyzed in a off-line work (hyperfine interactions magnet (3). Individual masses are then selected in solids, determination of shifts in X-ray by electrostatic deflection in the switchyard (4) energies, targets for nuclear reaction and distributed through the external beam-lines studies) (5) to the various experiments. These are: (11) Beta delayed particles (12) Range measurements of ions in gases detected when the number of neutrons finding. The interpretation now accept­ volving a dye laser, pumped by a decreases from 107 to 105 (i.e. from ed is that of a shape transition from a pulsed nitrogen laser which serves as a 187Hg and 185Hg). This discovery was slightly oblate nuclear shape for 187Hg tunable light source. As laser action achieved using the RADOP (Nuclear to a strong prolate deformation for cannot be obtained in the ultra-violet Radiation Detected Optical Pumping) 185Hg. region, it is necessary to start with a technique described in the November Information on the charge radii of beam of visible (green) light. The fre­ issue 1971 and it was possible the even-even mercury isotopes for quency is then doubled by using the because of the intense beams of these 192Hg and below was still missing non-linear effects in a crystal and the short-lived isotopes available at because the RADOP method is not resulting ultra-violet beam can induce ISOLDE-1. Since then extensive ex­ applicable to nuclei with zero spin. To transitions between the electron perimental and theoretical work has find this information, a laser ex­ energy levels of the mercury isotope been concentrated on this unexpected periment was set up at ISOLDE-2 in­ under investigation. If the frequency of 16 The equipment of the tunable laser experiment A graph of the charge radius of mercury which is giving measurements of the radii of isotopes plotted against atomic number zero spin isotopes. The predecessor of this ranging from A = 205 (125 neutrons in the experiment, using optical pumping, discovered nucleus) to A = 181 (101 neutrons in the the abrupt change in radius of mercury nuclei as nucleus). The abrupt change at A = 185 was the number of neutrons is reduced. discovered by the optical pumping, RADOP, technique. The new laser spectroscopy (Photo CERN 324.10.76) experiment has already added some measurements and the interesting value for A = 184 will be published shortly. the laser (about 1015Hz) is adjusted so that the energy conveyed by the light coincides with the atomic transition energy, fluorescence radiation is observed. By simultaneously measur­ ing the resonance radiation of a stable mercury isotope in a magnetic field, the isotope shift is determined with an accuracy of about 100 MHz. The combined results of experi­ ments to date are shown in the diagram. As a function of decreasing neutron number, a regular shrinking of the charge radius is observed starting from the nearly doubly-magic nucleus 205Hg down to 188Hg. The three odd isotopes 185Hg, 183Hg,181 Hg exhibit the anomalous charge radius cor­ responding to that of 196Hg which has 10% neutrons more. A measurement of the isotope shift of 184Hg is being analysed to see whether the charge radius jumps to a value similarto those of the odd neighbouring isotopes or, as indicated by gamma spectroscopy on 184Hg, continues to follow the trend of the heavier isotopes. The latter would mean a drastic odd-even staggering O GROUND STATE corresponding to an adding and sub­ tracting of about eleven neutrons. * ISOMERIC STATE / Such an effect is completely unknown in other mass regions. • * CLASSICAL OPTICAL SPECTROSCOPY / ©• RADOP 2. A double focusing mass spectro­ ©• LASER SPECTROSCOPY / meter has been installed to measure isotope masses with an accuracy better than 100 keV (i.e. a relative ac­ curacy of 10"6 for A = 100). It is a con­ tinuation of the very fruitful spectrometry measurements carried out for many years by the Laboratoire René Bernas from Orsay (see, for ex­ ample, June 1973).
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