KESONANCE CAPTURE OF PROTONS BY Mg2^ AND Mg25

DISSERTATION Presented In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy In the Q-raduate School of The Ohio State University

By WARREN E. TAYLOR, B. A.

The Ohio State University 1952

roved by:

Advls ACKNOWLEDGEMENT

I wish to express my appreciation to Dr. John

K. Cooper for the interest and encouragement exhibited during this research and for the cooperation shown during ay whole association with the electrostatic generator program, I am indebted to Dr. J. C. Harris for many excellent technical suggestions, and to the other members of the group, both technicians and graduate students, for considerable assistance before and during the investigation of this problem. Sincere appreciation is expressed to the Development Fund and the hesearch Foundation of the Ohio State University for their aid during the last five years. I also wish to acknowledge the loyal encouragement of my wife during this period.

S09684 TABLE OF CONTENTS

Page I INTRODUCTION 1

II MODIFICATION OF THE E-uECTROSTATIC GENERa TOR

The Pressure System lu

HiNu Voltage Changes 12 Focussing thj Ion Beam 20

Effect of the Analysing Magnet on the Beam 25

The Lens System 32

The Proton Source 35

III ENERGY CALiB-ttATION 40

IV ASSOCIATED MEASUREMENTS a MB PROCEDURE

Gamma-ray Counting 44

Positron Counting 53

V PROTON CAPTURE BY NATURAL Mg 54

VI PROTON CAPTURE BY Mg25 68

VII PROTON CAPTURE BY Mg24 82

VIII SUMMARY 95 IX BIBLIOGRAPHY 100

X APPENDIX 105

11 RESONANCE CAPTURE OF PROTONS BY Mg24 AND Mg25

I. INTRODUCTION

Considerable work has been done In the last twenty

years toward investigating the structure of light nuclei by bombardment with charged or uncharged par­ ticles accelerated by various machines. Since a given nucleus can usually be formed by means of a variety of reactions, the avenue of approach can often be adapted to the equipment and elements available. This Is fortunate, for much lest information is known

concerning the core of an atom than about Its outer

structure. Experimental evidence shows that theie are discrete nuclear energy states in which a nucleus may exist and with which are associated a given angular mo­ mentum and spin. The positions of the.e levels may some­ times be predicted on the basis of various models. The correlation of observed and predicted states is much poorer than in atomic physics, however, for the poten­ tial function differs greatly from the fairly simple Coulomb potential. Similarly, the average level spac- ings and widths depend on the resultant of nuclear and Coulomb forces from a number of nucleons. The average spacing of the levels usually varies considerably from element to element. In general the levels become broader and closer together toward higher energies until there is effectively & continuum.

Experimentally, the investigation of these discrete

levels by bombardment involves resonance processes. That

is, the probability of forming any given compound nucleus is large for those energies of the incident particle

correlated with an allowed energy state and small else­

where. When a nucleus is formed by partlole capture, the

position of the levels above the ground state is dependent on the mass excess of the original particles over the

compound nucleus, and on the fraction of the kinetic energy of the accelerated particle available for excita­

tion. A small fraction of the latter energy is associated with motion of the center of mass of the system. The mass excess when converted into the equivalent amount of energy

is known as "the Q of the reaction. The value is usually not accurately known and makes uncertain the absolute position of the levels. However, it is the same for all

levels determined by a given reaction, so the relative

spacing can be w

As the compound system must return eventually to a state of minimum energy, one can hope to check the position of the states of the compound nucleus under

consideration by measurement of the released energy. Here again, nuclear spectroscopy is much more complex than the atomic subject. If energetically possible and if not forbidden by transition rules, protons, neutrons, deuterons, tritons, or alpha particles may be ejected. The lifetime for gamma-ray emission alone is much longer than for particle emission so pure capture processes occur usually for low excitation energies or when particle, emission is not permitted for heavy particles of any type other than the incident type. After the primary radiation, the resulting nucleus may release the remaining excess energy by emission of one or more gamma-rays or by pair emission. The problem of determining the energy expended in thiB way depends on identification of the process and measurement of the energies released in the various transitions.

Completion of the transformations Just described do not necessarily terminate the possibilities for informa­ tion. Quite often the resulting nucleus is unstable with respect to a neighboring isobar and can decay by

K-electron capture or beta-emission to some state of a residual nucleus. The former process is identified by x-rays characteristic of the residual atom and often by gamma-rays identified with excited states of the residual nucleus as well. The latter case may give a simple beta spectrum whose maximum energy is equal to the nuclear mass excess less the beta mass or it may give a complex spectrum*

For other then simple decay, the less energetic beta-

rays are accompanied by gamma-rays which represent the energy released by the residual nucleus in returning

from excited levels to the ground state. Thus information gained In such processes depends very much on identifica­ tion of Just what occurs as the parent decays to a daughter nucleus.

The particular light element studied in this dis­

sertation is magnesium, which is composed of three stable isotopes with the relative abundances

24 , ^ 25 26 Mg 77.^% Mg ^ 11.5% Mg 11.1% Bombardment of magnesium by protons could conceivably produce a number of reactions. Of all the proton-induced reactions in magnesium, the possibilities were immediately limited to two for each isotope by the energies that were available for this investigation and for the earlier experiments of others. The Q*s of the reactions, which are shown on page 5, are computed from the mass values of Mettauch and Flammersfeld1 when available. Otherwise 2 the values from Bethe's table are used. All reactions other than proton capture arid inelastic scattering are endothermic and require at least 2 Mev. Consequently, thejjffehould not be initiated in the present energy range of the electrostatic generator. Proton-Induced heactlons On Magnesium Isotopes

M g 24 + P — ♦ Al25 + 2.4 Mev

M g 25 + P — P Al26 + 5.2 Mev Proton capture w 26 Mg + P — t Al2? + 8.5 Mev

24 24 Mg + P Mg + P 1 * iitr Inelastic scattering M g 25 + P M g 25 + P' p hIf to* 4 Incident proton energy MS 26 + P M g 26 P* p h *

MS 2 4 + P — ♦ N a 21 + a - 6.5 Mev M « 25 + P Na22 + a - 5.4 Mev Alpha emission 26 Mg + P — 4 Na2''* + a - 2.0 Mev

24 24 Mg + P —* Al + n - 15.8 Mev

MS 25 + P Al25 + n - 4.8 Mev Neutron emission

+ MS 26 ♦ P r Al26 n - 6.1 Mev

24 2;> Mg + P —* Mg + d - 15.5 Mev 25 Mg + P - - t M g 2^ + d - 5.0 Mev Deuteron emission , 26 Mg + P — 9 M g 25 + d - 9.1 Mev

-5- 24 25 The proton capture reactions with Mg and Mg result in the radioactive nuclei Al2^ and A l ^ reepec- 26 tively, while the capture reaction with Mg produces 27 25 26 the stable nucleus Al . Al and Al are both positron emitters which decay with half-lives of approximately seven seconds. The maximum energy of the positrons is about 2 Mev. Inelastic scattering produces only a proton, one or more gamma-rays, and the original nucleus. The latter is stable, and in this case no activity is present after a fraction of a second. Since the yield curves of this dissertation come from measurements of positron activity, it is safe to assume that only the capture 24 25 25 26 reactions Mg (p,Y)Al and Mg (p,Y)Al were observed. The capture process was first excited by Curran and

StrotherB^ who used targets of medium thickness and 200 to 1000 kev proton energies. Those gamma-ray resonances for which positron activity was observed were assigned to Mg25(p,Y)Al26, and all others were assumed to be 26 , 27 caused by the reaction Mg (p,Y)Al . The production 25 24 of Al , which Is also radioactive, from Mg was not 25 considered probable since Al was unknown. Three 26 25 resonances were assigned to Mg and five to Mg .

TVro of the latter can be shown to belong to a reaction OA with Mg • Because of their fairly thick targets and poor resolution, they did not resolve close resonances -6- and a large number of the weaker resonances were missed.

The next group to work on this element was Hole, II. Holtsmark, and Tangen who made use of thin targets of

the natural metal and protons In the region of 200 to

500 kev. Only the garnma-ray Intensity was measured. No

attempt was made to separate out effects of the three

Isotopes by means of the radioactivity accompanying

some of the resonances. In all, eight resonances were

determined which can now be shown to belong to reactions 2 4 26 with Mg and Mg • Their numerical values are somewhat

in error owing to a faulty calibration. 5 Tangen duplicated these measurements, but with

greater accuracy and sensitivity. An anticoincidence

arrangement was used to detect the gamma-rays emitted

as a result of each disintegration. The positron activity

was checked at each resonance in order to assign it to

an isotope. On this basis, seven resonances were assigned

to Mg2^(p,Y)Al2^ and seven to either Mg2^(p,Y)A12^ or

Mg2^(p,Y)Al2^ . Since the energy excess, which was emitted

in the form of gamma-rays, was much less for Mg (p,Y)

than for Mg2^(p,Y), he concluded that two of the re-onanoes

which had accompanying radioactivity belonged to the former reaction after measuring the gamma-ray energy.

This conclusion was Boon confirmed by Orotdal, et al.^ by bombardment of small quantities of isotoplc M g 2if

-7- obtained from a mass spectrograph. The work of Tangen provides excellent data in the low energy region.

Early in 1950, it was decided that accurate data above 500 kev should be supplied by the Ohio State elec­ trostatic generator which works best in the region of 500 to 1600 kev. The Van de Graaff generator is par­ ticularly well adapted for resonance work because it provides a beam of particles of well defined, but easily varied energy, and it has comparatively good resolving power. Thin targets of natural magnesium were made and bombarded with protons of reasonably monochromatic energy.

The gamma-ray intensity was obtained at each point in the energy range of 700 to 1600 kev. Seven resonances were assigned*^ in this region. Below 700 kev yields were low for a good analysis. Fluorine contamination also masked certain regions because of its very high yield compared to the magnesium. However, the data indicated a great deal of activity which Curran and Strothers^ had not resolved, and also revealed a number of resonances for proton energies above 1000 kev.

The availability of separated Isotopes of magnesium from Carbide and Carbon Chemicals Division, Y-12 Area,

Oak Ridge National Laboratory Immediately suggested the possibility of resolving the complex spectrum by proton bombardments of the separate isotopes. A ninefold -8- pc increase in the yield could be uxpected from Mg and

Mg . Upon allocation by the Isotopes Division of the Atomic Energy Commission, small amounts of the oxides of the separate isotopes were obtained and were made

into thin targets. The reaction Mg2^(p,T)Al2^ is the 8 subject of a thesis by L. N. iuissell while the reactions

Induced by the proton bombardment of Mg2if and Mg2^ are the principal topics of this thesis. The gamma-ray yield from the two lighter lBotopes was considerably 26 weaker than that from Mg (p,Y), and was masked by gamma-rays from a heavy fluorine contamination. To eliminate these difficulties, the equipment was modified to count positrons from the radioactive nuclei Al2^ and Al2^ which decay to Mg2^ and Mg2^. This increased the sensitivity since the counter was many times more sensitive for positrons than for gamma-rays. It also eliminated the interference from the fluorine.

A great deal of the preparation for the work discussed here involved modification and maintenance of the machine in order to produce stable operating conditions and an ion beam of sufficient strength. To attain this end a number of experiments were tried in an attempt to deter­ mine optimum conditions for operation of the ion source, for controlling the drain of charge from the high potential cap, for producing a stable path for the beam, and for focussing a resolved proton beam. These are discussed in detail in the next chapter. XI. MODIFICATIONS OF THE ELECTROSTATIC GENERATOR THE PRESSURE SYSTEM

The basic construction of the electrostatic generator Q 10 used in this work has been described elsewhere so only a brief sketch will be given here. Modifications up to early 1950 have also been covered^ as well as adaptations 12 for special experiments . Consequently, only significant changes made Bince that time will be dealt with. The Ohio State Van de Graaff generator is of the horizontal, pressure-insulated type, being pattexned on a smaller scale after the Wisconsin and Notre Dame machines.

By utilization of a single high voltage cap and pressures up to 80 lba/in^ it has attained a voltage of two million volts. The highest potential at which satisfactory work has been done is 1700 kilovolts. Mixtures of 60# nitrogen and 20# carbon dioxide have been found to have excellent dielectric propex'ties for many machines of this type.

Pure nitrogen is supposed to be unsatisfactory because it tends to suppress the corona needed for control of the cap potential almost up to the point where undesired sparking sets in. Ordinary air, if well-dried, seems to have better dielectric strength than nitrogen, but is not used since the oxygen may support combustion.

The pressure of the insulating gas has an appreciable -10- effect upon the intaMity of the proton beam produced by the Van de Graaff generator. In general, the maximum amount of charge 1 b accelerated for a pressure Just sufficient to suppress sparking to the tank walls. This is apparently associated with optimum distribution of potential down the corona rings to ground. In the range of 3C0 to 500 kilovolts, a pressure of 20 lbs/in2 provides the best beam, although it is more than adequate to suppress sparks to the tank wall. The presence of appreciable moisture mixed with the gas in the tank will lower the sparking voltage seriously at any given pressure. If the tank has been open to air at high humidity for an extended period of time, the cotton charging belt and the surface of the insulating supports retain considerable water vapor. In this case, charge leaks off as fast as it is accumulated and no very high voltage will be built up. Proper operating conditions can usually be obtained in about two hours time if the belt is driven and charg# sprayed on as in actual operation while the enclosed gas is circulated through the drying trap. In this way the belt is dried both by electrical heating and by rapid exposure to new volumes of gae.. -11- HIGH VOLTAGE CHANGES

In an attempt to raise the maximum potential avail­ able from the electrostatic generator, a number of exper­ iments were tried. An attempt was made to eliminate various irregularities in construction in order to provide better performance at all voltages and especially at high voltage where the effects appear to be more critical. A second attack was to vary the present system of corona leaks and drains Into patterns which appeared more logical for high potentials.

The spray comb which provides the belt charge is composed of steel needles 0.25 in. apart pressed into a brass bar over a length slightly le-s than the width of the belt. It is fed by a 55 kv rectified power supply with a built-in current limitation of 8 ma. In order to attain high cap voltages at correspondingly high pressures of the Insulating gas, the potential on this spray comb must be increased to spray sufficient charge on the moving belt. Quite often, however, this results in spark break­ down from some of the needle points through the belt to the metal roller below. In turn this causes surges in the current fed to the comb and thus irregular and inadequate spraying of charge, on the belt. If the spray needles are placed at a point whole the belt 1b several Inches

removed from the steel driving roller, this condition is

worsened. Thus the corona discharge is most effective when the spray comb is directly over the steel roller.

The later arrangement Increases the electric field between the two, encouraging the flow of charge toward the charging belt. As a further experiment, the spacing between the points of the needles and the belt surface was set at two dif­ ferent values and the results noted. The spark discharges occurred with about equal frequency for spacings of three eights and five eights inches for the same cap voltages.

Obviously, the potentials applied to the comb Itself were different in the two cates since the requirement was for equal amounts of charge to be carried up to the cap. The former spacing has been used for some time in regular operation. The last experiment on the spray comb was to replace the phonograph needles, which had fairly coarse points and which projected about three quarters inch from the bar in which they were pressed, with No. 9 sewing machine needles which have rather fine points. These project

1.25 in. from the bar and are spaced 0.25 in. apart as before. This change eliminated a great deal of the irregular breakdown from the needle points and provides

-13- a positive step toward attaining higher working poten­ tials for the machine.

The next points of attack were the two strings of

corona drain needles. One set is connected to alternate

corona rings down the length of the tube while the second

set is connected to the metal focussing plates which

separate the porcelain sections of the accelerating tube. The two sets ere connected together every 18 inches to

insure electric potential gradients down the tube similar to those down the corona rings. The physical arrangement of the needles is such that there is about 0.5 in. spacing between a one inch square copper plate fastened at one of the given positions and the corona point fastened to the next successive position. Phonograph needles, originally used for corona points, were replaced with No. 9 glovers needles. Some improvement in performance was noted.

The main effect of the change seemed to bo a displacement in the focusaed position of the beam spot, apparently caused by a different distribution of potential drops along the accelerating tube. As expected, the beam spot gradually returned to normal position as the points of the needles were burned off through corona discharge.

Much of the focussing of the beam is done by varia­ tions of the first and second corona gaps at the high voltage end of the electro-atatlc generator. Since the field -14- inside the accelerating tube between the steel shields that are electrically connected to the terminals of the

variable gaps acts like a lens in focussing the ion beam,

these are called the controls for the first and second lenses. As might be expected, the first lens exerts a much greater degree of influence than the second. The corona gap of the first lens consists of a single needle

fixed at right angles to the end of a brass bar supported

by a single pivot. The other end of the bar is then

fastened to the fishline control coi'd which is moved

against spring tension by remote operation. The spacing between the point and a fixed plane can be varied up to

1.25 in. For a while, two No. 5 glovers needles were used to replace the original needle. This resulted in

poorer focussing of the beam under all conditions.

The point to plane gap for the fixed elements of the corona string was next increased from 0.5 in. to 0.625 in. This produced no noticeable gain in useable voltage nor did it alleviate the flat sparks, i.e. sparks other than to the tank wall, s u c h as down the length of the corona string. It is likely that tills was a move in the wrong direction. Possibly the gaps should be decreased for higher voltages and higher pressures so that current could drain down the corona string to ground equivalent to thst at lower voltages.

-15- The fact that the corona leak needles are fastened only to alternate rings give* rise to a series of possible discontinuities. Open air tests, made by charging the cap

to about 200 kilovolts and then shorting various corona

rings to ground, revealed that flat spark discharges set up in this wa| very often Jump to an intermediate ring which is electrically floating. The large drop in potential over a small number of rings provided by this experiment simulates actual conditions when working with volttges in the neighborhood of 1500 kilovolts. In normal operation, occasional sparking to these intermediate rings may produce such discontinuities in the potential gradient

from cap to ground that the sparks continue down the whole corona string. In the high voltage electrode itself the charge brought up by the belt is picked off by a set of phono­

graph needles pressed 0.25 in. apart into a brass oar which is electricalconnected to the belt roller but not directly to the cap Itself. An induction type voltage doubler, like that of Van de G-raaff, Compton, lb and Van Atta , is used to spray charge of opposite sign on the belt for its return run. A plate fastened to the pick-off comb faces a set of needles shorted to the cap. These needles can be moved toward the plate to obtain the desired potential drop. Below the pick-off comb is the -16- return comb which Is fastened electrically to the cap. If positive charge is originally brought up by the belt, the pick-off comb and roller remain positive with respect to the cap although the whole system rises with respect to ground potential as more charge is accumulated. Thus

the return comb faces the belt going over a roller which

is positive with respect to the comb. Negative charge leaves the return comb in the direction of the roller

but is intercepted by the belt and carried back to ground.

In previous tests made several years ago, it was found

that the doubler circuit did not have much effect up to about 600 kilovolts. Measurement of the return current on the belt showed the charge had the same sign as that carried up, though the charge density was much smaller. At higher voltages, charge of the opposite sign was returned to ground, but the charge density was small. Many of the flat sparks at high voltages can be identified by a ringing note which is apparently characteristic of the arx . ngement of the support where the spark was emitted. This type of spark was essen­ tially eliminated by closing the doubler drain needles, whereupon the advantages of the doubler system were lost.

It was thought that sparks were Jumping inside the high voltage electrode from the return comb to the steel belt pulley which was occasionally exposed to the end needles -17- of the return comb by the lateral shifting of ihe charging belt. The fact that the end needles were considerably

more burned by corona than the centrally located ones

seemed to confirm this. The end needles were removed so that no point of the return comb was less than an inch away from the exposed metal face of the pulley. A new

set of No. 9 sewing machine needles was installed in the return comb and its spacing from points to belt surface was varied from 0.15 to 0.4 in. None of these changes eliminated this particular kind of flat sparks.

The doubler drain system formerly consisted of about

twenty phonograph needles pressed into a three inch brass bar which was pivoted toward a plate on the pick-off comb

Oy an externally controlled fishllne. This was replaced by six needles of the same type mounted aiound a disk of one inch radius fastened in turn to a shaft that slid horizontally toward the plate of the ; ick-off comb.. In both thete cases changing of the doubler gap had little eifect on the cap voltage until the gap across the doubler was greater than the gap from the return comb to the belt. At this point the voltage rose abruptly and was then relatively unaffected by further opening of the doubler drain. Tests made in open air at about 200 kilovolts potential showed that the points of the return comb abruptly lit up from corona at the critical point,

- 18 - Three of the needles of the doubler drain were then re­ moved. This appears to decrease the sharpness of the discontinuity. A long single sharp drain needle, perhaps

in parallel with a fixed resistor, might give somewhat better characteristics.

-19- FOCUSSING THE ION BEAM

The problem of how to obtain high voltage and stabil­ ized conditions at all voltages is intimately connected with the problems of a proper type of ion beam. In fact, one of the limitations on voltage is present only when the ion beam exists, i. e. when the accelerator is in practical use. Beyond pressures of ; x 10 mm. of Hg.the mean free path of ions in the accelerating tube is such that a continuous discharge is set up at usual operating voltages.

Immediately the cap voltage falls considerably because of the excessive drain. Visually the critical point is obvious since a blue glow can be seen inside of the accelerating column instead of bei *g Jnst along the path of the ion beam. Conversely, a minimum pressure in the system is conducive for attainment of high voltages. This, however, must be balanced against a minimum hy­ drogen flow required for stable operation of the arc In the ion source and the fact that the ratio of protons to total ion beam increases with the hydrogen excess. A further consideration is the possibility that the increased numbers of electrons knocked off by the beam in its passage down the tube will produce a greater x-ray intensity around the cap, which may in turn promote sparking. -20— The position of the beam on the exit glass plate is dependent on voltage because of the fact thst the axes of all focussing sections do not coincide. One factor which contributed to this difficulty was a serious bending of the textolite supports. These were replaced in 1949*

Although the new textolite tubes were internally braced, the end of the cantilever is dropping slightly as shown by the following table:

Sag from original position Time from original in Inches construction

i/8 2 months 1/2 4 " 9/16 6 " 11/16 9 " 15/16 12 " 1 1/4 j O "

Starting in the summei' of 1950 other measures w d e attempted to align the porcelain and steel sections of the vacuum tube with the exit port which is part of the fixed base. These changes were made during a period of about

9 months. First the bases of the textolite tripod, which forms the cantilever system, were shimmed in an attempt to compensate for the amountof sag in the cap accruing after theoriginal installation. Then the ion source end of the accelerating tube was shimmed about 0.75 in. relative to the supporting frame in the cap end. Also this end was mechanically shifted to the side by insul­ ated guy wires to see if the supports were maladjusted horizontally. Finally the yoke which transfers the lateral -21- compression forces exerted by the porcelain tube to the

brass collars on the three textolite supports was tilted

around a horizontal axis to raise or lower the compression

springs for the tube. The results of the^e teats were for

the most part disappointing, for a change of the types

listed above was usually compensated for by a contrasting bend in the tips of the textolite tubings. Thus the tube

retained nearly its original position.

The accelerating tube is composed primarily of three

large sections, each formed by a number of hollow porcelain cylinders separated by steel accelerating electrodes.

The section nearest the cap was rotated 160° around its own axis. Tne position of the beam on the exit port

was now low instead of high, but it was still off to

the same side. The vertical displacement was apparently conaected with the first section or its Junction with the

ion source, and the horizontal displacement was conaected with misalignment of the whole tube. The first accelerating section was closely inspected for imperfections, some minor correctipnB were made, and it was then replaced as before. There was no change

in beam position. As a consequence of this result, the

Junction of the steel end of the secticai with the ion

source was next considered. Various deliberate misalign­ ments were made on this end or in other parts of the ion source itself. The most Important lenses re the probe lens and the first lens. The successive lenses have increas­ ingly lesu effect since the time that any ion spends in

any given lens field is less as the velocity of the ion

increases. It was found that tilting the ion source about

2°, changing the first lens spacing about 0.5 cm., tilting

of the probe canal about 2 °, or shifting of the probe

canal about 0.2 cm. with respect to the first lens had no noticeable effect on the beam path. Translation of

the ion source 0.5 cm. seemed to produce a corresponding shift of the beam spot and in the same direction. The next point of attack was the alignment of the one

steel and the three porcelain sections of the tube. For vertical mea.urement a mark was made on the center of the tube about every 18 inches. The top of the sections could not be used because the individual porcelains vary in diameter slightly. A water level was tr.en constructed

from two graduated tubes connected by a very long length of rubber hose. One meniscus was set on successive center marks while the displacement of the othor meniscus from a given fiduciary mark was recorded. It was impossi­ ble to make the whole accelerating tube horizontal because of the previously indicated reasons, but it could be given a uniform slope with no discontinuities by proper adjustment of supporting straps at the Junctions -22- of the sections and careful shimming where the steel section Is Joined to the fixed base plate. Horizontal alignment of the tube was then made by measurements from a string stretched alongside. A small slope of the tube is of no Importance since the analyzing magnet and recording equipment can be adapted to a certain amount of vertical tilt. When a beam thi't leaves the last section asymmetrically Indicates an electrical or mechan­ ical misalignment of the column, one may expect the posi­ tion of the beam to vary appreciably with voltage.

Optimum conditions have not yet been achieved and a terminal plate with an exit hole slightly off center is being used. One control which seems to have an effect on the spot position is the cap corona drain. To a set of seven needles equally spaced on a small brass disk is coupled a threaded rod that can be screwed in and out of the insulated plug in the tank wall above the cap. Its influence may be related to the relative distribution of voltage down the accelerating column for various charging currents brought up by the belt. When the drain is screwed inward the beam image is moved vertically in the same direction that it moves under the influence of greater cap potential. At high potentials the cap corona drain must be screwed outward in order to keep the -24- potential gradient low enough to ;.rev^nt sparking. This change has little influence at these voltages. However, the voltage as a whole may be more unstable because of the reduction in drain current. The external apparatus dealing with magnetic separation and definition of the beam should be fixed into position at high energies.

Then optimum conditions for vertical centering can probably be attained over most of the voltage range of the machine.

-25 EFFECT OF THE ANALYZING MAGNET ON THE BEAM

The beam as It appears before entering the analyzing

system forms a round image with a core in the center.

About 90% of the total charge available exists in the

focussed core of the beam. The rest consists of Ions

that started acceleration at such a wide angle that they

were never focussed or else were scattered by molecular

collisions. This core must pass first through a coarse hole to eliminate the very widely scattered Ions. It Is

then analyzed by a magnetic field, whereupon the separated proton core must pass through a 1/32 In. slit before

entering the target chamber. Contrary to what might be

expeoted, a core that Is very sharply focussed on the defining silt has not proven to be Ideal, even though

the resolution Is reduced by defocusslng. Due to small rapid fluctuations In the cap potential and In the magnet

current, the beam sweeps back and forth a millimeter or

so on either side of the slit. For a sharply focussed core, such operation results In bursts of charge being

recorded by the Integrator. All the charge passed by the slit except when the core Is exactly centered usually

leaks off without being recorded. This procedure also

Increases the strain on the operator who Is attempting

- 2 6 - to correct for the slow fluctuations of the beam by

adjusting the voltage when the audible clicks from the

Integrator counts die off. On the other hand, a slightly

defocuBsed beam gives a fairly steady counting rate and

a leakage loss that can be measured with some confidence.

So far all discussion of a focussed fluorescent

spot on a port of the analyzing magnet had assumed that

this spot is circular since all preceding defining aper­

tures , if the proton defining slit is neglected, are

round. This is not the case. The Images on the 0° and

15° ports are apparently circular, but the beam through

the 30° port is not. This is caused by the fact that

this fraction of the beam passes through a great deal

of the fringing field of the magnet. The effects of the

fringing field from a magnet of comparatively small pole

face are manifold and difficult to predict with accuracy.

Quantitatively, the forces depend on the relative strength

of the field components along any given line (1) in the direction of the central field, (2) in the direction of

the beam Itself, and (3) perpendicular to the other two directions. They also depend on the flux density at various points.

The component In the direction of the central field

just deflects the beam to one side, which corresponds to

the Intended function of the magnet. The component in the direction of the beam itself has no effect. The third component pushes the ions upward or downward, depending on its direction. For any given flux line this component is directed inward at one pole face and outward at the other. Now the fringe field has little net effect on those ions entering and leaving the magnet nearly perpendicular to the center of the pole edges.

This is represented by the almost undlstorted molecular beam at the 15° port. The proton beam through the 30° port, however, passes through the pole faces fairly close to one edge. Consider the beam entering the magnet to be circular in shape with a central core. It is deflected toward the right, as one faces the proton source, by the vertical component of field. Since the fringe field bows outward, the ions at 9 o'clock on the image pass through flux of greater density than those at 3 o'clock. Consequently, the former are deflected further, and all together tend to form an ellipse, with a vertical major axis, at this stage. If all the lines that bow out terminate on the pole faces near the edge, the flux density there will be greater at the faces than at a point midway between. As a result, that portion of the original beam at 12 o'clock and 6 o'clock is deflected even further than the other portions. The image is now a crescent moon, open to the

- 28 - right. Finally, if the fringe field component perpen­ dicular to the beam and to the central field is at all strong, the beam may be made to diverge or converge in a vertical direction. The image can now vary from a short flat oval with its major axis horizontal to a long vertical crescent line. On the basis of this infor­ mation, a desirably shaped image is selected* The one found to be most suitable for the work done thus far is Bhaped roughly like a blunt equilateral triangle, with the core two-thirds of the distance from one corner at left of the port aperture to the opposing flat side at the right. This picture does not necessarily give the most total proton curient, but it does provide

the largest current through a slit, and was selected on this basis. bince the residual beam behind the core does not cut off sharply on either side, the integrator c counting rate is roughly symmetrial for minor fluctuations.

Such an image may not be suitable for use with the automatic voltage control whose split slit accepts all the current on either side. The asymmetric charge distribu­

tion would tend to force the core off the slit to the low voltage side. This might be corrected by use of a small circular aperture after the beam leaves the magnet.

For future reference the present position of the magnet with respect to the analyzing chamber is given

-2 9 - In Figure 1. In this ca^e the magnet apparently has three degrees of freedom, two of horizontal translation and one of rotation. These are not all Independent, however, since it is also required that the analyzed proton beam must fall along the axis of the 20° tube. For conven­ ience, three coordinates are given here. The point on the magnet frame immediately above the intersection of the 0° and the 30° port pipes will be called the ref­ erence point* The angle of the magnet frame taken counter-clockwise from the 0° port pipe will be called the reference angle. It is assumed that the neutral beam is lined up with the 0° port*

-;>0- -VIEWING SOX WITH VAOUUM VALVE REFERENCE ANGLE • 2.9* MAGNET ASSEMBLY

REFERENCE MOLECULAR SEAM PORT POINT PROTON BEAM PORT

V OL T AS E STABILIZING SLIT 0.1 ON

BEAM SHUTTER VIEWING 0 WIN00W8

GLIDING LEAD SHIELD

TARGET HOLDER ANALYZING SYSTEM AND

0 M COUNTER POSITRON COUNTING ASSEMBLY LEAD FIGURE I. BOX -31- THE LENS SYSTEM

At the final stage, the conditions under which the

controls can focus the beam are of Interest. Whether

a sharp focus can be attained should depend only on the

first two lenses and the probe. Actually, however,

unstable conditions in the ion source, slight sparking

of the probe, or too high a hydrogen pressure may give a multiple, flickering or diffuse spot as the best obtain­

able image. This results in a considerably weaker beam.

Filling of the probe canal with particles from the

filament coating also has been a source of trouble. An

excellent article on probe operation and electrostatic generator performance has been published by Bailey and

Williams14.

The equipotential surfaces between each successive

electrode inside the accelerating tube may be crudely

approximated by a spherical shape. They form an

electrostatic focussing system. A force exists on the

ions in the direction of the electric field lines. Thus the ion is pushed toward the axis as it enters the field

and outward as it leaves. Since it is moving more

slowly as it enters, the net result is to push the ion toward the center of the tube. The probe and the series

-32- of electrode pairs form a long and complex focussing system, of which the first few elements have the most control.

Tests on this focussing system show that the resul­ tant Image is erect. Three holes were drilled in the probe in the form of an Isosceles triangle, the largest being centered. The image focussed upon a glass viewing plate was not inverted. It was also found that for any given cap potential a sharply focussed core could be obtained for two positions of the first lens, provided

the probe potential was above a minimum voltage. Curves

for three different values of cap potential are shown

In Figure 2. The second lens was fixed In this case with

a corona gap of 0,5 in. Each point represents a probe voltage and a first lens voltage (proportional to the gap of the first lens corona drain) for a sharply focussed

spot. For probe voltage below any given curve, a spot

of minimum size could be obtained by a proper setting

of the first lens, but not a sharply focussed spot. The sharpness of the focus also dependB somewhat on

the size of the probe canal. In general, the larger the

hole, the more total current present from the core,

while the smaller the hole, the finer the focus. Holes

drilled with number 56,58,60, and 63 drills have all

proven satisfactory.

33- -14000 CAP POTENTIAL

3 0 0 0

02000 g 1000

DISTANCE ACROSS FIRST CORONA GAP (MM ) CONDITIONS FOR FOCUS OF THE ION BEAM FIGURE 2 THE PHOTON SOURCE

The proton source Is of a type developed by Dr. W. 15 E. Shoupp at the Westlnghouse Research Laboratories.

It consists of an oxide coated filament about 0.25 In. distant from an anode. The latter is at 150 volts potential Initially, but drops to about 70 volts as an arc Is struck to the filament because of a series bal­ last resistor. The filament Is operated at a temperature of approximately 850°" C. (Incipient bright red heat), but may be operated at an even higher temperature in order to obtain an arc late In the lifetime of the filament. Since the Ohio State machine does not have a differential pumping arrangement, the pressure In the accelerating tube Is closely connected with the pressure

In the source. The connecting channel Is the probe canal. Consequently the latter must be of such a size that a satisfactory pressure can be maintained In the source for the arc to exist. At the same time, the speed of the pumps must be enough to keep the pressure -5 in the accelerating tube less than 5 x 10 mm. of Hg. so that there is no gaseous discharge in the tube. It is further required that the probe canal be large enough

-35- so that & satisfactory beam current can be obtained. These conditions appear to be met by using a hole formed by a number 60 drill, a National Research Corporation

Type 114 diffusion pump, and a type CVD 556 Kinney mechanical pump* The Bystem, when it has been pumped for some time,

and with the use of a liquid air trap, should have a —7 —6 vacuum of 8 x 10 ' to 2 x 10 mm. of Hg. without hydrogen. Under the conditions specified above, the machine is put into operation and the current supplied to

the palladium leak heater is increased until a pressure

of about 1.5 x 10 J mm. of Hg. is reached at the dif­ fusion pump intake. The filament current is then increased

up to three amperes for a fairly new filament. At this

point an arc should strike and should be set between 150 and 250 ma. by adjustment of the ballast resistor.

The filament current and arc current may be read from meters visible in the high potential cap. A third meter indicates the primary of the probe voltage transformer.

On occasions when an excessively large probe canal

was used, pressures of the order of 5 x 10”^mm. of Hg.

were needed to start the arc. It has been found that

that the probe voltage is Instrumental in helping an arc

strike. Sometimes, the sudden electrical changes set

up by high voltage sparks from the cap to the tank wall

-36- have also been of aid. The filament has been one of the weakest points of the Westinghouse type Ion source. A description will be given here of the activation procedure for oxide coated

filaments, which should have a lifetime of well over a hundred hours. A tight helix is wound with 10 mil tung­

sten wire on a 32 mil mandrel. The ends of the sprial

are spot welded to supporting electrodes under enough

tension to just separate the turns. Under this tension

the spiral is approximately 0*5 in. long with a pitch

of 0.02 in./turn. The bare filament is then heated to

about 1200° C. (incipient white heat) in a hydrogen atmosphere to remove surface oxides and impurities.

Then it is sprayed with the filament coating, using a

small spray nozzle and compressed nitrogen. A standard

mixture composed of complex molecules of Ba CO^ • Sr CO^

suspended in amyl acetate in which has been dissolved a

few percent of nitrocellulose is commonly used.

If the sprayer is too close to the filament, a cake

is formed over the spiral before the solvent has time

to evaporate. This is undesirable. Usually a fuzzy

coating is formed on each turn and is allowed to grow

until it reaches about a half millimeter thickness.

The excess moisture is then removed by placing the

filament before a fan for at least an hour. It is then Installed In the Ion source, which is returned to the vacuum system. After a normal pressure of 2 x 10~^mm. of Hg. has been attained, the activation commences. An anode po­ tential of 200 volts is supplied by an external power supply. The filament is gradually heated to a temper­ ature of 500° G. to drive off remaining amyl acetate; this should correspond roughly to a current of 2.0 amperes. This value has been used successfully but there is every indication that heating to 200° C. or

1 . 2 amperes works Just as well. The main requirement at this stage is that the heating be slow enough that the coating is not cracked off by escaping vapor bubbles.

The filament current 1b then increased to 2.5 amperes. This drives off the binder very rapidly and the pressure in the system usually rises to 5 or 5 x 10”^ mm. of Hg.

At such a pressure the arc strikes. The applied positive potential is now set at +150 volts, which corresponds to about +70 volts at the anode itself because of the drop through the ballast resistor.

The filament current is set at 2 . 5 5 amperes and the arc current at 200 ma. by use of the ballast. The arc at this stage is in its second phase, where it operates

Independently of any hydrogen supply. However, to prevent hot spots from forming on the filament as the vapor from the binder decreases, a hydrogen flow is provided by the palladium leak such that the pressure

— S at the diffusion pump is retained at 2 x 10 ^ mm. of Hg# A note of warning should be included here. No attempt should be made to strike the arc by increasing the applied positive potential above + 2 0 0 volts. In such case a false arc may be formed between the anode lead and various parts of the source housing. The vapor formed as a result of this action is detrimental to the filament# Operations in the second phase should be continued for six to ten hours or longer if desired and the initial settings kept constant. During this time the high temperature ate probably converts the carbonates to oxides and partially reduces the oxides to barium and strontium metal. When the existence cf the arc becomes fully dependent on the hydrogen su.ply, the filament is in its final phase and is ready for operation.

5 9 Ill. ENERGY CALIBi\ATKN

The determination of the energy for the resonances in the proton bombardment of magneElum is baaed on the calibration of the analyzing magnet by well known lithium and fluorine resonances. The cross section for these resonances is comparatively large, so that it is simple to determine points of maximum gamma-ray inten­ sity. The energies and spacings of the levels have been determined accurately by Fowler and Lauritsen^and by 17 C. Y. Chao et al. . The relative gamma-ray yield as a function of proton energy is a maximum for the lithium resonance at 4 4 1 . 4 kev and the fluorine resonances at

340, 478, 598, 669, 843, 873*5, and 935*3 kev. No attempt is made to determine the exact current corresponding to the known energy. This would corres­ pond to the current at the peak less one half the target thickness at that energy. Since the thickness of the magnesium targets is reasonably close to that of the

LiF calibration target, the corrections for target thickness nearly cancel, and so the values for magnesium are read directly. However, the calibration for the

Mg2^(p,Y)Al2^ energies is exact since the target, com­ posed of M g 2 ^F2, was itself used as a calibrator. The

-40- gamma-ray yield from M g 2^ls of the same order as the background and cannot be Recognized. This yield curve

is given in Figure 3 . It ie of the same form as that from the LIF target used for other calibrations except that

the 441 lithium resonance iB missing. However, the outline of this peak has been indicated by a dashed line for reference purposes.

A plot of the square root of the excitation energy versus the magnet current gives a straight line up to 1 2 0 0 kilovolts with a Blight bend thereafter. This is

to be interpreted as operation on a linear portion of the

hystereses curve for the region considered (300 to 1200 kilovolts), i.e. a constant permeability of the iron.

This linear relationship was found to hold well and to

permit reproduction of settings within 10 kilovolts

providing the magnet was cycled between runs. The

straight line curve ie of the type E ® kl+c where k = 35.54 and c * 0.2,59 for the curve in Figure 4. E

is given in kev and I in amperes.

-41 RELATIVE GAMMA-RAY40 YIELD 60 KEV 340 AM- A YED RM FLUORINE FROM YIELD RAYGAMMA- .55 .60 AN T URN (AMPERES) CURRENT MAGNET .65 I U E 3 FIGURE 978 KEV .70 669 KEV .75 80 973 KEV .85 KCV 930 V PR OTON ENERGY 25 IN KEV .50 /F = 55 I 0.239 + I 35.54 = F 1 / OTG CALIBRATION VOLTAGE A N T URN (AMPERES) CURRENT MAGNET .60 I U E 4 FIGURE .65 .70.55 7 .0 .85 .80 .75 i 17. A33~3j.1T JD h-JiSb.-.-h ..h'b . ...1 xhbQCiSDURE

g a m m a -nA* JubhTii-;g

The capture oroces? oy the .nr re3iun isoto pes is characterized oy the emission of one or more (Camiita-rays.

The yield, or number of disinte r r trms per p rot or., may t’ en be com a-ted f r t: t. e nimo:r '•f y m r.-r: ys emitted e r pr^t ) s for er cl value of proton en:.r.py, if tr. ' resultant no c 1 eo g is re ?. inactive , r positron ->v beta particle may als) be emitted for each d i s int e ;;;r •; 11 on.

In this c.v se the yield may also oe represented by a number of positrons or beta o.articles or '.uhhih proton. An the v;orh re.ore '-ented by t: is the si - e >th mo--; na of detec- ti >n wo'-e st 'lcycd . The do si 11 nr; of t . e rvs o.r.njes for s. 'r'ton cs stare re' c 11 m '‘U '..lie • e r-.a. nesium i so to ms re ore sorts :-rl y part of tie iof oreat 1 m tv - t c ' r be obtained. Fro o the width at half-, uniu.u;. • ...d r u . tj.ve y laid vs.i-.ies can oe obtvi:ed the cross sect! ri nt resonance, and the true maximum thlci taruat yield.

These det'.:r nine ti no. 3 depend on t "e call ori.tirp. - f th~ equipment tru't is used. TV. i s first sec ti "-a h ; is v: i th the ca librat 1 r..s :,..b t ’. s ecpuipnent used in conjunction w itij the measurement of rrmma-ray s •

The se ran t :d pr ;t u. c o m Is fcca.ss.-d u.-ns a s.. Lt -44- 1/32 in. v/ide t nd 3/3 Lu. ri jn. it.e- trims ,:i ttod na i I: te..y nr b*.r n Farr di-.y cr -o . .1 1... i ;; r. ti .•= t- f e Iso; ;t; if t c cj,: Leo t nr is : jj.t 12 in.

3v tin t sec >rdary ol-3tr);:3 trv: b -re e n.itted fr:-r: tie

V • • et '■’"V" :: vrry srml. on A: esn1; ;3i:ii- riy, tie yeor ?try ie s.-ci ti • t vex-; f cv/ eicatams ejected

:‘r-on tie r ."ice ■ o-onnd ■“'no slit c . .. enter tin e cltrye a-*' Hector. C'ro e - d. H t •. o Forocay ca; e is c.v; -osed f

•• err- s-s d isi v;‘ .lei is ield oetv'cen tneo Ionov; insnio. tiny

■i'. ■ s dlsis. 'i ese ds'"S m v i d s i* r-; s i r; t n ^ o of f 'rty f ''.'s- nd ne ;:-i:ns to nr-- -id. nr: _ i.;_j , ti e or- r- ' risk r m ' old oy st-ais ' ;sni t d mti ro._o nne, o..t t s

•' • • s t i c d ' d rot s ■ t i is-.'* n tie ^osixod s-.rf: oo resistiv­ ity In vrrr::; ■: or1 ~ i c we-tier. _ -s liny ti is r n ’/: r to / ^ c-^te-no sor-y • is' m o ft-.. eo-d t f'nd c. e ... on i-nis.

T: et- r -;et s "-.re sir ted .-n “ i .73 in. tmtmo:-! disk

of 5 nii ti. iciness. A to :--c-t is ' .-rid t nroso c. y wi i.c!. Vs o- silv f' otered to ti end of ti. o L* 0 . ~d by r. _ine: r C'**riny; seal. ^ of ti-o moily zi re syster vni t’ ie closed in order to briar tire ext nrer.i o >.rc.l :n r to n troscioric msssere for ra'ic cionyiny of tr.r rts.

Ills is si twin in Fiyure 1. A n:.-of t or i vices a o-rns of ro: jc ii; tie r r.ir, .• n t of . m Vrside t'. e a m iy zly ci-bsr so It c m be r.tomod t .■ ti e rnui*r vacm.n s /ster;,.

-4b- ILe cb^rue is transferred by ? cr.xiil cr ole 1;;

rd -•>•• t'-' be measured r f yratron c.cr nt ’. .t i _:• • ter

. ,c rcccrder si. II n t-:. t u t j f «,u:j in.;1" . _y .is-- of

rr diff- scale^, a s' • i e r te uf 0 .01 to 10

:i.cro: : e s coi be me- s.;rud r: v :•• t ‘ y. b err Is •_

crt" lr: - rent ■'>x‘ le.° lt?\' e o .'rr^rt vt Is? '•duce s n ■

4- O ♦ s o bo■■ in fiv.ct o,t c , 11 o v;eai-: curovnt

.> Jt- load o' en a: a c o e Is .lot on t: i .t may not

>. rocerded o‘ r.i .. It I s v ..ry diff ici.lt to O.-t n . .iue t •- -o ••'.tor -t of so cV. i lo 3 3 sir.c-. cr.siti i.s vr- ry nib. toe st-bility of f e u- s' ine ■- .o' to. • r o b b ; -of t.. o ■ oi tor.

.'V-V'1!1, sV.e Lit: yu v l.i n i :oo co _ lor' o vd or: t..a scale

•: 3 >a.:. ly used ayrinct y ivooo ::t o' .:. .-von so o o iviiiy. 1 " -L’ 'v'* c -y " ' tj ; ^ ^ "j * J* ^ ^ ", "L ^ 1 , t r sec x.d. Ii‘ t: . nivc is u : t m ,ci‘ tod to :_ero c..int3

.or sec )id, t: e leaks, e c... o;-; f ,-r u is.,a 1 a r.Doot 0.029

I microcoulombs w?. i 1 e tie Into: r-lor i s s 1 :t^-. . I.. e

lo r- i cr it self V. n. s? -u _e. r e s m d 0.117 I'icroc.nloius o j i" count. I. ’3 10 -. total C;o iw o v. '.en so. 1.. t .. r t- -r - 3 count mu is 0.097 C*t* 0.029 T icicrcc nbji.ijg, v:V .ue 0 is t? e number of counts a no T is t: ^ ties. bo cri.lc a re re carded for a current under about 0.075 .r,icroa:: .ores. bn t. is be 3:l3 it is estimated t'.at tV e u-.v ,r re current

er. t' e iote. rotor Is n^t c ruotin; is no r 1.350 niCro­ at o -eros , a nd t1 = a t f rs eond 1 ti on exists r bo t 2u V of t. e

-4 5- tl-ne. Consequently, t- e lerhiu ve sharpe o.verr a .... 3 t ah

’•033 T, p. ivinp a <3 t’ e reor r'ertrtive equation 0.0? 7 Z f

0*033 T :.iici\Tcoulo[.iJ9. Fhr :>n individual. run t..:s S - . U a -i- d

-3 coiTGCt to leg3 than 5,-*

The T&.:rna-ray3 produced > >0 :.- on ra . + e r oded by a. Trc cerlao TA-S1 0-3ip; r tu oe which Is

’.r-o tec ted frorr.ndoo radio tic:* 0 ? t ^proxi.ortely z': re e inches of ie-d except for a cyI indrical hole wh ore the

a e ent-’rs* A Si.eet of 1/1 in. lead is aced between the target and ti 0 go.niter to eaiminate tie soft x-rndis ti on produced by ionics, ti on in t.*e t ! i. p. et.

T' o tube is corrected by s-. ’-rt, v e h s. j o'. dad. lead s to a •r"3-rmplif ier which is then connected to a Tracer lab, ire. An to scalar, hodel 53-1.

A fair estivate :f t , effioi- ucy of t i : c. n t . r nay do pained bp an indirect process in r :c: the cot. cuted max i-.cut yield free a thich t 1 y t curve f 0 r t..e t .1 be 11 '•w in use in a 1 ven :'e j:. a tr ice. 1 u • '• a: rent 3 co •’ pored wit’ •• a.. :'.b30 lnte vol e .• f rani :..,v. t he? t h St yield obts Lned by uth r rsans. .nco; i n_ to how a ort )

Louri t3en, end hour it sen"1'" the yield fro a a t.. r.jet V/( ? P o tv iciness is co e -nrc ole to t- e na.trrr 1 v 1 dt’ of tie r. - sonance can ot obtained by into r. ti -rei t o ^ ’.rinner*"^ disoorsion forTi.ila ov .r t . - t r\". cor-'-es 'oaf 0 t ?! t' e loss in eh:’ t- p a

-47- - s£-€. €

n Tor o.otons

jer dislnteyrcble r.ucleu-;

^ - loss of ener j in Vu. target

The dispersion eqi.ation, ne_lectin v.ave length and

penetration factors, i3

^ r v y « ( e f 1 ^ 7 y

“ cress section at resonance

' - full width of resonance at half maximum

intensity Y - J* f arctan ^ ^ arc tan ^ ^ *7 a # /% ' r/i J lor a known ^ or loss of ener y, the yield has a maximum

-t 3 = 3R ♦

-‘-max = ■ * * .£ .- arc tan

For an Infinitely thick target this becomes

= ^ —

Thus the ratio in the step in a thick target curve to

the maximum yield from some target of thicknessf is

'i-.gxpf) . n __ Y Ti,x '•$) 2 arctan f . /j*

The 340 kev resonance of fluorine is now selected from

d'^ta taken with known geometry. Its maximum yield, cor-

, -11 rected for background and current leakage is 9*20 x 10 -43- disintegrations ;er proton for a beam of ionized hydropen s toma. The target thickness can be found from tue rula- ti onshio

r * * y * * ^ p * s measured half width = 12.5 kev

•( = voltare spread due to use of a finiteslit

and fluctuations in the analyzing magnet

3.3 kev (see Appendix)

^ m natural half width = 3-2 kev asriven by

- I s ' Fowler and hauritsen

3o the target thickness at the 34-2 kev fluorine resonance

^ 3 11.5 kev

The maximum yield due to the 34-0 kev resonance if the tar et were infinitely thick is then

V jfT* Q Op V '1 ,''>'"11 ' m a x ™ = -i. --- ' • -J - x 2 arc tan 1^.. 6/ 1.5

r 2.20 x 10 disintegrations/jroton

The maximum thick tarret yield obtained in this way compares with the absolute yield by

YiTi3^(

The relative solid angle for a cylindrical counter used 21 in such work has been determined by Norling

^ r _i_ ?r c t a n ______j f J X ______+ 4 / 1* Z ^ * -A v ~ small correction trat is neglected

r = radius of t.-..e counter -43- 1 = ler.rth of the sensitive volume

a = distance from source of radiation, located

symmetrical'.,/ vrith respect to the counter,

to toe axis of the counter.

Althou h the target in t.,is case is not a point source and is too close to the tube for- this relationship to hold exactly, the computed value v/ill be used here and it turns out to be 0.125 for the relative solid an_le. bsinp Yniax^) - *L- ^ I* - 2.04 x 10"® disint./proton

^ 3.4 x 10"*^ cm2 (Fowler and Eauritsen1;^)

J* Z 3.2 kev(Fowler and ^auritsen)

target and 1.20 x 10~17 kev cm2stop-

pin^ power for- air (divineston and

a--, 2?\ riethe- )

Inserting this into a former relations i

Eff. of counter . ______Y-^ax^j exp. Y rnaxfetf) knevn • Rc 1. soli^ V-. "Ie

= 8 . 2 / for 6.14 Mev gamma-rays

(Ho rny a k e t a a . 2 ^)

fhe graph.3 of counter efficiency for gamna-rnys of various energies as given by Fowler, ^auritsen, and 1 o lauritsen '' provide a juide to tire relative ef hciency of the presently used counter. For the ran e of gamma-rays expected from the magnesium isotopes anu the _iven effic-

-50- ioncy for 5.14 Mev yamna-rays a table is formed

Zr.er y in Mev Efficiency of Counter

1 • _> l»o 3/ •> 2.0 2.3 2.3 2.0 3.0 3.65 4.0 5.1 5-0 5.4 ' ^ o ^ ^

Thu: sequence of operations in countin'- pamma-rays from proton bombard "tent of various 11, it nuclei is rela­

tively simple and 13 outlined on pape 52. The distance between data points corresponded to 7 kev at 1 Mev,

though on resonance peaks and at points of uncertain

resolution they were often taken e, out 3.5 kev apart.

Each point was usually recorded two or- n >re times dvriny

Vue run, dependin.. on hovr well the values a; reed . Most

ro.-.ions of data we ■ e covered uy sevcx-tl rum. An averaye

backjround rate due to cosmic rays, residual radiation,

and spontaneous counts in the tube was determined at inter­

vals. The number of lei eu counts taken durinw ;o iven run

was thus corrected accordina to t..e lenptk of th run. ho

attemot was made to subtract out background regultlr^ from

the molecular and other beams which struck netal or lass

surfaces, nor to subtract the x-rays which increased in

intensity at hiyh enerpies. Hovievex- th.is was reduced to a

minimum by increased shielding. The tantalum backin^ for

tl ; tar et has a low yield over the re; ion covered and is

not an important f a c ;.or,

- 51- *'V| • > A r1 T ^ O ^ VT, ‘ ' P w jL . - ± . j . » -J A _ ' f U I 'I ju b u i' li, v

Lackrround counts per second of Seiner tube determined ’’under same conditions as when operating to determine time dependent correction for data.

Marnet current corresponding to volta; e for start of run determined from LiF calibration curve and bridge set at this value.

lie-net cycled, tv,en brought to current v.’l.ere the bridge lances .

la ■; potential increased until molecular oeam falls in 15° iort, correspondin_ to proton beam in 30° port.

Balientine meter, reading generatin0 voltmeter current, should have a readine proportional to cap voltage Ballentine volts = 250 kilovolts/volt

Fine voltage control varied until oscilloscope and inte- rator indicate steady charge count. note exact B&llen- tlne meter readin..j.

bridge cv ocx.’d against standard cell. Fine magnet control varied to keep brid.ee galvanom- te . ralanced.

Record: ballentine reading, Kagnet settin^ indicated on the bridge, initial integrator reading, and preset num­ ber of Geiger counts.

Turn on the scalar and clock, and the integrator simul­ taneously .

Monitor Ballentine meter, integrator count rate, and na - net balance to hold beam on target.

At the preset count the scalar turns off. Integrator is turned off at the same time.

Record new Integrator reading and elased time. Compute yirld from v m G-eiger Counts - Background Integrator Counts

Set bridge foi* next higher current interval; decrease mag­ net series resistance until bridge balances. Check standard cell.

Increase cap potential to correspond to higher magnet cur­ rent until oscilloscope Indicates steady beam. hake new run. PC31:

3d. f*o --U - -1 - J x_ ^ x '**»■? -*- O . -J- ^ -

■ j M i v x :nt t t i ..itx:_ i.n :,:.-ii.y3 ix:e,, t f >r

3 0 VS OOVl ' ■ -w. X I 5x..cc- t o ■::i:-;i.::an Indaced r"d1 - x t 1 vit; i e xi..' ■: T1' Jr * TL . .iCi. !S/iS t ■ r o-1 13 o : ■- c. . ■ o. i\.e, 2

. 11 r - . -X 1{ VI ■f ' c ii jL-.ne­ tt :■'! v;it'.' i'..x Fnr-ni'V ca_e.

3 "i n oo t’ -3 ra di m o t :i v i t y O V . i 0

■, -p f ‘ • -■ c. *• 1 ... t ■ t 1 ,3 x . ec ■CXI-

f I vo h a 1 f -1 i vo 3 . ntnr-

1 ?n 3tivit ' to: t an inf ir:

„■ r ^ ton 03 3 O I .n'te r

11 3 f 3.ot.•••>ry bo:, b ff

3 .. '3c 11 v

t .r.i x : j O'

f e I d lift- n Voil^.- t'joe o t •• e; 1 " V J. to.id.3 t r not

13 ontsito t. e vac Gx to

f f 1 co.lt ie3, X 3

n.e o’.33-1 ter fits t.:t on. 3s oy i.. ■ • cxe.vronco 30 t'. t it 3 e 3 .to t.. O 313 t:l o x’o m, , , 1 ■ s n: oeo

■53' 2 \t or ’JOiitiy ; Oj 3 no i:lf^,L^"'O0 ■' : 0 ; r;r, O

.. ■ ...- t c v o f *-\o o'" 'T-t o', i toly

‘ t

CIO CM 1 t o' 30 +3V v > oiiod :: i on-

t r.liy. T j o orlof 1 3 30 • j.„.e .30 o x

or. .t3 oetorT.i..ed by i ■- t r r~ i or to e r : leys. j 1r3t f o :e o~r to.je 1 tlvo, o

"■' o Tr 3 rloo 3 orior, t' ro .o j r.

- r 4 O' -L _ tf VO t'.o

- ^ oroi:t

S C .L. t .10 o I ■_ dec* i.se of

s or.!es r .. ’vev -'r-, t o/ e

+ ■ ; o^o: ve v o r *

r ■

-34- •' t1 ve V'' ■)n oo' - 3 m •a V

t • '-7 n ’ s ch " r e :;s .h .d

t r -rt 7 ■ _V1- ■ •- Jrr t f ’ fj x~

?et holder -h t. e I/ .’. La. f • . <3 I'.'f ; 2 O

T'T'-n: ibl? at. r'or

- ^ t. laa, the

■::ter v;al ; , ' a'- •. v • '. .. ■ : ; ■ ra>v i d “

t tl e ■ as o r s t i . (It Is a -m; pd 1 a _ i 1 rs 3 11-

a 'o ^ ' ::t. la r- ze f r f e

t rn i s e j-acl-iro-.md c •> -a.t. to sevfit'i tines

m s t ^ i ;uut 'us it ran s. ) Since a.

- "1 r%-,+ 5 • rn f-a ran’ •• . th-. t. ->e If It : 3

’■;al ’ ■ v; f ’' a s ; vaae g

. l^e 01:.. 3 1 e c f’ anc r r .

^ t. j ,.3 :j 3 van ■ v : c • L. r of the r

1 cl i s s a t 1 > ' Bi'i- ut -r. dead e..ei at any fl oy

' m l svh t

O 1 :.it :r ci: ■a O L It

■ T* 0 ’1' C “',‘in* L lV ‘ n ^ ; 4 • 1 f o r t ’ a f l a ~ t few 3 a •j..do,

IV: i s n.>eno7!3no is -i ' ■ t y not jy ■ re recast tests . nwever, as an ai t am:., 1Ive :.:et..ad of o ’7r" than, a shutter v/as ca.str cf . c 7f ; s..a t :f

3/12 in. ' -s.-.- 5 \f. i cl. s .id vortico L..y in a crass fr . .e !■ cod. iomedi:. toiy in f r m t of t o i e i .i

field - r c n d l y I *. f tc-d. •n-t -'f nr. t n ny u oo^oorold

•; U: vr o u r I -d 1.: V o erne roii\ o ; .•:• .ct. t..:. - fnr-

r\y c' Iv:ted tie volt' e on tfo c u t i : t t o c t rt

f n run. T o co n t - n 1, under t. to 3 ' . J 0 1V?

^■ "— •~ J -• ■*■ i - • .1 ^ " -'i.r>. - v ’ - » Vv -'• 1T‘'' j ' —" 4‘ + - ■• --■ - ^ ^

u,- or - n * :i • o re3ult3 ?f t : •; iLi.;■ t. I

' ’ .it j. ■. : b ■ .t from t' ' t f-' r c "> eit' v : v v. rr'r von • t t. - end of

i t(r. - *vi •

1 ‘, t . A

3 j . . . •: v - f

• « r '1

t

z. tie in 0. oil t- ntolu;: o o m ;

on oor oxco it one r ■ ■ i-

+ • , o + x = thi chnes o f - xs ji her if. era o_ A 1

fk - adsorption coefficient - 3.653/d

d - Vu-'lf v; ue th i 3 ime s s o - o The maxiaua onerpy of the.- positrons from Al^ J anc! a.--1- u\

Is close to 3*0 Ilev in hoth cases. From the ran^e e-ier^ taeles riv.n oy Siri this Is equivaleno to a rar._e of

1.45 for alumin.' * n or .541 cm of alurnin on. 15 sn

-57- - " . 2 7 4 c:n o f A 1 A = . S ? 7 / . 27 4 - 2.77 m ~r t~, “ 2 . ; 3 r - * 0 •'

2he 'j,rticl"3 pass throu- 1. .? -.7 nil I n . t n l a a °oll,

'02 1 to 2.5 cm of oirdepaud 3 nv on Q ^ V ' o r tube with a wall thickness of _ • ^ 3 2 om/c• fart of the po3itron3 pass tl.rou h the wall of the tube at an

on le also, so that the average t h ‘. okness is est iinatcd to p be 2.235 7 m/cm b y takin into ' ccount the relative num­ ber of part icles at each an pie. Similarly, the average

11 stone e in air is probably about 1.7 cm. ..'hen convert­

ed to centimeters of aluminum t; me ore:

Tantalum C.221 ;m/c:n^ 2 • 2-273 cm of _-il

Air 2.022 2.0227

lei er tube 2.23^ -2.2174

Thu f •-action of initial particle s pone t rat In thw 2-el _.er

tube is its actual efficiency in the _iven __ eomotry:

Efficiency = I__ _ -.2554 _ 2 4 7 T - e i o The relative solid an ;le is completed ^ocordirp to t .e 2" eq-.ation of ITorlin. ^ as "ivc-n .Teviou3l;; a n d is

Relative solid anpie - j . 0 7 3

Several factors must oe evaluated before absolute

computation can be made for cross secti n or maximum

yields at resonance. The calioration of the needs and

Horthrup galvanometer which indicates the proton current,

rovided a valuof 0 . 2 4 8 / i a / y . n for tho coarse sens! ti v It, A shunt used v/ith the ■“j^vanomcter modi i- d thi u value

to C . '32 ytfa/.nn.

Tho second factor involves th-. determination or

the number of positrons per radioactive nucleus which would be counted unc r the present procedure, aasu:rin_

1 0 0 * efficiency for all positrons in trie solid - n 1 ..

represented by the counter. Inis includes deterrr,inin.y

the ratio of radioactive nucuei produced bombard ..ent

for a yreat many half-lives as compared to a bombardment

for a few half-lives, th fraction count ui duriir. a

limited interval, ancVtho frac:.ion mistakenly c minted as

:ac Icy round. The opera tiny procod ire on va.ich these coi-

pu tat ions are based is: (1 ) fonn.nrduo: ■ t w 1 th the beam

cverayiny 35 seconds, (2 ) uno-f :urtl seome delay etv/ecn

time beam is cut off and start of o.n,rt, (3 ) 2 ,. sec nd

countiny time, (4) 10 second dead tine, and (1 } 2^ se.

bach p round c-^unt which was then su otr- cted fro.: t-c read*

in for the initial 25 secants.

!' e half-life for both Al2 - -nd Ai2 ° is very cl,mo

to 7.3 sec ands so the '.nrh d_-ne i.one is applies.--!© t„- - cc

h (t) = nun .er of &to :m present nftei turn t of

m-,nt

3 - no- cr of ut,.ns fo i- .:«d ..er sec .nd by h e .ma...

d A. ^ ^ - t tr = ^ - o*

WT f* ^ ~ 'X t* r ' v 's + w - , ■-> - - O ■-> *■ -» -> IT m 2 ; i _ ift */ — -'c ■"- tkj '- ~ i " I • ^

end oT ' 3 T 3'jc :v" bo ^rzr:: n .

= 1 - = -..-5

T at i: , si1:’. n bomb:: rdron.t ;.. od-ce: - 5 ; os rush rn d i e - activity as an infinite boa.curd menu. The amount re­ gainin'* o fter the next 0.25 second , I" exists at the

-» 4- ->o +■ » h> o - U • »t — —0(t _ -, ^,<7 7 — j c ,L i 1, — ^ b ~ r • „' J i • • •>! CJ " ■ . J f— ^ • so that 57 • o i of the available r.-- II nc tivi c,v ro.nins e fter th - counts in the f irst sV.-rt ti...e interval arc discorded. The a count ucunuod chnn. ". the first 2 ~ sec —

■:tnc- s of opera tin • tine , inci the laiti "1 . 2" sec - nod is:

:: = ::,;i - e-*Te = ..-.7 ::0 T- = O'

To th a so’ m 1 ac '■ually count vd 5* th fii st 25 so sends is ?? .75 loss the 2 .4.5 of f c :’irst 5.23 second Ivin _

77.53-• At the end of 35 seconds v;h on the bach ground i.: to b recorded, trn: tot 1 rad ioac tivi oy re on i:;hy i • ;

■*T — V a 4- — T r end at th ^ end. of a 25 second oachyround. run, sakh\; u total of oO seconds, there retains: ’■ - *T c>“*(t _ *. -) -,TE + _ . 0 ^ A , — . r*) w — >•_ « ^ -C . O — J ' - J O v-» W • so that the 'ere nt counted as background 13 3 .35*

3ousequently , for the present precede re, only 55 • 2,5 of the rodiocactivity available at the instant the ... JL

-50- eut a co o (. f a of results actor cut off ic counted, counted, ic off cut ci’ol -3 activity that aould be counted counted be aould that activity rcino atvt ectd r excited activity of Fraction

yj\% of th*=. saturation activity activity saturation unc'.er 8 " ) ( . ? = ? 5 )

;■

. j r ■ .jeonetryct . 2 ,J to : the of tal 82/g v;itli v:as xie, t excited, 100 ;

- 3 0 e try. jre 3E m tc ; j. ±U .. FOR i\ A ±*. BuBITldhS

Relay circuit reset to protect Helper tube. Manual protecting switch set to "Relay" position.

Magnet current corresponding to voltage for start of run determined from LiF calibration curve and bridge set at this value.

Magnet cycled and then brought to current where bridge balance s• rv,ap \ potential increased until molecular beam falls in 15 port, corresponding to proton beam in 30° port.

Ballentine meter, giving generatin. voltmeter current, should have reading proportional to cap potential. Ballentine volts « 260 kilovolts/volt

Fine voltare control varied until galvanometer inbicates maximum steady charge count. Exact Ballentine meter reading is noted.

.ridge checked against standard ce^l. Fine :aa;net control varied to keep bridge galvanometer balanced. The m.a.. i. e t setting is recorded.

Target is bombarded at constant current, for enou h half- lives to excite nearly all possible radioactivity.

Ballentine meter, beam current, and macnet balance are monitored to hold beam on tar,get at the proper volta e.

Solenoid which closes shutter to cut off the beom is activated. This automatically sets off th. relay se­ quence when the shutter strikes a microswitcr. Re^ru v-uage galvanometer current during bombardment period. (Beiger tube activated and scalar started automatically;.

At the time when nearly all activity should be Lone, the B-eiger tube is deedened manually and scalar counts re­ corded .

Beiger tube activated and background counts taken for an interval equal to the length of time for the positron count. Tube is then deadened manually.

Open solenoid circuit, dropping shutter and allowing the beam to hit the tar-et again. _ r o _ Reset relay sequence, which deadens the tube automat- 1cally•

Return nianufl dead onin^ switch to "Relay" since the feiger counter is now protected by a connection through the relays.

If the scalar shows no signs of counting an Intense x-ray beam, the relay is protecting the counter properly. Stop the scalar and zero the clock.

Compute the yield from y _ Positron Counts - Background Average Galvanometer Current

Set bridge for next higher current interval. Decrease the magnet series resistance until the bridge balances. Check the standard cell.

Increase the cap potential to correspond to the higher magnet current until galvanometer indicates a steady beam current. Make new run. _ -L A . i V . u' .ki.

The invcstiyation of th- resonances resultin^ from

;roton capture by rnrunoslua was first started ay Currc n r r ' . 3tv,others^ in the eneryy region of 200 to 1GJ0 kev. fi ht resonances wei e determined anc^:ere tentatively assiyn-d to reacf i^ns v/ith various isotopes on the oasis 4 of acccs'anyln^ positron activity. hole et al. then found a number of resonances oelov; i?30 kev jul did noi

^istin yuish between isotopes. larvs'rh, by so of rela­ tively sensitive detectin e . 1 .sent, deter.m'.nod res­ onances in the \"ine rcy ion and as si yn d th m, to vari.. us

Isotopes. It "as felt t’ at a nous r of res onances must e::l st in tne • eyi >n above ? ih kev vhh a:, were no; detect­ ed by Curran and Strothers because oi poor resolution und/fno use of relatively thick tar y- s. t..i.. m.sis

.an invest : atian was ms.de in tn.e re _ ion of tub to Ic JO k e v e arly i r 1 pT 3 .

In nuclear spectroscopy it it useful to ve tar yets that fall int., one of two catauori...s. h in targets a r

™c. _r.:nl"m \?ere easily plated in a vacuum on ba.ck.in_s of

ore 3 mil tantalum from an alumina coated oaoket of tire type d .-scribed by 11 sen, Smith, and Irittenden^-.

T’^e resu'tant yi.lds of _am na-rays per incident ckarre are shown on popes 55 and 57. 11^ re. ion bel. v;

713 kev is ouitt d b~cause intei-p r^tnti - n is difficult.

Several re i no are mask",d by flu : in cti^riey vol.ok vas present as a cor.t" v.bnan t. 1' e?e er- ncica.t 1 by thn das1’ . ? line 3. ,k’finl or resonant's crn be as 'i a. a at 727, ?2Q, 331, 543, 760, 9'^3, 1112, 1133, 113c,

1217, 1253, 1435, 1472, and 1311 k.-v. The v/o.k of lerr and Strothers skews only two we 11 -resolvef peaks in thi rep ion vfnich tney report >' at 523 0.110 13..0 kev. fro o- ably the peak at 323 kev io due to the 321, 331, and 34 resonances while the 293 and 1012 resonances prrs aably

••‘.vc- rise to the step th~y report at 1012 kev.

-J' RELATIVE GAMMA-RAY YIELD 80 40 60 0 2 5 80 5 90 5 10 1050 1000 950 900 850 800 750

AM-A YED FROM YIELD GAMMA-RAY / i i I i i R T N NRY (KEV) ENERGYPROTON IUE 5 FIGURE M g 1100 RELATIVE GAMMA-RAY YIELD 60 0 2 40 80 1200 AM-A YED FROM YIELD GAMMA-RAY RTN ENERGY PROTON IU E 6 FIGURE 1400 (KEV) M g 1600 13001500 VI PROTON CAPTURE BY Mg25

Although the work of the electrostatic generator r group upon thin taffeets of natural magnesium extended and improved upon the early results of Curran and Strothers^ and added a number of new resonanoes, it also indicated considerable structure that was not resolved.

This was to be expected sinoe three different Isotopes of magnesium were involved. A successful effort was made at this time to obtain small amounts of the separated isotopes in order to make an isotopio assignment of resonances. The samples received from Oak Ridge National

Laboratory were in the form of magnesium oxide. The isotope considered in this section is Mg2^ which was pro­ vided with the following chemical analysis: 24 m i Z 10.7 % 86.8 M g 2.5 B <0.015 Cu < 0.04 Fe <0.04 Mh <0.04 Mo <0.15 Si <0.05 V <0.08 Sinoe the isotopes were received in the form of MgO, they were not evaporated as easily as was done with the natural metal. Thin targets were formed by decomposition and evaporation of the oxide isotopes in a tantalum

- 68- basket at high temperature In a vacuum®. The Mg2^ deposit was chiefly Mg2^F2» A chemical analysis showed fluorine was present as a fairly rich contamination In the material received from Oak Ridge. Due to Its low melting point ( 1 4 0 0 * G) the fluoride plated easily in this process while the oxide with its high melting point (2500 - 2 6 0 0 ° C) did not. Target backings of both 5 mil and 0.5 mil tantalum were used. The average target thickness is an important quan­ tity in determining the natural half width of a level.

It is possible to compute the thiokness of the target at a given resonance directly if one knows the increase In yield of the thick target curve for the same resonance and the target composition. This was tried with an iso- topic MgO thick target but the results were not oonslstant with other data. The thickness of those targets which contained fluorine throughout the target material and not just as a surface layer was easily evaluated by using the known natural half widths of fluorine resonances in con­ junction with the measured half widths and the energy spread of the beam. It is suggested here that an easier method for most targets would Involve the use of auxiliary thin targets of some element with many narrow resonances, such as aluminum. One of an identical pair of these reference blanks would be plated again with the element to be

studied and in a comparable geometry with the regular

target. If a suitable aluminum resonance is picked, its shift in termB of energy as given by the double

plated reference blank compared to the blank singly plated with aluminum would give directly the thickness of the plating of target material.

The gamma-ray activity from these targets was first

Investigated. However, the activity from the fluorine

contamination masked out a considerable portion of the

region investigated. The other portions Indicated gamma-

ray activity that could be resolved only with difficulty

from the background fluctuations. An obvious alternative

to these difficulties was to count positrons from the

reaction Mg2^(p,Y)Al2^ 0 + )Mg2^* The experimental arrange­ ment for counting positrons is discussed in the preceding

chapter. Since proton bombardment of fluorine produces no activity of half-life more than a fraction of a second,

fluorine contamination would no longer matter.

A possibility did arise concerning radioactivity by other substances that may have been present in the orig­

inal sample or introduced later. No activity could def­

initely be assigned to any of the trace elements in the

sample although a small double peak near 450 kev with

about one minute half-life occurred in the positron -70- activity of both the lighter isotopes. Since the reaction

M g 26(p,Y )A12^ produces a Btable nucleus, the only expected activity due to impurities would come from Mg2if(p,Y)A12^ 0 +)Mg2^ which has a weak resonance at 420 kev and a com­ paratively strong one at 824 kev. The yield from these resonances, as determined by bombardment of the pure iso- . 24 tope, coupled with the 10.7% abundance of Mg in this target should produce noticeable irregularities on the high energy side of the 812 kev peak and around 420 kev.

Such discontinuities are indicated by the yield curve and marked with a dashed line in Figures 7 and 8 . A fair amount of ten minute positron activity was present in the 420 to 460 kev region due to proton capture by C^2 (p,Y)N^(fJ + )C1'^, which occurs in oil films deposited by the pumps. This activity was of long enough half- life that it could be subtracted out by background read­ ings taken Immediately after the count of the short half- life activity. That is, since the radioactive aluminum has a half-life of approximately seven seconds, measure­ ments need only be made for four halfllves to count near­ ly all (94^6) of the positrons that can be emitted. The intensity of the activity from the C12(p,Y)N1^(p+ )G1^ reaction has changed very little over this same period since the 28 seconds is much less than its 10 minute half-life. Thus a succeeding run for a second 28 seconds will count only a few percent of the positrons from aluminum but practically the same number from the carbon reaction as before. If the results of the second run are subtracted from the first, the net activity Is prac­ tically all due to the aluminum radioactivity. However, the presence of carbon does make the data more uncertain in this region. Identifications of Impurities, If the half-life can be measured, Is simplified by a table for radioactive isotopes listed according to half-life

The positron activity as a function of proton energy is plotted in Figures 7 and 8. Eighteen levels in the compound nucleus A1 are assigned to correspond with the following proton energies for a capture reaction: 388,

494, 510, 563, 588, 650, 683, 722 , 777, 812, 880, 928, 986, 1043, 1081, 1098, 1132, and 1197 kev. These are probably correct to less than 20 kev in absolute value and to 10 kev in relative spacing. The values for several of these levels agree well with the results of T a n g e n ^ who obtained resonances with positrons at 310, 392, 417,

492, 508, and 525 kev using ordinary magnesium. The 417 kev value was later definitely assigned to a reaction on

Mg2^ by Orotdal et al.^ The weak 525 kev resonance is indicated In the present work with the right intensity.

The weak 310 kev resonance is not noted. The largest discrepancy, in both position and relative intensity, -72- RELATIVE POSITRON YIELD 4 6 8 2 350 POSITRON R T N NRY (KEV) ENERGY PROTON 450 I U E 7 FIGURE IL FO M FROM YIELD 550 650 g 25 b 2

800 900 1000 1 1 0 0 1200 PROTON ENERGY (KEV)

FIGURE 8 Survey of Mg2^(p,Y) Proton Energies at Resonance

Curran and Strothers^ Tangen^ Thls wo rk

180 kev 310 kev

392 388 kev

480 492 494 508 510

525 563

575 588 650

683 722

777 812 880 928 986

1043 1081 1098 1132

1197

-75 la for the peak at 386 kev. The 494 kev an<^he 563 - 588 kev values agree as well as could be expected with the resonances at 480 kev and 575 kev which Curran and Strothers^ assigned to

Mg2^(p,Y)Al2^(£+) on the basis of positron activity.

They also assigned resonances at 410 kev and 825 kev which can be shown to belong to the reaction Mg2if(p,Y)

A1250*) Instead. On the basis of measurements and computations made

In an earlier part of this dissertation, an estimate can now be made of the cross section, natural width at half maximum Intensity, and maximum thick targes yield for a given resonance. Since the target used proved to be nearly pure Mg2^F2» measured width of the 669 kev fluorine line provided an accurate means of determining the target thickness

P sb 9.7 kev r *7.5 kev a = 4.7 kev

so the target thickness at 669 kev energy is ^ = 4 kev The 588 kev resonance of Mg25(p,Y) is selected as a suit­ able example with a measured half*width of 6.4 kev. This corresponds to a natural width at half maximum intensity of about 3 kev. The corresponding mean lifetime of the excited state is approximately *£■ be X/jrt * 2 x lO"*1^ seconds -76— The yield for the resonance is then found by applying the results of the fourth section to the measured yield

Current = 0,051 microamperes/mm. deflection

Relative solid angle = 0,073 Efficiency of counter = 94% Fraction of possible activity excited = 82%

Consequently, the measured yield is

*max^> * 3*39 x 10*^ disintegrations/proton Now the yield is related to the cross section at resonance

*max$F> “ ltotan £-

€ = 22.39 x 10” kev cm^/dlslntegrable

nucleus (Livingston and Bethe*^)

So the cross section at the 588 kev resonance is -27 2 = 2.7 x 10 cm The maximum yield for an infinitely thick target is then

computed to be Y fae) = n YmaxCp) m a x ^ 2 arctan ~ y r n ■ 5*75 x 10-10 disintegrations/proton

The cross sections and maximum yields of the other res­

onances are of the same order. The maximum energy and half-life of the positrons emitted by Al2^ were not studied. However, from various

compilations of nuclear data2^*28»29» ^ references

could be found for considerable early work on the sub­

ject, In general, the latest sources of collected

-77- Information on the subject^2* ^ assign a value of 2,9 Mev as the maximum beta energy and 6.3 seconds as the half-life. The latter value is due to work of Bradner and Grow-^ and seems to be open to que tion since it Is 0.7 second J.ower than any other previously deter­ mined value. They used protons from a linear accelerator for the experiment s^nd by using separated isotopes probably pi educed the reaction Mg2^(p,n)Al^. Presum­ ably previous experimenters obtained too high a value by using ordinary magnesium and thus producing mostly 7*3 2R 2*+ second A1 ^ from the abundant Mg 1-utope, The latest value for she maximum positron energy was ^ade by Allan and Wilkinson^ with absorption measurements. A table giving the history of these measurements is given on the following page. Theie is acpnr^ntly no evidence for suspecting a complex positron decay scheme^ for A1 and it was not investigated. However, an excellent discuss­ ion of the determination of decay schemes by coincidence measurements is given by Mitchell*' .

The position of the eneigy levels of Al^ can be determined from the energy available for excitation at each resonance. In particular,

E = Q + Mo E M 0 * mass of initial nucleus M 0+M r Mj.. = mass of incident particle# = particle energy at reson­ ance

-78- If a particle 1b emitted as a result of the reaction, its kinetic energy must be measured and subtracted from E^.

The Q, for proton capture by magnesium was given in the introduction as 5»2 + 0.26 Mev. It was computed from mass values given by Mattauch and Flammersfeld^ based on data up to the end of 19^8. In general, there have not been very many determina- 26 tions of the excited levels of A1 • A fairly recent 40 survey of the known levels lists only the results obtained by the proton capture work of Curran and Strotriers and of Tangen . More recent work by Swann, Mandeville and

Whitehead^ with the reaction Mg^^d,n)Al2^ has indi­ cated levels in the excited nucleus at 2.00+0.18,

.2.65 + 0.18, and 5.15 ± 0.18 'lev. A survey of the known levels is shown on page . Tne results of other groups are indicated by dashed lines. The 5.2 Mev Q-value was used for all measurements by proton capture. Note that the levels are quite densely spaced in the region covered by the (p,Y) work. Possibly many other levels exist at lower energies but are too wea<< to be detected easily. On the basis of preliminary measurements the gamma 26 ray intensity from the levels of A1 appeared to be too weak to determine a decay scheme with the present equip­ ment, either by absorption or coincidence measurements.

No other measurements of Intensity are known at this time. Half Life And Positron Energies Of Al26

Process Half-Life Maximum 3 Reference Energy

Mg2 6 (p,n) 6.3 sec. (37)

Al 27(Y,n) 7.0 (43) A l 27(Y,n) 7.2 (44)

Mg(d,n) 7.0+0.2 2.8 Mev (38)

A l 27(Y,n) 7.25+0.2 (45)

A12 ? (Y,n) 7.2+0.5 (46) A l 27 (Y.n) 7.2+0.5 (47) Mg(p,Y) 7 *4 ( 3)

N a 23(a,n) 7 + 1 1.5 (48)

Na2 3 (a,n) 7 4•6+0.46 (49) N a 2 3 (a,n) 7 + 1 1.8 (50)

-80 3 • oo 5.34 5*31 r O O ■r 5.72 . 5 5.57 5.55 5.75

O'! VA VI C’> C"l \J\ V . \ 3 i I' 3 G 'hi 3 .33 .27 .22 .24 .18 .13 .07 .03 .93 ♦ d - 5

Kev nrj Lvl o Al2° 2 l A of iner Levels jy 3 . 5 *35 5.13 3.48 3.63 -3

-81- o: P Ko2:V 5 .lB“ r- r \ ^ S

*il 25 ■4 •1 r’.e O V VII. PKOTON CAPTURE BY Mg2

The investigation of resonances from the reaction

M g ^ ( p , Y ) A l ^ ( 3 + )Mg was carried out Just aft~r the inves 25 tigation of Mg and was similar in nearly all respects. Since Mg is the most abundant of the isotopes (78$) little gain in yield over that indicated by targets of natural magnesium was to be expected. The composition of the 0.5l>2 g. oxide sample provided by Oak Kldge National Laboratory is as follows

99.52% 0.55 0.15 Cu 0 .04 Mn 0.04 Na 0.08 Si 0.05 A small amount of fluoYlne was also picked up by the target plated from Mg 0. This was enough to make 24 the Investigation of the gamma-ray yield from Mg rather uncertain. Also the yield proved to be fairly low.

Consequently most of the measurements weie made on the positron activity as discussed in the previous section.

Very little activity caused by contamination was detected. The yield curve,which is shown in Figure 9, indicates only a small double peak at 450 kev correspond­ 25 ing to a similar activity in the Mg data. This impurity of about one minute half-life, has not been identified.

The C12(p,Y)N1")(p + ) activity in the region 420 to 460 kev was subtracted out by taking advantage of Its compar­

atively long half-life. The targets were produced by

plating the sample on 0.5 rail and 5 mil tantalum backings at high temperature in a vacuum. This is discussed in more detail in the preceding chapter.

Only two resonances were definitely identified, one at 420 kev and the other at 824 kev. These are probably

correct to 10 kev. The resonances were first reported by Curran and Strothers at energies of 410 kev and 825 25 26 kev but were then assigned to the reaction Mg (p,Y)Al 24 since it was assumed that a proton capture by Mg was A improbable. Hole, Holtsmark, and Tangen determined

the yield from thin targets of natural magnesium but did not differentiate between isotopes by means of the resultant beta activity. However, their resonances at 24 228 kev and 4>5 kev correspond to Mg peaks determined 5 later by Tangen . He reported the corresponding levels at 222 kev and 417 kev bombarding energy and was able to 24 25 assign them to the reaction Mg (p,Y)Al because of the

soft gamma radiation. The deduction was definitely confirmed by Grotdal et Pl,^ by use of isotopic targets. Mooring et al.^, doing elastic scattering experiments 24 on Mg , missed the 420 kev resonance but found discon-

-85- RELATIVE POSITRON YIELD 9 3 6 400 24 RT N NRY (KEV) ENERGY PROTON 600 IUE 9 FIGURE 800 0 0 0 1 ;urv';v f i2:24 ( i,if) x‘r e to -•: -n j 2 - j 1 e s r.e so.:."..nee

.,-,3 Curr Hole4 .:ea -on-' Ho Drlnrv-1 iM3 v.r j rk 0 'Jl Holt ST.-: rk et al. 3trotkers i Ton^sn

228 kev 222kev

4lGkev 4 3 5 417 42 a GV

825 821 kev ’24

1420

1 520

± \jo0

2010

_ +0

-85 tinuities at 825,1490, 1620, 1660, 2010, 2400,5140, and

5660 kev. A proton group corresponding to Inelastic scattering was found at 2410 kev with the possibility

of inelastic scattering for several of the other values. The cross section and maximum yield of the 824 kev peak ca i now be determined fairly accurately on the basis of a number of runs. Both the positron and gamma-ray yields were measured for this purpose. The thickness of the different targets used and the natural half-width were determined by an indirect method. The target used for counting gamma-ray intensity contained a small frac­ tion of fluorine, although it was not pure MgFg as in the 25 case of Mg . However, the fluorine was inherent and not a surface layer since it showed up from the beginning. Thus it could be used to determine the thickness of the target. The 875 kev fluorine line was most suitable for this measurement ]* = 1 8 . 4 kev

J* = 5.2 kev Fluorine 875 kev 4^ = 6.4 kev

^ = J ft 'H. f* v -or*- = 9.6 kev Using this value of the target thickness, the natural half width of the level was computed m 18 kev

f = 1 6 . 5 kev -66- of =6.1 kev

r = = 4 kev Mg24 824 kev The mean lifetime of the exoited state is thus about -10 2 x 10 y seconds. Using the corrected calibration for charge count, the efficiency of the counter for a gamma- ray energy of 1.8 Mev, and the known solid angle, the maximum yield at resonance is found to be

x 10*^° disintegrations/proton The maximum yield for an infinitely thick target is then

Ymax**) = * ___ XmAi m 2 arctan y r f f = 2.0 x 10“9 disintegrations/proton

In order to find the cross section at resonance the follow­ ing equation is used

^ — Ymax4o) n p

The stopping cross section 6 is estimated from the tables 22 of Livingston and Bethe for a target composed of mag­ nesium and a small fraction of oxide or fluoride

6 * 8.5 x 10*1® kev cm2/disintegrable nucleus Consequently, the cross section at resonance Is

m 2.7 x 10*27 cm2 For comparison the yield is determined from a thick target of Mg^^*0. The powder was pressed into a brass cup to a depth of 0*3 cm. The backing of the holder had a thickness of 0.2 cm compared to a foil of negligible

-87- thickness for the work on thin targets. Consequently, about Q% of the gamma-rays are absorbed or soattered

out of the counter area. The thickness of the target brought about a smaller relative solid angle of value

0.090. The corrected thlok target yield Is computed to be

YnaxM = 7.5 x 10~10 dlslntegrations/proton Since every alternate atom In the target Is an oxygen atom, the yield for the pure metal should be approximately

^ m a x W * 1.5 x 10"9 disintegrations/proton This compares favorably with the yield found from a thin target.

The thin target values can also be determined from the yields of the positron curves. Presumably, values

found in the latter way will be more reliable than those found from gamma-ray data since both the gamma-ray energy and counter efficiency are uncertain. A different target was used for positrons but since a value for the natural half width has been obtained, the thickness can be found

t* * 10.1 kev

f * * 4 kev Mg2if 824 kev H m 8.2 kev

So ^ 4.5 kev

Using the previously derived values

Current * 0.052 )ba/mm

- 88- Kelative solid angle = 0,073 Counter efficiency * 94%

Fraction of activity excited * 82^6

The maximum yield at resonance is

Ymax (£) - LO x lO”^ disintegrations/proton

For an infinitely thick target

Ym a x W = 5 ------. 2 arc tan tf/Jnr

* 1.9 x 10"^ disintegrations/proton and the cross section at resonance is thus

^ | — jt *max(«*)

* 2.6 x 10-27 cm2

The maximum yields deteimlned from the data on posi- 24 25 trons for both Mg and Mg appear to be high compared to the fluorine. This is possibly caused by underesti­ mation of the solid angle and relative efficiency of the counter.

Even less is known about the nucleus Al2^ than Al2^.

This is true for several reasons. The only common re- oc pA. 04 actions that produce A1 3 are Mg (p,Y), Mg (d,n), and Mg25(p,n). Each of these involves an isotope of magnes­ ium and unless separated Isotopes are used each type of reaction may produce three different nuclei. Identifi­ cation of the positron and gamma-ray energies belonging to Al25 is then difficult. The energy levels in the

-69- 2A pA region covered by the Mg (p»Y) and the Mg (d,n) reactions p A may also be expected to be widely separated sinoe Mg is a very stable nucleus from the point of view of the

alpha particle model and another particle should pene­ trate the nucleus with difficulty.

Gurran and Strothers^ determined a half-life of 7.25 seconds at 850 kev which is probably due to this isotope. Their corresponding maximum positron energy was 2.2 Mev. White et al.^2f exclbsAlng a (p,n) reac­

tion on natural magnesium, found an activity of 7 +0.5 25 seconds and 2.99 Mev energy which they assigned to A1 because it agreed with a predicted value from the Wigner

formula. No anomalies could be found in their half-life or energy curves which at least indicated the values for Al2® and Al2® might be very nearly the same since both

could be produced by the reaction. Allan and Wilkinson^®, working with (d,n) and (p,Y) reactions on natural magnes­ ium also found no anomaly, again Indicating the possibil­ ity of very close agreement between the two isotopes.

However, they tentatively assigned the 7.0 +0.2 second half-life and 2.8 Mev energy to Al2®, although if both are produced, the lighter isotope should be seven times as abundant for the same cross section. The latest meas- urement made by Br&dner and Gow^'57 on separated Isotopes gives a value of 7*3 seconds for the half-life. This -90- agrees fairly well with the value of 7*11 seconds for the 824 kev resonance resulting from independent inves­ tigation in this laboratory53. The accepted maximum

positron energy is 3,0 Mev. It appears probable that

there is a simple beta spectrum^. The main assignments of the energy levels of the compound nucleus Al2^ were made by Koester^^ using 2.32 pA Mev as the Q for the Mg (p,Y) reaction, which is the only source of information thus far* This value was p determined from Bethe's table , using the equation

Q » Mc + MR - M pit M0 — mass of magnesium nucleus Mg

M r * mass of the proton M — mass of compound nucleus Al2^ The position of the level is then given by

®T1 * Q + Mo M 0 + Mh * proton energy at resonance

* excited level of aluminum 24 If a later, more precise value for the mass of Mg is n taken from the tables of Mattauch and Flammersfeld , the excitation energy is 2.40 +0.91 Mev. On this basis the energy levels of Al2^ are shown at the end of this

section. The classification of the excited states was made by Koester from the elastic scattering data of Mooring et al.^ Casson^ attempted to measure the

-91- gamma-ray energy from the 222 kev reson&noe by bombarding thin targets of natural magnesium with protons and by use of a crystal spectrometer. The value 2.3 Mev Indicates that there Is probably a single transition to the ground state. Tangen^ also made a measurement on the gamma-rays from the excited states of Al25 by use of a coincidence method. A value of 1.5 Mev was determined for the 222 kev resonance. The radiation from the 420 kev resonance was also relatively soft, being less than 1.5 Mev. The shapes of the resonances shown in this dissertation were not accurately enough defined for any information to be deduced. The mirror nucleus Mg2^ is shown on the same energ, level scheme as Al2^ for comparison. This data was obtained by Schelberg, Sampson, and Cochr a n ^ the Al2*^(d,a)Mg2^ reaction and Is quite representative. Reasonably good correlation is obtained for many of the levels of the two nuclei shown.

-92- Energy Levels of the Mirror "uclel U & 2d and AX25

ev A l25

Scheiberj Tanpen ini s Sampson & Thesis Cochran 2•51 Mev

2.JO Mev

3.19 1.53

1.97 Mooring, Koester, 2.74 Colaberg, Saxon & Kaufmann

4.01 3.S3 .lev

4.81 ^ • j 3/2 48 4.00

4.33

,4.71

3.42

o . 32

25

-93- Half-Life And Positron Energies Of Al‘

Process Half-Life Maximum fj Keference energy

24 Mg (p,r) 7.xl sec. (5>)

M g 25 (P , n ) T o (^7) Mg (p,n) 7 0 2.99 Mev (52) Mg (p,n) 7.0 + 0.5 2.2 6) Mg (p , Y) 7.25

-94- VIII s u m m a r y

The relative yield for the resonance capture of protons

Mg2^ + p — ► Al25 + 2.4 Mev Mg25 + p — ♦ Al26 + 5.2 Mev was Investigated for protons of energies 300 kev to 1200 kev. Two resonances for the first reaction and eighteen for the seoond were assigned. The maximum thick target yield and the cross section at resonance were computed for the 824 kev resonance of Mg2^(p,Y) and the 588 kev resonance of Mg2^(p,Y)

YmaX {•*) — 1-9 x 10”^ disint./protnn ^ ^ . 0 7 O Ms (p.Y) •f = 2.6 x 10 cm

Ymax(s) * 5.75 x 10-9 disint ./proton 2C- _ -27 2 M6 (P »Y ) ^ = 2.7 x 10 ' cm

The spectrum represented by the gamma-ray activity from natural magnesium in the region 700 to 1200 kev oan now be compared with the results^7»58 from the three separated isotopes. The peaks assigned to fluorine con­ taminant are eliminated from the data for natural magnes-

-95- lum and Mg2^ and replaced by an assumed curve. The

curves obtained from positrons are corrected to the proper yield by use of the available information on

the corresponding gamma-ray yield for at leaot one peak. In addition, the peak for is multiplied sevenfold

in order to account for its relative abundance with

respect to the other isotopes.

The resulting composite curve, shown in Figure 10,

correlates quite well with the original non-isotopic

curve, although the latter is slightly shifted in energy

because of the use of a thicker target. Mg and Mg

provide most of the contribution. It is impossible to assign all the resonance peaks

to a capture reaction with any certainty, although there

is some supplementary evidence. The more intense peaks cl6 resulting from proton bombardment of Mg have been found to be caused by gamma-rays of too great an energy gl to have come from Inelastic scattering. Mooring et al.^ found no inelastically scattered protons from Mg24- in this region. No data is available on inelastic scattering

fro. Mg2^. Thus weak scattering peaks due to Mg2^ and 26 Mg may be present in the curve for natural magnesium and 26 in the isotopic curve for Mg • The isotopic curve

for Mg2^ is due to positrons definitely from Al2^ so no

inelastic scattering resonances can be present. A comparison of the lsotoplc curves with the curve for natural magnesium Indicates a possibility of activity at 795 kev 25 and 970 kev which could be ascribed to Mg (p,p*).

Considerable information can still be gained by 2 4 25 investigation of these two isotopes Mg and Mg .

Determination of the spectrum could be carried at least 400 kev higher and still be within range of the machine. 51 The elastic scattering work of Mooring et al. partic­ ularly indicates odd-shaped resonances for Mg24 in this region. Only a few measurements have been made on the gam/na-ray energies resulting from either reaction and no gamma-ray decay schemes are known for the levels in tnis region. Available data on the maximum positron energy is scarce and even the values for the half-lives are not too well fixed.. Consequently, a great deal of work remains to be done before a complete picture of proton capture and subsequent positron decay is obtained.

-97- 5 NATURAL 4 Lii Mo > 3 — 4- > 2 s I I < vo 5 • 5 < 5 O

UJ 4 > H 3. < 2 UJ fr

800 900 1000 1100 PROTON ENERGY (KEV) FIGURE 10 Comparison Of Energies Of Protons At resonance

M 24 Mg MS 25 M g 26 Mg

420 kev 568 kev 545 kev 727 lcev 324 494 450 820

510 662 851 56 5 720 848

588 815 960 650 840 995

685 954 1012 722 992 1056

777 1015 1158 812 1056 1188

880 1127 1217

928 1295 1265 986 1425 14^5

1045 1464 1472 1081 1511 1098 11^2

1197

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Grove, Phys. Rev. 80. 131 (1950).

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the Ohio State University Research Foundation (1949).

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Phys. Soc. Mar 1952 p. 20. X. APPENDIX

The relative spread in the energy dE/E is given in

terms of the width of the slit and the variation in the

magnet current for an analyzed beam of protons by

dE _ dl - _ Ctn # d* 2E I

I — magnet current

e = angle between deflected and undeflected paths of

beam viewed from center of the magnet (30°). de * angle subtended by half of the slit from the center of the magnet. This is inherently negative.

As a check, a one mm. slit was used to measure the 669 kev and 87>.5 kev resonances of fluorine from the Mg2^F2

target. The current for the 87.?. 5 kev resonance was

0.825 amp. with a variation of 0.002 amp. r i o o g + .0 0.242 7 LTB 25 2 J Next the half-width of the resonances wexe measured and the natural half-width obtained from Hornyak et al.

669 k e v p ' * 9.7 k e v p = 7.5 kev 873.5 k e v 8 . 8 k e v r * 5.2 k e v Assuming that the energy spread is a direct function

-105- of the voltage a = k*E and that the target thickness 1 ® ^ at 669 kev and 0.9 at 873.5 kev, use of r " * r u * ♦ f - x gives two equationo

(9.7)2 * (7.5)2 + k2 (669)2 + $ 2 (8.8)2 * (5• 2)2 + k2{873)2 + (O.gfi2

The simultaneous solution of these equations glveB £ = 4 kev for the target width

k * a£ the energy spread

kE * 6.1 kev at the 873-5 kev resonance which agrees

well with the value derived from the formula.

-106 AUTOBICO^APHY

I, Warren E. Taylor, wa8 born In Colorado Springs, Colorado on November 15,1920. My primary and secondary training was received In the Public schools of South Bend, Indiana. Undergraduate training was taken In the

Extension Division of Indiana University and at Kalamazoo College, Kalamazoo, Michigan from which I received the degree of Bachelor of Arts, Magna Cum Laude, in 194-7• From 1943 to 1946 my position was that of a communications officer in the Air Force. Since July 1947 I have been a graduate student in the DePa *tment of Physics at Ohio State University, and have been employed at the same time as a hesearch Assistant on the electrostatic generator group. While competing the requirements for the Doctor of Philosophy degree, I have held the position of Research Fellow*

-107-