Science, Faculty of
Physics and Astronomy, Department of
DSpace
UBCV
Critoph, Eugene
Master of Applied Science - MASc
University of British Columbia
The energy of gamma rays from the reaction H² (p,γ)He³ has been measured as a function of incident proton energy in the range of proton energies from 1 to 2 Mev. The results were consistent with the assumption that the He³ is left in the ground state. The Doppler shift in gamma ray energy was measured for incident protons of energies 1 Mev. and 1.7 Mev. and was found to be consistent with the value calculated assuming that the He³ was not slowed down before emission of the gamma ray. This puts an upper limit on the lifetime of the intermediate state of 2 x 10⁻¹² sec. The angular distribution of gamma rays at an incident proton energy of 1.75 Mev. was of the form sin²θ + (.025 ± .007), and at incident proton energy of 1.00 Mev. was of the form sin²θ + (.043 ± .008). The excitation function, over a range of incident proton energies from 0.25 to 1.85 Mev., is fitted by a function of the form AEp⁰•⁶⁵.
I. A STUDY OF THE GAMMA RAYS PRODUCED BY THE BOMBARDMENT OF DEUTERIUM WITH PROTONS by EUGENE CRITOPH A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of PHYSICS We accept t h i s thesis as conforming to the standard required from candidates f o r the degree O f MASTER OF APPLIED SCIENCE Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA November, 1955 I l l ABSTRACT The energy of gamma rays from the reaction H 2 (p,Y)He^ has been measured as a function of incident proton energy i n the range of proton energies from 1 to 2 Mev. The results were consistent with the assumption that the He^ is l e f t i n the ground state. The Doppler s h i f t i n gamma ray energy was measured fo r incident protons of energies 1 Mev. and 1.7 Mev. and was found to be consistent with the value calculated assuming that the He^ was not slowed down before emission of the gamma ray. This puts an upper l i m i t on the l i f e t i m e of the intermediate state of 2 x IO""12 sec. The angular d i s t r i b u t i o n of gamma rays at an incident p proton energy of 1.75 Mev. was of the form s i n 6 + ( . 0 2 5 ± .007), and at incident proton energy of 1.00 Mev. was of the form s i n 2 6 + (.043 ± .008). The e x c i t a t i o n function, over a range of incident proton energies from 0 . 2 5 to 1.85 Mev.,is f i t t e d by a function of the form A E p 0 , 6 5 . . IV ACKNOWLEDGrMENTS I would l i k e to acknowledge the receipt of a National Research Council Bursary f o r the year 1951-52, without which t h i s work could not have been done. I would also l i k e to thank Dr. J.B. Warren and Dr. CA. Barnes, under whose supervision t h i s work was done, and Dr. G-.M. G r i f f i t h s , who collaborated on t h i s work, f o r t h e i r help and guidance. II TABLE OF CONTENTS Page I INTRODUCTION a) Reasons for interest in the Reaction H2(p,lOHe3 1 b) Previous Work 2 c) Reasons for the Present Study 3 II THEORY OF EXPERIMENT a) Outline of Experiments 4 b) Theory 5 III APPARATUS a) Van de Graaff Generator 10 b) Single Scintillation Crystal Gamma-Ray Spectrometer 10 o) Target Assembly & Target Chamber 18 IV PROCEDURE & RESULTS a) Calibration, Checks, and Lining-up of . Apparatus 24 b) Determination of Gamma-Ray Energy 85 o) Doppler Shift 28 d) Angular Distributions 30 e) Excitation Function 34 V SOURCES OF ERROR 40 VI CONCLUSIONS 42 VII BIBLIOGRAPHY 44 APPENDIX A 45 •1 ?A Study of the Gamma Rays Produced by the Bombardment of Deuterium with Protons" I INTRODUCTION a) Reasons f o r the Interest i n the Reaction H 2(p>y)He One of the biggest problems i n physics to-day i s that of the nature of nuclear forces. At present there i s no theory which gives a comprehensive picture of nuclear forces. As a r e s u l t many problems dealing with n u c l e i cannot be solved and most predictions concerning nuclear reactions must be based on experience and empirical r e l a t i o n s . The function of the experimental p h y s i c i s t i s to obtain as much data as possible on nuclear reactions i n order to help the t h e o r e t i c a l p h y s i c i s t constract a use f u l theory of nuclear forces. In so doing, the experimental p h y s i c i s t should attempt to accumulate that data which w i l l be the most usefu l to the theoreticians. I t i s reasonable to assume that the most useful data w i l l be that obtained from experiments deal-ing with the l e a s t complex p a r t i c l e s and systems of p a r t i c l e s . Consequently, most of the experimental work at present i n t h i s f i e l d concerns reactions involving elementary p a r t i c l e s and l i g h t n u c l e i . The reaction, H 2(p,tf)He 3, involves only fundamental p a r t i c l e s and very l i g h t n u c l e i . Hence i t i s l o g i c a l to obtain as much information about t h i s reaction as i s f e a s i b l e . This accounts l a r g e l y f o r the i n t e r e s t i n t h i s reaction. 2 There are, however, other reasons for studying this reaction. The f i r s t of these is that, based on present knowledge, this reaction seems to provide a source of polarized gamma rays over a range in energy of several Mev. Such a source might be of considerable use i n future experiments. Therefore i t i s desirable to check whether or not this i s actually the case. Secondly, this reaction lends i t s e l f to the detection and measurement of the Doppler shift i n gamma ray energy. a Such a shift has been observed once before in^nuclear re-action (1) and while there i s no fundamental importance attach-ed to such an observation i t i s nevertheless reassuring to check the theory. b) Previous Work The reaction, H 2(p,y)He 3, was studied by Fowler, Lauritsen and Tollestrup in 1949. (2) They bombarded a thick target of deuterium with protons and detected gamma rays produced i n the target. They measured the energy of thfise rays as 6.3 0.3 Mev. when the target was bombarded with incident protons having an energy of 1.42 Mev. They obtained an excitation function for the reaction f) 72 which yielded a value for the oross-section of 0.74 E x 10" 2 9 cm2 for protons of incident energy E. Incidentally this low cross-section aocounts for most of the d i f f i c u l t i e s encount-ered i n studying the reaction. They also measured the angular distribution of the gamma rays produced with a beam of 1.42 Mev. protons. Their result wa that, 3 I e a 0.85 s i n 2e + 0.15, where 1^ i s the r e l a t i v e f l u x at the angle 0 and, Q i s the angle between the d i r e c t i o n of the incident proton beam and the d i r e c t i o n of observation. They f e l t considerable doubt as to the size of the i s o t r o p i c component and a f t e r more careful work they f e l t that a better value f o r the r e l a t i v e i s o t r o p i c component was 0.04 *" .08. c) Reasons f o r the Present Study Since 1949 there has been considerable advance i n the equipment and techniques of gamma ray spectrometry. S c i n t i l l a t i o n c r y s t a l s have l a r g e l y replaced Geiger counters f o r detection and energy-measuring purposes, r e s u l t i n g i n vast improvements i n energy resolution and e f f i c i e n c y . Hence i t was f e l t that with these new tools and techniques, a more oomplete and detailed study of the reaction could p r o f i t a b l y be made. I I THEORY OF EXPERIMENT a) Outline of Experiments The experiments performed i n connection with t h i s reaction f a l l into four main classes: determination of gamma ray energy, measurement of the Doppler s h i f t , measurement of angular d i s t r i b u t i o n s , and measurement of ex c i t a t i o n function. The gamma ray energy was measured as accurately as possible f o r several d i f f e r e n t incident proton energies, using a single s c i n t i l l a t i o n c r y s t a l gamma ray spectrometer. Gamma rays from the reaction F ^ 9 (p»«»y) 0 1 6 were used to cal i b r a t e the spectrometer i n t h i s region. These c a l i b r a t i o n gamma rays have well known energies based on a study.of the associated p a r t i c l e s . (3) The Doppler s h i f t i n gamma ray energy was measured f o r several d i f f e r e n t energies of incident protons. The resol u t i o n of the spectrometer was good enough to permit t h i s provided considerable care was exercised. An e x c i t a t i o n function was obtained i n the range, 250 kev to 1.8 Mev., to cheek that there were no i r r e g u l a r i t i e s which had not previously been detected. F i n a l l y , a oareful study of the angular d i s t r i b u t i o n of the gamma rays was made for two incident proton energies; 1 Mev. and 1.7 Mev. P a r t i c u l a r attention was paid to the d i s t r i b u t i o n close to the d i r e c t i o n of the incident protons. I t was f e l t that t h i s was the most important part of the work and therefore the most time and e f f o r t was spent on i t . 5 b) Theory i ) Gramma Hay Energy. The reaction under consideration i s represented as follows: H 2 + p -> He 3 +• V Both the Law of Conservation of Energy and the Law of Conservation of Momentum must hold i n t h i s reaction. Assume that the deuterium atom i s i n i t i a l l y at rest, and l e t Ep be the k i n e t i c energy of the incident proton, E3 be the k i n e t i c energy of the assembly of proton plus deuteron, B^ be the Jf-ray energy, B r be the r e c o i l energy of the He 3 atom, m be the mass of the proton, M 2 be the mass of the deuterium atom, M 3 be the mass of the helium atom, Q be the accurate mass difference between He 3 and H 2+ p (assuming both are i n the ground s t a t e ) . Then by the Law of Conservation of Momentum |2mEpl s J2M3EIP ' E3 s £ Bp By the Law of Conservation of Energy E / s 0, t-Ep -mEp - E r M 3 SjT - j 2M3E. 1 c or B r = E * 2M3c 2 6. The formula, t o a s u f f i c i e n t degree of ac c u r a c y f o r our purposes, i s then, E v a Q+(l-m)Ep-fa Y M 3 - nfc ]2 S M 3 o « ® P u t t i n g i n the a p p r o p r i a t e v a l u e s f o r G; ( 4 ) , m, M 3 we get, E s 5.493+(2/3)Bp - (5.493+(2/3)Ep}2 2M 3o 2 where S ^ , S p and M 3 c 2 are i n Mev. T h e r e f o r e by measuring E ^ f o r s e v e r a l v a l u e s of Ep we can get a check on the mass d i f f e r e n c e and v e r i f y t h a t the H e 3 nucleus i s l e f t i n the ground s t a t e , i i ) Doppler S h i f t When monoenergetic photons are e m i t t e d from a body which i s moving w i t h respeot to an observer, (the photons are monoenergetic i n the c e n t r e of mass c o - o r d i n a t e s of the body) there i s a Doppler S h i f t i n the energy of the photons. That i s , the energy as measured by the observer depends upon the angle between th e d i r e c t i o n of motion of the source and the d i r e c t i o n of o b s e r v a t i o n . S i n c e i n our work we w i l l be d e a l i n g w i t h v e l o c i t i e s which a r e s m a l l w i t h r e s p e c t t o c we can use the n o n - r e l a t i v i s t i c formulae f o r the v a r i a t i o n of observed energy w i t h a n g l e . These are B y ; 8 S ^ (1 - % cos©) - - ( i ) cos© a cos© / - - - - - - - ( 3 ) where V i s the v e l o c i t y of the source w i t h r e s p e c t to the observer, o i s the v e l o c i t y of l i g h t , ,7 6 i s the angle between the v e l o c i t y vector and the di r e c t i o n of emission i n the centre of mass co-ordinate system, ©''is the angle between the v e l o c i t y vector and the di r e c t i o n of emission i n the observer's system, Ey i s the photon energy i n the centre of mass system, E ' i s the photon energy i n the observer's system. In our case, the combination of deuteron and proton has one-third the k i n e t i c energy of the incident proton. Assuming that no energy i s l o s t by c o l l i s i o n before emission of a i f-ray, we have V2 - 2B p e2* 9me2 where Ep i s the incident proton energy m i s the proton mass and me2 s 931 Mev. Therefore by measuring the gamma ray energy at various angles from the dir e o t i o n of the incident proton, for. various values of Ep, we can check t h i s formula. This would enable us to say something about the time elapsing between the c o l l i s i o n with a proton and the emission of a gamma ray. I f we detect a Doppler S h i f t of the magnitude predict-ed i n equation @ we w i l l know that the gamma rays were emitted from the bodies before they were slowed by c o l l i s i o n s with other atoms i n the target. This would enable us to put an upper l i m i t on the l i f e t i m e of the intermediate state e x i s t i n g between the 8 absorption of the proton and the emission of the gamma ray. This upper l i m i t would be of the order of the average time between c o l l i s i o n s i n deuterium, i i i ) E x c i t a t i o n Function Of i n t e r e s t i n a l l nuclear reactions Is a measure-ment of r e l a t i v e oross-section as a function of incident p a r t i c l e energy. This information i s often useful to experimentalists who wish to use a ce r t a i n type of r a d i a t i o n i n experiments and are searching f o r a suitable source. Also i n many cases where resonances occur the information may be used to determine energy l e v e l s of the compound nucleus. The previous work done on t h i s reaction showed no resonances and the eross-section increased monotonically with energy over the range studied. In a reaction of t h i s type i n which both the target nucleus and incoming p a r t i c l e are charged we would expect the non-resonant eross-section to increase with energy at low energies. This i s due the higher p r o b a b i l i t y that the incoming p a r t i c l e w i l l penetrate the coulomb bar r i e r of the target at higher energies. When the energy of the incident p a r t i c l e i s high enough to insure penetration of the coulomb b a r r i e r the cross-section should l e v e l o f f . F i n a l l y the cross-section should drop o f f at high energies, where the higher v e l o c i t y of the i n -coming p a r t i c l e reduces the time spent i n the neighbourhood of the target nucleus and hence the p r o b a b i l i t y that the reaction w i l l take place. iv) Angular D i s t r i b u t i o n s I t i s not within the scope of t h i s thesis to discuss 9 the general problem of in t e r p r e t a t i o n of angular d i s t r i b u t i o n s i n nuolear reactions. The reader i s referred to French's notes ^ f o r such a discussion and to M. Verde f o r a treatment of d i r e c t radiative t r a n s i t i o n s . 10 I I I APPARATUS a) Van de Graaff Generator The beam of monoenergetic protons used i n t h i s experiment was obtained from the Unive r s i t y of B r i t i s h Columbia's Van de Graaff generator, which has been described elsewhere. (8,9,10,11) This machine has several o h a r a c t e r i s t i c s which make i t p a r t i c u l a r l y suitable f o r studying t h i s reaction. A proton beam ourrent up to 30^/camps, i n the energy range from 200 Kev. to 2 Mev. i s obtainable. This high current i s a necessity due to the low cross-section f o r the reaotion. The energy range i s e s s e n t i a l l y that i n which we are interested since background troubles become very serious above 2 Mev. and the y i e l d i s extremely low below 200 Kev. The proton energy may be held constant to within 1 Kev. using the electron gun s t a b i l i s i n g system. (10) While t h i s s t a b i l i t y i s better than i s a c t u a l l y needed f o r t h i s work, i t i s nevertheless very convenient. The beam may be focussed i n a 1/8" spot or spread out evenly over a oiroular area i n diameter. This i s very s a t i s f a c t o r y f o r t h i s work also, since a small spot i s needed f o r doing angular d i s t r i b u t i o n s and a di f f u s e spot was found more convenient i n measuring the gamma ray energies. b) Single S c i n t i l l a t i o n C r y s t a l Gamma-Ray Spectrometer 1. Theory 1) General. Many d i f f e r e n t types of s c i n t i l l a t i o n c r y s t a l s may 11 be used f o r the detection of gamma rays, but sodium iodide c r y s t a l s with thalium impurity, Nal (Th), are the only c r y s t a l s , i n use at present, which are suitable f o r the accurate measure-ment of gamma ray energies. The following discussion w i l l be lim i t e d to Nal(Th) c r y s t a l s and i n the case of e x p l i c i t c a lculations and curves to one Nal(Th) c r y s t a l i n p a r t i c u l a r . This c r y s t a l , i n the form of a rig h t c i r c u l a r oyliner 2.02" long and with a diameter of 1.76rt, was the one used i n t h i s experiment f o r the determination of gamma ray energies and also i n c i d e n t a l l y as a detector i n other parts of the work. When a gamma ray i s Incident upon a c r y s t a l i t may pass through unaffected or may be absorbed* There are three mechanisms by which the gamma ray may be absorbed: photo-e l e c t r i c process, Compton process and pai r production. In a l l of these processes at le a s t part of thej'-ray energy i s impart-ed to an electron (also a positron i n the case of pai r production). The electron subsequently loses energy by the ion i z a t i o n and exc i t a t i o n of atoms of the c r y s t a l . These atoms; then de-excite and a number of low energy photons ("light" pulse) are emitted. The number of photons emitted (or the i n t e n s i t y of the l i g h t pulse) i s proportional to the energy of the electron. In turn the spectrum of electron energies a r i s i n g from a beam of incident monoenergetic gamma rays i s dependant on the gamma ray energy. I f the spectrum of electron energies a r i s i n g from a beam of monoenergetic gamma rays i s known the gamma ray energy may be deduced. To show how t h i s may be done requires a closer study of the gamma ray absorption processes. Later i t w i l l be shown how the electron energy 12 speotrum a r i s i n g from the absorption of gamma rays i s measured. i i ) Photo-electric process When an incident gamma ray i s absorbed by the photo-e l e c t r i c process a l l i t s energy goes into the ex c i t a t i o n of an atom. The excited atom emits an electron with k i n e t i c energy equal to the gamma ray energy minus the i o n i z a t i o n p o t e n t i a l of the electron emitted. Therefore the electron energy spectrum a r i s i n g from t h i s process i s a peak at the inoident gamma ray energy minus the most probable i o n i z a t i o n p o t e n t i a l . This i s shown i n f i g . 1 f o r a beam of 2.62 Mev. gamma rays from RaTh. The resolution used i n p l o t t i n g these and sub-sequent t h e o r e t i c a l curves was that experimentally determined f o r the p a r t i c u l a r c r y s t a l mentioned previously. The reason for such a resolution i s discussed l a t e r . The areas under the curves are normalized to the cross-sections of the various processes at t h i s energy. The curves are only drawn approximat-ely to the expected shape. i i i ) Compton Process When absorbed by the Gompton process, the incident gamma ray undergoes a c o l l i s i o n with a free electron. This may be considered as an e l a s t i c , two-particle c o l l i s i o n obeying the Laws of Conservation of Energy and Momentum i n the r e l a t i v i s t i e sense. The electron energy spectrum a r i s i n g from t h i s process i s shown i n f i g . 1 f o r the 2.62 Mev. gamma rays from RaTh. The shape of the spectrum and po s i t i o n of the Compton edge depend on the gamma ray energy. iv) P a i r Production The t h i r d mechanism f o r absorbtion i s p a i r production. THEORETICAL PRIMARY ELECTRON ENERGY DISTRIBUTION FOR ABSORPTION OF 2.62 MEV / - RAYS IN Nal C R Y S T A L ENERGY in MEV. 13 The incident gamma ray can, i n the neighbourhood of a nucleus, form an electron and a positron. The energy of the gamma ray minus the energy equivalent of two electron masses (1.02 Mev.) goes into k i n e t i c energy of the positron and the electron. In los i n g energy these y i e l d a l i g h t pulse equivalent to that a r i s i n g from a single electron with the sum of t h e i r k i n e t i c energies. When the positron comes to rest i t annihilates with an electron, producing two 0.51 Mev. quantac. The electron energy spectrum due to t h i s process i s shown i n f i g . 1 f o r the 2.62 Mev. gamma rays from RaTh, assuming both a n n i h i l a t i o n quantae escape. This i s a peak at the gamma ray energy minus 1.02 Mev. Of course pair production takes place only f o r gamma rays having an energy greater than 1.02 Mev. v) Resultant E f f e c t of the Three Processes. I f a l l the secondary gamma rays produced i n the Compton process and a l l the a n n i h i l a t i o n quanta- from the p a i r production process escaped from the c r y s t a l , we would expect the resultant, energy spectrum to be the sum of the three spectra due to the three separate processes. In f a c t , a l l the secondary gammas do not escape the c r y s t a l . The p r o b a b i l i t y of absorption f o r an 0.51 Mev. an n i h i l a t i o n quanta i s approximately 0.6 f o r t h i s c r y s t a l . Therefore the pai r production peak i s divided into three peaks at (E - 1.02) Mev., (E - 0.51) Mev., and E • The r a t i o of the areas under these peaks i s then roughly J.6; .48; .36 as shown i n f i g . 2. The l a s t peak e s s e n t i a l l y adds on to the photo electron peak. Some secondary gammas from the Compton process are also absorbed, s h i f t i n g the Compton edge to a THEORETICAL LIGHT P U L S E INTENSITY DISTRIBUTION FROM Nol C R Y S T A L E X P O S E D T O 2.62 MEV. y^- R A Y S INTENSITY in arb. unils 14 s l i g h t l y higher energy. The expected resultant electron energy spectrum i s shown i n f i g . 2 a f t e r Including these refinements. 2. Measurement of Electron Energy Spectrum As was stated previously, the number of photons (i n t e n s i t y of l i g h t pulse) emitted, when an electron i s stop-ped i n a Nal(Th) c r y s t a l , i s proportional to the k i n e t i c energy of the electron. Therefore i f the i n t e n s i t y spectrum of l i g h t pulses coming from a c r y s t a l , which i s exposed to gamma rays, i s measured, i t w i l l have the same shape as the electron energy spectrum. Therefore i f we have one or more c a l i b r a t i o n points (preferrable i n the neighbourhood of the gamma ray energy to be measured) we can deduce the gamma ray energy from the l i g h t pulse i n t e n s i t y spectrum. The i n t e n s i t y of the l i g h t pulses was measured using a photo-multiplier tube ( i n our case, EMI 6262). The c r y s t a l was mounted on the end of the photo m u l t i p l i e r tube (see f i g . 4 and the discussion i n the following section) and the assembly made l i g h t - t i g h t . The photo m u l t i p l i e r tube then converts the l i g h t pulse into a voltage pulse and amplifies i t . The voltage pulses were taken from the l a s t or second-to- l a s t dynode and af t e r suitable a m p l i f i c a t i o n were fed into an 18-channel, Marconi ki c k - s o r t e r . Here the voltage pulses were sorted and recorded. The re s u l t i n g spectrum represents the electron energy spectrum. Therefore from t h i s information the gamma ray energy may be deduced. A sample experimental spectrum, as displayed on the kick-sorter f o r the 2.62 Mev. ^ -rays from RaTh, i s plotted i n f i g . 3 and may be compared with expected curve in::fig. 2. The 3 0 0 0 E X P E R I M E N T A L S P E C T R U M , AS D ISPLAYED ON KICK SORTER, FOR THE 2.62 MEV. y - R A Y S FROM R a T h oo 3 o o 2500 2000 1500 1000 500 (counts in each channel plotted at lowest voltage in t h a t channel) 18 20 22 24 26 K.S. V O L T S 28 30 32 34 T P = 31 I ? 15 difference i n the lower end i s due p a r t l y to 1 .8 Mev. gamma ray contamination. 3. The Technique of Mounting Crystals The problem of mounting the Nal(Th) c r y s t a l i s a major one. The c r y s t a l must be mounted on the end of a photo m u l t i p l i e r tube with a good o p t i c a l seal between the c r y s t a l and the tube* The whole assembly must be l i g h t - t i g h t so that the only l i g h t which the tube sees i s that a r i s i n g i n the c r y s t a l . The d i f f i c u l t i e s are increased by the f a c t that Nal(Th) c r y s t a l s cloud up when exposed to traces of water vapour. This means that the o r y s t a l must be kept free from water vapour before, during and a f t e r mounting. "Wet" Mount A "wet" mount was the f i r s t type t r i e d . The c r y s t a l was polished well under o i l and then put i n a th i n , l o o s e - f i t -t i n g aluminium can and the space f i l l e d with l i g h t mineral o i l . The can was then taped securely to the end of an EMI6262 photo m u l t i p l i e r tube so that one face of the c r y s t a l pressed t i g h t l y against the end of the tube. Care was taken to use enough tape to prevent any o i l leaking out. The photo m u l t i p l i e r tube was then taped up to make i t l i g h t - t i g h t . The re s o l u t i o n obtained with t h i s type of mount was approximately 15% at 2.62 Mev. and was not considered good enough f o r t h i s work. "Dry" Mount The second type of mount t r i e d was a "dry" mount. A dry box was used i n which the f i n a l steps of mounting could be carried out free from water vapour. The d e t a i l s of . FIG.4. l+o £ol low) p , / ^ ) E x p l o d e d view >nmunn)»>) iDtnirrm f aluminium cap cry s ta ! posit ion I I Y,, J / \-I / / / I t f -Aluminium can rubber ring fe lucite plate photomultiplier tube aluminium ring DRY MOUNT FOR No I C R Y S T A L 16 the mount are shown i n f i g . 4. The c r y s t a l was given a pre l iminary p o l i s h i n g , out-side the dry box, under o i l . Then the remainder of the procedure was c a r r i e d out ins ide the dry box. The o i l was wip-ed from the c r y s t a l and a f i n a l p o l i s h administered us ing b l o t -t i n g paper. Then the c r y s t a l was placed on a l u c i t e d i s k which had been coated with s i l i c o n e o i l of v i s c o s i t y 2 x 1 0 5 centistokes ( f i g . 4). A good bubble-free contact was made by press ing the c r y s t a l f i r m l y down and r o t a t i n g i t . The aluminum cy l inder was then put i n place-and the space along the s ides of the c r y s t a l packed with MgO powder. A l ayer of dry MgO powder was put on the top of the c r y s t a l and the aluminum cap screwed down. The j o i n t s were made l i g h t - t i g h t with Black Duco p a i n t . This formed a permanent un i t of a sealed c r y s t a l which could be subsequently mounted on any EMI 6262 photo m u l t i p l i e r tube using s i l i c o n e o i l as a l i g h t seal between the l u c i t e and the tube. Black tape and p las t l cene were used to make the assembly of c r y s t a l mount and tube l i g h t - t i g h t . MgO powder has a high r e f l e c t i o n c o e f f i c i e n t f o r l i g h t and i t was used to maximize the proport ion of the l i g h t from the c r y s t a l entering the photo m u l t i p l i e r tube and a l so to keep th i s proport ion constant as the p o s i t i o n of the l i g h t source changed i n the c r y s t a l . I t also served, i n c i d e n t a l l y , to absorb any water vapour entering the mount, thereby prevent-ing the c r y s t a l from c louding . This type of mount gave a r e s o l u t i o n of approximately l^fo at 2.62 Mev. ft"-ray energy, which was considered adequate for t h i s work. 17 4. C i r c u i t r y A block diagram of the electronics used i n the spectrometer i s shown i n f i g . 5. The H.T. supply, whieh provided the high voltage f o r the photo m u l t i p l i e r tube, was variable i n the range 0 to 2200 v o l t s and was stable to 1 v o l t i n 2000 v o l t s a f t e r a half-hour, warm-up period. The kick sorter was the Chalk River type produced by Canadian Marconi of Montreal. 5. L i n e a r i t y . Resolution, and E f f i c i e n c y The l i n e a r i t y of the system was checked over a range of photo m u l t i p l i e r H.T. v o l t s and a range ofX -ray energies. An operating voltage of 1100 v. was chosen as a suitable voltage f o r measuring Jf-rays up to 7 Mev. While the l i n e a r i t y over the complete range was only good to a few percent, t h i s was ample since several c a l i b r a t i o n points were always used close to the gamma ray energy being measured. Over such a r e s t r i c t e d range deviations from l i n e a r i t y could not be detected. The resolution of the spectrometer i s defined as the width of a peak (oaused by monoenergetic electrons slow-ing down i n the crys t a l ) at half maximum divided by the energy at the peak. There are several factors contributing to t h i s peak width. Due to t h e i r f i n i t e range some of the electrons escape from the c r y s t a l before they lose a l l t h e i r energy. This i s c a l l e d "wall e f f e c t " and i s of l i t t l e importance i n cry s t a l s as large as those used i n t h i s study. S t a t i s t i c a l f l u c t u a t i o n s , i n the number of photons a r i s i n g from the slow-ing down of an electron of given energy and also i n the number of these photons which are e f f e c t i v e i n producing electrons i n H.T Supply c 3 -3 °\ cathode Iran smis*sion line ar amplif. kick-sorter fol lower cable BLOCK DIAGRAM OF ELECTRONICS A S S O C I A T E D T H E / - R A / S P E C T R O M E T E R W i T H 6 18 the photo m u l t i p l i e r tube, are the most important causes of peak width. The e f f i c i e n c y of the c r y s t a l i s of course a function of energy, since the cross-sections f o r the various absorption processes are functions of energy. The e f f i c i e n c y of t h i s p a r t i c u l a r c r y s t a l i s about 33$ at 6 Mev, c) Target Assembly and Target Chamber 1. Tube Assembly The beam of protons passed from the Van de Graaff proper into an analysing magnet, and then through the tube assembly to the target. The whole system was evacuated, with the pressure being kept below IO"*5 mm. of mercury. The positioning and design of the components of the tube assembly i s shown i n f i g . 7. The section adjacent to the magnet box was a 12" length of i r o n pipe. The purpose of t h i s pipe was to protect the beam from the f r i n g i n g f i e l d of the magnet and henoe to eliminate the necessity f o r realignment of the tube sections and r e c a l i b r a t i o n of s l i t system as the proton energy was changed. Next came a sylphon bellows which provided a means of centering the beam on the target. The bellows permitted the target chamber to be moved i n any d i r e c t i o n perpendicular to the beam. The next section contained the " s n i f f e r s " , an a u x i l i a r y target (IT 1 9), and a quartz shutter. The " s n i f f e r s " provided the e r r o r signal f o r the electron gun, s t a b i l i z i n g unit which kept the beam on the target. They merely consisted 19 of two plates between which the beam passed. I f the energy upper of the beam dropped, the beam struck the lewor plate, and t h i s sent a signal to the electron gun which lowered the number of electrons sprayed on the upper terminal of the Van de Graaff, thereby r a i s i n g the voltage and bringing the beam back i n lower po s i t i o n * The plate, of course, sent the reverse s i g n a l when struck by the beam. The " s n i f f e r " plates were f i r s t made of molybdenum but were l a t e r changed to gold to cut down the background. The a u x i l i a r y F 1 9 target was used f o r c a l i b r a t i o n purposes. Since f l u o r i n e i s a serious contaminant due to i t s high proton cross-section, precautions were taken i n the design to allow i t to be completely withdrawn from the beam and protected from any scattered beam. The shutter was made of quartz and was included to permit examination of the beam immediately before i t entered the target chamber. I t was also used to keep the beam off the target while minor adjust-ments were being made. The target chamber f i t t e d onto the end of t h i s section* 2. Target Chamber A detailed drawing of the target chamber i s shown i n f i g . 6 . The design of the chamber was dictated by the following requirements: i ) A t h i n deuterium target was required which was convenient to use and to make. Gas and l i q u i d targets were eliminated on t h i s basis, and a s o l i d , t h i n heavy water target decided upon. A s o l i d deuterium target was eliminated due to i t s low freezing point. _ Fi&.t . . T A R G E T C H A M B E R V E R T I C A L — POSITIONING RING, T A R G E T - R O T A T I O N S V S T K N O U T S I D E P O T 7/ / > > ? / S > > > f_ 7V-7-3 2 : ft G E T -j / / / / I 3 ••5^c.c.C0W-mtpvJE^ ^ STo?» C6-" Tf?%T Tu©e Z7T 3 "7 i N ^ U L A T I M e "N V L I Q U I D AIR V A C U U M B E A N E"KJTR/ T U B E C E N T E R I N G Pc ? ^ r 2.0 i i ) The entering beam had to be we l l defined and the point at which i t struck the target well defined. i i i ) P rovision had to be made f o r rota t i n g the target so that e f f e c t s due to absorption i n the backing could be eliminated i n the angular d i s t r i b u t i o n s . iv) I t was desirable to obtain as large a v a r i a t i o n i n angle of observation as possible f o r the angular d i s t r i b u t i o n s . v) The background had to be as low as possible since the cross-section of the reaction being studied was very low. How these requirements were met can be seen from the drawing i n f i g . 6 and the discussion which follows. Some of the problems which arose i n s a t i s f y i n g the requirements are also discussed. In order to make the target i t was proposed to i n j e c t a known volume of D 20 vapour i n the v i c i n i t y of a cold metal backing. A f r a c t i o n of the vapour would then freeze on the back-ing and form the target. The thickness of the target could be regulated by the amount of vapour injected. Therefore a t h i n copper sheet was soldered to the end of a c y l i n d r i c a l l i q u i d a i r container and the whole assembly was surrounded by a larger evacuated chamber. The design was such as to allow a one hour run on one f i l l i n g of the l i q u i d a i r container. The general shape as shown i n f i g . 7 was necessary because the counters had to be very close to the target f o r some parts of the experiment. A window was inserted to allow the beam positioning and target thickness to be check-ed. An opening was provided, with a flange and 0-ring s e a l , 21 f o r the i n j e c t i o n of D 2 0 vapour. The D 20 dispensing unit i s shown i n the inset diagram i n f i g . 6. The double O-ring seal at the top of the chamber allowed the target to be rotated and also to be moved v e r t i c a l l y over a short range. The p a r t i c u l a r arrangement of t h i s seal was necessary to insure that the O-rings did not get too cold, since t h i s would prevent movement and possibly cause leaks. The outer chamber was made i n two parts f o r two reasons. In order that the target could be rotated, the diameter of the upper double O-ring seal had to be as small as possible. Once t h i s had been achieved, the outer chamber had to be i n two parts so that the target could be put i n place and removed e a s i l y . Also the target mount had to be insulated from the outer case i n order that the current to the target could be measured. This was done by i n s e r t i n g a l u c i t e disk between the two sections of the outer chamber. O r i g i n a l l y i t was planned to make the lower section of the outer chamber of aluminium to keep absorption of gamma rays between the target and the counters a minimum. Owing to f a b r i c a t i o n d i f f i c u l t i e s t h i s scheme was dropped and t h i n brass was used. The diameter of the tube through which the proton beam entered the target chamber was made small so that as wide a range of angles as possible was a v a i l a b l e f o r the angular d i s t r i b u t i o n work. For the same reason the tube was made long enough so that the flange at the end would not seriously r e s t r i c t the angles av a i l a b l e . There were stops at both ends 22 of t h i s tube. The aperature which the beam encountered f i r s t was i n i n diameter and the one at the target end of the tube was 5/16" i n diameter. In order to keep the background as low as possible, a preliminary study was made of the K-rays above 2 Mev. a r i s i n g from the proton bombardment of various materials. Rough exc i t a t i o n functions were run f o r gold, tungsten, platinum and copper. Tungsten and gold were found to be the best materials as f a r as background was concerned. Platinum was s l i g h t l y worse and copper was the worst by a fact o r of ten. Previous experiments which had been done i n t h i s laboratory showed that s i l v e r and molybdenum were worse than platinum but f a r superior to copper. In the l i g h t of t h i s information, the stops were made of pure gold, and the entrance tube to the target chamber was l i n e d with platinum f o i l . Eventually the " s n i f f e r " plates were also made of gold but t h i s was only f o r a few of the. l a t e r runs. At f i r s t a sheet of tungsten was attached to the copper backing, on which to deposit the target. However, the heat transfer through the tungsten was so bad that, f o r beams over 5^/tamps. at 1.5 Mev. the target melted and disappeared. There-fore a .020" gold button, 7/8" i n diameter, was soldered onto the copper backing and the target was deposited on t h i s button. 3. Counter Holder and Angular D i s t r i b u t i o n Apparatus The counter holder and angular d i s t r i b u t i o n apparatus are shown i n f i g . 7. A central pin was s o l i d l y f i x e d to the bottom of the target chamber and t h i s was the axis about which the moving counter rotated. A large angular scale and pointer /4RRAMGE.MEMT O F EXPERIMENTAL APPARATUS MOVABLE COUNTER F IXED MONITOR COUNTER T A R G E T C H A M B E R FIX E © S U P P O R T R O T A T I N G S U P P O R T I If h 31 1 J fc. SfLPHOM ( BELLOWS SHUTTER ( W i r * RIGID rMOLVBErJUM) ADJUSTMENT) AUXILIARY T A R G E T ( F N ^ Q U A R T S ^ 4-o - f N BOX IRON T U B E 2 3 were a f f i x e d as shown so that the angle between the d i r e c t i o n of observation and the beam could be read d i r e c t l y . The rota t i n g arm, on which the counter sat, was maintained horizontal by a brace and could be clamped i n any desired angular p o s i t i o n . Adjusting sorews allowed the whole stand to be raised or lowered during the lining-up procedure. The counter could be screwed to the counter holder at three d e f i n i t e distances from the centre l i n e of the target. This apparatus allowed an angular accuracy of 1 ° to be e a s i l y maintained. A monitor counter was f i r m l y f i x e d to the target chamber as shown i n f i g . 7 . The counter was s i m i l a r to the main counter but the c r y s t a l was somewhat smaller. 4 . Arrangement of Apparatus The arrangement of a l l the apparatus i s shown i n f i g . 7 . Lead shielding between the counting equipment and the Van de Graaff generator was inserted wherever possible to keep down the background. .24 IV PROCEDURE AND RESULTS a) Ca l i b r a t i o n , Checks and Lining-Up of Apparatus The s c i n t i l l a t i o n counters were f i r s t tested and suitable plate v o l t s and amplification f a c t o r s chosen by studying the X -rays from RaTh. These were then put i n t h e i r regular positions with a RaTh source nearby. Spectra were taken f o r various f i e l d s i n the Van de Graaff magnet. A s l i g h t s h i f t downward of the spectrum with respect to kick-sorter voltage was noted f o r high magnet f i e l d s . Therefore soft i r o n shields were placed around the photo m u l t i p l i e r tubes and the tests run again. No observable s h i f t s occurred f o r magnet f i e l d s up to the maximum i t was planned to use. In a l l the succeeding work the s o f t i r o n shields were always around the photo m u l t i p l i e r tubes. Lead shielding was wrapped around the sides of the cr y s t a l s not fac i n g the target, to protect them from sof t gamma rays from the Van de Graaff machine. This improved the background quite appreciably. Part of the equipment associated with the Van de Graaff i s a current integrator which was used to measure the t o t a l current to the target during any run. Since i t was suspected that there might be some electron emission from the target varlous-ibias voltages were t r i e d with a steady beam. The current was found to increase with negative bias and decrease with small+bias, thereby substantiating the fear of electron emission. However f o r p o s i t i v e bias over+90 v o l t s the registered beam current was constant i n d i c a t i n g that the + 90 v o l t s was enough to prevent electron emission. Therefore 2 5 the target was biased to 4-90 vo l t s during a l l phases of the work which required a knowledge of the integrated beam current. Before a l l runs the following procedure was follow-ed to insure a well-defined and well-centred beam. The quartz shutter was raised and the beam was focussed on i t u n t i l a uniform spot approximately 4" i n diameter was obtained. The tube assembly was then moved about, using the adjusting screws, u n t i l the beam looked as though i t would pass through the entrance stop* A thick heavy water target was then l a i d down on the target backing to provide a means of seeing the beam on the target (the thick DgQ target glowed blue where the beam struck i t ) . The quartz shutter was then removed and the spot on the target examined. The l i n e of the target tube was then adjusted, using the clamping screws on the vacuum seal , at the end joining the shutter tube, u n t i l the spot was well-defined and c i r c u l a r . The beam focus was then adjusted f o r maximum current to the target consistent with good d e f i n i t i o n and shape. b) Determination offr-yRay Energy Method The target assembly was l i n e d up following the procedure previously outlined. A DgO target which was estimated to be 60 Kev. thick was then l a i d down on the target baoking. The proton beam was then set to the desired energy by set t i n g the magnetic f i e l d i n the analysing magnet to the appropriate value and adjusting the Van de G-raaff v o l t s u n t i l the beam s t a b i l i s e d on the target. Beam currents of the 26 order of 10-15 microamps. were used. The gamma ray speotrum from the main c r y s t a l was then displayed on the kiok-sorter and recorded. Immediately a f t e r t h i s the f l u o r i n e target was slipped i n to intercept the beam. The f l u o r i n e gamma ray speotrum was then recorded. In a l l the cases done, the proton energy was close enough to a f l u o r i n e resonance to get adequate counting rates and spectra without changing the proton energy. The proton energy was then changed to the nezt desired energy and the process was repeated. Results Graphs of the DgO and f l u o r i n e spectra are shown i n f i g . 8 f o r an incident proton energy of 800Kev. Similar spectra were obtained f o r incident proton energies of 1 Mev. and 1.50 Mev. The energies and r e l a t i v e abundances of the gamma rays from three of the most important resonances i n f l u o r i n e are shown i n table 1. (16) TABLE 1 Resonance E-» 6.13 Mev. 6.9 Mev. 7.1 Mev. (Bp) I . 873 kev. 0.65 0.24 0.11 1355 kev. 0.53 0.12 0.35 340 kev. 0.96 - 0.04 Using t h i s data and the method previously desoribed, a rough t h e o r e t i c a l curve of the expected f l u o r i n e spectrum was drawn. This enabled the peaks i n the experimental spectrum to S C I N T I L L A T I O N S P E C T R U M O F & A » 4 M A R A Y S 27 be i d e n t i f i e d and energies assigned to them. ( f i g . 8). As can be seen from table 1, the 6*13 Mev. gamma ray i s the most abundant. Since the peaks from t h i s gamma ray were also i n the most suitable energy range they were the ones used i n deriving the energy of the fr'-ray from deuterum. The r e s u l t s obtained f o r Bp - .800, 1.00 and 1.50 Mev. are summarized i n table 2. TABLE 2 Bp By Targ. Thick. Cor rH B r Corr. 0 . Mev. Mev. Mev. "Mev. M .800 6.00 *• .04 .03 ± .01 6.03 t .05 5.50 t .05 1.00 6.16 ± .04 *03 t .01 6.19 ±" .05 5.52 - .05 1.50 6.46 i .04 .03 - .01 6,49 - .05 5.49 t ,Q5 Ave. Q, 5.50 - .05 * Mev. Sample Calc. f o r Ep = 800 kev. As can be seen from f i g . 8 the positions and energies of 2 f l u o r i n e peaks are: 6,13 Mev. at 26.5 K.S. v o l t s , and 5.11 Mev. at 22.4 K.S. v o l t s . Hence 1 K.S. v o l t = 0.249 Mev. ( i n t h i s range) The positions of the three pair peaks f o r the DgO spectrum are: Peak K.S. v o l t s E ^ 1st P a i r (E^- 1.02) 21.8 5.98 * .06 2nd P a i r (E^,- .51) 23.9 5.99 ± .06 3rd P a i r (E^ ) 26.0 6.01 ± .06 e.g. Ey as determined from the 1st pa i r peak i s then: E„ - 1.02 - 5.11 - (22.4 - 21.8) (.249) 9 a 4.96 Mev. 28 E^ s 5.98 *" .06 Mev. Where the error quoted i s § a kick sorter channel. The other values f o r E yshown in , t a b l e 2 are calculated i n a sim i l a r manner. The best value for the Jf-ray emitted from deuterium when bombarded with 800 kev. protons, uncorrected f o r target thickness i s 6.00 ^ .06 Mev. Q, i s calculated from formula 1, Section I I b ( i ) . The target thickness was estimated using the following method. The volume, pressure and temperature of the heavy water vapour injected into the system was measured. Erom t h i s the mass injected can be calculated. I t was assumed that the vapour spread uniformly over a sphere whose radius was the distance from the i n j e c t o r to the target. Hence the target thiekness i n mg./cm2 could be calculated. Using the figures f o r stopping cross-section per DgO molecule from Wenzel & Whaling the target thickness i n Mev. was estimated f o r the incident proton energy. (An upper l i m i t on the target thickness could be determined also by the width of the peaks.) The target thickness estimated i n t h i s manner fo r the above runs was 90 kev. A correction of 45 kev. should be subtracted from the proton energies or 2 (45) a 30 3 . kev. added to the gamma ray energy. In table 2, 30 kev. - 10 kev. has been adddd to theY -ray energies to correct f o r the target thickness. c) Doppler S h i f t The apparatus was l i n e d up as previously described. 29 A f a i r l y t h i n target was then l a i d down ( <a 40 kev. t h i c k ) . The proton beam was then adjusted to the desired energy. A spectrum was taken with the counter at 45° to the d i r e c t i o n of the beam. When good s t a t i s t i c s had been obtained, the counter was s h i f t e d to the 135° p o s i t i o n and another spectrum taken. This was repeated several times with read-ings also being taken once or twice with the counter i n the 90° p o s i t i o n . As soon as enough readings had been accumulated, the target was removed by blowing the l i q u i d a i r out of the pot and warming the target backing. Backgrounds were then taken with the same beam conditions, f o r the counter i n the 45°, 135° and 90° p o s i t i o n s . The integrator was used to normalize the background to the regular runs. Care had to be taken i n setting the K.S. channels. Usually the channel width had to be adjusted to show the s h i f t to best advantage. For wide channels the re s o l u t i o n was too poor and f o r very narrow channels the s t a t i s t i c s were bad and also s l i g h t errors, i n adjusting the channel width showed up more. The Doppler S h i f t was measured at 1 Mev. and at 1.7 Mev. Sample curves are shown i n f i g s . 9 and 10 (the back-grounds have been subtracted). The r e s u l t s are tabulated below. Ij 3oor DOPPLER SHIFT AT E P * 1 M e v . ' SPECTRUM # r % ^ S P E C T R U M A T 8 zz J D O P P L E R S H I F T /rr E P « L 7 Mev. .30 TABLE 3 Doppler S h i f t from 45° to 135° Proton Energy Calculated Value Observed Value 1 Mev. 133 kev. 135 * 10 kev. 1.7 Mev. 187 kev. 195 * 15 kev. Sample Calcul a t i o n f o r Ep - 1 Mev. The curves i n f i g . 9 have had the backgrounds subtracted and are normalized so that the corresponding p a i r peaks are approximately the same height. The displacement between the curves was measured at the Compton edges since t h i s i s the steepest part of the curve. In f i g . 9. Displacement at Compton edges s 0.55 v o l t s . Energy scale: £3.2 v o l t s corresponds to 5.66 Mev. i . e . 242 kev./volt. Doppler S h i f t : (0.55)(242) = 134 kev. The probable errors assigned were estimated. The calculated values i n the second column of table 3 were obtained using e q u a t i o n ^ i n section I I b. d) Angular D i s t r i b u t i o n s Method In taking angular d i s t r i b u t i o n s , the main counter was moved, i n a horizontal plane, i n a c i r c l e about the target. The p o s i t i o n d i r e c t l y i n l i n e with the beam, such that the target was between the beam and the counter, was l a b e l l e d 0°. A l l other positions were l a b e l l e d by t h e i r angular displace-ment from the 0° p o s i t i o n as measured either north or south. 31 The monitor counter was used f o r t h i s part of the experiment, as well as the movable counter. The pulses from the monitor counter fed into a scaler which was equipped with a descrlminator. The descriminator was set at about 4 Mev. so that only pulses above t h i s l e v e l were recorded. The monitor counter was f i x e d at 90°S. The pulses from the main (movable) counter were fed into the kick-sorter and also into a scaler with descriminator. The descriminator was set so that the count recorded on the scaler was the i n t e g r a l of the spectrum above 4 Mev. Cosmic ray backgrounds were taken i n both the main and monitor counters before the Van de Graaff generator was switched on. This was done by recording the number of counts i n both scales during a period of 300 seconds. The target backing was then put at an angle of 45° to the d i r e c t i o n of the beam so that the side facing the beam also faced the monitor counter. A proton beam, of the energy which was to be used f o r the p a r t i c u l a r angular d i s t r i b u t i o n being done, was then focussed on the target backing (using the procedure previously outlined). Background readings were then taken with the movable counter i n positions 30° apart between 150°S and 150°N. These readings consisted of time, integrated ourrent, monitor scaler count, main counter scaler oount, and kick-sorter readings. A DgO target, about 100 kev. thick, was then deposited on the backing. To insure steady beam conditions, the shutter "was inserted during t h i s operation rather than s h i f t i n g the current i n the analysing magnet or changing the ,32 Van de Graaff voltage. The beam was then allowed to s t r i k e the target and readings of monitor counts, main counter counts, integrated current, time, and kick-sorter counts were taken f o r the main counter i n positions spaced every 15°, between 150°S and 150°N. The above readings were repeated f o r the main count-er i n the 90°S and 90°N po s i t i o n s . The target was then r o t a t -ed thru 90° and the readings at main counter positions of 90°S and 90°N were repeated. This was done to check the amount of gamma ray absorption i n the backing. The DgO target was then removed by warming the target backing and the background readings were repeated f o r the main counter i n positions 15° apart between 150°S and 150°N. The shape of the curve around 0° was obtained with better s t a t i s t i c s . This was done by laying down a 100 kev. thick, DgO target and putting the target perpendicular to the beam. Readings were taken every 5° between 45°S and 45©N. The target was then removed and background readings were taken over the same range of angles. Results The cosmic ray background was obtained i n counts per unit time f o r both the monitor counter and the main counter. Machine background was defined as a l l background other than cosmic ray background. I t was expressed f o r each counter i n counts per unit integrated current. I t was i n general a function of angular p o s i t i o n f o r the main counter. 33 For each set of readings the cosmic ray and machine backgrounds were subtracted from the recorded monitor counts and main counter counts. The corrected main counter read-ings were then normalized to a given monitor count using the corrected monitor counts. F i g s . 11 and 12 show some t y p i c a l r e s u l t s . The points i n F i g . 11 have been corrected f o r target absorption. This was done by c a l c u l a t i n g the percent-age absorption of gamma rays t r a v e l l i n g through the target backing at 45°, from the 4 readings taken at 90°S and 90°N with the target i n the two d i f f e r e n t p o s i t i o n s . I t was then assumed that the correction varied as the thickness of target backing traversed i n going from the target to the counter. This correction was small and a f t e r t h i s p a r t i c u l a r set of r e s u l t s the amount of target backing was reduced. For a l l subsequent re s u l t s the absorption i n the backing was neglected. The experimental re s u l t s are tabulated i n Table 4. The f i r s t set of readings taken f o r each proton energy was used only to v e r i f y the general shape of the curve. The value quoted f o r the i s o t r o p i c component was obtained from the readings which were taken around 0° with better s t a t i s t i c s . There are two corrections shown i n the table which have not yet been explained. The s o l i d angle correction i s derived i n Appendix A. The neutron correction arose from work done by J . Sample and G.M. G r i f f i t h s . They showed that 6 -P S a < «* ID p D 2 U F I G . 11. O B S E R V E D A N G U L A R D I S T - A T L O H£V. ( C O R R E C T E D FOK T'^R^KT 4 B S O R P T i D M ^ t X S E T ^ i ( T f t i t t i E T Oftl L I N E 3 0 * 6 - I S * / * ! ^ f T A * s * £ T S C . ' * ! - f £ ~ 0 * $ ^ 0 3 3 £ > -#* S.OBO S l t 4 * £ 1 1 M.0. 5 A M P L E T A f e S A R E S T A T I S T I C S O N L Y . 150*5 i a O ^ S 9 0 ^ 6<*5 3 ^ 6 O * S O 5 * 9 0 * N I 2 0 ° N I50*H ANGLE o r OBSERVATION FIG. 12.-AHaVLHR DlSTRIBUTiOISi AT 1.7 MEV. 7 r 6h p > or * A (X 4 2 h 2 :> 02 O \ •t CURVE 0 . 6 O * J2.4 S IN 2 6 M B . T ^ C - S A R E tH»U STATISTICS I' \ V f / 45*5 30*5 15*5 ©• fi*.v 30°N 45*N ANGLE O F O B S E R V A T I O N TABLE 4 Proton Experimental Angular S o l i d . Neutron Corrected # Isotropio Energy D i s t i l Flux/Unit Angle CorrS Angular Y i e l d S o l i d 0orr£ DistS , 1.75 Mev, sin 2©+(.047 £ .005) -.010 -.012 £ .005 sin2e+( .025 £ .007) 3.6 t 0 .9 1.0 Mev. sin 2©+(.069 - .004) -.010 *.016 t .007 sin 2«+(. 043 * .008) 6.1 *1.0 34 25% ^ 1 0 % of the counts i n the 0° d i r e c t i o n were caused by-neutrons. The neutrons arose from bombarding the target but no complete explanation for the mechanism of t h e i r production has yet been advanced, e) E x c i t a t i o n Function The main counter was placed i n the 90°S p o s i t i o n . The target and beam were then l i n e d up as previously described. A known mass of D 20 was injected i n t o the chamber, forming a s o l i d iee target on the backing. A proton beam of energy 1.25 Mev. was focussed on t h i s target. The time, kick-sorter spectrum, and integrated number of counts above a bias l e v e l of 4.25 Mev., were recorded f o r a know integrated beam current s t r i k i n g the target. The energy of the beam was raised and the same data recorded at the higher energy. Three or four sets of readings were taken at energies above 1.25 Mev. and then a set of readings was again taken at 1.25 Mev. This procedure was continued u n t i l sixteen readings at nine d i f f e r e n t energies ( i n addition to those at 1.25 Mev.) had been obtain-ed. The highest was at 1.86 Mev. Then two sets of readings were taken at 1.25 Mev. and one at 1 Mev. For readings below 1 Mev. the point at 1 Mev. was used as a check point and i t was repeated every fourth time or l e s s . The lowest proton energy used was 250 kev. This was obtained by using the 500 kev. singly-charged hydrogen molecules (mass 2 beam). One run was done with the 1 Mev. mass 2 beam to check the consistency of measurements made with the mass 2 beam and 35 the mass one beam at h a l f the energy. Of course the read-ings taken with the mass 2 beam were normalized to § the integrated current used f o r the other readings. At a l l points other than those mentioned above the mass 1 beam was used. The target was then removed by heating, and back-ground were taken over the same range of energies. Backgrounds were taken at the same energy as most of the points but there were a few exceptions which w i l l be noted l a t e r . For the back-grounds, the time, integrated current, kick-sorter spectrum and Integrated counts above 4.25 Mev. were recorded. Five counter checks, using a RaTh source, were spaced evenly throughout the other readings to insure that the energy c a l i b r a t i o n did not s h i f t appreciable. One small s h i f t occurred and was immediatly corrected. Since the check points were used to normalize the counts no ad d i t i o n a l corrections were required due to t h i s small s h i f t . Cosmic ray backgrounds which had previously been measured f o r the counter were used i n tr e a t i n g the r e s u l t s . Treatment of Data The cosmic ray background associated with the time taken f o r each reading was subtracted from the integrated counts above 4.25 Mev. f o r a l l readings. The set background ( i . e . a l l background other than cosmic ray) was subtracted from the D 20 readings at each energy, the integrated beam current being used f o r normalization. 56 I t was found that the counts at the check points decreased as more points were taken. Presumably t h i s was due to a loss of DgO as the target was bombarded. I t was assumed that the loss of DgO per point between any two check points was a constant. Corrections were determined at each check point to make the readings at a l l check points consist-ent. The intermediate points were then corrected i n l i n e with the assumption made above. The r e s u l t s appear i n table 5 under experimental results and are plotted i n f i g . 13. Results TABLE 5 Relative Y i e l d Ep dE Ep E Xffo Exp't'l Corrected Mev. Mev. Mev. Mev. Results Results 0.250 0.17 .17*.06 5.60 29.2% 0.412 0.470 0*500 0.111 .44*.04 5.79 31.1 2.05 2.17 0.750 0.087 .714.03 5.96 32.2 5.25 5.56 1.00 0.070 .97*.03 6.14 35.5 4.00 4.00 1.26 .058 1.23*.05 6.51 34.2 4.90 4.77 1.32 .055 1.29 6.57 54.5 4.86 4.69 1.40 .053 1.57 6.42 54.7 5.70 5.47 1.48 .052 1.45 6.46 54*9 5.29 5.05 1.53 .050 1.50 6.49 55.1 5.68 5.59 1.58 • 049 1.56 6.52 55.5 5.62 5.30 1.66 .047 1.64 6.58 55.6 5.74 5.57 1.75 .045 1.75 6.64 55.9 6.15 5.69 1.80 • 044 1.78 6.68 36.1 6.48 5.98 1.86 .043 1.84 6.72 56.5 6.52 5.80 Column one shows the incident proton energy. This was determined using an energy scale set up by a study of f l u o r i n e resonances. ( 1 0) Column two i s the target thickness i n Mev. at eaoh energy. This was determined as i s shown l a t e r . F I G . 1 3 , E X C I T A T I O N FUNCTION FOR THE REACTION M A s s 1' s y s s / / / / .%s .rfo .75" — i « o o /.ar /-75"" £ p IN M E V . 37 Column three i s the average proton energy, while i n the target, f o r protons going through the target. Column four i s the average gamma ray energy associated with the average proton energy. Column f i v e i s the counting system e f f i c i e n c y . This i s normalized at 33.3% at 1 Mev. which i s the approximate absolute value at that energy* A sample ca l c u l a t i o n i s given below showing how these e f f i c i e n c i e s were obtained. Column s i x shows the exp't'l r e s u l t s as treated above. Column seven shows the f i n a l r e s u l t s corrected for counter e f f i c i e n c y . Sample Calculations i ) Target thickness as a function of energy 250 o.c. of DgO vapour at 1.3 cms. of mercury press, was injected. Mass of DgO injected s 250 1*5 (20) - ,00388g. 22,000 76 I t i s assumed that t h i s spread over a hemisphere of radius 2 cm. Thickness - .00588 - ,00032g/cm2 (2)2 At Ep a 500 Kev. ( 1 5) £ = stopping X" a 11.6 x I O ' 1 5 ev-cm2 At 500 kev. dE = 6 NdX - (11.6 X 10~ 1 5) X .00032 x 6 x 10 20 = 11.1 x 10 4 ev. = 0.111 Mev. 23 38 Stopping numbers were used i n ranges where stopping cross-sections were not tabulated. Counter e f f i c i e n c i e s The cross-sections f o r the various absorption processes as a function of energy were obtained by in t e r p o l a t i n g between published values. The r a t i o of the e f f i c i e n c y at Ep a 1.86 Mev. to that at Ep * 1.00 Mev. i s as obtained i n the following manner. ?,p °CoaP« °Tot. Mev. Mev. b a r n s barns. 1.86 6.72 3.534 7.714 1.00 6.14 3.820 7.676 Since the bias i s set at 4.25 Mev. many of the counts a r i s i n g from Compton absorption do not r e g i s t e r . An estimate of the proportion of those which are absorbed by the Compton process which do not re g i s t e r i s given Ey - 4.25 E f f i c i e n c y at Ey a 6.72 Mev. _ 7 7 1 4 . _ 4.25 E f f i c i e n c y at K ,6.14 Mev. ~ 6.72 (5.554) 7.676 - £ g ( 3 # 8 2 0 j 3 1.089. I f the e f f i c i e n c y at 6.14 Mev. i s 33.3% Then the e f f i c i e n c y at 6.72 Mev. i s 36.3% An approximate estimate of the e f f i c i e n c y at E - 6.14 Mev. i s obtained as follows: E f f e c t i v e macroscopic x£ at E y - 6.14 Mev. a JJ7.676 - 4^ 25 ( 8. 8 8 0j] x lO-34J( 6 .o 2 ^10 2 g) (3.67) - .088 cm-1 39 Capture mean free path - 11*4 cm. Approx. av'g. effective path in crystal = 4 cm. -4 Efficiency x 1-e 11.4 ?Z » 3 3 3 .40 V. Sources of Error i ) Angular D i s t r i b u t i o n s There was one correction which was not applied to the angular d i s t r i b u t i o n as i t could not be done accurately. How-ever i t was checked that i t s magnitude was small. For a given incident proton energy, the gamma ray energy was a weak function of angle of observation due to the Doppler S h i f t . Hence the counter e f f i c i e n c y was also a weak function of angle. This was accentuated by the f a c t that a fix e d bias was used. I t was estimated that the counter e f f i c i e n c y changed by about 3$ between 0° and 150° at Ep s 1 Mev., and by \% between the same angles of observation at Ep = 1.7 Mev. The d i r e c t i o n of the correction would be to lower points at angles l e s s than 90° and raise them at angles greater than 90°. I t i s i n t e r e s t i n g to note that such a correction would tend to make the agreement between the experimental points and a curve of the form, a+b sin 2©, better i n f i g u r e 11. However i t w i l l not seriously change the r a t i o of a to b. The r e l a t i v i s t i c correction r e l a t i n g angles i n the observer's system to angles i n the centre of mass system would also tend to better agreement. This correction would tend to s h i f t the peak of the d i s t r i b u t i o n to about 87° at 1 Mev. This correction has not been shown i n d e t a i l since i t i s small compared to the s t a t i s t i c s on the points, i i ) E x c i t a t i o n Function This part of the work was the lea s t accurate f o r two r e a s o n s .41 F i r s t l y t here were f a i r l y l a r g e changes i n counting e f f i c i e n c y over the r e g i o n s t u d i e d , which had to he estimated. Secondly the c u r r e n t i n t e g r a t o r had t o be r e l i e d upon i n order t o norm a l i z e r e s u l t s to the same numbers of i n c i d e n t p a r t i c l e s * I t was found when working a t constant i n c i d e n t p a r t i c l e energy, where the monitor counts were a l s o a v a i l a b l e f o r n o r m a l i z a t i o n , t h a t t h i s was not always completely r e l i a b l e . T h i s was probably due t o s l i g h t s h i f t s o f the beam on the t a r g e t to a s l i g h t l y d i f f e r e n t t h i c k n e s s of t a r g e t . However, enough check p o i n t s were taken to guarantee 5 $ accuracy p r o v i d e d the c u r r e n t i n t e g r a t o r n o r m a l i z a t i o n was as good as experience a t constant i n c i d e n t p a r t i c l e energy l e d us to b e l i e v e . 42 VI. Conclusions 1. The gamma ray energy in the reaction, H 2(p, 5f)He3, is given by, E p a-vfE/dJEp where E g is the gamma ray energy Ep is the incident proton energy and Q, - 5.50 - 0.03 Mev. Since this value for Q, agrees, within experimental error, with the latest published figures for the mass difference between H24~p and He 3, the gamma ray transition must lead directly to the ground state of He 3 . 2. Since the measured and calculated values for Doppler Shift in gamma ray energy agree, the time between proton absorption and gamma ray emission must be short compared to the average slowing down time of a deuterium molecule in deuterium. ( i .e . < 2 x IO*"12 sec.) 3. There is an isotropic component (or at least a component at 0°) to the angular distribution of gamma rays. Therefore the gamma rays are not completely polarized. The percentage of isotropic component is a function of energy. The measured distribution functions were: At 1.75 Mev. y(©) - s in 2 © 4- (.025 * .007) At 1.00 Mev. y(©) - s in 2 © +(.043 * .008) 4. The excitation function shows no obvious resonances but is a oontinually rising function over the range of incident proton energies studied. Over the range from 500 Kev. to 1.85 Mev. 43 the experimental points are f i t by a function of the form, y = AE°' 6 5» where A i s an undetermined constant, E i s the incident proton energy* 44 VII. BIBLIOGRAPHY 1. Wilkinson, D. P h i l . Mag., 43, 659, (1952) 2. Fowler, Lauritsen and Tollestrup. Rev. Mod. Phys. 78, 1767, (1949) 3. Fowler et a l . , Rev. Mod. Phys. 22, 291, (1950) 4. Ajzenburg and Lauritsen Rev. Mod. Phys. 24, 321, (1952) 5. H e i t l e r , W (1944) "The Quantum Theory of Radiation" (Second E d i t i o n ) , Oxford U n i v e r s i t y Press, London... 6. French, A. Mimeographer Lecture Notes, Cavendish Laboratory, Cambridge University, (1950) 7. M. Verde Helv. Phys. Acta 23, 453, (1950) 8. D. Aaronson, M.A.Sc. thesis, UBC, (1950) 9. F. Flack, Ph.D. thesis, UBC, (1951) 10. D. Aaronson, Ph.D. thesis, U.B.C, (1952) 11. S. Woods, Ph.D. thesis, U.B.C, (1951) 12. Davisson and Evans, Rev. Mod. Phys. 24, 79, (1952) 13. R. Azuma, M.A. thesis, U.B.C, (1951) 14. Moody, N., Howel, W., B a t t e l l , W. >, and Taplin, R. Rev. S c i . Inst. 22, 551, (1951) 15. Wenzel and Whaling, Phys. Rev., Aug. 1, 1952, P. 501 16. Hornyak, Lauritsen, Morrison and Fowler. Rev. Mod. Phys. 22, 357, (1950) 17. Herschfelder and Magee. Phys. Rev. 73, 207, (1948) 45 Appendix A S o l i d Angle C o r r e o t i o n a t Q m 0° for a sin2© Distribution. Suppose the actual flux distribution of gamma rays, If the counter used in measuring the distribution subtends a f i n i t e angle at the souree, the measured distribution w i l l not be zero at 6 s 0°. Let F be the ratio of counts recorded at 0° to those recorded at 90°, R be the effective radius of the detection crystal 1 be the effective distance from the source to the detecting crystal. y(©), is given by, y(©) a A sin2© SOURCE t If R i s small compared to one, 1 Then F where ©(R) = tan" 1 R 1 For small © sin © In our angular distributions, ©(R) s tan" 1 .87 = .144 radians 6 „ . F * .0104 46 More a c c u r a t e c o n s i d e r a t i o n s , i n which the assump-t i o n t h a t O(R) i s s m a l l i s not made, show t h a t t h i s v a l u e i s good t o b e t t e r than 3%. Hence the above va l u e was used i n our work f o r the s o l i d angle c o r r e c t i o n f o r © » 0°,
Thesis/Dissertation
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Physics
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A study of the gamma rays produced by the bombardment of deuterium with protons
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