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A search for the photodisintegration of neon with the University of British Columbia Van de Graaff generator Woods, Stuart Brownlee 1952

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T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A F A C U L T Y OF G R A D U A T E STUDIES P R O G R A M M E OF T H E F I N A L O R A L E X A M I N A T I O N F O R T H E D E G R E E OF D O C T O R OF P H I L O S O P H Y S T U A R T B R O W N L E E W O O D S ' B .A. (University of Saskatchewan) 1944 . M . A . (University of Saskatchewan) 1948 MONDAY, F E B R U A R Y 18th, 1952, at 3:00 P.M. IN R O O M 301, PHYSICS BUILDING of C O M M I T T K E IN C i i A R C r . : Dean H . F. Angus, Chairman Professor J. B. Warren Professor G. M. Shrum Professor A. R. Clark Professor S. A. Jennings Professor H . C. Gunning Professor I. McT. Cowan Professor T . M . C. Taylor Professor J. G. Hooley PUBLISHED PAPERS The Resonance Method of Measuring the Ratio of the Specific Heats of a Gas, Canadian Journal of Research A21, 27, 1949 The Resonance Method of Measuring the Ratio of the Specific Heats of a Gas, Canadian Journal of Research A27, 39, 1949 T H E S I S A SEARCH FOR T H E l ' H O T O D IS I N T E G R A T I O N OF N E O N W I T H T H E UNIVERSITY OF BRITISH C O L U M B I A V A N D E G R A A F F G E N E R A T O R The construction of a radio-frequency electrodeless discharge type of positive ion source and ils installation in the Van dc Graaff generator are described. An ion current of 15 microamperes, approximately one-third protons, has been obtained with 1.5 MeV of energy. This performance is sufficient for the production of gamma-rays by the proton bombardment of light elements such as fluorine or lithium. A search for the pholodisintegration of Nc-° by the 0 and 7 MeV fluorine gamma rays and the 17 MeV lithium gamma rays has been carried out using a gridded ionization chamber. The equipment was calibrated by measuring the photodisintegration of deuterium for which the cross-section is known at these gamma ray energies. It is concluded that unless unsuspected systematic errors exist, the probability is 97 per cent that the photodisintegration cross-section for Nc-" by the fluorine gamma rays is less than 4x 10—30 cms". Similarly the proba-bility is 65 per cent that the cross-section for Nc"" by the 17 MeV lithium gamma rays is less than 7 x 10—30 cms-. G R A D U A T E STUDIES Field of Study: Physics X-rays and Crystal Structure—Prof. H. Johns Molecular Spectra—Prof. W. Pctrie Astrophysics—Prof. VV. Petrie Advanced Mechanics—Prof. R. N. Haslam Introductory Nuclear Physics—Prof. R. N. Haslam Electromagnetic Theory—Prof. L . Katz Quantum Mechanics—Prof. C. M . Volkolf Electronics—Prof. A. Van der /.iel Electron Optics—Prof. K. R. More / Special Relativity—Prof. VV. Opechowski Nuclear Physics—Prof. G. M. Volkoff Other Studies: Theory of Functions of a Real Variable—Prof. V. Scherk Theory of Functions of a Complex Variable—Prof. W. H . Simons Differential Equations—Dean VV. H. Gage ff i ' V fit A. SEARCH FOR THE PHOT ODISINT EGRATI OH OF NEON iTTH THE UNIVERSITY' OF BRITISH COLUMBIA VAN DE GRAAFF GENERATOR by STUART BROWNLEE WOODS A. THESIS: SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY JM PHYSICS: We accept t h i s thesis as conforming to the standard required from candidates f o r the degree of DOCTOR OF PHILOSOPHY IN PHYSICS Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA February, 1952. ABSTRACT The cons tract ion. of a. radio-f req.uency eleetro&eless discharge type of positive i o n source and i t s i n s t a l l a t i o n , i n the Van de G-raaff generator i s d e s c r i b e d . An Son current of 15 microamperes, approximately one t h i r d protons., has been obtained w i t h 1.5 MeV of energy* This performance i a s a f f i c i e n t f o r the production of gamma raya. by the proton-bombardment of l i g h t elements such as f l u o r i n e or l i t h i u m . A search f o r the p h o t o d i s i n t e g r a t i o n o f Ne 20 by the 6 and 7 MeV f l u o r i n e gamma rays and the 17 MeV l i t h i u m gamma raya has been c a r r i e d out using a gridded i o n i z a t i o n chamber. The eq.aipm.ent was c a l i b r a t e d by measuring the p h o t o d i s i n t e g r a t i o n of deuterium f o r which the c r o s s - s e c t i o n i a Imown at these gamma ray energies. I t i s concluded that, unless unsuspected ayatematic e r r o r a e x i s t , the p r o b a b i l i t y i s 97 percent t h a t the p h o t o d i s i n t e g r a t i o n erosa-seetion. f o r l e 2 ^ by the f l u o r i n e gamma raya i s l e s s than 4 x 10~3® cma 2. S i m i l a r l y the p r o b a b i l i t y i s 65 percent that the c r o s s - s e c t i o n f o r l$e " by the 17 MeV l i t h i u m gamma raya i s l e s s than q x 1 0 ~ 2 0 cms 2. TABLE OF-CONTENTS Page I". INTRODUCTION 1... 1. PARTICLE ACCELERATORS ... . 5 2 * NUCLEAR PHO TODISINTEGRATIONS . T > . . • .... 8 3. THE REACTION Ne^° ( i , OL ) 0 l b . . . i 9 PART I THE PRODUCTION OF A HIGH ENERGY POSITIVE: ION BEAM I I . THE UVBVCt FOUR MILLION VOLT VAN DE GRAAFF GENERATOR. 12 * X. GENERATION OF THE ACCELERATING VOLTAGE ' 12. a. THE: HIGH VOLTAGE: COLUMN 13. 3. THE HIGH VOLTAGE: ELECTRODE AND GAS INSULATIOH.. . 13 4. THE VACUUM -COLUMNS. . < 14 5. THE PUMPING ARRANGEMENT 15 6. COMPONENTS IN THE TOP TERMINAL... . - 16 I I I . THE POSITIVE ION SOURCE .- 18 1. REQUIREMENTS 18 . 2. SUMMARY OF THE ION SOURCE DEVELOPMENT. . - 19 3. FORMATION: OF THE IONS 21 a) Excitant ion of the Discharge..... 21 b) Recombination i n the Discharge 22' 4. EXTRACTION, OF THE IONS . , . 23 si) :Exfr&&.t/i6h FlaTcL i ;.... 23 b) Ex i t Canal .. 24 5. THE HYDROGEN SUPPLY 25 6. INITIAL ACCELERATION AND FOCUSSING OF THE. BEAM. . 27 7; PERFORMANCE OF THE ION SOURCE. • 28 a) Color of the Discharge....." 28 b) Pressure i n the Discharge 29 c) Ion Current..... 30 d) Alignment of the Lenses 30 e) PercMrmance on the test unit 31 f) Performance i n the Van de Graai'f Generator.. 32 8. ENGINEERING FEATURES ; 34 PART II INVESTIGATION OF THE PHOTODISINTEGRATION OF NEON IV. f S g i i M i l M ? SURVEY OF THE, N.e.20( i, oC ) 0 1 6 REACTION.. 36 1. NUCLEAR MODELS a) .. The Central F i e l d Model..; b) The Alpha P a r t i c l e Model..................i c) INTEREST i n the Ne 2 0( V, 06 ) 0 l b Reaction. 36 37 39 40 2. PHOTODISINTEGRATION; OF NUCEEI 41 3. THEORETICAL STUDY OF PHOTO-ALPHA REACTIONS.... 42 ' 4. REPORTED EXPERIMENTAL RESULTS. FOR SOME PHOTO-ALPHA REACTIONS 46 V*. OUTLINE OF EXPERIMENTAL METHOD . 48 1. METHODS OF DETECTION . . 48" 2. IONIZATION CHAMBERS . 49 a) Two Electrode Ionization Chambers 49 b) Gridded Ionization Chambers 51 3. LINEARITY OF ENERGY MEASUREMENTS BY IONIZATION 52 4. SOURCES OF HIGH ENERGY GAMMA RAYS 56 5. COMPARISON METHOD FOR MEASURING REACTION CROSS-SECTION 61 VI. EXPERIMENTAL EQUIPMENT 64 1. THE GRIDDED IONIZATION CHAMBER 64 a) Construction of the Chamber......... 64 b) Operating Charac t e r i s t i c s of the Chamber.. 65 c) C a l i b r a t i o n Pulses. 66 d() Chamber Cathode Voltage 66 e) The Gas P u r i f i e r ' 67 f) Pulses from the walls and Insulators 68 2. PULSE AMPLIFICATION • 69 a.) The Pre^amplifier 69 b) The Main Amplifier. 71 c) Power Supplies 71 3. PULSE, AMPLITUDE ANALYSER 72 (KICKSORTER) 4. PRELIMINARY TESTS OF THE EQUIPMENT 73 a) Noise Measurement and Bandwidth 73 b) Spurious Pulses 74 e) Spread i n Pulse Size.. , 75 5. PROTON BEAM TARGETS 77 • 6. GAMMA RAY MONITORS 78 VII. THE PHOTODISINTEGRATION MEASUREMENTS: 80 1. CHAMBER FILLING . yO a) Method, .'..*.'.'." 80 b) JBMVM$„ of the Gasea 80 2. ROUTINE CHECKS OF THE EQUIPMENT DURING THE RUNS 81 3. DEUTERIUM MEASUREMENTS ' 82 a) Measurements wit£ the Fluorine Gamma Rays 82 b) Measurements with the Lithium Gamma Raya. 85 c) Background Pulses 87 d) Number of- Pulses i n the Photodisintegra-t i o n peaks 90 .4. NEON MEASUREMENTS. 92 a.) Meaau.remen.ta with the Fluorine Gamma Rays.. 92 b) Measurements with the Lithium Gamma Raya... 92 e) Upper Limit f o r the iNe?°( z, e*. ) 0 1 6 Reaction Groaa^-s ect ion............ 95 YTII DISCUSSION OF THE RESULTS . ...... 100 1. POSSIBLE SYSTEMATIC ERRORS 100 2. Li e 2 0 ( n, oC ) 0 1 7 EXPERIMENTAL CHECK 100 2. CONCLUSIONS ... 101 TABLE OF ILLUSTRATIONS Figure following page 1. The U.B.C. Van de Graaff generator. 11 2. A Section of Vacuum Column 14 3. The Ion Source 21 4. The 20 mc/sec. O s c i l l a t o r . . . . 22 5 . Extraction and Electrode Power Supplies 28 6. Ion Current as a Function of Extraction Voltage ii. 30 7. Extraction Canal and Safety Plug 35 8. <f(Ey) f o r 0 1 6 ( i,oC ) C 1 2 (Preston).. 44 9. Pulse Formation i n Ionization Chambers 50 10. Thick target Excitation, Curves fo r 59 F 1 9 ( p , °c 3 ) 0 1 6 and L i 7 ( P. V )Be 8 11. Electrode Structure of the Gridded.. 64 Ionization Chamber 12. The Pre-amplifier.. 69 13. L i n e a r i t y of the Amplifiers...... 72 14. Pulses from the Signal Generator through.... 75 the main amplifier only 15. Pulses from Signal Generator through both amplifiers 75 Pulses at least 50 x noise 16. Pulses: from Signal Generator through both amplifiers .............. 75 Signal to noise 14/l 17. Pulses from P Alpha P a r t i c l e s i n Two 75 Atmospheres of Argon 18. Deuterium Disintegration and Hydrogen Background 83 Curves with F 1 " Gamma Rays 19. Photodisintegration of Deuterium with L i 7 . . . 85 Gamma Rays , Q 20. Disintegration- of Neon with F Gamma Rays.. 92 21. Dsiintegration of Neon with L i 7 Gamma Rays.. 94 22. The Ne 2 0(r>,oC ) 0 1 7 Curve 101 ACOO WLEDGEMEN T S The research described in. t h i s thesis i s divided into two phases. The f i r s t part, which occupied the major portion, of the time spent "by the author at t h i s work, was larg e l y an engineering problem concerned with the construction of the Van de Graaff generator at t h i s u n i v e r s i t y . The efforts, of many people contributed, to this programme, those of the author being mainly confined to u he design and i n s t a l l a t i o n of a po s i t i v e ion source. Part I of t h i s thesis describes this part of the work with only s u f f i c i e n t d e t a i l about the remainder of the generator to give an understanding of the problems involved. Part II of the thesis describes, the inves t i g a t i o n of •a. problem i n nuclear physics.j the f i r s t to be c a r r i e d out with t h i s Van de Graaff generator. I am indebted to Dr. C. A. Barnes f o r the suggestion of t h i s problem, which was concerned with the photodisintegration of the neon nucleus. , It i s a pleas.ure to acknowledge the supervision of t h i s work, i n i t s early stages by Dr.K.R. More and l a t e r by Dr.J.B. Warren. Dr. CfA.Barnes has also been a source of invaluable suggestions and assistance, e s p e c i a l l y during that part of the work described i n Part II of t h i s thesis. Mr.F.C. Flack has been a continued source of a i d and encouragement during these researches. He i s responsible f o r the design and much of the construction of the gridded i o n i z a t i o n chamber used infthe photodisintegration work. It would be impossible to express my indebtedness to a l l of those s t a f f members, students and members, of the machine shop and technical s t a f f who have contributed to this programme of research. The author's part i n this work was made possible by scholarships awarded by the National Research Council and by the B r i t i s h Columbia Telephone Company. Without these awards, i t woald have been impossible f o r me to attend this university. i I. INTRODUCTION One of the chief problems, of nuclear physics today i s to understand how atomic n u c l e i are formed from the so c a l l e d fundamental p a r t i c l e s . The development of a useful quantitative theory of the nueleus i s hampered by the lack of a detailed knowledge of the nature of the forces between neutrons and protons, the only constituent p a r t i c l e s according to present ideas. IMore, and more accurate, experimental data are necessary to obtain information- about the magnitude, d i r e c t i o n and range of the fdrces between nuclear p a r t i c l e s . The experimental investigations of nuclear forces may be divided into two classes i l l u s t r a t i n g two approaches, to the problem. The f i r s t of these comprises the experiments involving the int e r a c t i o n of two nucleons, (neutrons and protons), for which detailed mathematical analyses are possible. Scattering experiments, and investigations of the ground state of the deuteron at present provide most of the information about t h i s i n t e r a c t i o n . I t turns out that the data derived from these experiments can be explained i n terms of two parameters which do not describe i n d e t a i l the forces between the nucleons. 1' l i e second class of nuclear force experiments includes the investi g a t i o n of the nuc l e i consisting of many nucleons. As much information as possible i s t a b u l a t e d about the properties of these n u c l e i and attempts are made to explain the main features of these results by a com-prehensive theory. The complicated, nature of the many body problem coupled with the uncertain quantitative part-i c u l a r s of nuclear i n t e r a c t i o n have resulted i n li m i t e d progress i n the t h e o r e t i c a l i n t e r p r e t a t i o n of the data obtained. '• In order to develop a'workable theory i t i s usual to adopt some simple model of the nucleus, the prop-e r t i e s of -which may be regarded as similar'to those of the actual nucleus. The forces between the in d i v i d u a l nucleons are replaced by a small number of a r b i t r a r y parameters which are empirically chosen to f i t the observed f a c t s . I t i s then the task of the experimentalist, to accumulate more data which w i l l . i n d i c a t e the range of v a l i d i t y of the proposed models. The experimental investigation of nuclei may be carried out by bombarding them with, energetic protons, deuterons or alpha p a r t i c l e s . When such a p a r t i c l e strikes a target placed i n i t s path, i t may be scattered by the target material,, or i f i t has enough energy i t may enter one of the. nuclei forming a "compound" nucleus vi/hieh has too much energy to be stable, and breaks up with the emission of a. nuclear p a r t i c l e an& perhaps, a gamma ray.> Although the compound nucleus has but a short l i f e ( ~ 1 0 - 1 8 seconds) this i s very long compared to the c h a r a c t e r i s t i c nuclear time (time for a. nucleon to t r a v e l across the nucleus which i s ^  1 0 ~ 2 1 seconds). Many of the energy 2. le v e l s i n which the compound nucleus exists are well defined and the reactions show strong resonances as the energy of the incident p a r t i c l e s i s varied. The properties of these energy levels, may "be determined from' the' measurements of the resonances with anincident he am of well defined and . accurately known energy. Any excited energy l e v e l s i n whi-ch the r e s i d u a l nucleus, i s l e f t when the compound nucleus "breaks up may also he found from the mass energy changes i n the reaction or from the energy emitted by the residual nucleus when i t subsequently decays by gamma, emission to i t s ground state. She efforts, of many experimenters have been occupied with the determination of the properties, of nuclear energy l e v e l s which i t i s hoped, i n analogy •with the energy-levels, studies: i n atomic-systems, w i l l lead to a more complete understanding•of nuclear structure. Another important way of producing nuclear reactions', with which this, thesis i s la r g e l y concerned, i s to i r r a d -iate materials with s u f f i c i e n t l y energetic photons. In this way Wi.e.denbeck has produced n u c l e i i n metastable states, i n which they have a. considerable l i f e t i m e , return-ing to the ground state by gamma emission. The process with which this, thesis, i s l a r g e l y concerned i s photodisintegra-tio n , i n which a. p a r t i c l e i s ejected from the nucleus by direct action of the photon without the formation of a. compound nucleus with an appreciable l i f e . This coupled with the fact that no s p e c i f i c a l l y nuclear force acts 4. between photons and. nucleons., makes i t possible in the t h e o r e t i c a l treatment of photodisintegration to omit some of the uncertain assumptions about nuclear forces which enter i n most calculations' concerning nuclear structure. The energy levels, of the residual nucleus can be investigated i n the same way as f o r p a r t i c l e induced reactions. S u f f i c i e n t l y intense sources of.photons f o r the i n -vestigation • of photodisintegrations may be produced i n two. ways. A target material may be bombarded with electrons to produce X-rays., which w i l l have a l l energies up to that of the electrons. However,, the measurement of the mass-energy changes and the i d e n t i f i c a t i o n of the photon energies which produce the disintegrations are made d i f f i c u l t by the large energy spread. Mono-energetic gamma rays, produced by the decay of.excited nuclei to lower energy states, are therefore more suitable f o r this type of work. .Excited nuclei twho:eh decay by.gamma emission result from the proton bombardment of several of the l i g h t elements. A; source of high energy nuclear p a r t i c l e s i s thus necessary for most investigations of nuclear structure. For many years the n a t u r a l l y radioactive elements, provided the only such sources.. Using natural alpha, p a r t i c l e s Rutherford established,the. nuclear model of the atom i n 1911, and f i r s t observed, a r t i f i c i a l .disintegration/bf a. nucleus, i n 1919. Other discoveries, made with the aid of natural alphas were the neutron by Chadwic'k. i n 1932, a r t i f i c i a l r a d i o a c t i v i t y by Curie and J o l i o t i n 1934, and nuclear f i s s i o n by Hahn and Strassman i n 1939. In spite of these impressive achievements, the need for laboratory accelerated p a r t i c l e s of controllable energy, i n t e n s i t y and d i r e c t i o n i s c l e a r . 1. PARTICLE ACCELERATORS';' A p a r t i c l e accelerator for use i n precise investigations of nuclear structure.mast satisfy, certain requirements. The beam of p a r t i c l e s must be well collimated so that the number of disintegrations caused by part of the beam impinging on materials other than the desired target i s small. The voltage or energy of the beam when i t s t r i k e s the target must be constant and accurately known f o r the determination of energy l e v e l s . Simple methods should be available f o r continuous adjustment of the voltage up to as high a value as possible, i n order to investigate many nuclear resonances. The beam must contain enough p a r t i c l e s so that an excessive length of time i s not required to obtain reaction r e s u l t s . The f i r s t , nuclear reaction produced with a r t i f i c i a l l y accelerated ions, was observed by Cockloft and Walton i n 1932, using equipment of a type that f u l f i l l s most of these requirements. In the Cockloft-Walton high tension seta the output voltage from a transformer i s m u l t i p l i e d and r e c t i f i e d by a cascade of condensers and diodes. The d.c. voltage obtained i n t h i s way. i s applied to a vacuum tube along which the ions are accelerated. The p r a c t i c a l l i m i t of these sets i s determined by corona from the high voltage terminal 1to the surroundings 6 which becomes excessive above one m i l l i o n v o l t s . Enclosing the machine i n a container of gas at high pressure -would; raise the voltage l i m i t but i s d i f f i c u l t to do. The e l e c t r o s t a t i c generator developed by Van de a 5,6 G-raaff , Herb and others f u l f i l l s the need f o r a machine with which precise measurements can be made at ion beam energies greater than one m i l l i o n electron v o l t s . An i n s u l a t i n g b e l t carries charge placed on i t by a corona process at the ground end, into the hollow high voltage terminal where i t i s removed. This charging process produces a voltage between the hollow terminal inside which the ion source i s located, and ground. The ions t r a v e l i n a focussed beam through an evacuated tube to the target at the ground end. The background of disintegrate ions due to high energy p a r t i c l e s s t r i k i n g elements other than those, of the target can be kept very low. The inhomogeneity of the beam energy i s very small depending mainly on the s t a b i l i t y of the generator voltage, which can be kept constant •7. to one part i n 10 . Changing the beam energy over a wide range requires an adjustment which takes only a few minutes. , The highest voltage yet obtained with; a Van de Graaff generator i s 5 m i l l i o n electron v o l t s , although one i s now being constructed whose designers have set 12 m i l l i o n volts as a goal. Many of the present machines have f a l l e n short of expect-ations by a large f a c t o r . Tie l i m i t of these machines i s determined by voltage breakdowns inside the vacuum tube f o r reasons which are not yet well understood. S i m i l a r l y no e x i s t i n g generator has operated with an ion current greater than 100 microamperes and many have been l i m i t e d to less than 7 10 mieroamperea f o r reasons which are not known. Magnetic resonance a c c e l e r a t o r s , such as the L a u r e n c e ancjL c y c l o t r o n developed hy/ L i v i n g s t o n i n 1932, produce much more energetic p a r t i c l e s hy a c c e l e r a t i n g them many times w i t h a . small r e p e t i t i v e v o l t a g e ^ A c y c l o t r o n i s able to d e l i v e r a much hi g h e r current than present e l e c t r o s t a t i c generators but can produce an emergent beam of only one voltage determined by the magnetic f i e l d and o s c i l l a t o r frequency, the l a t t e r of which i s very d i f f i c u l t to change. Reduction i n the p a r t i c l e energy may be obtained by p l a c i n g absorbers i n the beam but t h i s i n c r e a s e s the energy spread which i s al r e a d y l a r g e due to the manner of a c c e l e r a t i o n . The energy spread may be p a r t l y overcome w i t h a sa s a c r i f i c e i n i n t e n s i t y by s e l e c t i n g the p a r t i c l e s from a narrow energy r e g i o n . I n t e r n a l t a r g e t s , placed i n the path of the beam before i t a t t a i n s i t s f u l l energy, are inconvenient f o r i n v e s t i g a t a t i o n s of n u c l e a r r e a c t i o n s due to the h i g h magnetic f i e l d s and l a r g e background of gamma rays and neutrons from which i t i d d i f f i c u l t to s h i e l d d e t e c t i n g equipment c l o s e to the " c y c l o t r o n . Such magnetic resonance a c c e l e r a t o r s have extended the f i e l d f o r research to very high energies but do not y i e l d p r e c i s e r e s u l t s concerning n u c l e a r s t r u c t u r e a t energies below. 4 m i l l i o n e l e c t r o n v o l t s as conve n i e n t l y as do e l e c t r o s t a t i c generators. In order to c a r r y out a program of n u c l e a r s t u d i e s at the U n i v e r s i t y of B r i t i s h Columbia, pr.J.B.Warren and Mr.T. Mouat designed a f o u r m i l l i o n v o l t Van de G-raaff generator. a* Conservative ratings were used f o r a l l components i n an e f f o r t to avoid the d i f f i c u l t i e s encountered by other workers. Attempts have not been made to obtain the ultimate voltage the machine w i l l produce but the expectations of the designers have so f a r been met. A poten t i a l of over 2 m i l l i o n vdilts has been produced and a beam current of 100 microamperes, has been obtained at reduced voltages. The generator runs consistently at 1.5 m i l l i o n volts with 25 microamperes of positive ion current directed on the target. A voltage s t a b i l i z a t i o n system i s required before the beam can be used d i r e c t l y f o r nuclear energy levelsmeasurements and this i s now being i n s t a l l e d . Howeverj at the present stage of construction the beam may be used f o r the production of mono-energetic gamma raya. Thus photodisintegration experiments may be conveniently performed. 2 iNUCLEAR PHOTODISINTEG-RATIONSf P Chadwick and Goldhaber succeeded* i n 1934* i n disi n t e g r a t i n g the deuteron into a proton and neutron by i r r a d i a t i o n with s u f f i c i e n t l y energetic gamma rays. Since that time other examples of nuclear photodisintegration have been discovered. The minimum gamma ray energy or threshold energy f o r t h i s process corresponds to the binding energy within the nucleus of the p a r t i c l e which emerges. The prob a b i l i t y of the occurrence of the nuclear reaction under given conditions * which i s measured by the reaction cross-9 section, i s also an important fa c t o r i n determining the nature of nuclear forces. A photodisintegration process of in t e r e s t , an investigation of which i s described i n this The Ne nucleus can he represented hy a model that has been investigated p a r t i c u l a r l y i n the case of nuclei which can be decomposed into an i n t e g r a l number of alpha p a r t i c l e s (Be 8, G 1 2* 0 1 6, We 2 0 e t c . ) . The so-called alpha p a r t i c l e model suggests i t s e l f due to the saturable character of the nuclear forces. The binding energy per nucleon i n the alpha p a r t i c l e i s approximately 7 MeV, which i s a-large proportion of the mean binding energy per nucleon for most n u c l e i ; e s p e c i a l l y those with a mass leas than 30. Thus, i f the nucleons are grouped i n alpha p a r t i c l e configurations within a nucleus,the binding energy.of the alpha p a r t i c l e s to each other should be r e l a t i v e l y small. An i n v e s t i g a t i o n of the photodisintegration of the; nuclei which lead themselves to this treatment would therefore seem useful. Some work has already been done on 0 ^ and C"^2 but as yet no reports have appeared concerning Ne 2^. p a r t i c l e model i s that there are few low energy excited states i n the nuclei investigated. This prediction i s i n q u a l i t a t i v e agreement with the experimental results so f a r obtained, but thesis, i s the reaction K e 2 0 ( V, OL ) o 1 6 i One of the predictions of studies using the alpha 10. more detailed information would be of value. Observations of energy releases i n photodisintegration experiments could y i e l d such information concerning the product nucleus. In p a r t i c u l a r , the Ne 2 0(j,°L ) 0 1 6 reaction should provide an excellent method of investigating the existence, of excited states i n 0 1 6 . The weak i n t e r a c t i o n between photons and nu c l e i makes the cross-section f o r photodisintegration reactions very low (10 ' cms. or less) so that experimental techniques to observe the process are l i m i t e d . ( ?/,o£ ) reactions i n C 1 2 and 0 ' have been observed using photographic emulsions containing these n u c l e i but d i f f i c u l t y would be encountered i n preparing emulsions containing Ne 2^ n u c l e i . Photodisintegration reactions which r e s u l t i n unstable product n u c l e i have been investigated using betatrons. This method i s l i m i t e d to the i d e n t i f i c a t i o n of products by the residual a c t i v i t i e s , due to the high background of events other than those under investigation when the, betatron, i s i n operation. The use of an ionization.chamber to s^udy photodisintegration i s p a r t i c u l a r l y applicable to neon since i t i s a gas from which fr e e electrons may be co l l e c t e d on a detector by an e l e c t r i c f i e l d without appreciable losses due to recombination. The electrons produced i n the track of an i o n i z a t i o n p a r t i c l e may be collected to detect the passage of the p a r t i c l e through the gas.. The p a r t i c l e s produced i n the photodisintegration of the gas contained i n an ion i z a t i o n chamber may be detected i n t h i a 11. way and the number of electrons c o l l e c t e d used as a measure of the energy released by the reaction. Part II of this thesis describes the investigation of the Ne20( j/, oC Jo1^ reaction using t h i s method. PART I t THE PRODUCTION OF A HIGH ENERGY POSITIVE ION BEAM. 12 I I . THE U.B.C. FOUR MILLION. VOLT- VAN DE GRAAFF' GENERATOR The U.B.C. Van de G-raaff generator, shown in F i g . l , follows the main design features of Herb's horizontal pressurized machine 6, but i s i n s t a l l e d with a v e r t i c a l column to s i m p l i f y the construction. 1. GENERATION OF THE ACCELERATING VOLTAGEt The ion accelerating voltage i s developed between the insulated top terminal and ground. A be l t composed of a rubberized f a b r i c (fabreeka) or cotton t r a v e l l i n g with a l i n e a r speed of approximately 5000 feet per minute, conveys charge to the top terminal. A row of needles j u s t above the lower pulley points at the belt surface and i s connected to a power supply which can be adjusted to d e l i v e r any voltage up to 60 k i l o v o l t s . Corona, from the needle points, sprays charge on the i n s u l a t i n g belt surface, from which i t i s removed by a s i m i l a r row of needles at the upper end. The voltage attained by the upper end i s determined by the balance between the charge carried up by the belt and that l o s t to ground by a l l processes (corona, leakage and ion current). 2. THE HIGH VOLTAGE COLUMN. The upper electrode i s supported by four columns each made up of sixty-four 3-inch long porcelain i n s u l a t o r s . Each group of four 3-inch insulators, i s separated from the groups above and below by equlpotential plates of aluminum. A 450 13. megohm r e s i s t o r connects each equipotential plate to the plates adjacent to i t i n the high voltage column or stack. The current flowing through these r e s i s t o r s from the top electrode to ground di s t r i b u t e s the v e r t i c a l voltage gradient evenly along the column. Losses of charge from the stack by other agencies disturb the voltage gradient and these must be minimized. Each equipotential plate has a highly polished aluminum hoop attached to its. periphery so that no sharp points, ex i s t which might i n i t i a t e corona discharges* 3. THE HIGH VOLTAGE ELECTRODE AND GAS INSULATIONV The top terminal i s a highly polished s t a i n l e s s s t e e l electrode which acts as a Faraday cage. The f i e l d free region within the electrode contains the ion source and the equipment attendant, to i t s operation. The voltage'gradient outward from the surface of t h i s electrode i s the largest i n the machine. It i s important that the gradient here be made such that e l e c t r i c a l breakdown w i l l occur f i r s t through the gaa rather than down the support column where i t might cause considerable damage to the insulators or the b e l t . In order to a t t a i n the highest voltage from the machine an intermediate s h e l l w i l l be placed around the upper part of the stack which w i l l divide the voltage between the upper electrode and the grounded pressure tank. This reduces the gradient near the electrode and increases i t toward the outer tank where i t i s lower. Dry nitrogen i s used as the i n s u l a t i n g gas with small amounts of Freon IE (C Clg Fg) 14 gas added to increase i t s breakdown s t r e s s 9 . The function of the freetn molecules i s to capture any free electrons thus reducing t h e i r m obility so that they cannot reach a s u f f i c i e n t v e l o c i t y to ionize the; gas and cause e l e c t r i c a l breakdown. The outer tank which encloses the machine i s designed to withstand an i n t e r n a l pressure of 200 pounds per square inch (p.s.i.') but a pressure of 100 p . s . i . without the use of the intermediate s h e l l has been found s u f f i c i e n t to a t t a i n 2 m i l l i o n v o l t s of accelerating p o t e n t i a l . 4. THE VACUUM COLUMNS';' Two vacuum columns, one f o r ion acceleration and one for d i f f e r e n t i a l pumping, extend from the outer tank through the stack to the upper electrode. A short section of one of the columns i s shown i n Fig;2. Each s t a i n l e s s s t e e l Y," x electrode i s connected by a section of moulded s l i g h t l y conducting rubber to the adjacent equipotential plate from which i t derives i t s voltage. The inner part of the electrode i s shaped to s h i e l d a beam passing down the centre of the column .from the ef f e c t s of f i e l d s due to charges c o l l e c t e d OM the insulators while providing as large an area as possible for passage of gas to the pumps. The e l e c t r o s t a t i c f i e l d obtained within the column by electrodes of this shape has a very weak focussing e f f e c t so that the ions entering the . .. column i n a beam are accelerated along i t s axis without the beam lo s i n g i t s i d e n t i t y . r u b b e r g a s k e t s t a i n l e s s s tee l e lectrode D D -top e l e c t r o d e of n e x t s e c t i o n F o u r e l e c t r o d e s a n d p o r c e l a i n s g l u e d t o g e t h e r w i th v i n y l s e a l a d h e s i v e t o f o r m one s e c t i o n . S c a l e i n c h e s FIG. 2. A S e c t i o n of V a c u u m C o l u m n 15 5. THE P M I I G ARRAJSTGEHENTt Each column i s evacuated hy two o i l d i f f u s i o n pumps with a combined pumping speed of 1000 l i t r e s per second at a pressure of 10"^ millimeters of mercury (mmSiHg). The ultimate vacuum which has been attained i s 2 x 10~6 mms.Hg indicated, by commercial i o n i z a t i o n type pressure gauges connected to the system at the lower ends tfif the columns. A measurement taxen with a hydrogen flow of 0.15 cms? per minute from the ion source indicated that the pressure at the top of the. accelerator column i s 2.8 x 10~ 5 mms.Hg when that at the bottom i s 1.6 x 10"5mms.Hg. This measurement was made with the flow of gas through the accelerator column only, whereas i n normal operation some of the gas i s pumped down each column r e s u l t i n g i n the attainment of lower pressures. It i s necessary to maintain as low a pressure as possible i n the accelerator column i n order that th© number of ions scattered from the beam by c o l l i s i o n with gas molecules w i l l be small. A small c o n s t r i c t i o n through which the ion beam may'pass w i l l r e s t r i c t the flow of gas into the accelerator tube and cause the other column to operate more e f f e c t i v e l y as a d i f f e r e n t i a l pumping tube. In order to minimize the i n i t i a l d i f f i c u l t i e s of soLligning the ion source with the column a 1.5 inch aperture i s s t i l l i n use where the beam enters the accelerator tube* With t h i s arrangement a pressure of 1.0 x 10~ 5 mms.Hg can be maintained at the bottom of the accelerator tube when the ion source i s operating. A 16. n e g l i g i b l e f r a c t i o n of the ions w i l l experience c o l l i s i o n s w i t h gas molecules while t r a v e l l i n g the l e n g t h of the column at t h i s pressure. 6. COMPONENTS IN THE' TOP TERMINAL*;1 A l l power u n i t s f o r running the i o n source are housed i n the top t e r m i n a l . The u n i t s are compactly arranged to f i t the l i m i t e d space. Since access to the top t e r m i n a l i s d i f f i c u l t to achieve, temperatures are high due to poor c i r c u l a t i o n of the gas from w i t h i n the e l e c t r o d e , and pressure i s h i g h , a l l components have been very c o n s e r v a t i v e l y r a t e d to achieve r e l i a b i l i t y . The use of equipment c o n t a i n i n g p i t c h o r o i l which might be a f i r e hazard or damage other p a r t s of the machine i n the event of breakdown or overheating, has been avoided. F i v e v a r i a c a provide the o n l y i o n source c o n t r o l s . These are connected by nylon s t r i n g s to s e l s y n motors at the bottom of the stack which are c o n t r o l l e d from the o u t s i d e . A. periscope hole through which f i v e meters are v i s i b l e i s provided f o r monitoring purposes. The e l e c t r i c a l power f o r u n i t s i n the top t e r m i n a l i s s u p p l i e d by two generators, V-belt d r i v e n from the top p u l l e y of the main Van de Graaff b e l t , which can supply 2000 watts a t 115 v o l t a and 400 c y c l e a per second. The output i a r e g u l a t e d by carbon | i l e a so t h a t the voltage v a r i a t i o n i s - 2 percent from the mean valu e , which i s a d j u s t a b l e , as the l o a d i s changed f r o m z e r o to 1000 w a t t s p e r g e n e r a t o r . A. 2 4 - v o l t d i r e c t c u r r e n t o u t p u t i s a l s o a v a i l a b l e but i s used o n l y f o r f i e l d e x c i t a t i o n o f the a l t e r n a t o r s and f o r l i g h t i n g m e t e r s o v e r the p e r i s c o p e . 18 I I I . THE POSITIVE- -IOH SOURCE? 1. REQ.UIREMMTS? The function of an ion aoarce i s to pro dace the desired ions, usually protons or deuterona, and i n j e c t them into the high voltage column* Hydrogen or deuterium gas i a admitted to a discharge chamber where eleetrona are accelerated to produce the successive processes H 2 HH + HH + ^ H +• H-*". The ions formed hy the i o n i z a t i o n and d i s s o c i a t i o n of the hydrogen (or deuterium) are then removed from the discharge and formed into a suitable- beam for introduction into the accelerating column, General c h a r a c t e r i s t i c s which are desirable i n an ion source f o r i n s t a l l a t i o n i n an e l e c t r o s t a t i c generator may be outlined. The atomic ion current, which i s the useful part f o r nuclear studies, should be aa large as -possible, yet consistent with a low gas consumption so that the pressure i n the accelerator tube may be kept low. This necessitates a high e f f i c i e n c y as measured by the r a t i o of the number of emergent ions to the number of gas molecules consumed and also by the r a t i o of the atomic ion current to the t o t a l beam current. The beam should be aa monor-energetie as possible and well focussed f o r i n j e c t i o n into the accelerator tube. O v e r a l l r e l i a b i l i t y , s i m p l i c i t y and low power consumption are d i c t a t e d by the f a c t that the i o n source must operate i n the top t e r m i n a l of the generator. 2. SUMMARY OF IOH SOURCE DEVELOPMENT. E a r l y i o n sources were of the canal r a y type developed f i r s t by Oliphant and Lord R u t h e r f o r d ^ • In the can a l ray sources ions are formed i n a hi g h v o l t a g e , c o l d cathode discharge from which they emerge through a hole i n j the cathode. The most o b j e c t i o n a b l e feature of these sourcea i s the la r g e energy spread of the emergent i o n beam. This a r i s e s because each i o n i s a c c e l e r a t e d by a f r a c t i o n of the t o t a l voltage across the discharge that depends on the po i n t at which i t i s formed. To overcome t h i s d i f f i c u l t y low voltage c a p i l l a r y arc sourcea were d e v e l o p e d - ^ a n d are used i n many a c c e l e r a t o r s today. These employ a fi l a m e n t to emit e l e c t r o n a which are a c c e l e r a t e d through a r e s t r i c t e d space ( c a p i l l a r y ) toward a low voltage anode. A l a r g e gas pressure and high power input are necessary to keep the c a p i l l a r y arc o p e r a t i n g . The engineering problems r e s u l t i n g from the heat developed i n the c a p i l l a r y by t h i s l a r g e power make sources, of t h i s type d i f f i c u l t to use i n a p r e s s u r i z e d generator. In a t h i r d c l a s s o f i o n sources ( " r e f l e c t i o n type) an e l e c t r o n beam* s u p p l i e d by e i t h e r a heated f i l a m e n t or a c o l d cathode e m i t t e r , i s constrained to o s c i l l a t e along 20 the axis of source hy a magnetic f i e l d , l i m i t e d filament l i f e i s the most serious drawback of the filament type and the cold cathode type gives rather modest currents, although i t s construction and operation i s extremely simple. Recently (1950) Kisljmaker and co-workers » have described r e f l e c t i o n type sources with filaments which give very large ion currents with low gas consumption and e f f i c i e n t power u t i l i z a t i o n . To overcome the d i f f i c u l t i e s associated with a filament,radio-frequency electrodeless discharges have been investigated as ion sources-^* 15,16il7^ operation of these i s fundamentally the same as that of the r e f l e c t i o n sources, except dm the manner of e x c i t a t i o n . The radio-frequency discharge i s excited i n a chamber of glass or quartz on which surfaces, recombination of the ions i s made much l e s s than on metals so that high atomic ion percentages may be obtained. The main d i f f i c u l t y with these sources i s the choice and construction of an o s c i l l a t o r which w i l l operate r e l i a b l y without frequent adjustment under the varying load conditions presented byaa discharge tube i n which the pressure and ion density are not constant. Starting the discharge i s es p e c i a l l y d i f f i c u l t due to the large change i n impedance that occurs when the discharge lights.. The performance of the sources reported before 1949 has been reviewed by Hoyaux and D u j a r d i n 2 3 . Table I gives a comparison of the operating c h a r a c t e r i s t i c s reported f o r t y p i c a l sources of each type. "The success of workers TABLE I CHARACTERISTICS OF VARIOUS TYPES OF ION SOURCES. Type of Souroe Canal Ray Capillary Arc Reflection Tgrpe Radio Frequency "Oliphant Metal Metal Glass Source" Probe Capil-Capil-lary lary Cold Cathode Filament Reference Craggs Zinn ' Lamar' Lamar Ward 19 20 21 21 221 Kistemaker Rutherglen Bayly Thonemann B a l l 14 15 16 17 18 Pressure i n Discharge 150 [microns) 30 20 20 20 1.0 15 10 10 100 Power Input (watts) 400 235 385 135 15 900 30 450 150 60 Maximum Ion Current 1800 (microamperes) 4300 1000 1500 250 5000 400 750 500 400 Orifice Size Diameter 1.5 (itms) Length 2.5 1 0.8 0.8 1.5 6 0 0 12 5 0 3 14 2 12 2 20 1 0 Atomic Ion Percentage 45 20 20 60 25 60 60 80 60 Maximum Energy Spread (electron volts) 2x10 9x10 13 30 low 100 low low 20 low GasCon sumpt i on (cm /minute at 760 rams.) 6 « 5 0.16 0.4 0.4 0.15 0.03 0.5 0.24 0.20 0.5 21. using the radio-frequency discharge sources prompted the choice ofathat type f o r use i n the Van de G-raaff generator i n this laboratory. The e s s e n t i a l features of t h i s ion source are shown i n F i g . 3. Hydrogen gas i s passed through the pyrex tube i n which a discharge i s excited with.a high frequency o s c i l l a t o r . Md.e. extraction voltage across the tube removes the ions from the discharge through a canal i n the dural probe. The emerging ions are then focussed and accelerated by the two c y l i n d r i c a l electrodes through which they pass. 3. FORMATION OF THE IONS. aj Excitation of the Discharge. The discharge always contains some molecular ions due to incomplete d i s s o c i a t i o n of the gas. A large atomic ion content i n the discharge i s associated with a large density of electrons with s u f f i c i e n t energy to dissociate the gas molecules and molecular ions. A voltage which w i l l accelerate the electrons to the req.uired energy between c o l l i s i o n s can be produced i n a discharge containing a large ion density only by an o s c i l l a t o r with a large power output. The f i r s t o s c i l l a t o r used here provided a maximum power of 150 watts at a frequency of 210 megacycles per second, I t was abandoned because of d i f f i c u l t y i n matching even a f r a c t i o n of t h i s power into the discharge and the necessity f o r frequent adjustment. A new operating frequency of about h y d r o g e n in let pyrex O-r ing seals-t i l t ad jus t (3) p o r c e l a i n (3) d i f f e r e n t i a l pump porce la in ring a c c e l e r a t i o n to 4 M e V * 1 » osc i l l a to r t a n k e x t r a c t o r cone 5 »— c e n t r e adjust (3) 1st e l e c t r o d e p o r c e l a i n (3) 2nd e l ec t rode i n c h e s o i 2 3 4 5 FIG. 3. I O N S O U R C E 22 20 megacycles per second was chosen and several successive designs have resulted i n a r e l i a b l e o s c i l l a t o r with a maximum power output of 400 watts. Eimac 4X150 tubesJ; are used i n the o s c i l l a t o r and metal 5T4 r e c t i f i e r s i n i t s power supply because of the high pressure they w i l l withstand. The tank c o l l i s wound around the discharge tube to minimize coupling d i f f i c u l t i e s . Once1 adjusted t h i s o s c i l l a t o r has run f o r more than 100 hours i n the Van de Graaff generator without1: attention. F i g . 4 shows the c i r c u i t diagram of t h i s o s c i l l a t o r and i t s power supplies. The dimensions of the discharge tube have been chosen large enough to prevent overheating og the glass due; to the large amount of power dissipated within i t and also to prevent a large l o s s of electrons from the discharge due to c o l l i s i o n s with the wall. b) Recombination i n the Discharge. Recombination which taxes place mostly on the walla of the chamber reduces the atomic ion content of the discharge. For t h i s reason metal surfaces exposed to the discharge are kept small. Pyrex glass i s used f o r the discharge tube because of the small amount of recombination which occurs on 18 i t s surface. Some workers have used quartz instead because A The manufacturers report that these tubes w i l l operate i n an external pressure of 450 p . s . i . • 0 0 h>H~f~ rnQ^ 4X 150 4 X 150 6 X 5 2 4-0 V. 3 h e n r y s N , O O O s c i l l a t o r and S c r e e n V o l t a g e S u p p l y 0 - / 2 0 0 v o l t s , 5"oo m a x . m il/i a m p e r e s o u t p u t 0 . 5 h e n r y I h e n r y 4 0 0 ~ t o v a r i a c 115 V. -o A 3 U S r^ t T J^. c o n d e n s e r s 0 . 2 5 2 0 0 0 v o l t s each P l o t e V o l t a g e S u p p l y FIG. 4. T h e 20 m c / s e c . Osc i l la tor 23 of the high temperatures to he withstood hut they obtained reduced atomic ion percentages, i n their beams. This i s to be expected since the recombination rate f o r atomic hydrogen i s much lower on pyrex surfaces than on quartz, . Cleanliness of the discharge i s also important i f a large atomic ion current i s to be obtained. The discharge tube and the glass cone covering the dural probe require cleaning with hydrofluoric a c i d at i n t e r v a l s to remove the coating of sputtered metal which forms on them. Organic materials such as rubber and vacuum grease must also be excluded from the.discharge tube. 4. EXTRACTION OF THE IONS, a) Extraction F i e l d . The ions are removed from the discharge by a d.c. voltage applied between the hydrogen i n l e t and the extracting cone through which the exit canal extends. A very low voltage gradient e x i s t s acroaa the discharge tube except close to the exit canal. Most of the p o s i t i v e ions w i l l therefore have approximately the same energy -when they leave the discharge. Thon&emann at a l x , using a radios-frequency source, report that 90 percent of the ions have an energy spread of - 20 electron v o l t s . The shape of the.extracting cone i s such that i t provides an e l e c t r i c a l f i e l d converging toward the exit canal along which the pos i t i v e ions t r a v e l out of the discharge tube. 24. The f i n a l geometry of the region was ar r i v e d at hy a process of t r i a l and error aimed at improving the p o s i t i v e ion current emerging from the discharge f o r a given current drawn by the extraction supply. The glass cone covering the extractor reduces the metal exposed to the discharge and also has an effect,. dependent on i t s length, on the ion current ontained. b) The Exit Canal. Several factors influence the choice of dimensions for the exit canal. O p t i c a l l y speaking, the accelerator tube acts as a lens system which focusses the ion beam on a target at i t s o u t l e t . Often the beam must t r a v e l a considerable distance a f t e r leaving the Van de Graaff generator so a p a r a l l e l or s l i g h t l y converging p e n c i l of ions i s desired. This can be achieved i f the angular spread of the beam i s small when, i t enters the accelerator. A small angle of emergence from the discharge i s also desirable to reduce a b e r r a t i o n s i n the focussing and prevent a large number of ions from s t r i k i n g the walls. The size of the image formed on the target i s d i r e c t l y dependent on the diameter of the exit canal, so i t should be made as small as possible without l i m i t i n g the t o t a l ion current too severely. The rate of hydrogen flow necessary to maintain the optimum pressure i n the discharge tube i s also determined by the dimensions of the canal. The flow should be kept small i n order that the pressure i n the accelerator tube may be kept low and also so that a small store of hydrogen w i l l not be r a p i d l y depleted. 25 Ex i t canals of several sizes were t r i e d , from which one of 1.5 mms. diameter and 12 mms. length was chosen. Canals that provide smaller flow rate and decreased angular apertures allowed l i m i t e d beam currents. The flow rate using the present canal with a pressure of 20 microns of Hg i n the discharge tube i s 0.15 cm^ per minute of hydrogen measured at atmospheric pressure. These are approximately the conditions obtained for the best performance of the ion source. 5. THE HYDROGEN SUPPLY'7 In the Van de Graaff generator the hydrogen i s stored at a maximum pressure of 100 p . s . i . i n a cylinder of 2.3 l i t r e s volume. A palladium thimble i s mounted inside the chamber covering the e x i t hoihe. A heater element of nichrome ribbon wound around the palladium tube i s supplied with a maximum power of 50 watts from a filament transformer. The sate of flow of hydrogen through the palladium i s controlled by the temperature to which i t i s raised. The f i r s t palladium tubes, used by us were obtained from Johnson Matthey Mallory Co., Toronto". " They were made i n the form of a thimble 3 inches long, with a diameter of 1/8 inch and 0.10 inch wall thickness. These tubes withstood the pressure without damage but t h e i r useful l i f e was l i m i t e d . At the temperatures necessary to obtain a s u f f i c i e n t flow of hydrogen (about 300° C) the structure of the palladium metal changed and small holes appeared i n the tubes. The palladium 26. appeared to be c r y s t a l l i n e i n form and very d o l l i n color. These c h a r a c t e r i s t i c s have been observed by others. Another d i f f i c u l t y was experienced i n obtaining reproducible behavior from these palladium leaks. After being heated i n the hydrogen atmosphere of the storage bottle i t was found that the flow of hydrogen was not completely stopped when the temperature was reduced to 20° C. This could be remedied temporarily by heating the palladium i n a i r . To prevent a recurrence of the trouble a i r was mixed with the hydrogen i n the storage b o t t l e . It was then necessary to evacuate the bottle and heat the p a l l -adium a f t e r several hours' of operation because the hydrogen flow gradually decreased. The only explanation of this i s that the action of the palladium i s gradually slowed by the presence of the a i r which i s removed by heating the tube i n vacuum. The tube now i n use was obtained from American Platinum Company, of lewark, Nfw Jersey. It i s 3 inches long, l / 4 inch in diameter and has a wall thickness of 0.012 inches. After more than 100 hours of operation the d i f f u s i o n of hydrogen through the leak at thiies operating temperature gradually slowed. This may have been due to impurities i n the gas slowly i n h i b i t i n g the action of the palladium metal. Heating the palladium under vacuum caused i t to pass hydrogen at room temperature when i t was again i n s t a l l e d i n the ion source. On the adviee of JtD? Cow of the university of C a l i f o r n i a , Berkeley, this trouble was eliminated by heating the tube with helium on one side while the other side i s evacuated. The tube i s performing well a f t e r 27 65 hours, of subsequent operation. 6. INITIAL ACCELERATION AND FOCUSSING OF THE BEAM. Before entering the accelerator column of the Van de Graaff generator the ions must be formed into a well defined beam t r a v e l l i n g with a v e l o c i t y large enough that i t w i l l not be.serious-l y deflected by small r a d i a l f i e l d s . Acceleration of the ions J as soon as they leave the discharge tube i s also e s s e n t i a l because c o l l i s i o n s with gas molecules are most serious at low v e l o c i t i e s . Such c o l l i s i o n s not only scatter the ions out of the beam but result i n attachment or i n i o n i z a t i o n of the gas molecules. In t h i s way the proton content of the beam may be reduced and ions of d i f f e r e n t energies introduced into the beam. Two mollow c y l i n d r i c a l e l e c t r o s t a t i c electrodes through which the ions pass provide acceleration to a maximum energy of 50 k i l o - e l e c t r o n v o l t s . This accelerating f i e l d also has the property of focussing the ions. By proper choice of dimensions and r e l a t i v e voltages the accelerating electrodes can be made to act as l e n s e s ^ 0 which provide a narrow p e n c i l of ions. The properties of lenses possessing a very simple geometry can be obtained from a n a l y t i c a l s o l u t i o n s ^ 6 . For more complicated forms i t i s necessary to choose the shape using as a guide lenses of which the properties are known and then to obtain a numerical s o l u t i o n ^ 7 . Since t h i s c a l c u l a t i o n i s laborious many people prefer to construct the proposed lens or a model of i t and 28. adjust the dimensions using i t s performance i n operation as a guide for the changes. Both methods were used i n this laboratory to .design the lens system shown i n Fig.3. rThe f i r s t lens has a very weak focussing action serving mainly jjo accelerate the ions as soon as they leave the discharge. With extraction voltages greater than 500 volts the ions leave the f i r s t lens as a s l i g h t l y divergent beam even with an accelerating po t e n t i a l of 15 k i l o v o l t s on the f i r s t electrode. The second lens i s strongly focussing and s u f f i c e s to pyoduee a convergent beam of ions with the maximum of 3 k i l o v o l t s extraction voltage. The action of the lenses of depehdsmnot only on the voltages of the adjacent electrodes but on the energy, of the beam when i t reaches them, so that both electrode.voltages were made independently adjustable to .allow focussing under a l l conditions. The 15 k i l o v o l t f i r s t electrode and 50 k i l o v o l t second electrode supply used i n the Van de Graaff generator are shown i n F i g . 5. 7. PERFORMANCE OF THE ION SOURCE, a) Color of the.' Discharge. The color of the discharge i s a good i n d i c a t i o n of "the atomic ion content. The spectral l i n e s of the hydrogen Balmer series are highly excited i n discharges containing a l a r number of protons. The predominant l i n e producer, a b r i l l i a n t ..red colored discharge. When f i r s t assembled and put into operation a new discharge tube behaves rather poorly i n this 2 henrys to variac 0-115 v. 3 W e s t i n g h o u s e B r a k e a n d S i g n a l 16 E H T 144 r e c t i f i e r s (four) 10 K v w w -0.1 uf y\/ 5 KV \ 2 0 K - \ A V W •€> + o 3 TO U3 c h o k e a n d n e o n s u r g e p r o t e c t i o n E x t r a c t o r V o l t a g e S u p p l y 0 - 3 kv a t 2 0 m a s . tuu o W. B.&S. 36 EHT 240 r e c t i f i ers to variac 0-115 v. 1 0 0 K •vww-O . 0 0 I ^ f / 2 0 k v \ 1 0 0 K - W W — 0 . O I fjf I O K V -© \ 80 meg F i r s t E l e c t r o d e V o l t a g e S u p p l y 0 - I S K V . e ight W. B.8. S. 3 6 E H T 2 4 0 r e c t i f i e r s C, 5O0 pf-if 2 0 k v c 2 50o yyf 30kv: C, to variac° 0 - I I 5 V . 1 -k c, v w w I 0 O . K O — s u r g e p r o t e c t i o n 4^ S e c o n d E l e c t r o d e V o l t a g e S u p p l y o.-5.o kv. FIG. 5. 29. respect. The discharge i s b l u i s h i n color and. delivers an ion current with a small proton content. A. gradual improvement occurs and a f t e r about an hour of operation a bright red color i s produced i f more than 300 watts of high frequency power i s a v a i l a b l e . At lower o s c i l l a t o r power a pink discharge i s the best that can be obtained. The sodium yellow l i n e i s also r e l a t i v e l y intense when the maximum o s c i l l a t o r power i s delivered to the source. This i s attributed to the pyrex glass becoming hot and sodium vaporising out of i t . Several tubes were punctured while operating i n atmospheric pressure^and others were discovered with collapsed areas a f t e r operation* For t h i s reason each discharge tube i s annealed before use and two blowers have been mounted.in the Van de G-raaff generator d i r e c t i n g streams of air, on i t . The present discharge tube i s s t i l l operating a f t e r more than 100 hours of running with this arrangement, b) Pressure i n the Discharge. The discharge operates well with hydrogen pressures of about 20 to 3& microns of Hg. Above t h i s range the pressure i n the vacuum "system of the test unit rose to where scattering of the ion beam was important and the results, were; e r r a t i c . The ion current from the source i s observed to increase slowly as the pressure, i s lowered u n t i l just before the discharge extinguish-es. A sudden decrease i n current occurs just below a pressure of 20 microns and the, discharge becomes, weak then disappears. For these reasons a hydrogen flow rate, which w i l l maintain about 30. E0 microns pressure i n the discharge has been used f o r a l l work. o:) Ion Current. The ion current depends d i r e c t l y on the extraction voltage (Fig.6 ) , which i s therefore used as a current cont r o l . The pressure i n the discharge, cleanliness of the discharge tube and geometry of the glass around the extracting cone; none of which i s exactly reproducible i n d i f f e r e n t ion sources, also a f f e c t the ion current so that the curve shown i n Fi g .6 represents only one t y p i c a l arrangement. A s l i g h t dependence of the o s c i l l a t o r power on the extraction voltage f o r optimum current has been noted. Observation shows that the dark space or "plasma" which e x i s t s between the red discharge and the extraction cone becomes l a r g e r as the extraction voltage i s increased. The position of the edge of the plasma also depends on the o s c i l l a t o r power and the radio-frequency c o i l which must be situated well above the extraction cone f o r optimum performance. Most of the d.e. voltage drop i n the discharge occurs between the extraction cone and the edge of the plasma,the shape of which helps focus ions into the exit wanal. It i s therefore not surprising that the adjustment of the o s c i l l a t o r for maximum current should be af f e c t e d by the extraction voltage. The ef f e c t i s small so the o s c i l l a t o r i s always set. to de l i v e r f u l l power, d.) Alignment of the Lenses. Great d i f f i c u l t y was experienced at f i r s t i n con-structing a suitable lens system. . I t was possible to focus 500 1000 /500 2000 2500 E x t r a c t i o n v o l t s F IG. 6. lon C u r r e n t as a F u n c t i o n of E x t r a c t i o n Voltage 21 the beam bat a l i g n i n g the lenses by i n s p e c t i o n d i d not produce a c e n t r a l beam. Furthermore as the energy was changed the p o s i t i o n of the beam on the t a r g e t v a r i e d . Such movement could not be t o l e r a t e d i n the long a c c e l e r a t o r column of the Van de Graaff generator where i t threw the beam o f f the t a r g e t . To remedy t h i s , t h e discharge tube, e x t r a c t i o n cone, f i r s t e l e c t r o d e and the i n i t i a l p a r t of the second electrode were a l l mounted from one p l a t e as shown i n F i g . 4 . This p l a t e i s p r o v i d e d w i t h a d j u s t i n g screws so that i t may be moved r a d i a l l y and t i l t e d w i t h respect to the a x i s of the a c c e l e r a t o r column without opening the vacuum system. A mandrel was made which f i t s through the e x t r a c t i o n c a n a l and the f i r s t and second l e n s e s so that; they may be a c c u r a t e l y a l i g n e d d u r i n g assembly. This arrangement permits c e n t e r i n g of the beam on the t a r g e t by simple and e a s i l y performed adjustments. e) Performance on the Test U n i t . A vacuum system and magnetic analyser were constructed so that the performance of the i o n source could be t e s t e d outside the Van de Graaff generator. & t o t a l i o n current of 350 microamperes has been c o n s i s t e n t l y obtained on a t a r g e t placed 6 inches beyond the second electrode on t h i s u n i t . The f o l l o w i n g operating c o n d i t i o n s are t y p i c a l . E x t r a c t i o n voltage 3,000 v o l t s E x t r a c t i o n c u r r e n t 20 milliamperes Discharge pressure 20 microns" of Hg F i r s t e lectrode voltage 6 k i l o v o l t s 22. Second electrode voltage 34 k i l o v o l t a Beam l/8 inch diameter at target E x i t canal to target distance 40 inches Analysis of a beam from this source was c a r r i e d oat by K i r k a l d y 2 8 and gave the following r e s u l t a . Protons 125 microamperes HH^iona 130 " HHH*" iona 75 « Higher mass ions 15 rt Total current 325 " These r e s u l t s werei obtained with a magnetic analyser and may be s l i g h t l y i n error due to poorly defined beam geometry. The presence of the high mass components, and an observed improvement of the proton content with increased operating time indicate that impurities i n the discharge tube were p a r t l y responsible fo r the low proton percentage. A previous analysis using a lens system i n which the beam t r a v e l l e d 2.5 inches before entering the f i r s t electrode (compared to 5/8 inches here) showed a component corresponding to protons which had gained only one h a l f the extraction voltage. These were undoubtedly due to p a r t i c l e a which emerged from the discharge as HH + ions and were ionized by c o l l i s i o n s with gas molecules to form protons before they entered the f i r s t lens. This re s u l t shows the importance of early acceleration, f) Performance i n the Van de Graaff Generator. When the pre-aligned and tested ion source was 33. placed i n the Van de Graaff generator a new d i f f i c u l t y was encountered. When the machine i s operating at low voltages the f i r s t , electrode of the main accelerating column has a voltage, of much les s than 50 k i l o v o l t s with respect to the ion source. This means that the ions experience a deacceleration or that the pre-aligned electrodes must he operated at a reduced voltage. In either case i t i s found that a current of a few micro-amperes i s a l l that can he obtained on the target. Also the f i r s t electrodes of the accelerating column have a considerable focussing e f f e c t on the low energy beam and caused i t to s h i f t on the target as the operating voltage i s varied. Overfocussing of the beam by these electrodes, with the consequent ion bombardment of l a t e r electrodes, can only be prevented by very c a r e f u l adjustments of the lens voltages of,the ion source. This adjustment i s made d i f f i c u l t by the fluctuations i n the accelerating voltage that occur when ions s t r i k e the electrodes. I t would be preferable to introduce the beam into the accelerating column with an energy of 50 k i l o - e l e c t r o n v o l t s and eliminate these e f f e c t a . To do that i t would be necessary to place the discharge tube and extraction cone at a potential of 50 k i l o v o l t s with respect to the upper terminal of the Van de Graaff generator. Providing the necessary i n s u l a t i o n would be d i f f i c u l t and has. not yet been considered worthwhile. An improved operation i s obtained with an accelerating potential greater than 750 k i l o v o l t s . The p o s i t i o n of the beam on the target i s no longer dependent on the voltage and l a r g e r 34 target currents aan be obtained. The extraction current and the second electrode supply voltage are the only ion source factors v i s i b l e on meters throughthe periscope so approximate values, judged by the position of the selsyn controls, are given for other parameters i n the following table of t y p i c a l operating conditions. Extraction voltage 1000 volts Extraction current 6 milliamperes F i r s t electrode voltage. 500 volts. Second electrode voltage 10 k i l o v o l t s Van de Graaff generator voltage 1.5 m i l l i o n s v o l t s Target current 15 microamperes Beam diameter 1/8 inch at the target A target current of 25 microamperes, coilld e a s i l y be obtained but the target becomes so hot that i t would have to be replaced a f t e r slfew minutes of operation. Coils have been \ . . . mounted above the target through which a l t e r n a t i n g current i s passed to deflect the beam r a p i d l y about a small c i r c l e on the target. This prevents l o c a l heating of one spot and allows the targgt to remain i n use for longer i n t e r v a l s without replacement. ' ' s 8. ENGINEERING FEATURESf The" extraction voltage and two accelerating electrode voltage supplies used «a&rS!§n source i n the Van de Graaff generator are shown i n Fig.5. Selenium r e c t i f i e r s are used i n these supplies to eliminate vaceum tubes which might breax 35 under pressure. An economy i n space and power i s also r e a l i z e d by the elimination of the filament transformers, necessary with tubes.. Si l i c o n e l i q u i d f i l l e d "glassmike" capacitors manufactured by Condenser Products Company, Chicago, were o r i g i n a l l y used in a l l the power supplies. Several breakages occurred so those i n the 50 k i l o v o l t supply, for which equivalent ceramic capacitors could be obtained, were replaced. The ceramic condensers are manufactured by Centralab under the trade name "Hi-Vo-Kap". A small hole d r i l l e d i n the wall of each G-lassmike condenser which i s oriented with the metal and upward would prevent s t r a i n on the glass when pressure i s applied without allowing the l i q u i d to escape. This would necessitate care i n handling the ehasses during overhaul i n order to a v o i d - s p i l l i n g the s i l i c o n e and i s possible with only a small number of the capacitors. A. safety mechanism* shown i n Fig.7, has been designed to keep gas out of the vacuum system i f the discharge tube breaks; The s t e e l b a l l i s pushed out of the sidearms by the inrush of gas and seals the exit canal s u f f i c i e n t l y to prevent damage to the pumps. Raising the pressure on the discharge side of the canal- to 1/2 atmosphere w i l l move the b a l l into the e x i t canal; 4- X F U L L S C A L E FIG. 7 E x t r a c t i o n C a n a l a n d S a f e t y P l u g PART I I . INVESTIGATION OF THE PHOTODISINTEGRATION OF NEON. 36. IV. PRELIMINARY SURVEY OF THE Ne 2 0( j y 06 ) O l 6 REACTION. 1. NUCLEAR MODELS. In attempting the study of any but the simplest nuclei.the actual nuclear system i s usually treated i n terms of a model i n which the nuclear forces are represented by a small number of a r b i t r a r y parameters chosen to f i t observed facts . While no e n t i r e l y adequate model has been found, several models have been proposed each of which describes co r r e c t l y , at least i n a q u a l i t a t i v e manner, some experimentally observed aspects of nuclear structure. The formulation of a more comprehensive model i s hampered partly by mathematical d i f f i c u l t i e s but more seriously perhaps by the lack of detailed experimental data with which to f i x the a r b i t r a r y parameters involved. The aim of the experimentalist i s to provide these data. A model which has received much attention recently 29,30,J and which gives a reasonably detailed account of nuclear structure i s the central f i e l d or "quasi-atomic" model, developed p a r t i c u l a r l y by M.G. Mayer Another model, i n which the nucleus i s v i s u a l i z e d as a group of v i b r a t i n g and ro t a t i n g alpha 34 p a r t i c l e s , was f i r s t extensively developed by Wheeler . This "alpha p a r t i c l e " model although more r e s t r i c t e d i n i t s application, has been esp e c i a l l y useful i n the study of l i g h t nuclei which contain 4n nucleons, where n = 2,3 ,4,5(Be&,c-*-2} 0-^, Ne 2 0.) Hafstad and Teller33 have considered i t s extension 37. to the 4 n i l n u c l e i . These two models, es p e c i a l l y the former, at present provide the most adequate description of nuclear strue: ture. a) The Central F i e l d Model. The central f i e l d model i s an independent p a r t i c l e model, that i s , each nucleon i s assumed to'move i n a f i x e d p o t e n t i a l which represents the average e f f e c t of a l l the nucleons The t o t a l nuclear wave-function i s the product of the wave-functions of the i n d i v i d u a l nucleons and the energy the sum of the energy eigen-values corresponding to the i n d i v i d u a l p a r t i c l e wave-functions. In order to represent with reasonable accuracy and mathematical s i m p l i c i t y the average ef f e c t of the short range nuclear forces simple functions are used f o r the f i x e d p o t e n t i a l . Functions which have been used are; the i n f i n i t e p o t e n t i a l well V= - ° ^ f o r r < R and V = O 4or v ^ R ? the f i n i t e p o t e n t i a l well • V = - V 0 fov r <-R cxnd V - O £or r ^ R ? and the harmonic o s c i l l a t o r V = - V 0 C r *" . (C-o_ c o n s i s t ) V i s the pot e n t i a l energy, r the distance from the centre of ' the nucleus, R the nuclear radius or some related length, and V, a f i x e d value of p o t e n t i a l energy. The parameters i n these potentials can be adjusted so that s i m i l a r quantitative r e s u l t s are obtained f o r the single p a r t i c l e energy eigen-values of the lowest energy l e v e l s . By assigning spin angular momemtum, orbitalF-a^gular momentum and isotopic spin quantum 38. numbers to the nucleons, the energy l e v e l s may be f i l l e d , i n accordance with the P a u l i exclusion p r i n c i p l e i n the same way that the electronic energy l e v e l s are f i l l e d i n atomic structure. It turns out that this model c o r r e c t l y predicts the pa r t i c u l a r s t a b i l i t y of the "magic number" nu c l e i containing 2, 8 or 20 protona#r* neutrons. For these nuclei the models exhibits closed shells of nucleons similar to the closed electron s h e l l s of atomic structure. The heavier ilmagic number" nuclei are those containing 50, 82 or 126 protons or neutrons. For n u c l e i i n this region of the periodic table the p o t e n t i a l well functions no longer show a pronounced s h e l l structure and the harmonic o s c i l l a t o r function predicts the closed s h e l l s f o r the wrong numbers of nucleons. Several attempts have been made to obtain a l e v e l structure i n agreement with the observed 29 properties, of the heavy n u c l e i . Feenberg and Hammaek use a qu a l i t a t i v e physical argument fo r modifying the p o t e n t i a l 30 function f o r these n u c l e i . Nordheim discriminates against higher o r b i t a l momentum states, since strong nuclear interaotoiona would cause the nuclei to have low o r b i t a l angular momentoL;. 33 Mayer assumes a strong coupling between the o r b i t a l angular momentum and spin angular momentum to achieve a s h i f t i n g of the energy levels, predicted by a f i n i t e square well with no spin-orbit coupling. Each of these modifications r e s u l t s i n a model which shows a s h e l l structure i n agreement with the known s t a b i l i t y properties of n u c l e i . The Mayer model, i n 3 9 . particular, shows only a very few discrepancies between the predicted and observed nuclear spins; magnetic moments and • o quadrate moments of n u c l e i . In order to obtain estimates of the positions of the energy l e v e l s of a l l except the very l i g h t e s t n u clei i n excited states using the central fie<l:d model, i t i s necessary to make e x p l i c i t assumptions about the nuclear forces and the potential well to be used. Experimental evidence on which to base these assumptions does not exist so r e l i a b l e results cannot be obtained. Attempts: by Feenberg ahd co-workers^»^ have predicted several low. l y i n g excited states i n each of the 4 16 nuclei l y i n g between He and 0 i n the periodic table. Despite extensive investigation of these nuclei only a few lev e l s have been found which can be i d e n t i f i e d with the predicted l e v e l s . b) The Alpha p a r t i c l e Model. This model i s a special ease of the "resonating 34' group" model proposed by Wheeler . Wheeler attempts to analyse the states of a nucleus into configurations i n which the nucleons are arranged i n groupsSwhich are continually changing. From general considerations of the nuclear forces he then chooses^ from the unmanageable number of possible groupings, those that are most l i k e l y . The alpha p a r t i c l e i s a p a r t i c u l a r l y stable configuration with about 7 MeV binding energy per p a r t i c l e . Since this constitutes a large f r a c t i o n of the t o t a l binding energy per nucleon i n the l i g h t n u c l e i , the binding energy between the alpha groups i n Wheeler's model i s 40. r e l a t i v e l y small. Thus the inter a c t i o n between the alpha particles, i s small enough that the time spent hy the nucleus i n other configurations i s n e g l i g i b l e . These: conditions only occur when the exc i t a t i o n energy of the nucleus i s less than about 10 MeV 3 8; When the excitation energy i s higher, i n d i v i d u a l nucleons may receive enough energy to destroy the s t a b i l i t y of the alpha p a r t i c l e grouping. 8 IP On the alpha p a r t i c l e model, the nuclei Be , C , 16 20 0 ,and We appear as symmetrical arrangements of alpha p a r t i c l e s , i . e . dumb-bell, e q u i l a t e r a l t r i a n g l e , tetrahedron, bipyramid respectively. Considerations of the quantum-mechanically allowed r o t a t i o n a l and v i b r a t i o n a l energy states of these configurations leads to predictions concerning the I W B energy states of the excited n u c l e i . The method i s exactly s i m i l a r to that used i n locating the energy leve l s i n molecular theory* The special symmetry of these models and the reduction i n the number of degrees, of freedom due to the grouping of the rcic;jGi\',ec>riS. results i n a larger spacing of the energyolevels. than predicted by the central f i e l d model 3 9. c) Interest i n I e 2 0 { ^ c C ) 0 1 6 Reaction. 16 The 0 residual nucleus from the reaction 20 i r . 16 Ne (^,oC )0 may be l e f t i n an excited state. If mono-energetic gamma rays of known energy are used to produce the j • •disintegration and the k i n e t i c energy of the products, i s measured., the l e v e l i n which the residual nucleus i s l e f t may be calculated. Although the central f i e l d model i n i t s present form y i e l d s no r e l i a b l e predictions concerning excited 4 1 . ata.tea of the n u c l e i , the establishment of these states experimentally would, a i d i n the refinement of the model. The alpha p a r t i c l e model however does y i e l d predictions f o r the nuclei to which i t has been applied. Predicted l e v e l s i n 0 ^ have heen i d e n t i f i e d by Dennison 4^ with the observed l e v e l s ft* produced by the reaction F 1 9 ( p , oC ) 0 1 6 . Other assignments have been suggested 4^, but with any of these assignments no l e v e l s are predicted below 6 MeV. The experimental observation of such a state would therefore cast serious doubt on the v a l i d i t y of the alpha p a r t i c l e model. 2. PHO T ODI SIN TEG-RAM I ON OF- NUCLEI. Several photdisintegration processes have been observed and d i f f e r e n t theories advanced 4^'^> 4 4 to account f o r t h e i r features. More information about the r e l a t i v e p r o b a b i l i t i e s of. these processes and the v a r i a t i o n of the reaction cross-sections with photon energy should lead to the development of a more adequate theory and to discrimination among the various models. The most investigated photodisintegrations are ( $t r\ ) and ( it p ) processes. Much of the work on the reactions has been carried out using the X-radiation from b e t a t r o n s 4 5 ' ^ ' 4 7 ^ ^ ' The excitation functions for these reactions, which have been measured, f o r many n u c l e i , exhibit broad resonances of about 6 MeV half-widths with maxima at photon energies i n the v i c i n i t y of 20 MeV. The y i e l d s obtained from elements i r r a d i a t e d with photons of 320 MeV maximum energy 4^ can be a t t r i b u t e d e n t i r e l y to these single resonances observed at lower energy. The 42. measured threshold energies that e x i s t f o r these reactions, i n most cases, agree approximately with thdae calculated from mass v a l u e s 4 6 . Other photodisintegration processes which have been observed are ( 2?, <X) and photo-fission reactions. Fev/er investigations of these reactions have been performed so that general features are not obvious. The t h e o r e t i c a l treatments of these nuclear photo-effects are of a preliminary nature and show only q u a l i t a t i v e agreement with the experimental r e s u l t s . Measurements of the r a t i o of ( 2^ , p ) to ( ^ , ) c r o s s - s e c t i o n s ^ have indicated a l a r g e r value than expected on the basis of compound nuclear theories although at least one experiment has been performed the results of which are not inconsistent with the hypothesis of a compound nucleus . However the most favored theories postulate a d i r e c t i n t e r a c t i o n between the' photon and one 44 nucleon. This nucleon may then be emitted immediately or, i n some cases, may inter a c t with other nucleons producing a compound nucleus which w i l l , i n general, emit a neutron 4 3* 3. THEORETICAL STUDY OF PHOTO ALPHA. REACTIONS. 52 Preston has published a b r i e f summary of a t h e o r e t i c a l investigation of photodisintegration reactions in which an alpha p a r t i c l e i s emitted. He uses an alpha p a r t i c l e model i n th i s a n a l y s i s . He applies his r e s u l t s to the 0 1 6 ( oi )c 1 2 reaction but his assumptions are s u f f i c i e n t l y general to apply to the phtotdisintegration of any nucleus f o r which an alpha p a r t i c l e model may be used. His resu l t s give no information about ( i , n ) or ( it p ) processes but th e i r presence does not af f e c t the cal c u l a t i o n s . He assumes the ( i ( oe ) process to be due to a quadrupole i n t e r a c t i o n . The unpublished results of M i l l a r and Cameron, quoted by Preston, for the ir-aa&Mohioi 6( V, oC )C 1 2,and also the re s u l t s of Goward, Telegdi and W i l k i n s 5 2 f o r C 1 2 ( )2 He4, show that the reaction cross-sections as a function of gamma energy w)i i irh are sharply peaked at an energy well above the Coulomb b a r r i e r s . It i s reasonable therefore to treat the emitted p a r t i c l e as a plane wave applying a b a r r i e r p e n e t r a b i l i t y factor at lower energies. Preston writes the wave-function of the i n i t i a l nucleus as where <fto (r) i s a function of the coordinates of the ejected alpha.particle§ and l£ depends on those of the remainder. He considers the e f f e c t of the gamma ray on only the ejected p a r t i e l e and obtains matrix elements of the form where 6 t y i s one of the terms f o r the quadrupole in t e r a c t i o n of the photon with the ejected alpha, and "Xf i s wavefunction of the re s i d u a l nucleus. In order to evaluate these matrix elements he puts 44 With this assamption any s p e c i f i c reference to the 0 1 6 ( V, oC ) C 1 2 i s l o s t and only an apper l i m i t i s obtained f o r the magnitude of the reaction cross-section. For(# (r) he uses three forma: An exponential function _*7b e A gaussian function „ v > and a modified Wheeler function a and b are parameters which are adjusted to make the maximum cross-section predicted by the theory occur at the gamma energy at which i t i s experimentally observed. The l a t t e r function i s deduced from a wave equation written f o r 0 X^ using an alpha p a r t i c l e model but i t gives results very l i t t l e d i f f e r e n t from those ontained with the Gaussian function. F i g . 2' shows Preston's curves together with the results of M i l l a r and Gameron 16 J 12 f o r the reaction 0 ( o 7 oC )C . The t h e o r e t i c a l curves are normalized a r b i t r a r i l y to give the correct magnitude for. the cross-section. The shape of the cross-section curve as a function of energy however i s seen to be corre e t l y predicted. In view of this agreement an estimate may be made concerning the expected cross-section f o r the reaction He^( T?f oC JO 1^. It seems reasonable to assume the overlap of the wavefunctions f o r the i n i t i a l and f i n a l states i a approximately the same f o r both reactions so that J* • z E^ -MeV <p0m i. e ~ / b b = 8 x 10 ~'4 cm. 2. e ~ r / b * b = 2 . 4 5 x I O ~ ' 3 cm. w i e ~ r / b * x f (r,a/ b). t>/a = W2 b - 3 . 5 3 x i o ~ 1 3 cm. r c = 3 . 7 x / o -' 3 cm. w2 e~ r / b 2 x f (r,Q/b) b/a = 2 b = 5.19 X JO"'3 Cm. r 0 = 4-.7 x /o~'3 cm. j_ r 0 = 0 . 7 5 ( 2 a J + b 2 ) 2 FIG. 8 cr {E4) for ( P r e s t o n ) 0'6(i,0L 45 would be expected to have about the same value i n each ease* Since t h i s f a c t o r determines the magnitude of the predicted cross-section,the l a t t e r should be of the same order of magnitude f o r both reactions. The modified Wheeler function i s the only expression for ( r) that i s developed with reference to an 0 x 6 nucleus, and a s i m i l a r form with two adjustable §arameters would be expected f o r 20 this function f o r the He nucleus. The parameters a and b i n the expressions f o r depend on the radius and binding energy within the i n i t i a l nucleus of the alpha p a r t i c l e group which i s emitted i n the reaction. These parameters would be expected to be changed only s l i g h t l y for d i f f e r e n t n u c l e i , so the predicted shape of the curve showing the cross-section as a function of gamma energy f o r the He 2^ di s i n t e g r a t i o n w i l l be s i m i l a r to that f o r 0 1 6 . Small adjustments of these; parameters w i l l change the value of the gamma energy f o r which the maximum cross-section occurs. The threshold energy f o r the ejection of an alpha p a r t i c l e from a Ne 2^ nucleus i s 4.6 MeV, calculated from the mass values, which i s considerably lower than the value 7.2 MeV 16 obtained f o r the same reaction i n 0 . The Coulomb b a r r i e r experienced by the emitted alpha p a r t i c l e i s , on the other hand, 6.1 MeV i n the former reaction compared to 5.0 MeV for the l a t t e r , assuming a nuclear radius of 5 x 1 0 ~ x 3 cms. Thus f o r gamma energies below 10.7 MeV (threshold plus Coulomb b a r r i e r height) the reduction of the cross-section f o r 46 °C due to the p e n e t r a b i l i t y of the Coulomb ba r r i e r f o r alpha p a r t i c l e s w i l l have to be taken into account. The corresponding energy f o r 0-^( " 2 ? ) C 1 2 i s 1.5 MeV higher. It might therefore be expected that the cross-section f o r the neon reaction would be measurable at lower gamma ray energies than that f o r the oxygen reaction. Experimentally two reasons exist f o r attempting a measurement at low gamma ray energies. F i r s t l y , an intense source of 6 and 7 MeV gamma rays i s available from the reaction F"^ ( p, i JO1^ and secondly this energy i s below the threshold f o r and ( ~i, p ) processes i n most substances. With gamma rays of higher energy these processes produce an objeetional background from which the (S,oC ) events must be distinguished. 4. REPORTED EXPERIMENTAL RESULTS FOR SOME PHOTO ALPHA REACTIONS. To date the most extensive study of ( i , oL ) processes has been ca r r i e d out by the use of photographic 54 emulsion techniques, using t h i s method Waffler and Younis report a cross-section of (1.8-0.6) x 1 0 ~ 2 8 cms 2 for the reaction 0 1 6 ( 3, °C ) C 1 2 with gamma rays of 17.5, MeV energy. The resulta of M i l l a r and Cameron indicate that t h i s i s very l i t t l e below the energy f o r which the cross-section i s maximum. ,. Gow&M, Telegdi and W i l k i n s 5 2 have observed the reaction C 1 2 ( "J.oC )Be 8 , Be 8 ^ 8He 4 47. i n photographic emulsions. They obtain ajcross-seetion of 10~ 2 8 cms 2 at a gamma ray energy of about- 18 MeV. Other photo-alpha reactions observed i n photographic emulsions^ 5*^ 7 -29 2 have lower cross-sections (— 10 '.ems. ) and involve the ejection of several alpha p a r t i c l e s . Other techniques, which have.been reported are the use of cloud chambers to observe the p a r t i c l e tracks and the observation of an unstable product nucleus to i d e n t i f y the 58 reaction. G-aerttner and Yeatea have obtained cloud chamber photographs of reactions i n N 1 4 and O1^ which they believe are due to (^,°C ) processes. A large percentage of the disintegrations observed by them i n 0 x 6 could be attr i b u t e d to ( ) reactions although t h e i r method did not distinguish ( 1?, <*• ) and ( ^ , P ) processes. Haslam and Skarsgard 5 9 have i r r a d i a t e d samples containing Rb with the X-rays from a betatron and observed the residual a c t i v i t y due to the isotope 83 ft 7 1 83 Br reproduced by the reaction Rb°' ( T>, °C )Br . They obtain -29 p a cross-section curve with a peak value of 7 x 10 cms"* i at a gamma energy of 22.5 MeV. ^ The peak has a half width of 6.6 MeV. The experimental results so f a r obtained indieate that the cross-section f o r (^, oC ) reactions l i e i n the region -27 -29 " 2 10 - 10 ems . Smaller cross-sections may occur but are d i f f i c u l t to observe due to the infrequeney of the events and the backgrounds due to other reactions. 48 V. OUTLINE OF EXPERIMENTAL METHOD 1. METHODS OF DETECTION.. The reaction cross-section i d defined "by the equation where ^  i s the number of disintegrations per unit time, f\| i s the number of gamma rays per cm per unit time incident on the material under investigation, n i s the t o t a l number of nuclei i n the material under investi g a t i o n and CT i s the reaction cross-section. An estimate of the expected y i e l d (Y) may be made for the reaction 20 }^ 16 • —29 2 Ne ( 7>, °C )0 i f we assume 0 ~ i o ~ ems.* which from the res u l t s quoted f o r s i m i l a r reactions; seems to be/reasonable lower l i m i t * I f the neon gas i s contained i n a volume of 1500 cms. at a pressure of 5 atmospheres n ^ 2 x 10 2 2. With the sources of monoenergetic gamma rays o r d i n a r i l y available a f l u x N ^  10 2 i ( cm? / sec. can be obtained with a suitable experimental arrangement. Using these figures Y ^  7 disintegrations per hour. It i s therefore necessary to detect t h i s small number of" events and dis t i n g u i s h them from a large number of other reactions which may occur. Techniques involving the use of a cloud chamber; a proportional counter or an i o n i z a t i o n 49 chamber might be used to do t h i s i n an i n e r t gas such as neon. The p r o b a b i l i t y of obtaining a cloud chamber photograph of such a rare event without some a u x i l i a r y method of detecting i t s occurrence, i s so small that a prohi b i t i v e number of photographs would have to be taken. The voltage required to obtain gas ampli f i c a t i o n would be very Mghliin a proportional counter with the pressure and volume necessary to /'an n of 23 the order of 10 . There w i l l be a large number of single electron pulses due to Compton, photo-electric and pa i r processes produced by the f l u x of gamma rays through the gas. Even i f a reasonable amplifiaation could be obtained, with the high voltagea necessary these noise pulses would receive >, a larger than proportional a m p l i f i c a t i o n . A better signal to noise r a t i o should therefore be possible i n an i o n i z a t i o n chamber where no gas ampli f i c a t i o n i s employed. 2. IONIZATION CHAMBERS. a) Two Electrode Ionization Chambers. In an i o n i z a t i o n chamber the gas i s contained i n . a volume across which an e l e c t r o s t a t i c f i e l d i s produced between two electrodes. One of the electrodes i s insulated and connected to an ampli f i e r . When an io n i z i n g p a r t i c l e passes through the gas, the ions are c o l l e c t e d on the electrode and a pulse i s received by the amplifier. The amplitude of the pulse i s proportional to the number of ions c o l l e c t e d which i s nearly proportional to the energy of the io n i z i n g p a r t i c l e * Therefore, i n the case of a nuclear d i s i n t e g r a t i o n detected by the i o n i z a t i o n produced by the product n u c l e i , the pulse 50 amplitude i s proportional to the k i n e t i c energy, received by the i o n i z i n g products. Thus the disintegrations under investigation may be distinguished from other events, i n which a d i f f e r e n t amount of energy i s available for i o n i z a t i o n . fin In order to study the pulse formation consider an i o n i z a t i o n chamber consisting of two plane electrodes A and B as shown i n F i g . iQj. Suppose charges +-q. and -a. are separated somewhere between, the electrodes. I f there i s an e l e c t r o s t a t i c f i e l d between the electrodes s u f f i c i e n t l y great to prevent recombination, 1 the charges w i l l move to the plates with an approximately constant v e l o c i t y depending on t h e i r m o b i l i t i e s . If the' resistance, R, and capacitance* G", connected to B are of such value that the time i n t e r v a l between the separation and c o l l e c t i o n of the charge i s T « RG the voltage pulse developed on B w i l l have approximately the shape shown i n F i g . '&b. The i n i t i a l r i s e , LM, i s caused by h the movement of both charges, and the f i n a l f r i s e , MN;, by the movement of whichever charge i s c o l l e c t e d laterro. The voltage attained at I i s v = 1 f o r T « RC „ .... : ,, c ""' ' " ' . • •'•' The pulse then f a l l s toward zero again at a rate determined by the product "RC"i The exact pulse shape depends on the p a s i t i o n of the charges i n the chamber when f i r s t separated and on t h e i r m o b i l i t i e s , but the one shown i s t y p i c a l . When charges are nega t i v e vol ts + 0. to a m p l i f i e r R A B A p a r a l l e l e l e c t r o d e i o n i z a t i o n c h a m b e r t i m e P u l s e f r o m a s i n g l e c h a r g e p a i r t r a c k c l o s e t o A t r a c k c l o s e t o B a s s u m i n g t h e — i o n s h a v e t h e g r e a t e r m o b i l i t y t i m e P u l s e s f r o m i o n i z a t i o n t r a c k s F IG . 9. P u l s e F o r m a t i o n in an I o n i z a t i o n C h a m b e r 51. formed along the path of an i o n i z i n g p a r t i c l e the pulse i s obtained hy adding several curves s i m i l a r to the one shown. Typical pulse shapes produced hy tracks are also shown i n Fig.lQ). In order to amplify a l l of these pulses f a i t h f u l l y a large; bandwidth i s required and consequently a large resolving time i s obtained. Due to the gamma ray f l u x through the chamber i n a photodisintegration experiment, a large number of electrons and ions w i l l be formed i n randomly occurring events, and "build up,r pulses w i l l be caused by fluctuations i n the electron and ion current to the electrodes. The shorter the resolving time, the smaller the b u i l d up pulses that occur. In order to be able to use short resolving times i t i s necessary to obtain pulses which do not depend on the track p o s i t i o n or orientation. This may be done with a gridded i o n i z a t i o n chamber. b) G-ridded Ionization Chambers. In inert and i n e l e c t r o p o s i t i v e gases such as argon, hydrogen, deuterium and neon; the electrons formed along the path of an i o n i z i n g p a r t i c l e may be c o l l e c t e d without the formation of heavy negative ions. The i n i t i a l r i s e of the pulse, even at high pressures i f the gases are pure enough, i s of the order of one thousand times more rapid than the f i n a l r i s e because of the high mobility of electrons compared to that of heavy ions. It was F r i s c h ^ l who f i r s t suggested that a g r i d might be placed i n front of the c o l l e c t i n g electrode, so that fast r i s i n g pulses due to the electrons would be obtained. 52 Consider again the i o n i z a t i o n chamber shown i n F i g . iQ). I f the charge, -q, i s an electron, then, due to i t s high mobility i t w i l l be quickly collected on the positi v e electrode, B. The pot e n t i a l of B w i l l be les s than -q/c due to the presence of -+• q between the electrodes;. I f a g r i d of an appropriate potential i s placed between the Electrodes and close to B, the effeet of the positive charge on the potent i a l of B may be made n e g l i g i b l e . At the same time the number of electrons c o l l e c t e d by the grid may be made e s s e n t i a l l y zero. The r i s e timet"; of the pulse i s equal to the time i n t e r v a l from the induction of a charge on the c o l l e c t o r electrode by the f i r s t electron to pass the. g r i d , to the c o l l e c t i o n of the l a s t electron. This time depends on the orientation of the track i n the chamber but w i l l be only a few micro-seconds i n length. To ensure that pulses of a l l risetimes reach the peak value, -q/c, the decay time determined by 'RCf i s made long. Bunemann, Granshaw and Harvey 6^» 6 2 have conducted an experimental and th e o r e t i c a l study of the electrode arrangement i n gridded i o n i z a t i o n chambers. They obtain expressions fo r the selection of a suitable g r i d and proper electrode voltages i n a p a r a l l e l plate chamber, so that the gri d w i l l s h i e l d the c o l l e c t o r e f f e c t i v e l y without c o l l e c t i n g electrons i t s e l f . 53. 3. LINEARITY OF ENERGY MEASUREMENTS BY IONIZATIONS When a charged p a r t i c l e traverses a gas i t s e l e c t r i c f i e l d interacts with the atomic electrons of the material and i f the p a r t i c l e energy i s s u f f i c i e n t may eject some of them. Some of the ejected electrons may themselves have s u f f i c i e n t energy to cause i o n i z a t i o n . This w i l l generally take place i n the immediate v i c i n i t y of the i n i t i a l i o n i z a t i o n as w i l l any ion i z a t i o n produced by photons emitted by the parent ion. Thus the passage of a charged p a r t i c l e i s marked by a track of i o n i z a t i o n , the p a r t i c l e coming to rest when a l l i t s energy has been spent i n this process. Only r a r e l y does the p a r t i c l e lose energy by some other process, such as an i n e l a s t i c c o l l i s i o n with a nucleus, so a relationship may be obtained between the t o t a l i o n i z a t i o n and the energy of the p a r t i c l e i If a charged p a r t i c l e i s allowed to spend a l l i t s energy i n an i o n i z a t i o n chamber the r e s u l t i n g e l e c t r i c a l pulse may be made accurately proportional' to the nmmber of ion pairs formed. I f the amount of energy which the p a r t i c l e loses on the average to form one ion p a i r (the lWT value) i s the same at a l l v e l o c i t i e s of the p a t t i c l e , the amplitude of the pulse may be used as a d i r e c t measure of the energy. The problem of the constancy of the, W value i n various gases has received much investigation which i s discussed by Gray , J _ and 72 73 more recently by Bohr and Bethe. 74 Jesse, Forstat and Sadauskis have shown that the io n i z a t i o n i n argon i s proportional to the p a r t i c l e energy. $4« The experiments were conducted with alpha p a r t i c l e s from natural emitters i n the range from 5 to 9 MeV. The energies of these alpha p a r t i c l e s are accurately known from other experiments and t h e i r measurements of the t o t a l i o n i z a t i o n showed that i t was proportional within 0.5 percent except for one case where a pl a u s i b l e reason existed f o r t h e i r error of one percent. They also.measured-the t o t a l number of ions produced i n argon by the two p a r t i c l e s L i 7 and He 4 from the reaction B e 1 0 + n — L i 7 + He 4. The r a t i o of this number to the otherwise known energy release was found to be the same as the corresponding r a t i o ' f o r polonium alpha p a r t i c l e s within 0.3 percent. Similar experiments performed by Franzen, Halpern and Stephens 7^ for the i o n i z a t i o n produced i n argon by the p a r t i c l e s emitted i n the reactions; Ee3 + n — ^ + H and N 1 4 + n — — C 1 4 + H gave the same r e s u l t within 0.4 percent. It may be concluded from these r e s u l t s that unless strange accidental compensations exist, the W value i n argon i s independent of the energy and kind of p a r t i c l e producing the i o n i z a t i o n . Gray,71 analyzing the measurements of Gurney 7 7 made with slowed down alpha p a r t i c l e s from natural emitters found that the number of ions produced i n argon, neon, helium and hydrogen bore a constant r a t i o to one another f o r any alpha p a r t i c l e energy. Thus the W value must be independent of the 5 % p a r t i c l e energy f o r each of the gases. Fano^ has given very plausible t h e o r e t i c a l arguments to show that the W should be at least approximately independent of the p a r t i c l e energy f o r a l l the noble gases. A useful theory has not been developed for other materials but Fano's arguments i n conjunction with the experimental r e s u l t s , are convincing evidence that W i s constant for a l l four gases. Cranshaw and Harvey76 have measured the pulse s i z e from a gridded i o n i z a t i o n chamber f i l l e d with argon and found that i t varies l i n e a r l y , within 0.1 percent, f o r alpha p a r t i c l e s with energies i n the region from 5 to 9 MeV. However, t h e i r results indicate that' the ionization-energy r e l a t i o n must be curved at lower energies i n order that zero energy correspond to 76 zero i o n i z a t i o n . Hanna' confirms t h i s r e s u l t , but more accurate data are required t o e s t a b l i s h f i r m l y the form of the v a r i a t i o n . However, on the basis of these r e s u l t s , the error introduced by assuming that W i s independent of the p a r t i c l e energy i s , at most, about 2 percent. The constancy of W i s a l l that is" required i n order to make quantitative energy measurements i n an i o n i z a t i o n chamber, because a weak source of alpha p a r t i c l e s of known energy i s usually placed i n the chamber f o r c a l i b r a t i o n purposes. Of course capture and recombination processes must be avoided i n order that the pulse amplitude from a gridded chamber be proportional to the energy of the i o n i z i n g p a r t i c l e . Gases i n which free electrons do not combine to form heavy negative ions must be used i n gridded i o n i z a t i o n chambers, and 56. for these, recombination effect's are -negligible, except possibly st very high pressures . Electron capture* which i s only important in ; chambers where the negative ion so formed, i s not collected, i n the pulse, i s due to the presence of electronegative gases. Wilkinson''^ estimates that as l i t t l e as one part i n 10^ of oxygen i s s u f f i c i e n t to capture one percent of the electrons before they t r a v e l 6. centimeters i n a chamber f i l l e d with argon at one atmosphere. Even greater p u r i t i e s are required at higher pressures and i n other gases, e s p e c i a l l y hydrogen. Because- of the release of occluded gases from the walls over long periods, continuous p u r i f i c a t i o n of the chamber f i l l i n g i s usually necessary. 4. SOURCES OF HIGH ENERGY GAMMA RAYS. Not only are photodisintegrationacross-seetions small and d i f f i c u l t to observe, but sources of monochromatic gamma rays of adequate energy are few and weak i n i n t e n s i t y . The "bremsstrahlung", produced by such machines as the betatron, and used by'many previous i n v e s t i g a t o r s 4 ^ ' 4 8 exhibits a continuous spectrum below the maximum energy of the electron beam, which makes the int e r p r e t a t i o n of r e s u l t s 47 d i f f i c u l t : Several l i g h t n u c l e i , when bombarded with protons, fcvm highly excited compound nuclei which may emit energy i n the form of gamma rays. The spacing of the exalted states of these compound nuclei i s several MeY and the states themselves have well defined energies, so the gamma ra d i a t i o n i s mono-57. energetic, or at l e a s t consists of a few separate gamma ray l i n e s . The process hy which a compound nucleus de-oxeites i s determined hy the quantum mechanical properties of the states involved. The nucleus, i n each of i t s states possesses angular momentum which- may he thought of as due to o r b i t a l and spin motion i n the same way as that of an atom. The o r b i t a l and spin momentum must be separately conserved i n any t r a n s i t i o n from one state to another. P a r i t y , which i s odd or even depending on whether the wave-equation of the state changes sign or not when a l l coordinates are r e f l e c t e d i n the o r i g i n , must also be conserved, Whether or not a t r a n s i t i o n can take place i s determined by the spin, o r b i t a l momentum and p a r i t y which must be ca r r i e d away by the emitted p a r t i c l e or ra d i a t i o n . Selection rules, according to which the tr a n s i t i o n s occur, have been determined for each type of emission. When a proton i s captured by a l i g h t nucleus, the compound nucleus so formed may emit a gamma ray or i t may re-emit the proton, which w i l l always be i n accord with the selection r u l e s . If the ex c i t a t i o n energy i s s u f f i c i e n t , the selection rules may also allow the ejfimission of a neutron or an alpha p a r t i c l e . In general, i f p a r t i c l e emission can occur, i t has a higher p r o b a b i l i t y than gamma emission. The following reactions are well known sources of high energy gamma rays which may be produced with a Van de 58 Graaff generator. I. . H 3 + p — * He 4 — — • He 4 +- 3 . I I . j l i p - i . 8 * — 0 1 6* * He* o 1 6 * _ o 1 6 4 *" I I I . L i 7 ^ p — ~ B e 8 Be 8 +• "J. Reaction I has been investigated by Argo et a l f 3 It produces gamma rays of higher energy ( —- EOMeV) than those fcpom any other nuclear transformation and could prove useful for photodisintegration investigation, but H i s not yet available in t h i s laboratory. Reaction II i s the most p r o l i f i c source of gamma rays produced by proton bombardment. The'gamma ray energy i s inde-pendent of the incident pnoton energy which affects; only the energy of the p a r t i c l e s emitted i n the primary reaction. Many resonance l e v e l s f o r the emission of gamma rays have been observed^ the p r i n c i p a l ones occurring at the proton energies of 340, 669, 874, 900 and 1355 KeV. The gamma ra d i a t i o n has been shown to"consist of three components with energies of 6.13, 6.94 and 7.15 Mevf 5 the 340 KeV resonance the radiation consists almost e n t i r e l y of the 6.13 MeV component, the higher energy ones becoming more, important as the proton energy i s increased. With a thick CaF^ target and a bombarding voltage of 1400 KeV the results quoted by Chao et a l 6 4 indicate 'that the 59 Intensity r a t i o of 6.1b MeV to 6,94 plus 7.15 MeV gamma rays i s about 4.2 to 1. A thick target excitation curve, which was plotted fpom figures quoted inomhe same paper, i s shown i n Fig.I®. Reaction III provides l e s s intense but much higher energy gamma radiation than does Reaction IT. The only low energy resonance observed, occurs at a proton energy of 440 KeV. Walker and McDaniel 6 6 have shown that at resonance the spectrum consists of a shagp l i n e at 17.6 MeV and a l i n e about 2 MeV broad! at 14.8 MeV. Tangen 6 8 and others^ 9 have shown the width of the PZ.6 MeV l i n e i s 12 KeV. The width of the 14.8 MeV 8 l i n e i s accounted f o r by the fact that the Be nucleus i s l e f t i n a broad excited l e v e l which decays by the emission of two ft alpha p a r t i c l e s . The 17.6 MeV l i n e leaves the Be° nucleus i n i t s ground state. The energy of the gamma quanta i s observed to vary with the incident proton energy i n the manner expected for a reaction f o r which the radiation a r i s e s from the primary decay of the compound nucleus. The quantum energy when Be8 i s l e f t i n the ground state i s hjy = Q, +- 7/8 E p ) where Q, i s the energy available from the mass change and Ep i s the proton energy. From the ijiass v a l u e s 6 7 0=17.21 * 0.8 MeV, which i s i n good agreement with the value 17.2 - 6.2 MeV obtained by Walker and McDaniel from the po s i t i o n of the higher energy gamma l i n e , Although the energy of the gamma rays T 1 1 1 1 r i i I l l l I 4-00 44-0 4 8 0 5 2 0 P r o t o n E n e r g y ( K e v ) F I G . 10. T h i c k T a r g e t E x c i t a t i o n C u r v e s 60. increases with the incident proton energy the cross-section f o r gamma ray production i s low except at resonance, so that f o r "bombarding energies greater than 440 KeV on a thick target the 17.6 MeV gamma ray l i n e s t i l l exhibits a 12 KeV width. The r e l a t i v e i n t e n s i t y of the lower energy l i n e also increases with the proton energy. Interpolating between the results of Walker, and McDaniel f o r the in t e n s i t y r a t i o ~ 0.65 X17.6 at a bombarding energy of 550 KeV. The thick target e x c i t a t i o n curve obtained by Fowler et a l 6 ^ for this reaction i s reproduced i n Fig . K 0 . Devons and Hine'^ have investigated the angular d i s t r i b u t i o n of the gamma rays, from both r e a c t i o n II and III using thick targets. Their measurements indicate a small departure fa?pm isotropy f o r Reaction III and a.somewhat larger anisotropy f o r Reaction I I . These results have been corroborated by Lindsey and Barnes4-*- They a f f e c t our measurements only insofar as the choice of proper geometry obtains a s l i g h t l y l arger gamma f l u x , because the same arrangement was maintained throughout the experiments. 61. ft. COMPARISON METHOD FOR MEASURING REACTION CROSS-SECTION. The quantities Y, N and n defined on page 48 must be measured i n order to obtain an absolute measurement of the ieaetion cross-section. N, the gamma rajr f l u x , can be determined i n either of two ways. 1. Counting with a known e f f i c i e n c y , events associated i n a known way with the gamma rays. 2. Measuring the number of events produced by the gamma rays i n a reaction f o r which the cross-section i s known. Either method involves an accurate knowledge of the experimental geometry. The determination of n, the number of nuclei i n the sensitive volume of the i o n i z a t i o n chamber, i s d i f f i c u l t where the chamber does not possess simple geometry. Not only must the value of the sensitive volume be known; but the bounoary e f f e c t s due to particles, which lose only part of th e i r energy i n the sensitive volume must also be .calculated. A simpler method of obtaining the cross-section i s to compare the y i e l d with that from another reaction which has a known cross-section. Only comparative values, which are more e a s i l y obtained, aa?e: required fo r n and N with this method. The most accurately known photodisintegration cross-section i s that f o r the reaction H 2(l£, p )n. The best value for the cross-section, using the gamma rays obtained by proton bombardi-ment of f l u o r i n e , has been determined by Barnes, Stafford and 80 Wilkinson; They obtained. G^ l 5 = 21.2 - 1.2 x 10" 2 8 cml 2 62. for 6.IS MeV gamma rays. They also measured <J^ = 1.8 £ 1.5 x 10" 2 8 cms 2 f o r the 7.4 gamma rays from the proton bombardment of Be and (j- ^ 8.5 £ 1.2 x 10" 2 8 cms 2 for the 17.6 MeV gamma rays from the reaction L i 7 ( p, ^  )Be 8. Two other measurements fo r the 17.6 MeV l i t h i u m gamma rays are CT = 7.0 t 2.5 x l O - 2 8 , ^ by Waff l e r and Younis§l and (f * 7.2-t 1.5 x 10" 2I m S*' by Hough 8 2 The errors quoted by Barnes at a l and Hough are probable errors., that by waffler and Ybunis i s an estimated l i m i t of error. Treating these values as probable errors i n order to obtain a mean value for the cross-section, at 17.6 MeV w i l l , i f anything, attribute too small a weight to the result of Waffler and Younis. The mean value obtained i n this way 8 2 i s <J^t « 8.0 - 1.5 cJfcS^ O cms . For the comparison run i t was necessary to f i l l the chamber to a xnown pressure with deuterium and add s u f f i c i e n t 63. argon to give the photo-protons a range short enough that most of those produced i n the sensitive volume would lose t h e i r t o t a l energy there. Measurements were made using the gamma rays from a calcium f l u o r i d e target and from a l i t h i u m hydroxide target. The energy of the protons, produced, by the fluorine gamma rays was small enough that the undistinguishable back-ground due to other low energy events was considerable. An i d e n t i c a l measurement was made with the deuterium replaced by hydrogen i n order to obtain the extent of this background. The pressure with neon i n the ch&mber was such that the range of the expected alpha particles, would be about the same as that f o r the photo-protons from the deuterium. Thus the same boundary effeet.3was: obtained without the addition of another gas to increase the stopping power. 64. VTt EXP ERIMMTAL EQUIPMENT. 1. THE GRIDDED IONIZATION CHAMBER a) Construction of the Chamber. 60.62 The formulae given by Bunemann et a l have been used i n the construction of the chamber, the electrode structure of which i s shown i n F i g . I I . The chamber i s a s t e e l cylinder 6 inches i n diameter and 10 inches long with walls 1/4 inches thick. Endplates 1/4 inches/are bolted on. The electrode assembly i s fastened to one end with a brass bracket. Kovar seals through the endplates provide the necessary connections to the amplifier and power supplies. The c o l l e c t o r electrode i s 6 inches by 3 inches and surrounded by a guard ring with outside dimensions of 9 inches by 5 inches. The adjacent edges of the c o l l e c t o r and guard ring are l/32 inch apart on the surface facing the g r i d and are bevelled to a wider spacing at the other side. The small spacing minimizes, the e l e c t r i c f i e l d d i s t o r t i o n due to the charges c o l l e c t e d on the i n s u l a t i o n behind the c o l l e c t o r and the bevel reduces the capacitance between the c o l l e c t o r and earth, to which the ring i s connected. The g r i d consists of p a r a l l e l copel wires 0.005 inches i n diameter and 0.042 inches apart stretched across a brass frame 5 inches wide and 9 inches long. Lucite spacers, support the g r i d 0.6 inches away from the surface of the c o l l e c t o r . The negative electrode or cathode i s supported from the g r i d frame by four l u c i t e posts each 3 inches long. F IG . II. E l e c t r o d e S t r u c t u r e of. the G r i d d e d I o n i z a t i o n C h a m b e r . 65. It i s a rectangular "brass plate (5" x 9,T) curved with the concave surface toward the g r i d . The curvature was chosen, from measurements conducted i n an e l e c t r o l y t i c tank,- to provide the largest possible sensitive volume from which elections woald arr i v e on the c o l l e c t o r . b) Operating Characteristics of the Chamber The formulae given by Bunemann et al 6^»62 indicate that no electrons w i l l be c o l l e c t e d by the g r i d i f i t i s maintained at a negative voltage greater than one-third of the voltage d>n the cathode. It was found experimentally, by connecting the amplifier to the g r i d through a 0.01 microfarad capacitor and looking f o r pulses due to electaronSj that a s l i g h t l y higher voltage than they predicted i s necessary* This i s probably due to the curvature of the cathode. No electron pulses were v i s i b l e on the g r i d when i t was at 0.45 of the cathode pote n t i a l a t which pote n t i a l i t was operated during a l l further work i n th i s t h e s i s . The g r i d i n e f f i c i e n c y ; C7' , as de—fined by Bunnemann et a l i s 0.01. The charge induced on the c o l l e c t o r by a positive charge Q,, at a distance AQ, from the cathode and Q,G from the g r i d i s %_ = cT Ag x 0. Q.G / where AG i s the g r i d to cathode distance. This formula gives an estimate of the positige ion e f f e c t on the pulses. Assuming a. random d i s t r i b u t i o n of the p a r t i c l e tracks through-66 oat the chamber, 67 percent of the pulses should, r i s e to 96.7 percent or more of t h e i r f a l l value as soon as the electrons are collected. ISL other words, the standard deviation, i n the pulse si z e due to pos i t i v e ion e f f e c t w i l l be 10Q, /-, 9 6 • 7 •= 1.7 2 percent of the mean pulse s i z e . This performance; i s comparable with that of other gridded ionization, chambers. c) Calib r a t i o n Pulses. A source of polonium alpha p a r t i c l e s which gives about 20 pulses per minute i s located i n the centre of the cathode surface facing the g r i d i n the i o n i z a t i o n chamber. The energy of the polonium alpha p a r t i c l e i s known to be 5.298 MeV, so the pulses from these provide the c a l i b r a t i o n of the pulse height i n terms of p a r t i c l e energy. Frequent checks of the c a l i b r a t i o n were made throughout the experiment. The source was prepared by placing a drop of d i l u t e solution of radium D i n equilibrium with i t s products i n HC1 on a polished s i l v e r button f o r a few seconds. It was then washed o f f thoroughly with d i s t i l l e d water and c a r e f u l l y dried with a piece of clean absorbent tissue. d) Chamber Cathode Voltage. Some of the electrons may be c o l l e c t e d fromma track of i o n i z a t i o n with a very few volts on the cathode. However as the cathode voltage i s increased the number of electrons c o l l e c t e d does not increase monotonieally. This i s due to the fact that capture processes are dependent ontthe electron energy and therefore at certain f i e l d strengths more 67 electrons are l o s t due to heavy ion formation. ' With a pressure of two atmospheres of argon i n the chamber, the pulse siz e from the polonium alpha p a r t i c l e s r i s e s as the cathode voltage i s increased to about 800 v o l t s . The pulse size then remains constant u n t i l 1200 volts i s reached and thereafter starts to decrease. Iii neon at a pressure of s i x atmospheres the pulse size becomes constant with abcbut 700 volts on the cathode, but unfortunately, e l e c t r i c a l breakdown occurs within the chamber when more than 900 volts are applied to the cathode, so further investigation i s impossible. During the experiments the cathode voltage was adjusted so that the pulse size from the polonium alpha p a r t i c l e s was maximum, e) The Gas P u r i f i e r . A p u r i f i e r of the type used by Jentsche and P r a n k l 8 6 1 R 7 and described by Wilkinson 0 i s connected to the chamber by two copper tubes. It consists of a brass tube 6 inches ldmg and 1 inch i n diameter around which a heater c o i l i s wound. The temperature may be raised to approximately 300°C, the gas from the i o n i z a t i o n chamber then c i r c u l a t i n g by convection through the tubes. Turntiuigs of calcium metal are placed i n the tube to pu r i f y argon and neon. Galcium r e a d i l y forms s o l i d compounds with such gases as oxygen, hydrogen and nitrogen at temperatures above 250°C. These compounds are stable at room temperatures so i t i s only necessary to heat the p u r i f i e r for short periods whenever the pulse amplitude appears to have decreased a l i t t l e . 68 Since calcium combines with hydrogen r a p i d l y even at room temperature i t i s replaced w i t p l a t i n i z e d asbestos when hydrogen or deuterium are i n the io n i z a t i o n chamber. Platinum catalyses the combination of hydrogen and oxygen to form water. The water vapor i s removed fromfthe gas by a small tube of phosphorous pentoxide inside the io n i z a t i o n chamber. f) Pulses from the Walls and Insulators. The materials of which the io n i z a t i o n chamber i s constructed may give r i s e to energetic charged p a r t i c l e s which, i f they reach the sensitive volume, w i l l produce pulses. These p a r t i c l e s may bomemfrom the small amounts of radioactive material present i n most substances, or from photodisintegrations produced by the gamma rays which pass through the chamber. Carbon i n the form of aquadag i s very free of radioactive contam-inants and has a high threshold f o r photodisintegration. Erom the mass v a l u e s 6 7 ( i>} p ) and ( ~&>(K) processes i n C 1 2 are energ e t i c a l l y impossible for gamma energies below 15.8 MeV and 7.4 MeV respectively. A layer of aquadag was therefore painted on the inside of the ion i z a t i o n chamber to prevent most i o n i z i n g p a r t i c l e s emitted by the walls from reaching the sensitive volume. Another source of spurious pulses i s small discharges along the surfaces of insulators i n e l e c t r i c f i e l d s . The construction of the chamber i s such that currents along the surface of those insulators which are i n large e l e c t r i c f i e l d s cannot reach the c o l l e c t o r , so pulses, could only be prodaced 69 by induction. A l l the insulators were c a r e f u l l y cleaned and coated with General E l e c t r i c Drir-film 9987 to prevent surface leakage currents. 2. PULSE AMPLIFICATION; •In a well designed pulse amplifier most of the noise i s due to fluctuations i n the g r i d current and shot noise i n the tubes, and to thermal noise i n the re s i s t e r s 8 . 8 Special precautions aareetaken with the f i r s t stage to keep the noise voltages low. The best signal to noise r a t i o i s obtained when the gain of t h i s stage i s just enough to make i t s noise lihe main contribution to the t o t a l noise of the a m p l i f i e r . In order to simplify the shielding of the f i r s t stages and to keep the capacitance low where the pulse amplitude i s small, a pre-amplifier was constructed,which may be located at a distance from the main amplifier, a) The Fre-amplifier. The pre-amplifier, the c i r c u i t of which ,is shown in F i g . 12, i s mounted on the endplate of the chamber so that the connection i s as short and well shielded as possible; The high voltage connections f o r the g r i d and cathode of the chamber are also located i n a shielded compartment within the pre-amplifier chassis. Shielded leads connect the high voltage supply to these connections and are used f o r a l l pulse carrying cables. A l l chasses are connected together and to one earthing point. The heaters of the pre-amplifier are + 2 2 5 V. c, 0.001 30 K wirewoimd R l 4 3 OO -n_ c 2 10. II 5 K c 3 . 0 0 0 1 i \ R 7 1 K V, 6 A K 5 25. 1  Re 1 M c 5 0. 1 it R 9 8 2 O K V , V, V 6 A G 5 R. 6. 8 K 1 w 5 0 0 A / 5 K 1 w R„ 6 8 0 n. V 5 6 J 4 2 2 K 1 w 6 8 _n_ 2 2 K 2 w R |3 1 O M R e s i s t o r s £ wat t (w) un le s s o t h e r w i s e s p e c i f i e d K - l o 3 o h m s (Si.) M - 10^X1 V, V 2 V 3 - h e a t e r s s e r i e s c o n n e c t e d to 25 V. d. c. V s - 6.3 V. a. c. h t r . s u p p l y a t 3 5 0 V. cl.c. FIG. 12. T h e P r e - a m p l i f i e r 70. aeries connected and supplied from a d i r e c t current source. These precautions are designadtto eliminate, as f a r as possible, a l l noise from sources, other than the tubes and r e s i s t o r s . A 6AK5 was used as the input tube fd>r several reasons. As i s well known, when shot noise i s a major consideration, the noise from a triode connected tube; due to the absence of p a r t i t i o n noise, can be made lower than that obtained with pentode connections. A 6J4 triode was selected for the i n i t i a l t r i a l s but i t was found that, the available tubes of t h i s type produced large noise voltages due to t h e i r g r i d curren - The large input capacitance encountered with triodes lowers the voltage developed by a given charge on the c o l l e c t o r . Although t h i s does, not a f f e c t the signal to noise r a t i o of the stage under given gain conditions i t necessitates an increase i n gain which does reduce the signal to noise; r a t i o . On the other hand, the 6AK5 pentode has a low g r i d current and low input capacitance, and triode connecting i t (screen g r i d and plate connected, together) overcomes, the objection to a pentode. The suppressor g r i d of t h i s tube i s i n t e r n a l l y connected- to the cathode, but makes no contribution to the noise because no current flows to i t . The .grid current i s reduced by operating the plate at the low value of 40 volts., A choice was made from several 6AK5 tubes i n order to obtain the one which gave the lowest noise voltages. The 2500 ohm plate r e s i s t o r i s wirewound so that the noise due to the current 71. flowing i n i t i a small. The remainder of the pre-amplifier consists, of a three tube feedback loop, the output of which i s connected to the cathode follower. The current i n the cathode follower i s about 6 milliamperes. so that i t can produce a pulse r i s e of one volt / microsecond into a capacitance of 6000 micro-micro farads, which i s more than s u f f i c i e n t to Seed the cable l i n k i n g i t to the main ampl i f i e r . The measured gain of the pre-amplifier is.about 700, b) The Main Amplifier. The main amplifier i s a Northern E l e c t r i c R 17526 4 Type 1444 amplifier. Its maximum gain i s 10 and 33 decibels of attenuation are provided by a control with 3 decibel r' steps.. Bandwidth l i m i t a t i o n s are provided by two con-tybls with which the maximum risetime may be set at 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10 microseconds and the cl i p p i n g time at 20, 10, 5, 2.5, 1.0, 0.5, 0.2, 0.1 and 0.05 microseconds.V The maximum output pulse size i s 50 v o l t s . c) Power Supplies. Regulated voltage supplies, are used, f o r a l l the power requirements,. The supplies are fed from constant voltage sola transformers, a l l of which are loaded to between 85 and 100 percent of t h e i r rated output. Line "hash" f i l t e r a are connected between the sola transformers and the mains. The s u i t a b l i t y of this arrangement and the equipment used i s to be judged from the performance obtained. d) L i n e a r i t y of the Amplifiers.. For the me as. are men t of the l i n e a r i t y of the amplifiers the grid, of the io n i z a t i o n chamber was connected, to a tap on a potentiometer. Pulses of calibrated, amplitude were then fed to the potentiometer. Thus the c o l l e c t o r received pulses through the g r i d - c o l l e c t o r capacitance, of an amplitude which was a constant f r a c t i o n of that of the calibrated, pulses. The amplitude d i s t r i b u t i o n of the corres-ponding output pulses, from the amplifier was measured at several d i f f e r e n t input pulse sizes. The range of the pulse sizes covered corresponds approximately to that produced by p a r t i c l e s which lose from 1.5 to 12 MeV of energy i n the sensitive volume of the chamber. The same bandwidth and attenuation settings were used f o r this check and f o r the photodisintegration experiments. The results, of t h i s check are shown i n Fig.13. The errors shown are standard deviations due to the spread i n output pulse height produced by the amplifier noise. 3., PULSE AMPLITUDE ANALYSER (KICKSORTER). The pulses from the main amplifier are fed' to an 18 channel kicksorter designed by w'estcott and Hanna 8 9 and manufactured, by the Canadian Marconi Company. This instrument displays in. each channel the number of pulses with t h e i r amplitude i n a preset voltage i n t e r v a l . Thus curves are obtained showing the number of p a r t i c l e s as. a function of the energy they have l o s t i n the i o n i z a t i o n chamber. D 73 73 O x r> O -i ~\ fD CO •T5 O Q. =3' UD TJ P 10 • fD 2 1 CO 2 (tl < o c -¥ T3 C c 00 55 k 5"0 45 Q 40 3 "5.3S 8 • - V 1 r n 1 1 1 r / J L J L I 2 3 4 5 6 7 8 9 i n p u t p u l s e a m p l i t u d e l a r b i t r a r y un i ts ) F I G . 13. L i n e a r i t y of t h e A m p l i f i e r s . 73. The discriminate-ra which determine the voltage^ i n t e r v a l covered by each channel^ were adjusted before each ran by feeding c a l i b r a t i o n pulses of known and adjustable amplitude into the kic k s o r t e r . After each run the channel widths were checked, to be certa i n that none of them had changed by a s i g n i f i c a n t amount. With th i s method of setting e the channel widths any n o n - l i n e a r i t i e s of the electronic c i r c u i t s i n the kicksorter are compensated f o r , because the pulses to be analysed are fed i n at the same terminal as the c a l i b r a t i o n pulses. In those runs which are reported i n this thesis, the errors due to the d r i f t of the'discriminator settings between checks were always le s s than the s t a t i s t i c a l errors of the counted events. 4. PRELIMINARY TESTS OF THE EQUIPMENT, a) Noise Measurements and Bandwidth. T1jhe i o n i z a t i o n chamber was f i l l e d with argon at a pressure of 2 atmospheres and connected to the amplifier during these measurements. The noise voltage was observed v i s u a l l y with a c a l i b r a t e d oscilloscope;. It was found that an amplifi e r risetime of 2 microseconds and a cli p p i n g time of 20 microseconds gave the best signal to noise r a t i o and the peak to peak noise voltage, at the output of the main ampli f i e r was then 7 v o l t s with a/total amplifier gain of 6.3 x 10^. Referred to the input the peak noise was therefore 5.5 microvolts with this bandwidth. Applying the electrode voltages, to the 74. chamber did not increase the noise perceptibly. Frequent checks of the noise were made during the course of the experiment. A pulse generator was also available with which the gain of the amplif i e r system was checked frequently i Waen the c&fember was exposed to a f l u x of. gamma, rays the noise was observed to increase and a large number of "build up" pulses were observed. Equal ampl i f i e r c l i p p i n g and risetimes of 5 microseconds were, therefore used f o r the photodis-integration work to minimize the number of "build up" pulses and to obtain the best signal Jo noise r a t i o under these conditions. Also, the chamber was surrounded, by a sheet of lead 3/l6 inches thick i n an e f f o r t to s h i e l d i t from low energy gamma rays, b) Spurious 'Pulses. With argon i n the chamber pulses of p o l a r i t y corresponding to the c o l l e c t i o n of positive ions were observed. 90 Similar pulses were observed by Neilson i n a shallow i o n i z a t i o n chamber when i t was f i l l e d with argon. These positive pulses remained with the cathode and g r i d of the chamber earthed and also with pos i t i v e voltages: on these electrodes. .After c i r c u l a t i n g the gas over hot calcium metal for several hours the number of pulses decreased to about one per minute and t h e i r height to about twice the noise voltage. No convincing explanation of these pulses, has been found. The kicksorters w i l l not count p o s i t i v e pulses' unless they are large enough to produce negative overshoots i n the 75. amplifiers. After careful p u r i f i c a t i o n , of the gas no overshoots were v i s i b l e on the pos i t i v e pulses, and so few of them occurred that the chance of one being aoincident with a negative pulse and reducing i t s voltage was n e g l i g i b l e . To check the performanae of the equipment and watch f o r spurious pulses, the output of the amplifier was displayed on an oscilloscope throughout the experiments, c) Spread i n Pulse Size. The spread i n the size of the pulses, fromaa source of mono-energetic p a r t i c l e s may be attributed to f i v e causes: (1) thickness of the source (2) straggling of the i o n i z a t i o n (3) v a r i a t i o n of the pulse risetime (4.)variation, i n the induced p o s i t i v e ion e f f e c t s (5)amplifier noise , The f i r s t four of these causes aris e i n the chamber and do not affeiet pulses, fed to the amplifier from a sig n a l generator. That most'of the observed spread i s due to a m p l i f i e r noise may be seen by reference,to F i g s . 14, 15, 16 and 17. F i g . 14 shows the pulse height d i s t r i b u t i o n obtained from the signal generator using only the main, am p l i f i e r . This curve was obtained with a large s i g n a l to noise r a t i o so that the the most of/spread i n pulse height can be a t t r i b u t e d to the generator. Fo slow d r i f t of the pulse height due to i n s t a b i l i t y of the amplifier gain, was v i s i b l e . F i g . 15 shows the results, of a s i m i l a r test i n which both amplifiers 129 130 131 132 133 Pulse s i z e at k i ckso r t e r in volts. F I G . 14-. Pulses f r o m S i g n a l G e n e r a t o r th rough m a i n a m p l i f i e r o n l y . P u l s e s at l e a s t 5 0 x n o i s e . D u r a t i o n o f t e s t 10 m i n u t e s . 1 1 1 T Pulse s i z e at k i c k s o r t e r in volts F I G . 15. Pu l s e s f r o m S i g n a l G e n e r a t o r t h r o u g h b o t h a m p l i f i e r s . P u l s e s a t l e a s t 5 0 x n o i s e D u r a t i o n o f t e s t o n e h o u r l 1 1 ; 1 r • 48 150 (52 15+ /56 Pulse s i z e at k i c k s o r t e r in vo l t s F I G . 16. P u l s e s f r o m S i g n a l G e n e r a t o r t h r o u g h b o t h a m p l i f i e r s S i g n a l t o n o i s e ~ 14/I . 1_ 140 142 144 146 1 4 8 P u l s e s i z e a t k i c k s o r t e r in volts F I G . 17 . P u l s e s f r o m P o oc p a r t i c l e s i n t w o a t m o s p h e r e s o f A r g o n S i g n a l t o n o i s e — 15 / I. 76. were used. The signal generator was connected to the amplifiers through the g r i d - c o l l e c t o r capacitance of the io n i z a t i o n chamber i n order not to disturb the input capacitance of the pre-amplifier. To obtain the curve shown i n F i g . 16,the amplitude of the pulses was attenuated at the output of the sign a l generator u n t i l the signal to noise r a t i o was approximately the same as that observed with the pulses from the polonium alpha p a r t i c l e s . The pulse height d i s t r i b u t i o n from the polonium alpha p a r t i c l e s with two atmospheres of argon i n the chamber i s shown i n F i g . 17. The standard deviatibQn of the pulse height was estimated i n each case from a measurement of the width of the peak at 60.7 percent of i t s maximum. To f a c i l i t a t e comparison, the standard deviations are stated i n d i v i s i o n s , the sizes of which are chosen so that the pulse height at the maximum of each curve i s 100 d i v i s i o n s . TABLE IT Standard Deviations of Pulse Heights Figure Standard deviations, Si (divisions) 2 14 0.3 0.09 15 0.5 0.25 16 1.0 1.0 17 1.0 1.0 If we assume.that the signal generator i s responsible f o r the t o t a l spread i n pulse height of F i g . 1^ we may write 77 Sp.G-. 0.3 d i v i s i o n s for the standard deviation due to the pulse signal generator. The standard deviation f o r F i g . 16 i s made up of two parts, SJJ due to amplifier noise and Sp.Q.. We may therefore write <? 2_ Ct-i n\2 , n ^,.21 ( d i v i s i o n s ) 2 , . A Q-. . . . . 2 SN I i 1 , 0 ' " (0.3) J ' ~- 0.91(divisions) or S = 0.95 divisions,. N This i s observed to be equal to the standard deviation of F i g . 17 within the error of measurement. It corresponds to a h a l f -width f o r the polonium alpha peak of 130 KeV. With higher pressures i n the i o n i z a t i o n chamber the spread i n pulse heights was greater. This i s a well known ef f e c t and i s ascribed to impuritiea i n the gas capturing electrons before they are c o l l e c t e d . A. half width of 190 KeV was obtained f o r the polonium alpha peak with c a r e f u l l y p u r i f i e d argon at a pressure of 10 atmospheres. 5. PROTON. BEAM TARGETS. A two foot section of glass tube extended the accelerator tube below the Van de Graaff generator. A removable cup with a copper bottom plate l / l 6 inch thick and i n c l i n e d at 45 degrees to the d i r e c t i o n of the proton beam was attached to the end of the glass tube. The beam targets were deposited on the copper p l a t e . By closing a valve at 78. the top of the glass section., target changes were made without opening the main vacuum system. The glass section was re-evacuated before opening the valve again. Thick targets of calcium f l u o r i d e and of l i t h i u m hydroxide were, used for the production of the gamma rays. Calcium f l u o r i d e (CaFg) was obtained i n powder form and mixed with water from which i t was deposited by sedimentation. The water was then evaporated by gently warming the target cup which was held with the bottom plate l e v e l , A l a y e r of Ga Fr> about l / l 6 inches thick was obtained. Lithium hydroxide (LiOH) targets of the same thickness ware prepared by heating a small amount of LIOH i n a target cup u n t i l i t melted (450°C) and allowing i t to s o l i d i f y i n a layer dm the copper plate. The target cups were cleaned c a r e f u l l y to ensure that the Ca-F2 and LiOH never became mixed. 6. GAMMA RAY 'MONITORS; A geiger counter was used to eount the gamma rays from the Ga Fg targets. It was surrounded by a sheet of lead 3/l6 inches thick i n order to prevent low energy radiati o n r e g i s t e r i n g a large number of counts. Although the e f f i c i e n c y of the counter was not known, i t was assumed to remain constant during the experiments. The positions of the geiger counter and the io n i z a t i o n chamber were kept f i x e s with respect to the target on the Van de Graaff generator . Thus the number of geiger counts recorded was proportional to 79. the number of gamma rays that passed through the i o n i z a t i o n chamber. Measurements were made with a copper disc intercepting the proton beam just before i t reached the target to ensure that the background of geiger counts from sources other than the target was small. Only minor and necessary changes i n the arrange^ ment of nearby equipment were made during the experiments, so i t i s assumed that the errors due to gamma rays from the target being scattered into the geiger counter are the same f o r each measurement. With CaFg targets i t was v e r i f i e d , using a sodium iodide c r y s t a l s c i n t i l l a t i o n counter, that the most of the gamma rays had an energy of about 6 MeV. The amount of scattered, radiation, which would be of low energy, was therefore small. The geiger counter was also used to monitor the l i t h i u m gamma rays but i n t h i s case i t was found with the s c i n t i l l a t i o n counter that only a very small percentage of the geiger counts were due to the gamma rays of energy greater than 12 MeV. The s c i n t i l l a t i o n counter was available only during the l a t t e r part of the experiment so the measurement of the proton current which struck the target must be r e l i e d upon to determine the gamma f l u x . Due to fluctuations i n the current which were not noted, an estimated error of 20 percent may be introduced into these r e s u l t s . \ 80. VII. THE PHOTODISINTEGRATION MEASUREMENTS,. 1. CHAMBER. FILLING. a) Method The chamber was throughly evacuated before each f i l l i n g . The p u r i f i e r was heated to approximately 400° C u n t i l i t ceased, to outgaa perceptibly. A cold trap surrounded by l i q u i d nitrogen was connected between the pump and the chamber. The ultimate vacuum achieved each time was about 10 microns of Hg. Tests showed that a f t e r t h i s procedure the pressure i n the closed i o n i z a t i o n chamber rose to about double t h i s value in 8 hours. Each of the gases was passed into the i o n i z a t i o n chamber through a cold trap immersed i n l i q u i d nitrogen. This treatment removed any impurities which had a low vapor pressure at the temperature of b o i l i n g l i q u i d nitrogen. Argon i t s e l f i s l i q u i d at t h i s temperature so i t was the f i r s t liq.uef.ied. i n the" trap and then allowed to vaporize, as the trap was slowly warmedj u n t i l the desired pressure was attained i n the chamber. b) Pu r i t y of the Gasas. • Commercial welding argon and commercial hydrogen with stated p u r i t i e s better than 99.9 percent, and with exceediing l y small oxygen and water vapor content, were used. The neon was Matheson Company research grade gas f o r which they claim the highest possible p u r i t y . Its p u r i t y i s c e r t a i n l y better than 99.99 percent. The deuterium was obtained from 81. e l e c t r o l y s i s of. 99.8 percent heavy water. Both the hydrogen and deuterium, were passed fromathe storage bottles, through a palladium thimble into the chamber f i l l i n g system. A small amount of the deuterium gas was sealed into sample tubes a f t e r i t passed through the palladium thimble. These tubes were sent to Dr.H.G. Thode of McMaster University f o r mass spectrographs analysis of the contents. The seal on one of the tubes was defective and the sample became contaminated with a i r . The other sample yielded the following results'* Ho + D „ . - 98.23 percent " 2 H 20, DgO, HDO . 1.29 percent Higher mass, components©.38 percent Although an accurate determination of the hydrogen/deuteriurn r a t i o i s not possible with the instrument used to perform t h i s analysis, the lower l i m i t of the percentage of the atoms i n the sample which were deuterium was estimated to be 97.5 percent. 2. ROUTINE CHECKS OF THE EQUIPMENT DURING THE, RUNS. The noise voltage at the output of the amplifiers waa checked before and a f t e r each run from observations on an oscilloscope and at the same time the presence of any ^ I wish to express my appreciation to Mr.W.H. Fleming for carrying out t h i s a n a l y s i s . 82 spurious pulses was looked f o r . The size and d i s t r i b u t i o n of the pulses from the polonium alpha p a r t i c l e s was. checked frequently during the experiments to make sure that the energy resolution of the equipment did not change. The mean pulse amplitude from the polonium alpha p a r t i c l e s was determined within one percent by each measurement and i t s v a r i a t i o n between measurements with the same gas i n the chamber was. never greater than 3 percent. This s l i g h t change i n pulse amplitude may be p a r t l y due to changes i n the gain of the amplifiers, but was more l i k e l y caused by changes i n the amount of electronegative impurities i n the gas.affecting the number of electrons c o l l e c t e d . Yfaenever the pulse size, from the polonium alpha p a r t i c l e s dropped by more than 3 percent, the gas p u r i f i e r was heated. A l l readings have been corrected f o r the s l i g h t changg i n energy c a l i b r a t i o n due to the changes of less than 3 percent which occurred between runs, 3. DEUTERIUM MEASUREMENTS. The chamber was f i l l e d with deuterium to a pressure of 78.5 cms, of Hg at 70° F. The p l a t i n i z e d asbestos was heated 1 for 30 minutes, then allowed to cool. The pressure was measured again and found to be unchanged. Argon was then i n t r o -duced to bring the gauge pressure to 60 p?^s'.i. (5 atmospheres t o t a l pressure). a) Measurements with the Fluorine Gamma Rays 83. The photodisintegration peak obtained, with the deuterium i s shown i n F i g . 18. The kicksorter channels were, made narrow enough to spread, the dis i n t e g r a t i o n peak across several channels. Each curve represents the results of several runs i n each of which the kicksorter recorded only a part of the t o t a l energy region covered. S u f f i c i e n t overlap of these inte r v a l s was allowed to check the f i t of the results from each run with the ones covering adjacent i n t e r v a l s . The overlaps and the f i t s obtained are shown i n Fi g . 18 where two points on the same curve occur at the same energy. The results of each run have been normalized to the same number of geiger counts, that i s , to the same integrated gamma ray f l u x . Expressed i n terms of the p a r t i c l e energy, the standard deviation i n size of the pulses from the polonium alpha p a r t i c l e s was 75 KeV, determined with a probable error of 8 percent from the res u l t s of six measurements made between runs. The protons from the photodisintegration of deuterium by the fluorine gamma rays f a l l i n three d i f f e r e n t energy groups.. The deuterium binding energy i s 2.23 MeV,9-'- the gamma energies are 6.13, 6.94 and 7.15 MeV and neglecting the small momentum of the gamma quanta, the disi n t e g r a t i o n protons receive one half of the energy released by the reaction so they w i l l have energies, of 1.95, 2.35, and 2.46 MeV. Due to the momentum of the gamma quantum there w i l l be a variation, of the' proton energies, with the d i r e c t i o n of emission. The proton d i s t r i b u t i o n as a function of energy turns out to be a p a r a b o l a 9 2 1.0 2 . 0 3 .0 P u l s e E n e r g y ( M e V ) FIG. 18. Deuterium Disintegration and Hydrogen Background Curves with F 1 9 Gamma Rays 84. with width at half maximum of about 1E5 KeV for the upper energy groups and about 100 KeV f o r the lower energy group. Thus, i n theory, the upper two energy groups merge into one, and only two pi/ton groups are produced, separated by about 450 KeV. The r a t i o of the number of protons i n the lower energy peaks to that i n the higher, w i l l be equal to the r a t i o of the gamma ray i n t e n s i t i e s 16.13 * = 4.2, x7.0 mu l t i p l i e d by the r a t i o of the photodisintegration cross-section CT7.0 = 1.1 •• This l a t t e r r a t i o i s obtained from Bethe and F e i e r l ' s 9 3 theory." Therefore 82 percent of the protons should be i n the lower energy group. Clearly, a single peak was obtained i n F i g . 18 with a maximum at about 2.1. MeV and a width at half maximum of about 400 KeV. Failure, io resolvfe. the components ofjiithis i s a t tributed l a r g e l y to the boundary ef f e c t i n the i o n i z a t i o n chamber. The dimensions of the sensitive volume are approximately 15 cms. x 7 cms. x 7 cms. (the dimensions of the c o l l e c t o r plate x collector-cathode distance). The stopping power of the deuterium-argon mixture i s the same as that of 4.0 atmospheres of a i r , hence the photo-protons w i l l have ranges of 1.6 cms. q a and 2.0 cms. Since these ranges constitute a considerable 85. f r a c t i o n of the minimum dimension of the s e n s i t i v e volume of the chamber, many of the protons would l o s e only part of t h e i r energy i n the s e n s i t i v e volume. A much wider pulse d i s t r i b u t i o n would therefore he expected from the p h o t o d i s i n t e g -r a t i o n peak than from the polonium source from which the alpha p a r t i c l e ranges a l l l i e completely i n the s e n s i t i v e volume of the chamber. Due to t h i s spread i n the pulse s i z e the two photo-d i s i n t e g r a t i o n . peaks would merge i n t o one at some mean energy. The l a r g e number of counts i n the minimum between the noise and the peak at £,1 MeV i s a l s o l a r g e l y a t t r i b u t a b l e to the boundary e f f e c t . b) Measurements w i t h the L i t h i u m Gamma Rays. F i g . 19 shows the deuterium p h o t o d i s i n t e g r a t i o n peak obtained w i t h the gamma rays from a l i t h i u m hydroxide t a r g e t bombarded f o r 2 hours w i t h a 40 microampere i o n beam wi t h an energy of 550 KeV. The maximum of the peak produced by the 17.6 MeV gamma rays sh'oald l i e a t an energy of 17.6 - 2 . 2 3 MeV 7 > 6 8 M e Y 2 • and protons w i t h t h i s energy would have a range i n the chamber of 18 cms. The maximum of the peak shown i n Fig.-19 l i e s a t 7.6 MeV. These two f i g u r e s agree w i t h i n the experimental e r r o r of the determination. The 14.8 MeV gamma ray l i n e i s only 0.65 as intense as the 17.6 MeV l i n e and i s 2. MeV wide, so i t would produce a peak which would be d i f f i c u l t to 1 -I I .—I—-J "i r " i 1 r 40[ Z p o , -K 30h TJ c N (J) fD 20h 0 0 t-0 J I L 1 J J L J I L 6 . 0 7 . 0 8 . 0 P u l s e E n e r g y i n M e V F IG . 19. P h o t o d i s i n t e g r a t i o n o f D e u t e r i u m w i t h L i 7 G a m m a Rays . 86. distinguish from the background. Unfortunately ,the t a i l of the polonium alpha peak masks any peak due to the 14.8 MeV line.. Due to the gamma ray momentumj a parabolic d i s t r i b u t i o n w i l l again dear, with a width at half maximum of 1400 KeV for the peak due to the 17.6 MeV gamma rays. The peak obtained i n F&g. 19 has a width at half maximum of about 550 KeV only. This i s explained by the fact that the maximum .energy a/proton could lose. in. the sensitive volume of the chamber was 7.8 MeV. To produce a pulse corresponding to this amount of energy a proton would have to tra v e l along the longest diagonal of the sensitive volume which i s approximately 18 cms. long. Thus no proton pulses corresponding to an energy greater than 7.8 MeV could appear, but, due to the boundary e f f e c t , a large f r a c t i o n of the protons produced by the 17.6 MeV gamma - rays lose only part of th e i r energy i n the sensitive volume. Therefore due to the energy cutoff at 7.8 MeV, the peak i s made narrower and the background of pulses at energies below the peak, which may be contributed to by protons, i s larger than that above the peak. A peak at about 7.8 MeV might also arise from protons of higher energies i n the chamber. ' A l l of these protons would produce pulses corresponding to 7.8 MeV O B l e s s . I f the dimensions of the electrode structure were such that most of the higher energy protons l o s t about 7.8 MeV while crossing the sensitive volume then a peak would appear at t h i s energy. The uncertainties i n the dimensions of the sensitive volume are such that the energy corresponding to t h i s peak might be 87. that of the observed peak at 7.6 MeV. However, w i t h other gases i n the chamber, although d e f i n i t e changes have been observed i n the number of background pulses at energies, r corresponding to the maximum energy which could be l o s t by a proton c r o s s i n g the s e n s i t i v e volume, no peaks have been observed. Also i n a rect a n g u l a r chamber such as t h i s one, the energy which a proton l o s e s i n completely c r o s s i n g the s e n s i t i v e volume depends on the p o s i t i o n and d i r e c t i o n of i t s entry. Thus a f a i r l y continuous d i s t r i b u t i o n of the pulses from high energy protons might be expected s t a r t i n g , a t a maximum energy corresponding to the maximum dimension of the. s e n s i t i v e volume and c o n t i n u i n g down to zero energy. " A background of t h i s type, that suddenly decreased w i t h i n c r e a s i n g energy at about 8 MeV, was observed w i t h neon gas i n the chamber ( F i g . 23)). This corresponds to the maximum energy that could be l o s t by protons i n the neon gas i n the s e n s i t i v e volume. Thus, i t seems u n l i k e l y t h a t the peak i n F i g . 19 i s due to protons of higher energies, c) Background Pulses . ( i ) without gamma f l u x Th-e ' background of pulses from the i o n i z a t i o n chamber without an i o n beam on the Van de Graaff generator target, was n e g l i g i b l e compared to that w i t h a CaFg o r LiOH ta r g e t under bombardment. Runs were made w i t h the Ion beam s t r i k i n g a eoppe.r d i s c and i t was found that the background was the same, w i t h i n the s t a t i s t i c a l e r r o r s , as that w i t h the 88. Van de Graaff generator not running. This background could be accounted f o r p a r t l y by radio-active contaminating i n the chamber, but mestly by the t a i l s of the peak produced by the polonium alpha p a r t i c l e s . 19 ( i i ) with F gamma rays 1/ilhen.the ion beam was allowed to s t r i k e a GaFg target the background at low energies increased by a large amount. The energy of the fluorine gamma rays i s below the threshold f o r ( ~& , p ) reactions i n p r a c t i c a l l y a l l substances and the few (T},QC) reactions which are energet i c a l l y possible would produce very low energy alpha particles.. Only those alpha p a r t i c l e s produced i n ihe gas or i n the materials of • the electrode, structure could reach the sensitive volume. The cross-sections f o r these reactions are low enough that few.of the background pulses could be accounted f o r by them., The thresholds for ( ^ , n )reactions i n most substances also occur above the fluo r i n e gamma ray energies and the number of neutrons produces i n the target would be so small that the number of secondary reactions produced by them i n the i o n i z a t i o n chamber could not aeeountifor any appreciable f r a c t i o n of the background. The neutrons themselves, of course, would produee no io n i z a t i o n pulses. Most of the background must, therefore, be attr i b u t e d to electron "build up"' pulses and to photodisintegration pulses from tracks which lay p a r t l y outside the sensitive volume. To determine how much of the background was due to the l a t t e r e f f e c t , the chamber was f i l l e d with hydrogen o to a pressure of 78.2 cms. of Hg. at 70 G and argon was 89 introduced u n t i l a gauge pressure- of 60 p . s . l . was obtained. The same procedure- was followed as f o r the deuterium f i l l i n g . The background e f f e c t s with the two f i l l i n g s should "be the same except for the photodisintegration pulses. The background curve shown i n F i g . 18 i s that obtained from runs with the hydrogen-argon f i l l i n g i d e n t i c a l to those performed with the deuterium-argon f i l l i n g usihg a GaF„ target on the Van de Graaff generator. It i s seen that a large part of the background at energies just 'below the photodisintegration peak i s due to pulses which a r i s e only in the deuterium, therefore:, presumably due to photo-protons which l o s t only part of t h e i r energy i n the sensitive volume. 7 ( i i i ) with L i gamma rays The background i n F i g . 19 below p a r t i c l e energies of 7i8 MeV may be pa r t l y due to ( l l , p .) or ( ^ >, °C ) reactions i n the materials of the i o n i z a t i o n chamber but, as previously mentioned,is caused mostly by the edge of the polonium alpha peak and by the boundary ef f e c t on the photo-protons r from the deuterium. The small background at higher energies-must be due either to p a r t i c l e s which have a shorter range than protons of the same energy (e.g. alphas), or to two p a r t i c l e s contributing to time pulse. The chance of two separate reactions being coincident i s neg l i g i b l e and reactions i n which two p a r t i c l e s are emitted have small cross-sections or are energetically impossible with the l i t h i u m gamma rays, so the p r o b a b i l i t y of two p a r t i c l e s contributing to one pulse 90. i s remote. The reaction C12('3),3cX) i s an exception to this statement, hat the ccarbon on the inside of the i o n i z a t i o n chamber i s f a r enough from the sensitive volume that the r e s i d -ual range of the alpha p a r t i c l e s when they reached i t , ecu.ld not produce a high energy pulae. However, the most l i k e l y processes from which the high emergy pulses could arise are ( ^, oC ) reactions i n the electrode structure next to the sensitive volume and i n the gas, other than the deuterium. Also ( n, 06 ) reactions produced by high energy neutrons could contribute to this,background. A reaction which might produce enough high energy neutrons to give an appreciable number of ( n , « 0 processes i s L i (d,n)Be , since a small percentage of deuterons w i l l be present i n the ion beam. d) Number of Pulses i n the Photodisintegration Peaks, (i) with the F"*"9 gamma rays In order to avoid any d i f f i c u l t i e s concerning the inte r p r e t a t i o n of the boundary e f f e c t , the number of pulses corresponding to the symmetrical peak, part of which i s dashed i n F i g . 18, was estimated for comparison with the neon r e s u l t s . This estimate y i e l d s 2740 pulses i n the photo-di s i n t e g r a t i o n peak.. The corresponding number of events 5 recorded by the Geiger counter i s 6.13 x 10 . The reaction 80 cross-section f o r the 6.13 MeV gamma rays i s OT^„ - 21.2 -t 1.2 x 10" 2 8 cms? 6 .13 \ 91. while that for the 7.0 MeV gamma raya i s s l i g h t l y lower. Using the cross-section value for the 6.13 MeV gamma rays w i l l therefore give a value s l i g h t l y low for a cross-section obtained "by comparison. ( i i ) with the L i 7 gamma rays The estimated background curve i s indicated by a dashed l i n e i n F i g . 19. The number of counts i n the photodisintegration peak: due to the 17.6 MeV gamma rays i s 69, obtained with a t o t a l beam current of 45 microamperes om a LiOH target for 2 hours. This corresponds to a beam charge of 0.48 coulombs. The reaction cross-section f o r the 17.6 MeV gamma rays i s (page 62) '/rin , = 8.0 ± 1.5 x i o " 2 8 cms? ^ 17.6 ( i i i ) . estimates from the known cross-section values. An estimate of the y i e l d to be expected inthe deuterium, photodisintegration peaks may be made from the equation for which the quantities are. defined on page 48. The reaction cross-sections are known, the number of nuclei i n the sensitive volume may be calculated and the expected gamma flux/cm. through the chamber may be obtained from the thick target e x c i t a t i o n curves." of F i g . 9. Assume the beam 92. current i s 30 percent protons. The centre of the chamber was 32 cms. from the Ten de Graaff generator target daring a l l the runs. The calcium fluoride, target was bombarded for 3 hours with a 15 microampere 15 ion beam with an energy of 1500 KeV. The number of photodisintegrations expected i n the deuterium i s , therefore, 5.4 x 10^. S i m i l a r l y the number of photodisintegrations expected i n the deuterium with the 17.6 MeV l i t h i u m gamma rays i s 180. Within the errors of t h i s estimate and the los4s due to the boundary efi'eets i n the experimental measurements,the two show s a t i s f a c t o r y agreement. 4. NEON MEASUREMENT T S The chamber was f i l l e d with neon to a pressure of 473.5 cms Hg. No pulses were obtained from the chamber u n t i l . the calcium metal was heated i n the p u r i f i e r . The calcium was heated for 4 hours and allowed to cool before any readings were taken. The standard deviation of the pulse size from the polonium alpha p a r t i c l e s was then 85 KeV determined with a probable error of 10 percent from 6 measurements. Further heating of the calcium had no observable e f f e c t oh the pulses from the polonium alpha p a r t i c l e s . a) Measurements with the Fluorine Gamma Rays. F i g . 20 shows the resu l t s of four overlapping runs taken with a CaFg target on the Van de Graaff generator. A l l runs were normalized to the same number of Geiger counts and each run f i t s the adjacent ones within the s t a t i s t i c a l errors. 120-100-G Disintegration Points n Background Points o 80-o TJ c_ w 6 0 CO 40-2 0 -1.0 2.0 P u l s e E n e r g y in M e V 3.0 F I G . 2 0. D i s i n t e g r a t i o n o f N e o n w i t h F 1 9 G a m m a R a y s . 93. The pulses from a reaction suchaas Ne 2 0( oC ) 0 1 6 , where both produets may carry a charge, w i l l correspond to the t o t a l energy released(;page 54 and references 74, 75). The threshold energy for the reaction Ie 2^(A , oC )o l b i s 4.62 MeV, so the peaks should appear at 1.5, 2.3 and 2.5 MeV. Dae to the resolution obtained, with our equipment, the two higher energy groups would merge into one peak. The stopping power of 6.25. atmospherea of neon i s approximately the same ' 94 as that of 3.86 atmospheres of a i r so the combined ranges of the alpha p a r t i c l e and O-1-^  nucleus would be 0.9 cms. f o r 94 the lower energy peak and 1.2. cms. for the higher energy peak . Thus the boundary effect would be s l i g h t l y smaller than f o r the photo-protons obtained with these gamma rays i n the deut-erium-argon f i l l e d chamber. The background points shown i n Fi.g. 20 were obtained with the Van de Graaff generator not running. The points on the dis i n t e g r a t i o n curve are corrected to correspond to 5 6.13 x 10 counts i n the Geiger counter and the background points were obtained i n the same length of time as that required to record t h i s many counts during the photodisintegration runs. Obviously the two sets of points agree within the s t a t i s t i c a l errors. These counts must, therefore, be mostly due to radio-active contaminations i n the chamber. b) Measurements with the Lithium Gamma Rays. The results, of a 5 hour run with a t o t a l beam current/of 45 microamperes on a L i OH target on the Van de Graaff. 94. generator are shown i n F i g . 21. The maximum of the photo-disintegration peak due to the 17.6 MeV gamma rays should be at (17.6 - 4.62) MeV = 13.0 MeV; (4.62/is the threshold energy). The number of pulses i n the energy region above about 7.6 MeV, which i s the maximum energy a proton can lose i n the sensitive volume of the chamber i s seen to be much smaller than that at lower energies. The background above 8 MeV i s very small when the ion beam i s not directed on a target. Most of the background i n F i g . 21 above this energy must therefore be due to reactions caused by the beam on the LiOH target and i n which the energy i s released to particles: (e.g.alphas) which w i l l complete t h e i r range i i i the sensitive volume of the chamber. A possible reaction i s (1) L i 7 (d,n)Be 8 i n the targets, the neutrons reaching the chamber then causing (2) H e 2 0 (n, o c ) 0 1 7 With a deuteron energy of 550 KeV the maximum neutron energy 6 7 from reaction (1) i s 15.5 MeV. The energy release i n v 97 reaction (2) f o r neutrons of 15.5 MeV energy i s 14.7 "MeV. For an order of magnitude estimate of the y i e l d from this reaction chain, the deuteron content of the ion beam may be taken to bd> 0,02 percent, the natural i so topic abundance of deuterium i n hydrogen. The y i e l d from reaction (1) i s 7 9*5 A ; 2 x 10 neutrons per mi|rocoulomb of deuterons. The CM O LL C ^ O >> z a 0 c o d E E a o a en " co -5 5 N o . o f P u l s e s energy d i s t r i b u t i o n of these neutrons i s such that about 10 percent of them could produce reaction (2) with an energy 9 6 9 7 release of more than 12 MeV. * Although the cross-section of reaction (2) has not been measured f o r neutron energies -26 2 above 3.5 MeV, 2.0 x 10 cms does not seem l i k e an unreasonable estimate i n view of the non-resonant cross-section which has about this value below 3.5 MeV?7 Since the center 3 of the chamber was 22 ems. from the ion beam target, 3 x 10 2 rieutrona/em with energies between 13 and 15.5 MeV would be incident on the sensitive volume of the chamber during the time i n which 0.7 microcoulombs of charge struck the LiOH target. The number of neon nu c l e i i n the sensit i v e volume 20 of the chamber i s 10 so that the y i e l d from reaction (2) due to these neutrons would be x—/ 6 disintegrations, with energy releases between 12.2 and 14.7 MeV. The y i e l d from reaction (1) increases f o r lower neutron energies so that these two reactions could account f o r most of the background shown i n E i g . 21. Another reaction which i s energetically possible but which would produce smaller pulses i s Ne 2 2( i)(K)01&. The abundance of He 2 2 i n neon gas i s 9.7 percent. c) Upper Limit for the .Ne2^( 'i,o(-)0^ Reaction Cross-section. • " • • » 19 (l) with F gamma rays The energy of the ion beam during the runs, the results of which are shown i n F&g. 20, was 1.5 MeV so that the i n t e n s i t y r a t i o of the gamma rayslines. from F 1 9 would be 96 . 4.2. . i7.0 However, the r a t i o of the pe n e t r a b i l i t y of the Coulomb bar r i e r for the higher energy group to that for the lower -13 energy group, assuming a nuclear radius of 5 x 10 cms, i s CzL 28 P 6 1 2MeV .» so the ra t i o of the number of counts i n the peaks would be i G7.0 7 •& * C6.13 Peaks are not v i s i b l e i n F i g . 20, corresponding to either of the gamma l i n e s * Erom the deuterium results we found that f o r 6.13 x 10 geiger counts 2740 photodisintegrations were -27 2 obtained with a reaction cross-section of 2.1 x 10 cms. 20 The number of the He nuclei i n the sensitive volume was 2.7 times the number of deuterons (the r a t i o of the p a r t i a l pressures of the two gasea divided by two because each deuterium molecule contains, two deuterons). Thus i f the phoctodisintegration cross-section i n neon was 4 x 10"*30 cms we would expect to have registered (2740 x 3) 4 x 1 0 " o u , , . „ — l • Vn eoants. = 14 counts. 3.1 x 10-^7 i n the photodisintegrations peaks.. Twelve of these should l i e i n the higher energy peak. Since the bo.a.lfntd^ kr^ i; e f f e c t which contributed l a r g e l y to the width of the peak from the 19 deuterium d i s i n t e g r a t i o n with the F gamma rays i s smaller for the neon reaction, i t w i l l "be safe to assume the width 20 at half maximum of the peak from Me would, i f i t were vi s i b l e ^ be not more than 400 KeV which was .observed f o r the deuterium peak. The standard deviation for the points i n the region of 2.4 MeV i n FjLg. 20 where the peak i s expected, i s about 4 counts. From these considerations three points on the disi n t e g r a t i o n curve should each l i e one standard deviation above the background curve iff. the reaction cross-section was. 4. x 10~30 oms^« The p r o b a b i l i t y that such a peak would be hidden by the s t a t i s t i c a l errors i s the cube of the pr o b a b i l i t y that one point be lower than i t s true value by a standard deviation. Thus we: may say, i f no systematic errors exist, that the pro b a b i l i t y i s 97 percent that the 20 photodisintegration cross-section of Ne for the fluorihfl. gamma rays i s less than 4 x 10~30 cms^. The peak due to the 14.8 MeV gamma rays would be so low and broad as to obscure even i f the background at 10 MeV, where i t i s expected, wasjinueh smaller. The peak due to the 17.6 MeV gamma rays should, be narrower than the corresponding peak i n the deuterium because of the smaller boundary e f f e c t . 98. Also, "because the t o t a l energy release i s measured, i n the 20 p h o t o d i s i n t e g r a t i o n of Lie , there w i l l be no energy spread due to the gamma ray momentum such as.the re was i n the deuterium peaks. I t seems .reasonable to assume, t h e r e f o r e , thai; the peak: i n We^ would have a width at half-maximum of about 400 KeV, the same as that f o r the other peaks w i t h about the same boundary e f f e c t . 84 percent of the p h o t o d i s i n t e g r a t i o n pulses should then l i e i n the k i c k s o r t e r , channel w i t h bounaries of 12.75 and 1^.25 MeV. One count was recorded i n t h i s channel so the p r o b a b i l i t y i s 84 percent that the true number i n t h i s channel ( i . e . i n 84 percent of the peak) i s l e s s than two counts. Thus the p r o b a b i l i t y i s 70 percent t h a t the whole p h o t o d i s i n t e g r a t i o n peaK contains no more than two counts. Two counts correspond to a p h o t o d i s i n t e g r a t i o n c r o s s - s e c t i o n of IL x °- 4 8 x (°-9) 78.5 x 2 x 8.0 x 1 0 ~ 2 8 ems2= 4.8 x lO~ 3 0ems 69 • 0,70 4-73.5 where 69 i s the number of counts i n the peak obtained f o r the p h o t o d i s i n t e g r a t i o n of deuterium by 17.6MeV gamma rays, 0.48 0.70 i s the r a t i o o f the t o t a l charge c o l l e c t e d on the tar g e t f o r the deuterium and the neon runs, ( i t i s assumed that the number of gamma rays produced i s p r o p o r t i o n a l to the charge c o l l e c t e d } 99. (0.9) 78.5.x 2 i s the ra t i o of the number of nu c l e i i n ' 473.5 ' the sensitive volume of the i o n i z a t i o n chamber for the deuterium and He 2^ runs, 20 (neon gas i s 90 percent Ne ) -28 2 8.0 x 10 Cms i s the cross-section f o r the photodisinte-gration of deuterium by 17.6 MeV gamma rays. The fluctuations i n the beam current and other experimental errors together with the probable error of about 20 percent i n the value of the cross-sectionjfor the photodisintegration of deuterium indicate a probable error of about 30 percent i n the value 4.8 x lO-^O cms 2. Thus the p r o b a b i l i t y i s 94 percent that 2 counts corresrp-pnd to a neon photodisintegration cross-section of l e s s than 7.0 x 10" cms 2. We conclude, therefore, unless systematic errors e x i s t i n these measurements, that the p r o b a b i l i t y i s (70 x 94)=65 percent that the cross-20 section f o r the photodisintegration of lie by 17.6 MeV gamma rays i s less than 7 x 10~^0 cms 2. 100. ¥111. DISCUSSION OF THE RESULTS. 1. EOSSIBLE, SYSTEMATIC ERRORS. Careful consideration has been given to the sources of systematic errors which could oacur i n these experiments. Ytfhere possible an experimental • measurement to determine t h e i r magnitude has been carr i e d out (e.g. the background measurements i n F i g . 18 with the hydrogen-argon chamber). I f an estimate could be made of possible errors f o r which measurements were not available, this also has been done (e.g. background curve i n F i g . 19). Where estimates were necessary, they have been made i n such a way as to make the estimated l i m i t of the Ne 2^ photodisintegration cross-sections as large as the i n t e r p r e t a t i o n of the res u l t s w i l l allow. The p o s s i b i l i t y that the peak obtained f o r the photodisintegration of deuterium by the 17.6 MeV l i t h i u m gamma rays i s partly due to high energy protons has been discussed on page 86. Since no very r e l i a b l e measurement or estimate of thi s e f f e c t i s avail a b l e , i t probably constitutes the most l i k e l y source of error i n the r e s u l t s . I t does not, 1 Q of course, a f f e c t jihe results, obtained with the F gamma rays. 2. N e 2 0 ( r> , oC ) 0 1 7 EXPERIMENTAL CHECK, In order to be certain that the equipment was s t i l l performing with the neon gas i n the chamber a search was made for the ( h,OC ) reaction i n Ne2<^ which has recently 101. been reported.. ' A high, tension set i s availa b l e i n the laboratory^ 0 with which high energy neutrons can be produced by the reaction of H (d,n)He * Although no quantitative measure of the neutron f l u x through the chamber i s available with which to make an estimate of the expected-yield, the alpha particle- pulses from the reaction Ne^( n, oC have been observed. The threshold for.the reaction i s about 0,8 MeV 9 7 so that the alpha peak corresponding to the ground state should occur a t 1.95 MeV, for neutrons of 2.75 MeV energy, which i s approximately the energy cfif the neutrons to which the chamber was exposed. The peak obtained, shown i n Fig.22, occurs at an energy i n agreement with these f i g u r e s . Further studies, of t h i s reaction have indicated that other peaks may exist and i t i s proposed to continue the experiments using neutrons of greater energy which may be 2 obtained from the H (d,n) reaction using a deuteron beam from the Van de Graaff generator. Measurements of the energies of 20 17 the alpha peaks produced i n the Ne ( n, oC )Q reaction could prove useful as a method of determining the energy of greater mono-energetic neutrons with energies/than 0.8 MeV. 2. CONCLUSIONS. The most in t e r e s t i n g feature of the res u l t s i s the smallness of the photodisintegration cross-section for Ne 2 G n ft compared to that quoted by WaffIer and Younis f o r 0 . As was: pointed out (page 44) the Ne^^x cross-section i s not 102. expected to be smaller on the basis of the alpha p a r t i c l e model unless i s smaller f o r t h i s reaction. Therefore, a smaller cross-section f o r the Me20 reaction means on this basis, that the 20 s p a t i a l d i s t r i b u t i o n of the nucleons i n the He nucleus 16 i s more d i f f e r e n t from that i n the 0 nucleus than the d i s t r i b u t i o n i n the 0"^ nucleus i s from that i n the C"*"2 20 nucleus. I f the maximum cross-section f o r the He reaction occurs at a gamma ray energy greatly d i f f e r e n t than 17.6 He?, our .results might be accounted f o r . However then the 20 parameter 'b' (page 44) f o r the He model would have to be 16 much d i f f e r e n t than f o r the 0> model. Although neither effect by i t s e l f i s l i k e l y to account for the observed difference i n the cross-sections, a difference i n the wavefunction i n t e g r a l and i n 'bf might together explain the r e s u l t s . Before drawing further conclusions i t would be useful to measure the 0 1 & and He21""* cross-sections with the same equipment. 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