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UBC Theses and Dissertations

I. The suppression of Compton electrons in some photoelectron spectra. II. the double Beta decay of Sn124 Pearce, Robert Michael 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 F . 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 R O B E R T M I C H A E L P E A R C E B.Sc. (McGi l l ) 1947 M . A . (British Columbia) 1949 T H U R S D A Y , J U N E 12th, 1952, at 2:30 P.M. IN R O O M 301, PHYSICS BUILDING of C O M M I T T E E I N C H A R G E : Dean W. H . Gage, Chairman Professor F. A. Kaempffer Professor K. C. Mann Professor W. Opechowski Professor J. B. Warren Professor H . Adaskin Professor D. Derry Professor B. Savery Professor F. Noakes G R A D U A T E STUDIES Field of Study: Physics Nuclear Physics—Professor K. C. Mann Quantum Mechanics—Professor G. M . Volkoff Special Relativity—Professor W. Opechowski General Relativity—Professor M . Wyman Electronics—Professor A. van der Ziel Chemical Physics—Professor A. j . Dekker Quantum Theory of Radiation—Professor F. A. Kaempffer Spectroscopy—Professor A. M . Crooker Cosmic Rays—Professor J . B. Warren Theory of Measurements—Professor A. M . Crooker Electromagnetic Theory—Professor G. L . Pickard Other Studies: Differential Equations—Professor T . E. Hull Group Theory—Professor D. C. Murdoch Topics in Applied Mathematics—Professor E . Leimanis T H E S I S I T H E SUPPRESSION OF COMPTON ELECTRONS IN SOME PHOTOELECTRON SPECTRA A new method has been used to suppress the undesirable Compton electrons ordinarily present in photoelectron specta. This is accomplished by electronic cancellation of the individual Compton electron counts. The new method has been used with a thin-lens type of spectrometer, and has made possible the detection of new gamma rays in Ra (B C), Tai82, and Snl24. No new gamma rays were found in C 0 6 O . II T H E DOUBLE BETA DECAY OF Snl24 A research has been made for double beta decay in Snl24 using an energy dependent coincidence technique particularly suited to the detection of double beta events according to Majorana's neutrino theory. No events attributable to double beta decay were found. From this result, an upper limit of 0.3x1017 years was set on the half-life for the process. PUBLISHED PAPERS Note on the Change in Average Particle Mass During the Aging of Ammoniun Chloride Smokes. G. O. Langstroth, T . Gillespie, R. M . Pearce, Chem. Rev., May (1949). Double Beta Decay of Sni24. R. M . Pearce, E . K. Darby, Phys. Rev., June 15 (1952). I THE SUPPRESSION OF COMPTON ELECTRONS IN SOME PHOTOELECTRON SPECTRA I I THE DOUBLE BETA DECAY OF Snl24 by ROBERT MICHAEL PEARCE A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF. THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n PHYSICS We accept t h i s t h e s i s as conforming t o the standard r e q u i r e d from candidates f o r the degree of DOCTOR OF PHILOSOPHY. Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA MAY, 1952 ABSTRACT PART 1 A new method has been used to suppress the undesirable Compton electrons ordinarily present in photoelectron spectra. As much as 90% of the Compton electron intensity was removed. This was accomplished by electronic cancellation of the individual Compton electrons. The method has been used with a thin lens type of spectrometer and has made possible the detec-tion of new gamma rays at .391, .#57 and 1.00 Mev. in Ra(B +- C), at 1.01 Mev. in Ta l 8 2, and at .472 and .843 Mev. in Sbl24. No.new gamma rays were observed from Co6G. ABSTRACT  PART 2 A search for double beta decay in Sn^2^ has been made using a coincidence technique particularly suited to double beta decay under the Majorana form of neutrino theory. Negative results were obtained and a lower limit of 0.3 - 0.7 x lO 1^ years has been set on the half-life of the process. •i TABLE OF CONTENTS PART 1 THE SUPPRESSION OF BACKGROUND IN  SOME PHOTOELECTRON SPECTRA I INTRODUCTION I I EXPERIMENTAL PROCEDURE A . The D e t e c t o r C o u n t e r B. The S o u r c e C o u n t e r C . The A n t i c o i n c i d e n c e C i r c u i t s D. The S u p p r e s s i o n o f Compton E l e c t r o n Counts as a F u n c t i o n o f E n e r g y E . L o s s o f Photopeak I n t e n s i t y I I I RESULTS OF THE GAMMA RAY STUDIES A . Ra ( B + C) B. T a 1 8 2 C. C o 6 0 D,. S b 1 2 ^ IV SOME ASSOCIATED STUDIES IN T a 1 8 2 A . The B e t a Spectrum o f T a 1 ^ 2 B. Gamma-Gamma C o i n c i d e n c e s i n Ta- 1 -^ 2 I V CONCLUSIONS APPENDICES 1. A n a l y s i s of t h e Improved S t a t i s t i c a l A c c u r a c y . 2. I n t e r n a l R e f l e c t i o n i n L u c i t e . PART 2 ON THE DOUBLE BETA DECAY OF Snl24 INTRODUCTION SOME PREVIOUS WORK A . The T r i p l e t 5 0 S n 1 2 4 - ^ S b 1 2 ^ B. The T r i p l e t 4 6 P a 1 1 0 ~ ^ A g 1 1 0 C . The T r i p l e t 5 2 T e ^ ~ - 5 3 l l 3 ° D. The T r i p l e t 9 2 U 2 3 ^ - 9 3 N p 2 3 g EXPERIMENTAL PROCEDURE A . The P r i n c i p l e o f t h e E x p e r i m e n t B. The E x p e r i m e n t a l Arrangement C . C a l i b r a t i o n o f t h e K i c k s o r t e r D. The Optimum Source T h i c k n e s s E . O b t a i n i n g t h e D a t a F . Shape o f t h e Background S p e c t r u m G. C o m p a r i s o n w i t h P r e v i o u s Work RESULTS AND CONCLUSIONS TABLE OF ILLUSTRATIONS FIG-. 1 SCHEMATIC DIAGRAM OF THIN LENS SPECTROMETER. FIG. 2 . GENERAL SHAPE OF A. PHOTO ELECTRON SPECTRUM. FIG. 3 SCHEMATIC DIAGRAM OF CONVENTIONAL SOURCE. - HOLDER. FIG. 4 INTEGRAL BIAS CURVESTAKEN."AT VARIOUS ELECTRON ENERGIES. FIG. 5 SCHEMATIC DIAGRAM OF SOURCE - HOLDER USED WITH COMPTON.. SUPPRESSION-. FIG. 6 SOURCE - HOLDER & ANTICOINCIDENCE PHOTOMULTIPLIERi FIG. 7 BLOCK DIAGRAM OF.ANTICOINCIDENCE ARRANGEMENT. FIG. 8 DETECTOR.HEAD AMPLIFIER. FIG.: 9 SOURCE HEAD AMPLIFIER.. FIG. 10 FED- BACK AMPLIFIER USED IN THE DETECTOR CHANNEL. FIG. 11 DISCRIMINATOR.:AND DIFFERENCE. AMPLIFIER. FIG. 12 SOURCE COUNTER AMPLIFIER. FIG. 13 CANCELLATION AS A..FUNCTION OF ENERBY FOR Ta & Ra-. FIG. 14. PHOTOELECTRON SPECTRA FROM, GAMMA RAYS Ra(B+C). FIG. 15 PHOTOELECTRON .SPECTRA. FROM GAMMA.RAYS.Ta182. FIG. 16 PHOTOELECTRON SPECTRA FROM GAMMA RAYS Co 6 0. FIG. 17 PHOTOELECTRON SPECTRA FROM GAMMA RAYS S b 1 2 4 . FIG. 18 KURIE PLOT OF THE T a 1 8 2 BETA.-GROUP. FIG..19 COINCIDENCE MIXER. FIG. 20 THE MASSES OF AN ISOBARIC TRIPLET. FIG. 21 BLOCK-DIAGRAM OF COINCIDENCE "ARRANGEMENT, FIG. 22 COINCIDENCE SPECTRA OBTAINED ON KICKSORTER. FIG. 23 DIFFERENCE BETWEEN COINCIDENCE SPECTRA SHOWN IN FIG. 22-.. ACKNOWLEDGEMENTS The work d e s c r i b e d i n t h i s t h e s i s was s u p p o r t e d by a G r a n t - i n - A i d - o f - R e s e a r c h a l l o t t e d t o D r . K . C . Mann by t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada. I am i n d e b t e d t o D r . Mann f o r i n v a l u a b l e s u g g e s t i o n s and d i s c u s s i o n s d u r i n g t h e c o u r s e o f t h e r e s e a r c h . N a t i o n a l R e s e a r c h C o u n c i l awards t o t h e a u t h o r made p o s s i b l e t h e c o n t i n u e d e f f o r t f r o m 1949 t o 1952. The work d e s c r i b e d i n P a r t I I o f t h e t h e s i s was done i n c o l l a b o r a t i o n w i t h D r . E . K . D a r b y . B A R T 1 THE SUPPRESSION OF BACKGROUND IN SOME PHOTOELECTRON SPECTRA I INTRODUCTION I t i s t o be e x p e c t e d t h a t a knowledge o f t h e energy-l e v e l s o f r a d i o a c t i v e n u c l e i w i l l p r o v i d e t h e means o f t e s t i n g t h e o r i e s o f n u c l e a r f o r c e s , s i n c e any s u c c e s s f u l t h e o r y must p r e d i c t e n e r g y l e v e l s e q u e n c e s . F o r t h i s r e a s o n , t h e " d e c a y s c h e m e s " o f r a d i o a c t i v e n u c l e i have been t h e s u b j e c t o f c o n s i d e r a b l e 1 2 r e s e a r c h . ' By " d e c a y scheme" i s meant a c o m p l e t e d e t e r m i n a t i o n o f t h e n u c l e a r l e v e l s as t o e n e r g y , a n g u l a r momentum ( s p i n ) , and p a r i t y t o g e t h e r w i t h t r a n s i t i o n s between t h e s e l e v e l s . I n most c a s e s a knowledge o f t h e e n e r g y l e v e l s can come o n l y f r o m a s t u d y o f t h e r a d i a t i o n s f r o m t h e n u c l e u s , w h i c h r e s u l t f r o m t r a n s i t i o n s from one s t a t e t o a n o t h e r . The decay o f a n u c l e u s b y p r i m a r y e l e c t r o n e m i s s i o n t o t h e d a u g h t e r n u c l e u s i s u s u a l l y f o l l o w e d b y t h e d e - e x c i t a t i o n o f t h e d a u g h t e r n u c l e u s by gamma r a y s o r i n t e r n a l c o n v e r s i o n e l e c t r o n s . I t i s customary i n t h e s e i n v e s t i g a t i o n s t o measure t h e i n t e n s i t i e s and e n e r g i e s o f a l l d e t e c t a b l e gamma r a y s . T h i s may make p o s s i b l e t h e a s s i g n m e n t o f a s e l f - c o n s i s t e n t decay scheme. However i t may be p o s s i b l e t o p o s t u l a t e more t h a n one sequence o f e n e r g y l e v e l s w h i c h w o u l d l e a d t o gamma-ray t r a n s i t i o n s c o n s i s t e n t w i t h t h e e x p e r i m e n t a l e v i d e n c e . Because o f e x p e r i m e n t a l d i f f i c u l t i e s , i t i s h i g h l y p r o b a b l e t h a t A l o w i n t e n s i t y gamma r a y s w h i c h a r e a c t u a l l y p r e s e n t have e s c a p e d d e t e c t i o n . A knowledge o f t h e s e would u n d o u b t e d l y make e a s i e r t h e c h o i c e o f a u n i q u e decay scheme*. F o r t h i s r e a s o n , an a p p a r a t u s has been d e s i g n e d and c o n s t r u c t e d i n t h i s l a b o r a t o r y t o i n c r e a s e t h e p r o b a b i l i t y o f d e t e c -t i o n o f low i n t e n s i t y gamma-rays. Evaluated tube Baffl Defector H fa 1 s ource Current in coil focusces one energy. SCHEMATIC D/AfRAM Of THIN LEMS SPECTROMETER <*7 c Conventional bhofoelectron Sjpecirmn. Spectrum ivitK Pkoto-peaks Electron Momentum Fiq. 2 GENERAL SHAPE OF A PHOTOELBCTRON SPECTRUM. THE SHAPE DESiRZ® IS SHcWAf BELOW. 2 Gamma-ray energies are usually measured by the photo-e l e c t r i c technique, whereby photoelectrons are ejected from t h i n lamina by the gamma-rays. Such a lamina i s c a l l e d a "radiator" and usually consists of some high atomic number material, since the p r o b a b i l i t y of photo-electron emission increases r a p i d l y with the atomic number Z. In t h i s method, a beta-ray spectrometer i s used to study the photoelectrons. The thin-lens type of spectrometer used i n t h i s work has been described i n d e t a i l e l s e w h e r e . ^ I t s operation depends upon the focussing of electrons by the magnetic f i e l d due to a current i n a large iron-free c o i l . The electrons pass down an evacuated tube which l i e s on the axis of the c o i l and concentric with i t (see F i g . 1). B a f f l e s are arranged within the tube to define a path f o r the electrons. The momentum of the electrons so focussed at the detector i s determined by the c o i l current. In t h i s way the i n t e n s i t y of focussed electrons i s studied as a function of c o i l current or momentum and a spectrum i s obtained. I f photoelectrons are focussed at an energy E by the spectrometer, the energy of the gamma ray i s given by hx> = E k + E where h i s Planck 1s constant, \) i s the gamma-ray frequency, and E^ i s the K-shell binding energy of the radiator. In favourable cases photoelectrons from the L - s h e l l of the ra d i a t o r may be detected, although the cross-section f o r t h i s process i s smaller. The L-photopeaks are found at s l i g h t l y higher energies than the corresponding K-photopeaks because of the smaller L - s h e l l binding energy. SOURCE COMPTON TRAJECTORY PHOTOELECTRON TRAJECTORY RADIATOR ABSORBER FOR PRIMARY BETAS FIG. 3 SCHEMATIC DIAGRAM OF CONVENTIONAL SOURCE - HOLDER 3 F o r an e l e c t r o n d e s c r i b i n g a c i r c u l a r p a t h o f r a d i u s jp , t h e momentum i n a m a g n e t i c f i e l d H - ' i s g i v e n by p = e Ef . c ' •' Thus p i s d i r e c t l y p r o p o r t i o n a l t o H f i n g a u s s - c m : and t h e r e f o r e t o H , s i n c e i s a c o n s t a n t o f t h e s p e c t r o m e t e r . F u r t h e r m o r e H v a r i e s d i r e c t l y w i t h t h e magnet c u r r e n t , I , s i n c e no i r o n i s p r e s e n t , w i t h t h e r e s u l t t h a t t h e momentum v a r i e s l i n e a r l y w i t h t h e magnet c u r r e n t . Thus s i n g l e p o i n t c a l i b r a t i o n o f t h e momentum i n t e r m s o f c o i l c u r r e n t i s p o s s i b l e and some w e l l known l i n e i s measured t o c a l i b r a t e t h e i n s t r u m e n t . C o n v e r s i o n from momentum p t o t h e e n e r g y , , may be e a s i l y shown t o f o l l o w f r o m ' f . p - H f = i f Y E e <Ee + i - 0 2 * >• where Hy" i s i n g a u s s cm. , and E 0 i s i n Mev. I n a c t u a l p r a c t i c e , s u c h a c o n v e r s i o n i s made e a s i e r by p u b l i s h e d t a b l e s ^ w h i c h g i v e p , E , and Hj 3 f o r a l a r g e s p r e a d o f . e n e r g i e s . I n t h e c o n v e n t i o n a l p h o t o e l e c t r i c t e c h n i q u e 3 , t h e r a d i a t o r i s p l a c e d c l o s e t o t h e s o u r c e o f gamma r a y s . The p r i m a r y b e t a p a r t i c l e s a r e f o u n d t o be much more i n t e n s e t h a n t h e p h o t o -e l e c t r o n s p r o d u c e d i n t h e t h i n r a d i a t o r and must be removed t o make t h e d e t e c t i o n o f p h o t o e l e c t r o n s f e a s i b l e . The p r i m a r i e s a r e t h e r e f o r e a b s o r b e d i n a low a t o m i c number (Z ) m a t e r i a l p l a c e d between s o u r c e and r a d i a t o r , (see F i g . 3). A low Z a b s o r b e r i s chosen b e c a u s e t h e p h o t o e l e c t r i c c r o s s - s e c t i o n v a r i e s as Z , and o n l y t h e r a d i a t o r i s d e s i r a b l e as a p h o t o e l e c t r o n s o u r c e . 4 U n f o r t u n a t e l y , many gamma-rays s u f f e r Compton e n c o u n -t e r s i n t h i s a b s o r b e r , and some o f t h e r e s u l t i n g Compton e l e c t r o n s a r e f o c u s s e d and r e c o r d e d . The d i s t r i b u t i o n f r o m a s i n g l e gamma-r a y a p p e a r s as a smooth,- -almost s y m m e t r i c a l , s p e c t r u m f r o m z e r o e n e r g y t o a maximum w h i c h a p p r o a c h e s bv> - 0.255 Mev. f o r h,v> » 0 . 5 1 Mev. The Compton e l e c t r o n d i s t r i b u t i o n i s much l e s s i n t e n s e t h a n t h e p r i m a r y b e t a - r a y s p e c t r u m , b u t i s o f t h e same o r d e r o f magnitude as t h e p h o t o e l e c t r o n l i n e i n t e n s i t y . The p h o t o e l e c t r o n s a p p e a r i n t h e s p e c t r u m as peaks on t h e smooth Compton e l e c t r o n d i s t r i b u t i o n as shown i n t h e upper c u r v e o f F i g . 2. -Because o f t h i s b a c k g r o u n d , a> peak and i t s immediate v i c i n i t y has t o be more c a r e f u l l y measured t h a n i f no b a c k g r o u n d were p r e s e n t . D e t e c t i o n o f a peak becomes p o s s i b l e o n l y when t h e Compton d i s t r i b u t i o n i n t e n s i t y has been measured w i t h an u n c e r t a i n t y w h i c h i s l e s s t h a n t h e i n t e n s i t y o f t h e peak. S i n c e t h e s p e c t r u m i s o b t a i n e d b y t h e r e c o r d i n g o f random e v e n t s , t h e u n c e r t a i n t y o f t h e i n t e n s i t y can be made s m a l l o n l y by r e c o r d i n g a l a r g e number o f e v e n t s . Such a p r o c e s s i s t i m e c o n s u m i n g , and i n s t r u m e n t a l i n s t a b -i l i t y imposes a l i m i t on t h e a c c u r a c y p o s s i b l e . In t h e case o f s h o r t l i v e d r a d i a t i o n s , l o n g c o u n t i n g t i m e s a r e n o t a v a i l a b l e . To improve t h i s s i t u a t i o n , an a p p a r a t u s d e s i g n e d t o s u p p r e s s t h i s Compton b a c k g r o u n d has been c o n s t r u c t e d f o r use i n a t h i n - l e n s s p e c t r o m e t e r . I t c o n s i s t s , i n p a r t ^ o f a c o u n t e r a t t h e s o u r c e end o f t h e s p e c t r o m e t e r w h i c h d e t e c t s Compton e l e c t r o n s . T h i s c o u n t e r i s a r r a n g e d i n a n t i c o i n c i d e n c e w i t h t h e c o u n t e r at t h e d e t e c t o r end where t h e p a r t i c l e s a r e f o c u s s e d and r e c o r d e d . Thus a f o c u s s e d b e t a p a r t i c l e i s n o t r e c o r d e d when a s i m u l t a n e o u s event i s o b s e r v e d i n t h e s o u r c e end c o u n t e r . That i s t o s a y , a Compton e l e c t r o n p r o d u c e s a c o i n c i d e n t p u l s e i n t h e c o u n t e r s at each end o f t h e s p e c t r o m e t e r , and a c i r c u i t i s a r r a n g e d t o r e j e c t t h i s e v e n t . The i d e a l e f f e c t o f t h i s p r o c e d u r e upon a t y p i c a l p h o t o -e l e c t r o n s p e c t r u m i s shown i n F i g . 2 . The " s i g n a l - t o - n o i s e r a t i o " has been i m p r o v e d , and photopeak d e t e c t i o n has been made e a s i e r . P a r t 1 o f t h i s t h e s i s d e s c r i b e s t h e o p e r a t i o n o f t h e a p p a r a t u s and r e p o r t s on measurements o b t a i n e d on Ra (B + C ) , 1&2 60 12A. T a , Co and Sb ( . In a d d i t i o n , o t h e r measurements t a k e n on Ta a r e r e p o r t e d . I t i s t o be e x p e c t e d t h a t w i t h a b e t t e r method o f d e t e c t i o n o f weak gamma-rays, t h e u n c e r t a i n t i e s i n t h e assignment o f decay schemes s h o u l d be r e d u c e d . However, even t h e n u n i q u e n e s s i s not always p o s s i b l e i n t h e a s s i g n m e n t and o t h e r s u p p o r t i n g 7 e v i d e n c e must be u t i l i z e d . C o i n c i d e n c e measurements t o d e t e r m i n e w h i c h gamma-rays a r e i n c a s c a d e may c l a r i f y t h e s i t u a t i o n . . S u c h e x p e r i m e n t s have been p e r f o r m e d w i t h Ta . The b e t a s p e c t r u m o f T a ^ ^ has a l s o been m e a s u r e d . 6 I I EXPERIMENTAL PROCEDURE A . The D e t e c t o r C o u n t e r F o r t h i s work on Compton s u p p r e s s i o n , t h e s p e c t r o m e t e r was changed o v e r from G e i g e r c o u n t e r s t o s c i n t i l l a t i o n c o u n t e r s . T h i s change was n e c e s s a r y because a r a p i d r e s p o n s e was n e e d e d i n t h e m i x i n g c i r c u i t , (see S e c t i o n I I C.) B e t a p a r t i c l e s f o c u s s e d i n t h e s p e c t r o m e t e r a r e d e t e c -t e d by a t y p e RCA 5#19 p h o t o m u l t i p l i e r ^ u s e d w i t h an a n t h r a c e n e c r y s t a l . The m a g n e t i c f i e l d o f t h e s p e c t r o m e t e r i s as l a r g e as 50 gauss at the p h o t o m u l t i p l i e r p o s i t i o n and was f o u n d t o d e f o c u s t h e e l e c t r o n s i n t h e m u l t i p l i e r t u b e . To s h i e l d t h e p h o t o m u l t i p l i e r from t h i s f i e l d , i t was h o u s e d i n a m i l d s t e e l t u b e . T e s t s showed t h a t t h i s arrangement p r o v i d e d adequate s h i e l d i n g and d i d n o t a p p e a r t o i n t e r f e r e w i t h t h e f o c u s s i n g a c t i o n o f t h e s p e c t r o m e t e r . E n d p l a t e s t o t h e t u b e were made " l i g h t - t i g h t " by t h i n n e p p r e n e g a s k e t s . One e n d p l a t e c a r r i e s t h e head a m p l i f i e r and c a b l e c o n n e c -t i o n s . The o t h e r e n d p l a t e f o r m s the vacuum s e a l t o t h e d e t e c t o r end o f t h e s p e c t r o m e t e r . F o c u s s e d e l e c t r o n s p a s s t h r o u g h a h o l e i n t h e e n d p l a t e and s t r i k e t h e d e t e c t o r c r y s t a l . T h i s c o n s i s t e d o f a f l a k e o f a n t h r a c e n e mounted on l u c i t e . The p h o t o c a t h o d e o f t h e m u l t i p l i e r tube i s h e l d a g a i n s t t h e l u c i t e and adequate o p t i c a l c o u p l i n g i s e n s u r e d by a l a y e r o f t r a n s p a r e n t j e l l y between t h e l u c i t e and g l a s s s u r f a c e s . W i t h o u t t h i s p r e c a u t i o n , i n t e r n a l r e f l e c t i o n s o f t h e s c i n t i l l a t i o n s o c c u r a t t h e l u c i t e - a i r and at t h e g l a s s i n t e r f a c e s , (see A p p e n d i x 2 ) . DISCRIMINATOR VOLTS INTEGRAL BIAS CUMBS TAKEN AT VARIOUS £L£CTRON £NER$l£S. 7 Counts from cosmic r a y s and l o c a l c o n t a m i n a t i o n a r e k e p t t o a minimum by u s i n g an a n t h r a c e n e ' c r y s t a l o f s m a l l v o l u m e . A c r y s t a l 0 . 4 mm. i n t h i c k n e s s and a p p r o x i m a t e l y 7 mm. i n d i a m e t e r i s u s e d . I t was f o u n d t o be 100$ e f f i c i e n t f o r b e t a p a r t i c l e s , w h i l e h a v i n g a cosmic r a y b a c k g r o u n d o f t h r e e c o u n t s p e r m i n u t e . The e f f i c i e n c y was d e t e r m i n e d by c o m p a r i n g t h e e f f i c i e n c y o f t h e c r y s t a l w i t h t h a t o f a g e i g e r c o u n t e r w i t h a c o l l i m a t e d b e t a s o u r c e o f R a E . The l o w b a c k g r o u n d compared f a v o u r a b l y w i t h t h e u s u a l b a c k g r o u n d o f , s a y , 25 c o u n t s p e r m i n u t e i n a g e i g e r t u b e . The e v a p o r a t i o n i n vacuo o f t h e a n t h r a c e n e i s s u c h t h a t t h e c r y s t a l had t o be r e p l a c e d a f t e r s i x weeks o p e r a t i o n . The c o u n t e r p e r f o r m s s a t i s f a c t o r i l y i n t h e e n e r g y r e g i o n above 150 Kev. At low e n e r g i e s , t h e p u l s e h e i g h t i s no l a r g e r t h a n t h e r m a l n o i s e i n t h e m u l t i p l i e r . However, t h e u s e f u l n e s s o f t h e s p e c t r o m e t e r i t s e l f does not e x t e n d t o e n e r g i e s much b e l o w t h i s l i m i t , due t o s c a t t e r i n g i n t h e l o n g e l e c t r o n p a t h , a b s o r p t i o n i n t h e s o u r c e and from o t h e r c a u s e s . The p r o p o r t i o n a l i t y o f p u l s e h e i g h t t o e l e c t r o n energy^ can be seen i n F i g . 4 . The s e p a r a t e c u r v e s a r e i n t e g r a l b i a s c u r v e s f r o m t h e s c i n t i l l a t i o n c o u n t e r t a k e n w i t h t h e s p e c t r o m e t e r s e t t o f o c u s e l e c t r o n s a t t h e d i f f e r e n t e n e r g i e s i n d i c a t e d . The p u l s e s i z e d o e s not i n c r e a s e above 850 Kev. i n t h i s p a r t i c u l a r case s i n c e t h e e l e c t r o n s t h e n p a s s c o m p l e t e l y t h r o u g h t h e c r y s t a l . B . The Source C o u n t e r I n o r d e r t h a t t h e Compton e l e c t r o n s be s u p p r e s s e d and t h e p h o t o e l e c t r o n s be u n s u p p r e s s e d , t h e c o u n t e r a t t h e s o u r c e end must count Compton e l e c t r o n s but n o t p h o t o e l e c t r o n s , as was e x p l a i n e d i n t h e I n t r o d u c t i o n . T h i s was a r r a n g e d i n t h e manner SOURCE COMPTON TRAJECTORY ABSORBER FOR PRIMARY BETAS PHOTOELECTRON TRAJECTORY RADIATOR COUNTER CONNECTED IN ANTICOINCIDENCE FIG. 5 SCHEMATIC DIAGRAM OF SOURCE - HOLDER USED WITH COMPTON SUPPESSION SOURCE - HOLDER 8 ANTICOINCIDENCE PHOTOMULTIPLIER shown s c h e m a t i c a l l y i n F i g . 5. The a n t h r a c e n e c r y s t a l i s p l a c e d between t h e l o w Z a b s o r b e r and t h e r a d i a t o r so t h a t a l l Comptons a c c e p t e d by t h e s p e c t r o m e t e r must p a s s t h r o u g h i t . P h o t o e l e c -t r o n s on t h e o t h e r h a n d , o r i g i n a t e beyond t h e c r y s t a l and hence l e a v e no r e c o r d i n t h e s o u r c e c o u n t e r . Thus Gomptons p r o d u c e a n t i - c o i n c i d e n c e p u l s e s w h i l e p h o t o e l e c t r o n s do n o t . F i g . 6 shows how t h e arrangement d e s c r i b e d above was p h y s i c a l l y r e a l i z e d . The r a d i a t o r and a n t h r a c e n e c r y s t a l a r e mounted w i t h Canada Balsam on t h e end o f a l u c i t e r o d w h i c h p r o t r u d e s i n t o t h e s p e c t r o m e t e r vacuum s y s t e m . The r o d a l s o s e r v e s t o h o l d t h e s o u r c e and t o p r o v i d e o p t i c a l c o u p l i n g f r o m t h e c r y s t a l t o t h e p h o t o m u l t i p l i e r . The s o u r c e i s c o n t a i n e d i n a s m a l l h o l e d r i l l e d a l o n g a d i a m e t e r o f t h e l u c i t e r o d c l o s e t o t h e c r y s t a l . The r o d i s f i x e d t o t h e s p e c t r o m e t e r e n d - p l a t e w i t h A p i e z o n wax t o f o r m a vacuum s e a l . The g l a s s e n v e l o p e o f t h e p h o t o m u l t i p l i e r i s p r e s s e d a g a i n s t t h e end o f t h e r o d away from t h e vacuum s y s t e m . The l u c i t e r o d i s p o l i s h e d i n o r d e r t o p r o v i d e an o p t i c a l p a t h f o r t h e s c i n -t i l l a t i o n s i n t h e l u c i t e - making use o f i n t e r n a l r e f l e c t i o n s o f f t h e w a l l s o f t h e r o d . U n f o r t u n a t e l y ; t h e opaque body o f t h e s o u r c e was f o u n d t o i n t e r f e r e somewhat w i t h t h e o p t i c a l p a t h . T h i s d i f f i c u l t y was overcome by p u t t i n g s l o p i n g s h o u l d e r s on t h e l u c i t e (see F i g . 6) w h i c h r e f l e c t e d s c i n t i l l a t i o n s a r o u n d t h e s o u r c e . A t r a n s p a r e n t j e l l y i s k e p t between t h e l u c i t e and t h e g l a s s e n v e l o p e o f t h e p h o t o m u l t i p l i e r t o e n s u r e e f f i c i e n t l i g h t t r a n s f e r . Enough l u c i t e i s l e f t between s o u r c e and c r y s t a l t o a b s o r b t h e p r i m a r y b e t a s . I f t h i s i s n o t d o n e , t h e c o u n t i n g r a t e i n t h e c r y s t a l becomes e x c e s s i v e b e c a u s e o f t h e h i g h i n t e n s i t y o f t h e p r i m a r y b e t a s . F u r t h e r m o r e , many o f t h e p h o t o e l e c t r o n s w o u l d be c a n c e l l e d b y t r u l y c o i n c i d e n t p r i m a r y b e t a p a r t i c l e s . By t h e DETECTOR COUNTER AMPLIFIER DISCRIMINATOR S h a p e d f > u l t n MAQMET COIL DIFFERENCE AMPLIFIER DISC4/MWAT04 ANTICOINCIDENCE COUNTER AMPL IFIER 0*1 DSL SjbtctiHim of frultcs SCALER BLOCK DIAGRAM OF AHTICO/A/CIDMCC ARRANGEMENT 9 same argument i t might be e x p e c t e d t h a t some p h o t o e l e c t r o n s would be l o s t because o f t r u l y c o i n c i d e n t gamma r a y s . However, t h e gamma-ray e f f i c i e n c y o f t h e c r y s t a l ( i n t r i n s i c + g e o m e t r i c a l ) was e s t i m a t e d t o be much l e s s t h a n Q . l p e r c e n t , so t h a t t h i s e f f e c t may be i g n o r e d . As i n t h e case o f t h e d e t e c t o r p h o t o m u l t i p l i e r , a m i l d s t e e l c y l i n d e r and end p l a t e s p r o t e c t e d t h e t u b e f r o m t h e s p e c t r o m e -t e r f i e l d . C. The A n t i c o i n c i d e n c e C i r c u i t s The n e x t s t e p was t o use t h e p u l s e s from t h e s o u r c e c o u n t e r t o c a n c e l c o i n c i d e n t d e t e c t o r p u l s e s . The c o n v e n t i o n a l method o f d o i n g t h i s w o u l d be t o f e e d t h e o u t p u t p u l s e s from t h e two c o u n t e r s t h r o u g h a m p l i t u d e " d i s c r i m i n a t o r s " t o r e j e c t t h e r m a l n o i s e p u l s e s . A m i x i n g s t a g e w o u l d f o l l o w , and would be so a r r a n g e d t h a t p u l s e s from t h e d e t e c t o r c o u n t e r w o u l d be b l o c k e d when i n c o i n c i d e n c e w i t h s o u r c e c o u n t e r p u l s e s . However, a d i f f i c u l t y a r o s e due t o t h e p r o x i m i t y o f t h e s o u r c e t o t h e c r y s t a l (see F i g . 6). A l t h o u g h t h e gamma r a y e f f i c i e n c y o f t h e c r y s t a l i s v e r y l o w , as was p o i n t e d out a b o v e , t h e f a v o u r a b l e geometry p r o d u c e s a v e r y h i g h gamma c o u n t i n g r a t e i n t h e a n t i c o i n c i d e n c e c h a n n e l . T h i s was e s t i m a t e d t o be 2 x 10-* c o u n t s p e r second f o r a t y p i c a l s o u r c e , mc Ra(B + C) ) . T h i s r a t e i s o f t h e o r d e r o f 10^ t i m e s l a r g e r t h a n t h e r a t e i n t h e d e t e c t o r c o u n t e r . S i n c e a c o n v e n t i o n a l d i s -c r i m i n a t o r r e m a i n s i n s e n s i t i v e f o r s e v e r a l m i c r o s e c o n d s a f t e r p a s s -i n g a p u l s e , s u c h a d i s c r i m i n a t o r c o u l d n o t be u s e d i n t h e a n t i -c o i n c i d e n c e c h a n n e l w i t h o u t s e r i o u s c o u n t i n g l o s s e s . Hence i t was n e c e s s a r y t o adopt t h e u n c o n v e n t i o n a l arrangement shown i n t h e b l o c k d i a g r a m i n F i g . 7. The p r i n c i p l e - I 0 0 O V. OUTPUT INTO too SL coajr. RCA FIG. 8 DETECTOR HEAD AMPLIFIER + 2oo v. _ OUTPUT* WTQ V V /ooo A . COAT. TtCA SB19 TVf CAKS* FIG.9 {SOURCE HEAD AMPLIFIER IN34 6AK5 6AH6 6 J6 €AC7 + Zoo v IN FIG. 10 FED-BACK AMPLIFIER USED IN THE DETECTOR CHANNEL. DISCRIMINATOR DIFFERENCE AMPLIFIER OUTPUT + v OUT ANTICOINC. ** k INPUT 6A65 6J6 6AK5 6AG5 6AK5 6AG5 6AG5 1/2 6AL5 FIG.II DISCRIMINATOR AND DIFFERENCE AMPLIFIER . 10 of o p e r a t i o n i s as f o l l o w s : the de t e c t o r pulses are shaped by a d i s c r i m i n a t o r and brought t o the d i f f e r e n c e a m p l i f i e r . The many source counter pulses are a m p l i f i e d and t h e i r whole spectrum subtracted from the shaped detector pulses. Any de t e c t o r pulse which l o s e s amplitude i n t h i s s u b t r a c t i o n i s r e j e c t e d by a second d i s c r i m i n a t o r . Thus the a n t i c o i n c i d e n c e c i r c u i t i s arranged i n such a manner t h a t n e i t h e r of the two d i s c r i m i n a t o r s run f a s t e r than does the slower channel. A more d e t a i l e d d e s c r i p t i o n of the separate c i r c u i t f o l l o w s . The c i r c u i t diagram of the detec t o r head a m p l i f i e r i s shown i n F i g . 10; The l a t t e r i s a feed-back"1"^ a m p l i f i e r w i t h an observed r i s e - t i m e of 0.1 }i sec. and a maximum output of 120 v o l t s . I t i s provided w i t h an input attenuator. The pulses are l e d from the output cathode f o l l o w e r t o the d i s c r i m i n a t o r shown on the l e f t of F i g . 11. The shaped pulses from the d i s c r i m i n a t o r then go to the d i f f e r e n c e a m p l i f i e r a l s o shown i n F i g . 11. The spectrum of source pulses i s also f e d i n t o t h i s stage where they are subtracted from the d i s c r i m i n a t o r pulses (from the de t e c t o r counter). Those output pulses from the d i f f e r e n c e a m p l i f i e r which r e t a i n t h e i r f u l l amplitude are then accepted by the d i s c r i m i n a t o r o f a commercial •scaling u n i t and recorded. The pulses i n the source channel are delayed 0.1 fx sec. (see F i g . 7) t o allow the d i s c r i m i n a t o r o f the det e c t o r channel to f i r e . This delay i s provided by three f e e t of RG 65/tJ cable. The head a m p l i f i e r of the source channel i s shown i n F i g . 9. The p l a t e l o a d matches the 1000 ohm c h a r a c t e r i s t i c impedance of the delay cable i n order t o stop r e f l e c t i o n s i n the cable. The source or a n t i c o i n c i d e n c e a m p l i f i e r i s shown i n GAKS SOURCE FIG. 12 COUNTER AMPLIFIER Qa«t t -cm. CANCELLATION AS A RWCTKW OF ENERGY FOR Ta & Ra. 11 F i g . 12. I t has a gain of 50 which i s necessary i n that some o f the u s e f u l pulses are small (see s e c t i o n "D"). A l l pulses are l i m i t e d to 2 v o l t s by c u t t i n g o f f the f i n a l tube. In t h i s way the d i f f e r e n c e a m p l i f i e r which f o l l o w s can not be overloaded. To stop the source a m p l i f i e r i t s e l f from b l o c k i n g , c r y s t a l diodes are arranged as shown i n F i g . 12. The r i s e time of the source ampli-f i e r i s 0.04 ^i. s e c , and the i n s e n s i t i v e time a f t e r r e c e i v i n g a pulse as l a r g e as 1 v o l t was estimated to be 0;2^ns. D. Suppression of the Compton E l e c t r o n Counts as a Function o f  Energy. Suppression of the Compton e l e c t r o n counts i s not 1Q0% e f f e c t i v e over the spectrum. The percentage o f Compton counts not c a n c e l l e d i s shown as a f u n c t i o n of momentum f o r Ta-^ 2 and Ra(B + C) i n F i g . 13. Suppression e f f i c i e n c i e s are as high as 90% i n the c e n t r a l r e g i o n of maximum Compton i n t e n s i t y where c a n c e l l a t i o n i s most needed. However, the suppression i s poor i n the high and low momentum regions and some expl a n a t i o n should be given f o r the energy dependence of the suppression. Obviously, the most energetic Comptons i n a spectrum can have l o s t no energy t o the source c r y s t a l by the very f a c t that they r e t a i n t h e i r f u l l p o s s i b l e energy. I f no energy has been l o s t to the source c r y s t a l , no c a n c e l l i n g pulse can have been prodiced. Thus the most energetic Comptons cannot be c a n c e l l e d i n p r i n c i p l e . A c e r t a i n minimum energy l o s s i n the c r y s t a l i s necessary to produce a c a n c e l l i n g p u l s e , depending on the s e n s i t i v -i t y of the source counter. The magnitude of t h i s minimum energy l o s s determines how f a r the poor c a n c e l l a t i o n extends down from 12 the top of the spectrum. In t h i s investigation the source counter can detect a minimum energy of about 50 Kev., the l i m i t to the sen-s i t i v i t y being thermal noise i n the photomultiplier. So the top 50 Kev. of the spectrum i s not cancelled i n t h i s case. Actually some Comptons as much as 150 Kev. below the maximum energy are not cancelled, and an extension of the above argument may be used to explain t h i s . Let us consider the Compton d i s t r i b u t i o n from a single gamma t r a n s i t i o n of 1.2 Mev. It may be said that the most energe-t i c Comptons arise from encounters i n the outermost la y e r of the source holder which consists of the radiator and a portion of the anthracene c r y s t a l . In fact the top 50 Kev. of the spectrum must come from a layer of thickness 30 mg/cm2 since the rate of energy l o s s , i s 1.7 Kev at 1 M e v . C o m p t o n encounters i n ^ x mg/cm t h i s layer w i l l not provide cancelling pulses of s u f f i c i e n t magni-tude. To explain the changes i n cancellation e f f i c i e n c y over the spectrum by the concept of t h i s i n s e n s i t i v e layer, we must consider the various angles, 9, which occur between the d i r e c t i o n of the incident gamma and the scattered electron. The energy of a scattered electron i s given by E(Q) = 2 m 0 c 2 r 2 c o s 2 9 l+2r+r 2 s i n 2 9 ' where r = ^ % c 2 We have already dealt with the head-on ( 9 = 0 ) encounters i n the l a s t layer. Comptons from 9 = 10° encounters i n the l a s t layer have a good chance of acceptance by the spectro-meter b a f f l e s . Since E (10°) i s about 100 Kev. below the 13 maximum, E ( 0 ) , t h i s means that some of the electrons at 150 Kev. below the maximum of the spectrum come from the l a s t layer and are therefore not cancelled. The i n s e n s i t i v e - l a y e r concept i s also consistent with the good cancellation at intermediate energies. This can be understood by considering the o r i g i n of these electrons. They may come from deep within the s'ourceholder, i n which case they w i l l be detected i n the anticoincidence c r y s t a l . They may not come from the l a s t i n s e n s i t i v e layer since the angle 9 necessary i n a c o l l i s i o n giving r i s e to an intermediate energy electron i s too large f o r the spectrometer b a f f l e s to accept the electron. Thus a q u a l i t a t i v e picture b u i l t on the one assumption of the o r i g i n of the uncancelled electron explains the shape obtained i n the intermediate and upper regions of the spectrum. There i s also the p o s s i b i l i t y that the s c i n t i l l a t i o n s from the high energy Comptons are not properly o p t i c a l l y coupled to the photocathode because of the opaque body of the source. This might be the case since the high energy Comptons come from head-on c o l l i s i o n s which tend to occur, therefore, i n the plane of the source hole and the spectrometer axis. To test t h i s explanation of the poor suppression at high energies f a seemingly perfect o p t i c a l system was constructed. The source was mounted i n the l u c i t e rod by a pressure mold, thus abolishing the source hole. This was done at 145°C at a pressure of 3000 l b s . per sq. i n . The source used was Co°^ wire of 0.01 inch diameter i n the form of an open l a t t i c e located at the source posi-t i o n . The source was believed small enough so as not to i n t e r f e r e with the s c i n t i l l a t i o n s . However, the behaviour of t h i s arrangement was i d e n t i c a l to that of holders having large holes to carry the source. The l a t t e r type of source holder i s therefore to be preferred since the molding process i s time consuming. To quantitatively account f o r the shape of the curve, i t would be necessary to determine the o r i g i n of the electrons i n d e t a i l f o r a l l energies. This i s impossible because of such complications as the large range of possible c o l l i s i o n angles, the 3 presence of multiple scattering, and the d i f f e r e n t transmission of the spectrometer f o r d i f f e r e n t points on the source. A tentative explanation may be given f o r the poor cancellation at low Compton energies (see F i g . 13), the explanation being consistent with the in s e n s i t i v e layer concept used above i n the high energy region. In the case of a single 1.2 Mev. gamma-ray i t was stated that an intermediate energy electron could not ar i s e from a large angle encounter i n the very l a s t layer of the c r y s t a l , because the b a f f l e s would not accept i t . There i s a chance of it's being accepted, however, i f i t i s multiply scattered i n the l a s t layer, of the source holder. Furthermore t h i s chance of acceptance increases f o r low energy electrons since the root mean square angle of scattering i s proportional to i ^ . Hence low energy electrons have a better chance of ori g i n a t i n g i n the i n s e n s i t i v e layer and being focussed than do Comptons of intermediate energy. Thus the main features of the Ta-^ 2 c a n c e l l a t i o n curve i n F i g . 13, which resembles that of the hypothetical 1.2 Mev. gamma-ray, have been explained. F i n a l l y , x>re must pass to the case of Ka(B + C) with i t s many l i n e s (see Table I) extending to 2.4 Mev. At 1 Mev. i n the Ra spectrum the preponderance of Compton electrons comes from the highest energy gamma rays. So the poor cancellation of the Comptons from l i n e s whose energies are just above 1 Mev. i s 15 not noticed. For t h i s reason the form of the cancellation curve 1$2 of Ra resembles that of Ta (see Fi g . 13), except of course f o r a s h i f t i n energy. E. Loss of Photopeak Intensity The use of the anticoincidence method r e s u l t s i n a small loss of photopeak i n t e n s i t y . Almost a l l of t h i s l o s s i s due to chance coincidences, i . e . a photoelectron i s focussed but not recorded because of an accidentally simultaneous event i n the source counter. With a gamma-ray source of | mc. strength, the photopeak loss was measured to be 5$. Ah almost n e g l i g i b l e l o s s of photoelectrons r e s u l t s from events i n the source counter which are t r u l y coincident with the photoelectrons, i . e . Compton events from gamma-rays which are i n cascade with the gamma-ray causing the photoelectron. This l o s s i s estimated to be much l e s s than 0.1$, as was mentioned i n Section IIB. It might be thought that another cause of photopeak i n t e n s i t y l o s s would be the somewhat larger source-to-radiator distance necessitated by the presence of the source c r y s t a l . This distance i s about 2.5 mm. instead of say 1.5 mm. i n the conventional source holder without the c r y s t a l . However, the larger distance does not lower the photoelectron i n t e n s i t y since both the source and radiator are not points but are extended. Furthermore, the angular d i s t r i b u t i o n of the photoelectron leans so f a r forward"^ at the range of energies i n question that the source-to-radiator distance tends to lose i t s importance, i . e . i f the source touched the radiator, many electron t r a j e c t o r i e s would be at too large an angle to the spectrometer axis to be accepted by the b a f f l e s . The loss of photoelectron i n t e n s i t y i s considered i n the s t a t i s t i c a l treatment of Appendix 1. \ 1 2 0 0 2 4 0 0 4 8 0 0 9 6 0 0 Hp (Gauss-cm.) FIG. 14 PHOTOELECTRON SPECTRA FROM GAMMA RAYS Sa(BtC). 17 III RESULTS OF THE GAMMA RAY STUDIES A. Ra(B + C) The photoelectron spectrum of a 0.5 mc. source of Ra(B + C) with Compton electrons suppressed is shown in the lower curve of Fig. 14. The upper curve is a conventional spectrum 15 taken previously in this laboratory ' also using a thin-lens spectrometer. The radiator used in the present work was a uranium f o i l of 24 mg/cm2 thickness. In the earlier work-^, a lead f o i l of 40 mg/cm2 thickness was used. The energies of the gamma rays detected are listed in , Table 1, together with the results of Mann and Ozeroff1^, of Latyschev and co-workers, and of E l l i s . New lines have been found at0.391, 0.857 and 1.00 Mev. In addition, the lines at 0.450 and 0.781 Mev. reported by Mann"*"-' have been confirmed. A line has been detected at 1.55 Mev. which may be the unconfirmed line reported to be at 1.52 Mev. by Latyshev^. Substantial agreement is found between the separate investigators on the majority of lines. No photoelectrons were found for the line listed at 2.41 Mev. This line is calculated from the Compton endpoint of 2.15 Mev. B. T a l g 2 The gamma ray spectrum of a 1 mc. source of Ta 1^ 2 with Compton suppression is shown in the lower curve of Fig. 15. The upper curve is the spectrum taken in the conventional manner in the same geometry, i.e., with the anticoincidence counter turned off. TABLE 1  GAMMA RAYS OF RADIUM B 4- C IN Mev. Present Investigation E l l i s 1 7 Latyschev 16 Mann 15 .292 .350 .391 .456 .507 .607 .766 .737 .^57 .933 1.00 1.10 1.22 1.37 1.55 2.15 2.41 .2937 .3499 .4260 .4930 .6067 .766 .933 1.12 1.233 1.379 1.414 1.761 2.193 .606 .766 .933 1.11 1.120 1.-21 1.234 1.370 1.390 1.414 1.52 1.62 1.75 1.761 1.32 2.09 2.20 2.40 .291 .352 .426 .450 • 496 .607 .731 1.12 1.22 1.40 1.77 2.21 2.40 1200 1697 2400 3396 4800 6787 FIG. 15 PHOTOELECTRON SPECTRA FROM GAMMA RAYS Tal82. H P (Gauss-cm.) 1 {Jo Tantalum was included i n the present invest i g a t i o n because of considerable disagreement i n reported energy measure-ments above 300 Kev. O'Meara has reported fourteen l i n e s i n the region from 300 Kev. to 1.1 Mev. which were not found by other i n v e s t i g a t o r s . 1 9 , 2 0 ' 2 1 * In t h i s study, the three well k n o w n 1 9 > 2 0 j 2 1 l i n e s at 1.23, 1.24 and 1.13 Mev. have been i d e n t i f i e d . In addition, a new l i n e at 1.01 Mev. has been found. None of the l i n e s reported by 0'Meara are present. The smooth prominance i n the spectrum at approximately 1.0 Mev. r e s u l t s from the sudden l o s s of Compton suppression discussed i n Section II , D. This occurs where the Compton d i s -t r i b u t i o n has the steepest slope and gives r i s e to a c h a r a c t e r i s t i c hump. This hump i s quite d i f f e r e n t from the narrow, t r i a n g u l a r form t y p i c a l to a photopeak. Unfortunately, the new peak at 1.01 Mev. i s superimposed on t h i s prominence, (see F i g . 15). To make certai n that the 1.01 Mev. peak was not spurious, a lead radiator was substituted f o r the uranium radiator. The peak i n question s h i f t e d by exactly the difference i n K-shell binding energy between lead and uranium. C. Co 6 0. The photoelectron spectrum of a 0.5 me. source of Go 0^ taken with and without Compton suppression i s shown i n F i g . 16. The same source-holder geometry was used i n each spectrum. Only the two well-known 2 2> 2 3 l i n e s at 1.17 Mev. and 1.33 Mev. were found. No evidence appeared f o r any weak gamma t r a n s i t i o n s . A smooth hump occurs i n the C o ^ spectrum as i t did i n the case of Ta^^ 2 which has a s i m i l a r high energy spectrum. It i s i n t e r e s t i n g that GAUSS--.CM-. PHOTOELECTRON SPECTRA FROM GAMMA RAYS Sbl24. i s y y ^698 2400 3396 1=3 4300 c m . FIG. 16 PHOTOELECTRON SPECTRA FROM GAMMA RAYS Cc-60. 19 there i s no hump i n the upper part of the Ra(B + C) spectrum. Presumably t h i s spectrum i s the sum of the^ spectra from the many Ra gamma-rays. Each of these single spectra may i t s e l f have a hump, but the composite spectrum may well be smooth. D. S b 1 2 4 . Fi g . 17 shows the spectrum of photoelectrons ejected from a uranium r a d i a t o r by the gamma rays from 1 mc. of Sb 1 2^. Only the spectrum with Compton electrons suppressed i s shown, the ordinary spectrum being omitted f o r the sake of c l a r i t y . The percentage suppression i s s i m i l a r to that of the previous work, except that the gamma rays from Sb"''2^ are so d i s t r i b u t e d i n energy that smooth humps appear at both 2400 gauss-cm. and 5800 gauss-cm. A radiator of thickness 40 m /cm2 was used to take the spectrum above 4000 gauss-cm. and 24 m /cm2 was used below. The gamma ray energies so obtained are given i n Table 2 together with the values of some other workers. New gamma rays have been found at 0.472 Mev.' and 0.843 Mev. There i s some evidence f o r the existence of a gamma ray at 0.609 Mev., because of the large width of the photopeak from the 0.600 t r a n s i t i o n . The remaining energy values are i n good agreement with those of previous investigators. 20 TABLE 2 GAMMA RAYS OF S b 1 2 ^ IN Mev. Present Kern et a l 2 ^ " Cook et a l 2 ^ Iowa S t a t e 4 0 Investigation .472 - - -.600 .603 .608 .598 .609 - - -.650 .650 .654 .645 •714 .714 .732 .817 .843 -1.71 1.708 1.708 I.67 2.04 2.056 2.04 2.07 2.072 - -530 KEV. 400 KURIE PLOT OF THE T a " * BETA q R O U P . 21 IV SOME ASSOCIATED STUDIES IN Ta 1^ 2 A. The Beta Spectra of T a l g 2 Comparison of observed beta spectra with theory is usually made with the aid of a Kurie plot. The function fN(p}/Fp2J ^ is plotted against the energy E, where N(p) is the intensity of electrons of momentum p, and F is a factor to correct for the Coulomb field of the nucleus. From the Fermi relation 2 7 [Nlpi/Fp 2]* = E f f l a x - E , the plot should be a straight line intercepting the axis at Em_ . 182 The beta spectrum of Ta was taken in a thin lens spectrometer with a source of thickness ofJrag/cm . The Kurie plot for the spectrum obtained above 250 Kev. is shown in Fig. 18. A single beta group with an endpoint at 530 ± 3 Kev. can be seen. This endpoint is in agreement with the value of 525 Kev. given by 20 Beach et al . Many internal conversion lines occur below 250 Kev., making a Kurie plot at low energies extremely difficult. In this plot, the approximation to the Coulomb factor 28 F which is valid for large Z was used. It is given by F = l/p - 0.355 •where p is in units of m 0c t>ISCRiMtWAT0R MA Rossi PAIR - — - OUTPUT €*c? SAC? tsj? t/kcj INPUT, C O I M C I 0 € M C £ M I X f f R . 22 182 B. Gamma-gamma Coincidences in Ta The gamma rays of T a ^ 2 are seen to f a l l into two energy groups, one below 0.33 Mev. and one above 1.00 Mev. This made i t possible to observe the high energy group by itse l f . This was done by using large anthracene crystals with photomultipliers and accep-ting only the largest pulses, which were presumably caused by the high energy gammas. Approximately 10 microcuries of Ta was placed between the two anthracene crystals. The pulses from each of the photo-multipliers were lead from a headamplifier identical to that shown in Fig. 8 to an amplifier shown in Fig. 10. The output from the £wo amplifiers went to the coincidence mixer shown in Fig. 19. The coincidence mixer had a resolving time of 0.136 usee. No genuine coincidences were observed between the high energy gamma rays of Ta 1 8 2. It was possible to place an upper limit on the number of high energy gammas in cascade in the following way: coincidences were observed from the two gamma rays of a CoD® source in a similar An 4 22 23 geometry. The Co 0 0 gamma rays at 1.17 and 1.33 Mev. ' ' are in cascade and have energies close to that of Ta . More than 100 times as many coincidences per gamma were observed in Co°^ than in Ta . Thus less than 1% of the high energy gammas from Ta are in cascade. Presumably the three intense high energy gammas are three parallel modes of decay. 23 APPENDIX 1 AN ANALYSIS OF THE IMPROVED STATISTICAL ACCURACY It has been mentioned above that not a l l the Compton electrons are suppressed and that the photoelectrons suffer a slight suppression by the use of the anticoincidence method. The k question then arises as to whether the method makes any improve-ment in the sensitivity of photopeak detection. Obviously i t might make no improvement in the case of a very large photopeak on a very small Compton background. Let the events recorded in unit time with the conventional arrangement be N,p, Np and Nq for the total count, photoelectron count and Compton electron count respectively. Let the primed symbols refer to the counts obtained using the Compton suppression method. Since the photoelectron count is obtained from ,Np — N T - Nc , we may write for the standard deviation 2 9 of N„ T C ' where <X_, is the standard deviation of N T and C£ is the standard deviation of H . Since ~t^T e t c * f o r r a n d o m events, we have N T +• N y 2N„ -l- NT c P „_ 30 ^ Taking the usual figure of merit ~rr we have 1 IMp * Np7 ^ N p as the condition that the anticoincidence device represents an 24 improvement over conventional operation. p (i) Or ^ 2Wg + Nj/ ^ y' 2NC 4 Nj NP NP Let us define the r a t i o s of the new to the old counting rates f o r photoelectrons and Comptons respectively by . P = V , (2a) C - ^1 • (2b) The condition (1) then becomes N C ^ pTi r —py (3) Typical experimental values of p and c are 0.95 and 0.12 respectively. Using these values i n (3) , the method represents an improvement i n counting f o r a l l cases where 1J[P<;3C" This N C ^ i s always the case i n practice since such large peaks do not occur. It follows that the anticoincidence method i s an improvement i n a l l s i t u a t i o n s . APPENDIX II INTERNAL REFLECTION IN LUCITE The c r i t i c a l angle 0, f o r t o t a l i n t e r n a l r e f l e c t i o n of l i g h t at an interface i s given by 0 ^ s i n " 1 1/jb j where ji i s the index of r e f r a c t i o n . For l u c i t e ji - 1.50 . Hence the c r i t i c a l angle f ' 0 = 42°. Thus the shoulders of the l u c i t e l i g h t pipe (see F i g . 6) which are at an angle of 45°, are adequate to r e f l e c t most s c i n t i l l a t i o n s from the c r y s t a l to the photocathbde of the photomultiplier. P A R T 2 ON THE DOUBLE BETA DECAY OF S n 1 2 Z f 26 I INTRODUCTION The emission of single electrons from radioactive n u c l e i 1 2 has been the subject of many researches ' during the past f i f t y years. I t i s now well established that i n single beta-decay, the atomic number of the nucleus changes from ZJ to Z — 1 depending upon whether a negative electron (negatron) or a p o s i t i v e electron (positron) has been emitt ed31. If,however, the process involves only the emission of a single electron, i t can e a s i l y be shown that the laws of conservation of energy and angular momentum are violated, To avoid t h i s impasse, Pauli proposed h i s neutrino hypothesis which describes any single beta-decay process as being r e a l l y the simul-taneous emission of two p a r t i c l e s , the electron and a p a r t i c l e of negligeable mass and no charge. Then negatron decay i s described as the conversion of one of the neutrons i n the nucleus into a proton according to N — P +- p +- \) , where N and P represent neutron and proton respectively, ^ represents the emitted negatron and l) represents the "neutrino" with no charge and small mass. On the other hand positron decay i s described as the conversion of a nuclear proton i n t o a neutron according to P — - N ^ + \) , where ^ represents the positron and -J represents the " a n t i -neutrino" also with no charge or mass. By v i r t u e of the properties assigned to both \) and \) they are e s s e n t i a l l y undetectable. In the ordinary form of the neutrino t h e o r y 2 7 , neutrinos ATOMIC MASS i i i i Z Z + f z + z THE MASSES OP AN ISoBARIC TRIPLET. 27 are normally assigned to negative energy states which are almost always completely f i l l e d . A particle in such a state is called a neutrino. The few vacancies in this negative energy "sea" are called antineutrinos, a formalism quite analogous to that of Dirac's hole theory of the positron. Thus in this form, the neutrino and the antineutrino are distinguishable and represent two distinct particles. On the other hand, the Majorana form^2 of the neutrino theory makes no distinction between \) and l) and states that the two particles are indistinguishable. Both forms of the neutrino theory make the same predictions for single beta-decay processes. The condition whereby single beta-decay is energetically possible is simply that the nuclear mass of the parent atom be at . least equal to the nuclear mass of the daughter atom plus the mass of the emitted electron. If the nuclear mass difference be greater than this lower limit, then the excess mass is converted into kinetic energy of the electron-neutrino pair. Expressed in terms of atomic mass units (whereby the orbital electron masses are included), the criteria may be summarized by stating that single negatron decay is possible i f the mass of the whole parent atom is greater than the mass of the daughter atom. Fig. 20 shows the atomic masses which are predicted for certain triplets of isobars (same mass number A but different Z) by the semi-empirical mass formula, a formula proposed by Weizsacher-and others which predicts atomic masses with reasonable accuracy. Nucleus 2 will certainly decay by negatron emission to nucleus 3. It may or may not go by positron emission to nucleus 1, depending upon the mass difference between 2 and 1. Nucleus 1 can-28 not decay to 2 since mass considerations preclude this. To decay to 3, i t would have to change from £ to which would correspond to the simultaneous emission of two negatrons. Such a double beta process has not been definitely abserved, although i t is apparently energetically possible in several cases. The calculated half-life for double beta-decay depends upon a l l the factors which affect the half-life of the single beta-decay process. These are the energy available (mass difference) } the spins of the i n i t i a l and final states and the parity change involved, the latter being a mathematical term which describes the symmetry properties of the wave functions describing the i n i t i a l and final states. The calculations are based upon second-order perturbation theory. Since the transition probabilities calculated by the use of perturbation theory for second order processes are very small, the half-life of the double beta-decay process is predicted to be extremely large. If we assume that such calcula-tions are even approximately correct, then should double beta-decay exist at a l l , its activity would be very weak. Hence i t is not surprising that previously published work3^"^ describing searches for this process quote conflicting results and that the existance of the process itself is in doubt. Two different attacks have been made on the calculations of the probability of double beta-decay. In the ordinary theory, two neutrons decay in a double beta-process and the emission of two neutrinos is to be expected since N — * P -t- ^  +• 0 ' N — * P 4- ^  f J } 2N —=*> 2P -+- 2*-f-2\) . Thus, the double beta-decay process r e s u l t s i n the simultaneous emission of four p a r t i c l e s , (two negatrons and two neutrinos). Goeppert-Mayer-^ has calculated the h a l f - l i f e of t h i s process to be of the order of 10 2^ years. 32 The other neutrino theory, the Majorana fornr f makes no d i s t i n c t i o n between neutrino and antineutrino. Thus v) i s the equivalent of Of* Now consider the single process N P t f 1 ^ v) . This corresponds to the simultaneous emission of a negatron and a neutrino. But on the ordinary theory, an anti-neutrino f"0 fis gust 1 a vacant negative energy state of a neutrino, so that the emission of a neutrino i s exactly the same as the absorption of an antineutrino. Therefore the process could be written -J* ~h N ^ P , and according to Majorana t h i s i s the same as x) -+- N — ^ P -h In the Majorana picture, the f i r s t neutron decays emitting a v i r t u a l neutrino which i s absorbed by the second decaying neutron. Thus 2N 2P +- 2(? No neutrinos are emitted and the process involves the emission of two p a r t i c l e s only (two negatrons). As might be expected the calculated p r o b a b i l i t y of t h i s event i s greater than f o r the four p a r t i c l e theory. Furry-^ has calculated the h a l f - l i f e of the two p a r t i c l e process to be of the order of 10^ years. A lO2**" year a c t i v i t y (four p a r t i c l e concept) i s incapable of detection using ordinary techniques, but 1 0 ^ years may be just within reach. Thus double beta decay o f f e r s a means of deciding between the ordinary and the Majorana form of the neutrino theory. It very probably offers the only means since the di r e c t detection of \) and of appears to be equally impossible. Another consequence of double beta decay would be that the e l e c t r i c charge of the electron would be shown equal that of the proton. 3 6 KRemark by Oppenheimer E. L. Firemen Princeton Thesis. 31 II SOME PREVIOUS EXPERIMENTS A. The T r i p l e t Sn12/»- - * S b 1 2 4 - T e 1 2 Z f ) 50 51 52 Several e a r l i e r workers selected ^gSn 1 2^ as a source. Double beta decay has been reported by Fireman36 a s observed i n Sn 1 24 with a h a l f - l i f e of 0.4 x 1 0 1 d years. The process a l l e g -edly observed was 5 C S n ^ . . . » 5 2 T e 1 2 4 ^ 2 ^ ~ - . Fireman used two thin window Geiger counters on each side of a t h i n f l a t t i n source. Only coincident events were recorded i n order to i d e n t i f y the double beta process and i n order t o lower the background from cosmic rays and l o c a l contamination. The background rate was fur t h e r reduced by the use of lead shielding and by banks of Geiger counters connected i n a n t i -coincidence. The background rate reported was 14 counts per hour. With the t i n i n place a s l i g h t l y larger rate was obtained. Fireman interpreted t h i s difference to be caused by double beta decay, and calculated the h a l f l i f e from the mass of the source i? and the s o l i d angles subtended by the counters. Absorption curves showed the "beta p a r t i c l e s " had a maximum energy of 1.5 Mev. Libby and Kalkstein^? performed a s i m i l a r experiment on Sn ^ and obtained a negative r e s u l t . They were able to place a lower l i m i t of 1.7 - 2.4 x ( l O y years on the h a l f - l i f e . 32 B. The T r i p l e t ^ P . " * - ^ A g 1 1 0 - ^ C d 1 1 0 Recently Winter-^ investigated the reaction , n 110 ^ „ ,110 n r> ~. 4 6 P a ^ ^gCd 2 V* , i n a cloud chamber and obtained negative r e s u l t s . A lower l i m i t of 0.6 x 1 0 1 8 years was placed on the h a l f - l i f e of the decay. C. The T r i p l e t 5 2 T e l 3 ° ~ • 53 1' 1 3 0 ~ 5 4 X e l 3 ° An investigation of the process , T e 1 3 0 —=*• X e 1 3 ° -+ 2 ^  ) was made by Inghram and Reynolds^. A mass spectrometer was used to search f o r the presence of Xe"^® i n t e l l u r i d e ores. An excess of X e 1 ^ was found and was attributed to the double beta decay of Te 1-^. From a f a i r l y r e l i a b l e estimate of the age of the ore, a h a l f - l i f e of 21 1.4 x 10 " years was found. D. The T r i p l e t 9 2 U 2 3 8 - ^ N p 2 3 8 - 9l?u2^ A negative res u l t has been obtained by Seaborg et a l ^ i n an investigation of the process } 9 2 u * 94' The available energy i s known to be 1.1 Mev. from the decay schemes of neighbouring isotopes. A search was made f o r the presence of 90 day P u 2 3 8 i n pure U 2 3 8 . This was done by chemical separation of the P u 2 3 8 followed by a search f o r the 5.51 Mev. alpha p a r t i c l e s from Pu 2 3^. No such a c t i v i t y was found, and a lower l i m i t of IS 6 x 10 years was set on the h a l f - l i f e . 33 Since double beta decay was reported i n Sn-14^, t h i s isotope was chosen f o r our in v e s t i g a t i o n . Recently Duckworth^ 2 has measured the S n 1 2 ^ - T e 1 2 ^ mass difference to.be 1.5 + 0.4 Mev. by an accurate mass spectrometer. Our experiment was quite d i f f e r e n t i n nature from the experiments described above, and we f e e l , should lead to a more* r e l i a b l e i d e n t i f i c a t i o n of double beta decay, should i t e x i s t . PM. WAX R M kLectd C O I N C I D E N C E O U T P U T CONTROLS GATE PULSE COINCIDENCE > r GATE ADDER MIXER r PULSE T O > KICKSORTER 34 I I I EXPERIMENTAL PROCEDURE A. The P r i n c i p l e of the Experiment I f double beta decay i s observable at a l l , i t follows from the h a l f - l i f e considerations that i t w i l l very probably be a two p a r t i c l e process. For t h i s reason, the experiment was designed to make f u l l e r use of the properties of the two p a r t i c l e process than was made in previous work. A distinguishing feature of the two p a r t i c l e process i s that the sum of the energies of the two beta p a r t i c l e s has a constant value equal to the t o t a l energy available f o r the process. ' i This i s because the two beta p a r t i c l e s receive a l l the energy carried away. This i s not the case when neutrinos are emitted. This unique feature was used i n an experiment i n which the sum of the energies of coincident events i n Sn^ 2^ was recorded on a pulse amplitude analyser or "kicksorter". The energy spectrum of the double beta process thus displayed should consist of a sharp peak at an energy corresponding to the sum of the two beta energies.. The background spectrum, however, should appear as a smooth d i s t r i b u t i o n . B. The Experimental Arrangement The block diagram of the experimental arrangement i s shown i n F i g . 21. A source consisting of a 200 t i n f o i l was placed between two s c i n t i l l a t i o n counters i n a l i g h t - t i g h t box. The source was on loan from the Oak Ridge National Laboratories and was enriched to 95% S n 1 2 ^ by electromagnetic separation. Coincident beta p a r t i c l e s were detected i n thick anthracene c r y s t a l s (1 x 1 x J) which faced the t i n f o i l . The pulse amplitudes from the s c i n t i l l a t i o n counters were used as a measure of the beta energies. The sura of the energies of the two betas was represen-ted by the sum of the amplitudes of coincident pulses which was obtained i n the pulse adder. The pulse representing t h i s sum was then displayed on an 18 channel kicks o r t e r of Chalk River design. A gate controlled by the coincidence mixer allowed the output from the pulse adder t o reach the kicksorter only i n the case of a coincident event. C. C a l i b r a t i o n of the Kicksorter In order to ca l i b r a t e the energy scale on the kicksorter, the beta spectrum of Sb^ 2^ was observed i n each c r y s t a l separately. The endpoint of the most energetic beta groups of Sb"*"2^ at 2.4 Mev.2^1 was used as a c a l i b r a t i o n point. Using the same point, the o v e r a l l gains of the s c i n t i l l a t i o n counters were made equal by adjusting t h e i r high voltage supplies. D. ^  The Optimum Source Thickness The thickness of material between the two cr y s t a l s seemed to have an e f f e c t on the amount of scattering from c r y s t a l to c r y s t a l and hence on the coincidence rate. Therefore i t was found necessary to substitute a dummy f o i l f o r the t i n source while taking the background rate i n order to keep the background i n t e n s i t y equal to that from the t i n . Both the t i n and dummy f o i l were made the same thickness, 100 m^./cm2. This p a r t i c u l a r weight was chosen f o r the following reasons: (i) a t h i c k source reduces scattering of stray radi a t i o n from c r y s t a l to c r y s t a l , and |) 5 j K> O 6 i o I o 1 o + o o o i ? K> O N O «vj 1 o O • o 4 + •Si o 1 8-l c i. o —<r r —y 4 J i o « -o ry o o» 3? «» «M ro to IN* \s 00 * o ?3 ! 1 V-» -a. 9 t 29 v> \0 o r> « 0 • o 6 $ o V* .< o O a> K» * 0^ ro *> ? <* I — * »*> «•< ^> • + > 4-•« « «N <• «M * s 3 I/1 * #> ~~ « *> 3: *-1 p ? o o % 9 oo o v. k o o r» <P S • i ^ Ki o *•> <0 fO k © i 9^ 6^  5 •o i V> v* N o — k 5) o «/> <P 9 I * ?> *p — m ><• — "> N. 5f * &* U op k o o do ^* e v. IS. $ *-> •*> —-k 5j o w JD • — •1 S V* 00 6 •> o •9 rv. QO o eg CSJ u oo o ro C 2? _c AO o •*» V> «0 ( i i ) Multiple s c a t t e r i n g in a thi c k source tends to remove any angular correlation which might occur i n double beta decay and so af f e c t the measured coincidence rate. A compromise between the loss of transmission f o r betas with a thick source and the favourable e f f e c t s mentioned above was made at a thickness of 100 m^,/cm2 It i s to be expected that the source thickness w i l l not have much e f f e c t on -the width of the kicksorter peak. This i s because the rate of energy loss i n the region from 0.3 Mev. to 3 Mev. and the t o t a l path length of the two betas are both nearly constant. Thus the t o t a l energy l o s t i n the f o i l tends to be constant, and the peak remains sharp f o r thick f o i l s . E. Obtaining the Data Dummy fo i l s ' made of s i l v e r and of natural t i n were found t o be s l i g h t l y active and were discarded i n favour of aluminum. The t i n and dummy were interchanged p e r i o d i c a l l y during the run i n order to compensate f o r any instrumental d r i f t . They were mounted on a s l i d e with an external control. In t h i s way the source and dummy could be interchanged without disturbing the equipment. The counters were shielded by 16 cm. of lead from above, and by 8 cm. of lead i n other d i r e c t i o n s . The t o t a l number of coincidences were recorded on a scaler during each run. The data shown i n the portion of Table I I I c a l l e d "kicksorter readings" were obtained during a t o t a l counting period of 264 hours. The right side of the table shows the average counts per hour as calculated f o r each channel. The difference between the t i n i n t e n s i t y and the background i s shown on the extreme r i g h t . TO 34-3 " l o BACKGROUND * Sn ' 2 4 O '4a T o 1, - • 1 i T T " V T o ^ C * -1-i 1 * 1 T I |/1 10 1-5 20 2-5 SUM OF THE BETA ENERGIES - Mev. 30 F i g . 22. Coincidence Spectra Obtained on Kicksorter. 37 The spectra from t i n and from background so obtained are plotted in F i g . 22. Standard deviations are shown f o r the background points. the standard deviations f o r the t i n spectrum are not shown, but are s l i g h t l y . l e s s . The large number of counts i n channel 18 (energies of 3 Mev. and larger) are due to cosmic ra d i a t i o n . The counting rate of channel IS was large enough to provide an experimental check on the s t a b i l i t y of the kicksorter and of the pulse amplifiers. The rate i n channel 18 i s shown at the bottom of Table H i together with the t o t a l coincidence rate obtained on the external scaler. I t i s seen that the arrangement -.• was s a t i s f a c t o r i l y stable. F. The Shape of the Background Spectrum The f i r s t two channels received no counts because the coincidence mixer discriminators were set to accept only 0 .3 Mev. events. This was done since the acceptance of lower energy p a r t i c l e s increased the background i n a l l channels. The e f f e c t of discriminator setting on the background per channel can be seen from these measurements taken i n the 2 Mev. channel: the backgrounds at discriminator settings of 0.1, 0.2 and 0 .3 Mev. were approximately 2, 1 and J counts per hour. A compromise between having a low background and l o s i n g part of the double beta spectrum was taken by setting the discriminators at 0.3 Mev. Another f a c t o r t o which the background rate proved sensitive was the separation of the anthracene c r y s t a l s . The t o t a l background at 3 mm. c r y s t a l separation was 155 10 counts per hour, and at 1 cm. was 44 ± 1 counts per hour. So the 1 cm. separation was preferable notwithstanding the s l i g h t l o s s i n s o l i d angle. In an attempt to lower the background rate s t i l l further an anticoincidence device was devised. It consisted of a s c i n t i l -l a t i o n counter using a solution of terphenyl i n toluene as a s c i n t i l l a t o r . This solution was contained i n a large s i l v e r e d vessel placed above and around the two anthracene c r y s t a l s . This had the e f f e c t of removing one fourth of the counts from the l#th channel and leaving the important center channels untouched. For t h i s reason i t was discarded as an unnecessary complication. The f i n a l arrangement had a c r y s t a l separation of 1 cm., and no a n t i -coincidence device. The poor e f f i c i e n c y of the anticoincidence device was rather surprising. However, at the discarded 3 mm. separation i t successfully cancelled 30% of the t o t a l background. So the explanation of the poor cancellation must l i e i n a poor geometry f o r t r i p l e coincidences. Probably the anticoincidence device i s very s e n s i t i v e to penetrating showers and the geometry f o r showers i s best when the c r y s t a l s are close,. The low rate of 0.5.per hour per channel made experimen-t i n g with the geometry tedious. At l e a s t two days were necessary to test the background with each new arrangement. G. Comparison with Previous Work A f a i r comparison of background i n t h i s and,previous work can be made i f we consider backgrounds at the peak p o s i t i o n . It has been shown (part D) that absorption i n the f o i l would not widen the peak. So the peak width may be caused by the energy resolution of the s c i n t i l l a t i o n counters only. I f t h i s resolution i s 10$, then the peak would occupy one, or at the most two, channels. Since the background i s 0.5 counts per hour per channel, the 39 " e f f e c t i v e background!' i s no more than 1.0 per hour. This compared favourably with that of Fireman^ (14 per hour) and that of Libby37 (80 per hour;}. The second advantage of t h i s experiment was that i t was capable of unambiguous i n t e r p r e t a t i o n . It i s d i f f i c u l t to imagine a radioactive contamination which would have given r i s e to a peak. On the other hand, contamination e f f e c t s i n conventional counting experiments are indistinguishable from true e f f e c t s . The c o n f l i c -t i n g r e s u l t s of Fireman and Libby represent a case i n point. 1 1 1 1 + 0-2 ' + 0*1 c ) c — HOUR 0 0 c < c > f ( > PER 1 1 -L O 1 1 ( J COUNTS - 0*1 1 c c - c c — - 0-2 1 1 1 1 1 c )+07_ 0-5 IO l'5 2 0 2-5 3 0 SUM OF THE BETA ENERGIES - Mev. F i g . 23. Difference Between Coincidence Spectra Shown i n F i g . 22. I I RESULTS AND CONCLUSIONS F i g . 23 shows the coincidence spectrum derived from F i g . 22 by subtracting the background. The standard deviations are shown. No trace of a peak was found. In order to place a lower l i m i t on the h a l f - l i f e of double beta decay from t h i s negative r e s u l t , the smallest e f f e c t detectable i n F i g . 23 must be estimated. To do t h i s a knowledge of the peak width i s important.. The width was estimated i n Section IIIG to be one, or at the most two, channels. Thus a peak of 0.2 counts per hour would just be detectable i n F i g . 23. The f r a c t i o n a l s o l i d angle subtended by each c r y s t a l was s l i g h t l y l e s s than 0.5. The geometric e f f i c i e n c y f o r coincidence counting i s therefore close to 0.2, a consideration not noted by Fireman^ 0. The correct h a l f l i f e observed by Fireman i s 0.2 x 10^° years rather than 0.4 x 10"^° years. The losses from absorption i n the source may be taken int o account by a transmission fac t o r , F, which i s estimated to 43 be 0.4 at 1.0 Mev. and 0.7 at 3.0 Mev. This transmission factor includes an estimate of the counting losses incurred by biassing the coincidence mixers at 0.3 Mev. Thus a detectable d i s i n t e g r a t i o n rate i s dN _ 0.2 ™ dt * ' 0.2F counts per hour , . = I / 3,760 counts per. year . The number of Sn-1-2^ atoms present i n the source may be calculated by N = I j n £-where m (the mass used) = 200 md L (Loschmidt's number) 6 x 10 23 j f (the f r a c t i o n of enrichment i n Sn122*-) - 0.95, and Mji(the molar weight of t i n ) = 124 gms. So N - 6 x 1 0 2 3 x .2 x 0.95 124 r 0.92 x 1 0 2 1 atoms. Then the lower l i m i t of the h a l f - l i f e i s given by T - log 2 1 - — N dN 0.693 x 1 0 2 1 x 0.92 f S,760 0.3 x 1 0 1 7 to 0.7 x 10 1? years. The negative r e s u l t s cannot be construed as showing the Majorana theory wrong, f o r the t r a n s i t i o n studied might be forbidden by the change of spin or p a r i t y involved. The work i s i n agreement with the work of W.F. Libby 3? and J.S. Lawson 3 8. K The r e s u l t s are i n disagreement with the work of E.L. Fireman. 3 6 : :A similar experiment to the one reported here with e s s e n t i a l l y the same r e s u l t s has just been reported by J.A. McCarthy, B u l l . Am. Phys. Soc. 27, No. 3 18 (1952). REFERENCES 1. Nuclear Data, National Bureau of Standards, C i r c u l a r 499/ U.S. Department of Commerce. 2. G.T. Seaborg, I. Perlman, Rev. Mod. Phys. 20, 585 (194#). 3. M. Deutsch, L.G. E l l i o t t , R.D. Evans, Rev. S c i . Inst. 15, 178 (1944). 4. E.N. Jensen, L.J. L a s l e t t , W.W. Pratt, Phys.Rev. 25, 458 (1949) 5. W.F. Hornyak, T. Lauritsen, V.K. Rasmussen, Phys.Rev. 76, 731 (1949). 6. Planck's Radiation Functions and E l e c t r o n i c Functions, Mathematical Tables Project, W.P.A. f o r the City of New York, 1941. 7. M.L. Wiedenbeck, K.Y. Chu, Phys. Rev. 72, II64 (1947). 8. R.C.A. Tube Handbook, Tube D i v i s i o n , Harrison, N.J. 9. J.I. Hopkins, Phys. Rev. 7Z> 406 (1950). 10. W.C. Elmore and M. Sands, E l e c t r o n i c s , McGraw-Hill Inc., 1949, page 60. 11. D.J.X. Montgomery, Cosmic Ray Physics, Princeton University Press, 1949, page 356. 12. D. Halliday, Introductory Nuclear Physics, John Wiley and Sons, 1950, page 150. 13. Nuclear Physics, University of Chicago Press, 1950. 14. W. H e i t l e r , The Quantum Theory of Radiatipn, Oxford University Press, 1944, page 123. 15. K.C. Mann, and M.J. Ozeroff, Can. Jour. Res. A, 2£, I64 (1949). 16. A. Alechanov and G.D. Latyshev, Compt. rend. acad. s c i . U.S.S.R. 20: 429 (1948). 17. CD. E l l i s , Proc. Roy. Soc. (London), A, 138: 318 (1932) A, 143: 350 (1934) 18. F.E. 0'Meara, Phys. Rev. 79, 1032 (1950). 19. W. R a i l , R.G. Wilkinson, Phys.Rev. 71, 321 (1947). References (continued) 20. L.A. Beach, C.L. Peacock, R.G. Wilkinson, Phys.Rev. 76, 1585 (1949). 21. C.H. Goddard, C.S. Cook, Phys. Rev. 76, 1419 (1949). 22. D.A. Lind, J.R. Brown, J.W.M. DuMond, Phys. Rev. 76, 591 (1949). 23. L.C. M i l l e r , L.F. Curtiss, Bur. Stand. J. Research £3, 359 (1947). 24. B.D. Kern, D.J. Zaffarano, A.C.G. M i t c h e l l , Phys. Rev. Jit 1142 (1943). 25. C.S. Cook, L.M. Langer, Phys. Rev. Jl> H49 (1943). 26. Iowa State College Progress Report 46. 27- £. Fermi, Z. Physik 33, 161 (1934). 23. J.Y. Mie, Phys. Rev. 31, 237 (1951). 29. L.J. Rainwater, C.S. Wu, Nucleonics, Oct. 1947. 30. A.B. Thomas, Nucleonics, Feb. 1950. 31. E. Rutherford, F. Soddy, P h i l . Mag. 4., 370, 569 (1902); 5, 441, 576 (1903). 32. E. Majorana, Nuovo. Cim. 14, 171 (1937). 33. M. Goeppert-Mayer, Phys. Rev. j£, 512 (1935). 34. W.H. Furry, Phys. Rev. 56, 1134 (1939). 35. E. Feenberg, Rev. Mod. Phys. 19, 239 (1947). 36. E.L. Fireman, Phys. Rev. 75, 323 (1949). 37. M.I. Kalkstein, W.F. Libby, Phys. Rev. 3£, 363 (1952). 33. J.S. Lawson, Phys. Rev. 31, 299 (1951). 39. R.G. Winter, Phys. Rev. 35, 637 (1952). 40. M.G. Inghram, J.H. Reynolds, Phys. Rev. 1265 (1949). 41. C.A. Levine, A. Ghiorso, G.T. Seaborg, Phys. Rev. 27, 296 (1950). 42. Benjamin G. Hogg, Henry E. Duckworth, private communication. 43. E.L. Fireman, University of Princeton t h e s i s , 1943. 

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