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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 UNIVERSITY FACULTY  OF BRITISH  COLUMBIA  O F G R A D U A T F . STUDIES  P R O G R A M M E OF T H E FINAL ORAL EXAMINATION FOR T H E DEGREE OF D O C T O R  OF PHILOSOPHY  of  ROBERT  MICHAEL  PEARCE  B.Sc. (M c G i 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 B U I L D I N G  COMMITTEE  Professor Professor Professor Professor  IN CHARGE:  Dean W . H . Gage, Chairman F. A . Kaempffer Professor K . C. Mann Professor W. Opechowski Professor J . B. Warren Professor  H . Adaskin D . Derry B. Savery 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 . H u l l Group Theory—Professor D . C. Murdoch Topics in Applied Mathematics—Professor E . Leimanis  THESIS  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  II 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 in PHYSICS  We accept t h i s t h e s i s as c o n f o r m i n g t o t h e standard r e q u i r e d from candidates f o r t h e degree o f DOCTOR OF PHILOSOPHY.  Members o f t h e Department o f P h y s i c s THE UNIVERSITY OF BRITISH COLUMBIA MAY, 1952  ABSTRACT PART 1  A new method has been used to suppress the undesirable Compton electrons ordinarily present i n 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 detection of new gamma rays at .391, .#57 and 1.00 Mev. i n Ra(B +- C), at 1.01 Mev. i n T a Mev. i n Sbl24. Co G. 6  l 8 2  , and at .472 and .843  No.new gamma rays were observed from  ABSTRACT PART 2  A search for double beta decay i n Sn^ ^ has been 2  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  l O ^ years has been set on the h a l f - l i f e of 1  the process.  •i  TABLE OF CONTENTS PART  1  THE SUPPRESSION OF BACKGROUND IN SOME PHOTOELECTRON SPECTRA  I  INTRODUCTION  II  EXPERIMENTAL PROCEDURE  III  A.  The D e t e c t o r  B.  The S o u r c e  C.  The A n t i c o i n c i d e n c e  D.  The S u p p r e s s i o n o f Compton E l e c t r o n C o u n t s a Function of Energy  E.  L o s s o f Photopeak  Ra ( B +  B.  V  Counter Circuits as  Intensity  RESULTS OF THE GAMMA RAY STUDIES A.  IV  Counter  T a  C)  182  C.  Co  6 0  D,.  Sb  1 2  ^  SOME ASSOCIATED STUDIES IN T a A.  The B e t a  Spectrum o f  Ta ^  B.  Gamma-Gamma C o i n c i d e n c e s  1  1  8  2  2  i n Ta- -^ 1  I  2  CONCLUSIONS  APPENDICES 1.  Analysis  of the  Improved S t a t i s t i c a l  2.  Internal  Reflection  in  Lucite.  Accuracy.  PART 2 ON THE DOUBLE BETA DECAY OF Snl24  INTRODUCTION SOME PREVIOUS WORK A.  The T r i p l e t  5 0  Sn  B.  The T r i p l e t  4  P a  C.  The T r i p l e t  5 2  Te^~  D.  The T r i p l e t  9  U  6  2  2  1 2  1  3  4  1  -  ^Sb  1 2  ~  ^ A g  1  -  53  0  ^  -  9  3  l l 3  Np  2  ^  1  0  °  3  g  EXPERIMENTAL PROCEDURE A.  The P r i n c i p l e  of the  B.  The E x p e r i m e n t a l  C.  Calibration  D.  The Optimum S o u r c e  E.  Obtaining  F.  Shape o f t h e B a c k g r o u n d  Spectrum  G.  Comparison w i t h  Work  Arrangement  of the  the  Experiment  Kicksorter Thickness  Data  Previous  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  F E D - 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.Ta .  FIG. 16  PHOTOELECTRON SPECTRA FROM GAMMA RAYS C o .  FIG. 17  PHOTOELECTRON SPECTRA FROM GAMMA RAYS S b  FIG. 18  KURIE PLOT OF THE T a  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-..  182  6 0  1 8 2  1 2 4  .  BETA.-GROUP.  ACKNOWLEDGEMENTS  The work d e s c r i b e d by a G r a n t - i n - A i d - o f - R e s e a r c h  in this allotted  the N a t i o n a l Research C o u n c i l o f I  am i n d e b t e d t o  s u g g e s t i o n s and d i s c u s s i o n s  the  d u r i n g the  D r . K . C . Mann by  invaluable  course  C o u n c i l awards  continued effort  c o l l a b o r a t i o n with  supported  Canada.  f r o m 1949  The work d e s c r i b e d i n P a r t done i n  to  D r . Mann f o r  N a t i o n a l Research made p o s s i b l e  t h e s i s was  Dr. E . K .  II  Darby.  of the to the to  of the  research. author  1952. thesis  was  B A R T  1  THE SUPPRESSION OF BACKGROUND IN SOME PHOTOELECTRON SPECTRA  I INTRODUCTION It levels  i s to  be e x p e c t e d  that  of radioactive nuclei w i l l  theories predict  of nuclear forces, energy l e v e l  since  sequences.  a knowledge  o f the  p r o v i d e t h e means o f any s u c c e s s f u l For t h i s  testing  t h e o r y must  reason, the  schemes"of r a d i o a c t i v e n u c l e i have been t h e  energy-  subject  "decay  of  considerable  12 research. of  '  By " d e c a y  the nuclear l e v e l s  parity  together  the  as t o  energy,  o f the energy  levels  r a d i a t i o n s from the n u c l e u s ,  f r o m one s t a t e t o  another.  a complete d e t e r m i n a t i o n  a n g u l a r momentum ( s p i n ) ,  w i t h t r a n s i t i o n s between  c a s e s a knowledge of  scheme" i s meant  these l e v e l s .  I n most  c a n come o n l y f r o m a  which r e s u l t  The d e c a y o f  from  and  study  transitions  a n u c l e u s by primary  e l e c t r o n emission to the  daughter nucleus i s u s u a l l y followed  the  d a u g h t e r n u c l e u s b y gamma r a y s o r i n t e r n a l  de-excitation  o f the  conversion electrons.  It  measure t h e  and e n e r g i e s  intensities  T h i s may make p o s s i b l e t h e scheme. sequence  However i t o f energy  consistent  with the  difficulties,  it  is  customary i n t h e s e i n v e s t i g a t i o n s  assignment  which would l e a d to  experimental evidence. A  p r e s e n t have  For t h i s  constructed i n t h i s  the  decay  more t h a n one  gamma-ray t r a n s i t i o n s Because  of  intensity  escaped d e t e c t i o n .  t h e s e w o u l d u n d o u b t e d l y make e a s i e r  tion  postulate  to  gamma r a y s .  of a self-consistent  i s h i g h l y probable t h a t l o w  which are a c t u a l l y  scheme*.  of a l l detectable  may be p o s s i b l e t o  levels  by  experimental gamma r a y s  A knowledge  choice o f a unique  of  decay  r e a s o n , an a p p a r a t u s h a s b e e n d e s i g n e d and laboratory to  o f low i n t e n s i t y  gamma-rays.  increase the  probability of  detec-  Evaluated tube  Baffl  source  H  Defector  1 Current in coil focusces one energy.  fa  SCHEMATIC D/AfRAM Of THIN LEMS SPECTROMETER  Conventional bhofoelectron  Sjpecirmn. Pkoto-peaks  <*7  c  Spectrum ivitK  Electron Momentum Fiq.  GENERAL THE  2  SHAPE OF A PHOTOELBCTRON SPECTRUM. SHAPE DESiRZ®  IS SHcWAf BELOW.  2 Gamma-ray e n e r g i e s are u s u a l l y measured by the  photo-  e l e c t r i c t e c h n i q u e , whereby p h o t o e l e c t r o n s are e j e c t e d from t h i n lamina by the gamma-rays.  Such a lamina i s c a l l e d a " r a d i a t o r "  and u s u a l l y c o n s i s t s o f some h i g h atomic number m a t e r i a l , s i n c e the p r o b a b i l i t y o f p h o t o - e l e c t r o n emission i n c r e a s e s r a p i d l y w i t h the atomic number Z.  In t h i s method, a b e t a - r a y spectrometer i s  used t o study t h e p h o t o e l e c t r o n s . The t h i n - l e n s type o f spectrometer used i n t h i s work has been d e s c r i b e d i n d e t a i l e l s e w h e r e . ^ I t s  operation  depends upon the f o c u s s i n g o f e l e c t r o n s by the magnetic to  a current i n a large iron-free c o i l .  field  due  The e l e c t r o n s pass down  an evacuated tube which l i e s on the a x i s o f the c o i l and c o n c e n t r i c w i t h i t (see F i g . 1 ) .  B a f f l e s are arranged w i t h i n the tube t o  d e f i n e a path f o r the e l e c t r o n s .  The momentum o f the e l e c t r o n s  so f o c u s s e d at the d e t e c t o r i s determined by the c o i l c u r r e n t . t h i s way  In  t h e i n t e n s i t y o f f o c u s s e d e l e c t r o n s i s s t u d i e d as a  f u n c t i o n o f c o i l c u r r e n t o r momentum and a spectrum  i s obtained.  I f p h o t o e l e c t r o n s are f o c u s s e d at an energy E spectrometer, the energy  hx> where  by the  o f the gamma r a y i s g i v e n by =  E  +  k  E  h i s P l a n c k s constant, 1  \) i s the gamma-ray frequency, and  E^ i s the K - s h e l l b i n d i n g energy o f the radiator.  In f a v o u r a b l e cases p h o t o e l e c t r o n s from the L - s h e l l o f the r a d i a t o r may  be d e t e c t e d , although the c r o s s - s e c t i o n 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 h i g h e r e n e r g i e s  than the corresponding K-photopeaks because o f the s m a l l e r L - s h e l l binding  energy.  COMPTON TRAJECTORY  PHOTOELECTRON TRAJECTORY SOURCE 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 jp  , t h e momentum i n a m a g n e t i c  p  =  f i e l d H-'is g i v e n  e Ef  radius  by  .  c  Thus to  p  is  directly  H, since  varies  is  result  current.  a constant  that  coil  to  calibrate  current i s the  energy,  of the  Hf  i n g a u s s - c m : and  spectrometer.  -  E In a c t u a l tables^  radiator  is  0  practice,  is  known l i n e  is  shown t o  i f  Y  E  e  follow  <e E  i -  +  0  s u c h a c o n v e r s i o n i s made e a s i e r p,  E,  and Hj  for  3  a large  p  2  *  to  >•  placed  close  s o u r c e o f gamma r a y s .  to  the  of  published of.energies.  technique , 3  a r e f o u n d t o be much more i n t e n s e  produced i n the  by  spread  photoelectric  between s o u r c e  terms  i n g a u s s cm. , and  conventional  are therefore  magnet  from  In the  detection  the  The p r i m a r y  than the  photo-  t h i n r a d i a t o r a n d must be removed  photoelectrons  feasible.  and r a d i a t o r ,  (see  photoelectric  radiator is  desirable  as  Fig.  3).  A low  cross-section  to  The p r i m a r i e s  a b s o r b e d i n a l o w a t o m i c number (Z ) m a t e r i a l  chosen because the  present,  measured  C o n v e r s i o n f r o m momentum  =  is  s i n c e no i r o n i s  H  i n Mev.  which g i v e  particles  electrons  Hf Hy"  Furthermore  l i n e a r l y with the  and some w e l l  instrument.  f . p  I,  therefore  c a l i b r a t i o n o f t h e momentum i n  possible  where  only the  momentum v a r i e s  , may be e a s i l y  '  make t h e  the  Thus s i n g l e p o i n t  of  beta  proportional to  d i r e c t l y w i t h t h e magnet c u r r e n t ,  with the  the  ' •'  Z  varies  a photoelectron  placed  absorber as  source.  Z  is  , and  4 Unfortunately, ters  in this  absorber,  are focussed ray  appears  energy  to  many gamma-rays  and some o f t h e  and r e c o r d e d . as  for  symmetrical,  bv>  -  h,v>  »  magnitude appear  primary beta-ray as t h e  i n the  distribution  as  0.51  vicinity were  peaks  of this  Detection  is  l e s s than the  o b t a i n e d by t h e  the  intensity  events. ility  imposes  short  lived radiations,  a limit  source This  detector  is  a> peak  on t h e long  time  possible  peak.  consuming,  accuracy  electron  immediate background  o n l y when an  Since the the  possible.  situation,  It  In the  are not  part^of  number o f instab-  case  designed f o r use  a counter  Compton with the  are focussed  of  of  available.  constructed  which d e t e c t s  particles  spectrum  uncertainty  an a p p a r a t u s  consists,in  the  uncertainty  and i n s t r u m e n t a l  counting times  spectrometer  the  photoelectrons  and i t s  has been measured w i t h of the  of  2.  m e a s u r e d t h a n i f no  arranged i n anticoincidence  end where  The  smooth Compton  Compton b a c k g r o u n d h a s b e e n  end o f t h e  counter  intensity.  curve o f F i g .  intense  same o r d e r  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 is  spectrometer.  zero  much l e s s  o f the  o f a peak becomes  To i m p r o v e t h i s  a thin-lens  is  r e c o r d i n g o f random e v e n t s ,  Such a p r o c e s s  suppress t h i s  but  background,  intensity  c a n be made  gamma-  Mev.  on t h e  upper  Compton d i s t r i b u t i o n i n t e n s i t y which i s  line  h a s t o be more c a r e f u l l y  present.  spectrum from  distribution is  spectrum,  shown i n t h e  -Because  electrons  0 . 2 5 5 Mev.  photoelectron  spectrum as  Compton  approaches  The Compton e l e c t r o n than the  resulting  Compton e n c o u n -  The d i s t r i b u t i o n f r o m a s i n g l e  a smooth,- - a l m o s t  a maximum w h i c h  suffer  to in at  the  electrons. counter  and r e c o r d e d .  at  the  Thus  a focussed is  beta  observed  particle  i n the  is  source  not  end c o u n t e r .  e l e c t r o n produces a c o i n c i d e n t of  the  spectrometer,  spectrum i s  pulse  and a c i r c u i t  The i d e a l e f f e c t electron  r e c o r d e d when a s i m u l t a n e o u s  of this  apparatus Ta Ta  1&2  ,  are  60  and Sb  thesis  12A. . (  to  say,  counters  a Compton  at  each  reject  end  this  event.  p r o c e d u r e upon a t y p i c a l  photo-  The " s i g n a l - t o - n o i s e h a s b e e n made  describes  the  easier.  operation of  o b t a i n e d on  ratio"  the  Ra (B + C ) ,  I n a d d i t i o n , o t h e r measurements t a k e n  on  reported. It  detection  1 of t h i s  is  arranged to  detection  and r e p o r t s o n measurements  Co  i n the  shown i n F i g . 2 .  h a s b e e n i m p r o v e d , and p h o t o p e a k Part  is  That  event  is to  be e x p e c t e d  o f weak g a m m a - r a y s ,  of  decay  schemes  is  not always p o s s i b l e  the  t h a t w i t h a b e t t e r method o f uncertainties  s h o u l d be r e d u c e d . i n the  However,  assignment  i n the even  and o t h e r  assignment  then  uniqueness  supporting  7 evidence  must be u t i l i z e d .  w h i c h gamma-rays experiments of  T a ^ ^ has  C o i n c i d e n c e measurements t o  a r e i n c a s c a d e may c l a r i f y  have been p e r f o r m e d w i t h T a a l s o been measured.  .  the  determine  situation.  The b e t a  .Such  spectrum  6 II EXPERIMENTAL PROCEDURE  A.  The D e t e c t o r  Counter  F o r t h i s work on Compton s u p p r e s s i o n , t h e was  changed  over from G e i g e r  T h i s change was n e c e s s a r y the mixing c i r c u i t ,  (see  because  crystal. 50  the  electrons  that  arrangement  gaskets. tions.  of  the tube  a flake  coupling i s lucite  of anthracene  reflections the  forms the  is  as  large  the  i s held against  showed  spectrometer.  by t h i n  (see  the  nepprene  occur at 2).  connec-  detector  through a hole This  consisted  The p h o t o c a t h o d e  lucite  Without t h i s  Appendix  pass  crystal.  of transparent  scintillations  glass interfaces,  defocus  Tests  vacuum s e a l t o t h e  detector  as  photomultiplier  tube.  a c t i o n of the  mounted on l u c i t e .  surfaces.  o f the  detec-  s h i e l d i n g and d i d n o t  Focussed e l e c t r o n s  e n s u r e d by a l a y e r  and g l a s s  are  c a r r i e s t h e h e a d a m p l i f i e r and c a b l e  and s t r i k e  the m u l t i p l i e r tube  focussing  steel  were made " l i g h t - t i g h t "  spectrometer.  in  anthracene  To s h i e l d t h e  p r o v i d e d adequate  The o t h e r e n d p l a t e  endplate  spectrometer  spectrometer  was h o u s e d i n a m i l d  One e n d p l a t e  end o f t h e i n the  it  i n t e r f e r e with the  Endplates to  of the  i n the m u l t i p l i e r tube.  field,  appear t o  i n 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  from t h i s this  focussed  field  counters.  C.)  p h o t o m u l t i p l i e r ^ u s e d w i t h an  The m a g n e t i c  gauss at  the  RCA 5#19  scintillation  a r a p i d r e s p o n s e was n e e d e d  Section II  Beta p a r t i c l e s t e d by a t y p e  counters to  spectrometer  and a d e q u a t e jelly  between  precaution, the  of  optical the  internal  l u c i t e - a i r and  at  DISCRIMINATOR  INTEGRAL BIAS CUMBS £L£CTRON  TAKEN  VOLTS  AT VARIOUS  £NER$l£S.  7 Counts from kept  to  and l o c a l c o n t a m i n a t i o n  a minimum b y u s i n g an a n t h r a c e n e ' c r y s t a l  A crystal is  cosmic r a y s  0 . 4 mm. i n t h i c k n e s s  used.  It  and a p p r o x i m a t e l y  was f o u n d t o be 100$ e f f i c i e n t  was d e t e r m i n e d b y  crystal with that of RaE.  of a geiger  say,  25  comparing the  counts  efficiency  o f the  At l o w e n e r g i e s ,  than thermal noise the  spectrometer  this  limit,  i n the  due t o  does n o t  scattering  source and from o t h e r  the  seen i n F i g .  curves from the set  to  focus  s i z e does  since  the  not  However,  extend to  i n the  electrons  The S o u r c e  must  that  the  energies  no  region  larger  usefulness  of  much b e l o w absorption  curves  to  electron  are i n t e g r a l  different  energies  energy^  bias  spectrometer  indicated.  above 850 Kev. i n t h i s  particular  then pass completely t h r o u g h the  The case  crystal.  Counter  photoelectrons count  such  causes.  at the  In order t h a t the  usual  energy  long electron path,  The s e p a r a t e  increase  is  is  s c i n t i l l a t i o n counter taken with the  electrons  pulse  B.  4.  source  tube.  i n the  pulse height  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 can be  the  s i x weeks o p e r a t i o n .  i n the m u l t i p l i e r .  itself  of  with the  anthracene  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 Kev.  particles, per minute.  per minute i n a g e i g e r  c r y s t a l h a d t o be r e p l a c e d a f t e r  above 150  diameter  counter with a c o l l i m a t e d beta  The e v a p o r a t i o n i n v a c u o the  7 mm. i n  counts  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  background of,  s m a l l volume.  for beta  w h i l e h a v i n g a cosmic r a y background o f t h r e e The e f f i c i e n c y  of  are  the  be u n s u p p r e s s e d ,  Compton e l e c t r o n s  explained i n the  Compton e l e c t r o n s  but  Introduction.  not  the  be  counter at  s u p p r e s s e d and the  photoelectrons,  T h i s was  as  source  end  was  arranged i n the  manner  SOURCE COMPTON TRAJECTORY PHOTOELECTRON TRAJECTORY RADIATOR ABSORBER FOR PRIMARY BETAS  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 between t h e accepted  low  Z  by t h e  t r o n s on t h e  in Fig. absorber  other hand,  anti-coincidence  into the  the  photomultiplier. along is  to  the  vacuum s e a l . against rod i s  the  the w a l l s  difficulty Fig.  the  envelope  lucite  jelly  somewhat  is  kept  photomultiplier to  absorb  the  i n the  crystal  be  p r o v i d e an o p t i c a l  primary betas. becomes  primary betas. c a n c e l l e d by t r u l y  of  with the  is  left If  is  is  and t h e  light  not done,  F u r t h e r m o r e , many o f t h e primary beta  a  pressed  the  path.  around the  excessive because o f the  coincident  form  lucite  scinoff source  This  s h o u l d e r s on t h e  between source  this  The r o d  The  path f o r  the  drilled  crystal.  optical  lucite  efficient  to  opaque b o d y o f t h e  scintillations  ensure  crystal  hold  internal reflections  sloping  between t h e  protrudes  serves to  vacuum s y s t e m .  ;  6) w h i c h r e f l e c t e d  the  was  are  rod which  photomultiplier  Unfortunately the  interfere  above  w i t h A p i e z o n wax t o  of the  - making use  produce  i n a small hole  to  r o d away f r o m t h e  Enough l u c i t e  the  rod close  hence  crystal  The r o d a l s o  end-plate  and  described  a lucite  contained  Comptons  not.  c o u p l i n g from the  was overcome b y p u t t i n g  transparent of  The g l a s s  of the r o d .  is  lucite  spectrometer  i n the  end o f  optical  The s o u r c e  end o f t h e  was f o u n d t o  (see  provide  polished i n order to  tillations  arrangement  vacuum s y s t e m .  a diameter of the  fixed  do  placed  Photoelec-  T h u s Gomptons  photoelectrons  Canada B a l s a m on t h e  and t o  is  all  crystal  The r a d i a t o r and a n t h r a c e n e  spectrometer  source  while  so t h a t  beyond t h e  counter.  6 shows how t h e  realized.  radiator  crystal  must p a s s t h r o u g h i t .  originate  source  pulses  Fig.  mounted w i t h  and t h e  spectrometer  l e a v e no r e c o r d i n t h e  physically  The a n t h r a c e n e  5.  lucite  source.  glass  A  envelope  transfer. and c r y s t a l the  to  counting  rate  high intensity  photoelectrons particles.  of  would By  the  DETECTOR COUNTER  ANTICOINCIDENCE COUNTER AMPLIFIER  AMPLIFIER MAQMET COIL  DISCRIMINATOR  Shaped  DIFFERENCE AMPLIFIER  f>ultn  0*1 DSL  SjbtctiHim of frultcs  DISC4/MWAT04 SCALER  BLOCK DIAGRAM  OF  AHTICO/A/CIDMCC  ARRANGEMENT  9 same argument be l o s t  it  might  because o f t r u l y  gamma-ray  efficiency  estimated  to  may be  cylinder  The n e x t counter  to  cancel  However,  the  so t h a t  this  was  effect  counters  coincidence  from the with  Although the  p o i n t e d out  above,  gamma c o u n t i n g r a t e to  be  mc Ra(B + C) than the  rate  i n the  coincidence  a mild  spectrome-  the  i n the  pulses. the  output  detector  conventional  pulses  from  reject  source  to the  o f the  crystal  geometry  per is  so  counter. for  crystal is  very  channel.  o r d e r o f 10^  several  channel without  serious  was n e c e s s a r y  shown i n t h e  to  adopt  Fig.  low,  high  was  source, larger  a conventional after  dispass-  anti-  losses. the  b l o c k diagram i n F i g .  unconventional 7.  6).  as  a very  times  microseconds  counting  in  (see  This  a typical  Since  arranged  difficulty  produces  second f o r o f the  a  the  thermal  and w o u l d be  However,  anticoincidence  This rate  The  source  c o u n t e r w o u l d be b l o c k e d when  favourable  counts  from t h 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  Hence i t arrangement  from the  " d i s c r i m i n a t o r s " to  efficiency  c r i m i n a t o r remains i n s e n s i t i v e ing a pulse,  tube  pulses  counter pulses.  2 x 10-* ).  the  the  feed  p r o x i m i t y o f the  gamma r a y  photomultiplier,  stage would f o l l o w ,  detector  source  use  detector  w o u l d be t o  A mixing  due t o t h e  estimated  s t e p was t o  through amplitude  pulses. pulses  detector  Circuits  coincident  method o f d o i n g t h i s  was  would  ( i n t r i n s i c + geometrical)  and end p l a t e s p r o t e c t e d  The A n t i c o i n c i d e n c e  arose  gamma r a y s .  crystal  case o f the  C.  that  coincident  some p h o t o e l e c t r o n s  ignored.  field.  noise  that  be much l e s s t h a n Q . l p e r c e n t ,  ter  two  expected  of the  As i n t h e steel  be  The p r i n c i p l e  -I00O  V.  OUTPUT  INTO  too SL c o a j r .  RCA  DETECTOR  FIG. 8 HEAD AMPLIFIER  + 2oo v.  _ OUTPUT* WTQ V V /ooo A . C O A T .  TtCA  SB19  TVf  {SOURCE  CAKS* FIG.9 HEAD AMPLIFIER  IN34 6 A K 5  6AH6  6J6  €AC7 + Zoo v  IN  FIG. 10 FED-BACK AMPLIFIER USED IN THE DETECTOR  CHANNEL.  DIFFERENCE AMPLIFIER  DISCRIMINATOR  OUTPUT +  v  OUT  ** 6A65  6J6  6AK5  6AG5  k  ANTICOINC. INPUT 6AK5  1/2 6AL5 FIG.II  6AG5  DISCRIMINATOR AND DIFFERENCE  6AG5  AMPLIFIER .  10 of operation  i s as f o l l o w s :  the detector  p u l s e s a r e shaped by a  d i s c r i m i n a t o r and b r o u g h t t o t h e d i f f e r e n c e a m p l i f i e r .  The many  source c o u n t e r p u l s e s a r e a m p l i f i e d and t h e i r whole spectrum subtracted  from t h e shaped d e t e c t o r  pulses.  Any d e t e c t o r  pulse  w h i c h l o s e s a m p l i t u d e 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 discriminator.  Thus t h e 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 o f t h e two d i s c r i m i n a t o r s r u n f a s t e r t h a n does t h e s l o w e r c h a n n e l . A more d e t a i l e d d e s c r i p t i o n o f t h e s e p a r a t e c i r c u i t follows.  The c i r c u i t diagram o f t h e d e t e c 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" "^ a m p l i f i e r w i t h an 1  observed r i s e - t i m e o f 0.1 }i sec. and a maximum output o f 120 v o l t s . I t i s p r o v i d e d w i t h an i n p u t a t t e n u a t o r .  The p u l s e s a r e l e d from  t h e o u t p u t cathode f o l l o w e r t o t h e d i s c r i m i n a t o r shown on t h e l e f t o f F i g . 11.  The shaped p u l s e s from t h e d i s c r i m i n a t o r t h e n go t o  t h e 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 o f  s o u r c e p u l s e s i s a l s o f e d i n t o t h i s stage where t h e y a r e s u b t r a c t e d from t h e d i s c r i m i n a t o r p u l s e s (from t h e d e t e c t o r  counter).  Those  output p u l s e s from t h e d i f f e r e n c e a m p l i f i e r w h i c h r e t a i n t h e i r  full  a m p l i t u d e a r e t h e n a c c e p t e d b y t h e d i s c r i m i n a t o r o f a commercial • s c a l i n g u n i t and r e c o r d e d . The  p u l s e s i n t h e source c h a n n e l a r e d e l a y e d 0.1 fx s e c .  (see F i g . 7) t o a l l o w t h e d i s c r i m i n a t o r o f t h e d e t e c t o r fire. The  channel t o  T h i s d e l a y i s p r o v i d e d by t h r e e f e e t o f RG 65/tJ c a b l e .  head a m p l i f i e r o f t h e source channel i s shown i n F i g . 9.  The  p l a t e l o a d matches t h e 1000 ohm c h a r a c t e r i s t i c impedance o f t h e delay cable i n order t o stop r e f l e c t i o n s i n t h e cable. The  source o r 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 FIG. 12 SOURCE COUNTER AMPLIFIER  Qa«tt-cm.  CANCELLATION AS A RWCTKW OF ENERGY FOR Ta & Ra.  11 Fig.  12.  I t has a g a i n o f 50 w h i c h i s n e c e s s a r y i n t h a t some o f  t h e u s e f u l p u l s e s a r e s m a l l ( s e e s e c t i o n "D").  A l l pulses are  l i m i t e d t o 2 v o l t s by c u t t i n g o f f t h e f i n a l t u b e .  I n t h i s way  t h e 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 n o t be o v e r l o a d e d .  To  s t o p t h e 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 d i o d e s a r e arranged as shown i n F i g . 12.  The r i s e t i m e o f t h e source a m p l i -  f i e r i s 0.04 ^i. s e c , and t h e i n s e n s i t i v e t i m e a f t e r r e c e i v i n g a p u l s e as l a r g e as 1 v o l t was e s t i m a t e d t o be 0;2^ns. D.  S u p p r e s s i o n o f t h e Compton E l e c t r o n Counts a s a F u n c t i o n o f Energy. S u p p r e s s i o n o f t h e Compton e l e c t r o n counts i s n o t  1Q0% e f f e c t i v e o v e r t h e spectrum.  The percentage  o f Compton  counts n o t c a n c e l l e d i s shown as a f u n c t i o n o f momentum f o r T a - ^ and Ra(B + C) i n F i g . 13.  2  S u p p r e s s i o n e f f i c i e n c i e s a r e as h i g h  as 90% i n t h e c e n t r a l r e g i o n o f 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, t h e s u p p r e s s i o n i s poor  i n t h e h i g h a n d l o w momentum r e g i o n s and some e x p l a n a t i o n s h o u l d be g i v e n f o r t h e energy dependence o f t h e s u p p r e s s i o n . O b v i o u s l y , t h e most e n e r g e t i c Comptons i n a  spectrum  can have l o s t no energy t o t h e source c r y s t a l by t h e v e r y f a c t t h a t t h e y 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 t o t h e source c r y s t a l , no c a n c e l l i n g p u l s e can have been prodiced. principle.  Thus t h e most e n e r g e t i c Comptons cannot be c a n c e l l e d i n A c e r t a i n minimum energy l o s s i n t h e c r y s t a l i s  n e c e s s a r y t o produce a c a n c e l l i n g p u l s e , depending on t h e s e n s i t i v i t y o f t h e source c o u n t e r .  The magnitude o f t h i s minimum energy  l o s s determines how f a r t h e poor c a n c e l l a t i o n extends down f r o m  12 the t o p o f the spectrum.  In t h i s i n v e s t i g a t i o n the source counter  can d e t e c t a minimum energy o f about 50 Kev., t h e l i m i t t o the sens i t i v i t y b e i n g thermal n o i s e i n the p h o t o m u l t i p l i e r .  So t h e t o p  50 Kev. o f the spectrum i s not c a n c e l l e d i n t h i s case. A c t u a l l y some Comptons as much as 150 Kev. below t h e maximum energy are not c a n c e l l e d , and an e x t e n s i o n o f the above argument may be used t o e x p l a i n t h i s . Let us c o n s i d e r the Compton d i s t r i b u t i o n from a s i n g l e gamma t r a n s i t i o n o f 1.2 Mev. tic  I t may be s a i d t h a t t h e most energe-  Comptons a r i s e from encounters i n t h e outermost l a y e r o f t h e  source h o l d e r which c o n s i s t s o f the r a d i a t o r and a p o r t i o n o f t h e anthracene c r y s t a l .  In f a c t t h e t o p 50 Kev. o f the spectrum must  come from a l a y e r o f t h i c k n e s s 30 mg/cm  s i n c e t h e r a t e o f energy  2  loss,  is ^  1.7  Kev  at 1 M e v . C o m p t o n  encounters i n  mg/cm  x  t h i s l a y e r w i l l not p r o v i d e c a n c e l l i n g p u l s e s o f s u f f i c i e n t magnitude.  To e x p l a i n t h e changes i n c a n c e l l a t i o n e f f i c i e n c y over t h e  spectrum by the concept o f t h i s i n s e n s i t i v e l a y e r , we must c o n s i d e r the v a r i o u s a n g l e s , 9, which occur between t h e d i r e c t i o n o f t h e i n c i d e n t gamma and the s c a t t e r e d e l e c t r o n . scattered electron i s given E(Q)  =  by 2 m c r 2  r  cos  2  0  l+2r+r where  The energy o f a  =  2  sin  2  2  9 9  '  ^ %  c  2  We have a l r e a d y d e a l t w i t h the head-on ( 9 = 0 ) encounters i n the l a s t l a y e r .  Comptons from 9 = 10° encounters  i n t h e l a s t l a y e r have a good chance o f acceptance by t h e s p e c t r o meter b a f f l e s .  S i n c e E (10°) i s about 100 Kev. below t h e  13 maximum, E ( 0 ) , t h i s means t h a t some o f t h e e l e c t r o n s at 150 Kev. below the maximum o f t h e spectrum come from t h e l a s t l a y e r and are t h e r e f o r e not c a n c e l l e d . 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 c a n c e l l a t i o n at i n t e r m e d i a t e e n e r g i e s . understood  T h i s can be  by c o n s i d e r i n g t h e o r i g i n o f these e l e c t r o n s .  may come from deep w i t h i n t h e s'ourceholder, i n which case w i l l be d e t e c t e d i n t h e a n t i c o i n c i d e n c e c r y s t a l .  They they  They may not  come from t h e l a s t i n s e n s i t i v e l a y e r s i n c e the angle 9 necessary i n a c o l l i s i o n g i v i n g r i s e t o an i n t e r m e d i a t e energy e l e c t r o n i s too l a r g e f o r the spectrometer  b a f f l e s t o accept t h e e l e c t r o n .  Thus a q u a l i t a t i v e p i c t u r e b u i l t on t h e one assumption o f t h e o r i g i n o f t h e u n c a n c e l l e d e l e c t r o n e x p l a i n s t h e shape o b t a i n e d i n t h e i n t e r m e d i a t e and upper r e g i o n s o f t h e spectrum.  There i s  a l s o t h e p o s s i b i l i t y t h a t t h e s c i n t i l l a t i o n s from t h e h i g h energy Comptons are not p r o p e r l y o p t i c a l l y coupled t o t h e photocathode because o f the opaque body o f the source.  T h i s might be t h e case  s i n c e t h e h i g h energy Comptons come from head-on c o l l i s i o n s which tend t o occur, t h e r e f o r e , i n the plane o f the source h o l e and t h e spectrometer  axis.  at h i g h e n e r g i e s The  f  To t e s t t h i s e x p l a n a t i o n o f t h e poor s u p p r e s s i o n a seemingly  p e r f e c t o p t i c a l system was c o n s t r u c t e d .  source was mounted i n t h e l u c i t e r o d by a p r e s s u r e mold, thus  a b o l i s h i n g t h e source h o l e . o f 3000 l b s . per sq. i n . diameter tion.  T h i s was done at 1 4 5 ° C  at a p r e s s u r e  The source used was Co°^ wire o f 0.01 i n c h  i n the form o f an open l a t t i c e l o c a t e d a t the source  posi-  The source was b e l i e v e d s m a l l enough so as not t o 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, t h e behaviour  o f t h i s arrangement  was i d e n t i c a l t o t h a t o f h o l d e r s having l a r g e h o l e s t o c a r r y t h e  source.  The l a t t e r type o f source h o l d e r i s t h e r e f o r e t o be  p r e f e r r e d s i n c e t h e molding process i s time  consuming.  To q u a n t i t a t i v e l y account f o r the shape o f the curve, i t would be n e c e s s a r y t o determine t h e o r i g i n o f the e l e c t r o n s i n d e t a i l f o r a l l energies.  T h i s i s i m p o s s i b l e because o f such  c o m p l i c a t i o n s as t h e l a r g e range o f p o s s i b l e c o l l i s i o n angles, t h e 3 presence o f m u l t i p l e s c a t t e r i n g , and the d i f f e r e n t t r a n s m i s s i o n o f the spectrometer f o r d i f f e r e n t p o i n t s on t h e source. A t e n t a t i v e e x p l a n a t i o n may be g i v e n f o r t h e poor c a n c e l l a t i o n a t low Compton e n e r g i e s (see F i g . 13), t h e e x p l a n a t i o n b e i n g c o n s i s t e n t w i t h t h e i n s e n s i t i v e l a y e r concept used above i n the h i g h energy r e g i o n .  In t h e case o f a s i n g l e 1.2 Mev. gamma-ray  i t was s t a t e d t h a t an i n t e r m e d i a t e energy e l e c t r o n c o u l d not a r i s e from a l a r g e angle encounter i n t h e v e r y l a s t l a y e r o f t h e because the b a f f l e s would n o t accept i t .  crystal,  There i s a chance o f it's  b e i n g accepted, however, i f i t i s m u l t i p l y s c a t t e r e d i n the l a s t layer, o f the source h o l d e r .  Furthermore t h i s chance o f acceptance  i n c r e a s e s f o r low energy e l e c t r o n s s i n c e t h e r o o t mean square angle of s c a t t e r i n g i s p r o p o r t i o n a l t o i ^ .  Hence low energy e l e c t r o n s  have a b e t t e r chance o f o r i g i n a t i n g i n t h e i n s e n s i t i v e l a y e r and b e i n g f o c u s s e d than do Comptons o f i n t e r m e d i a t e energy. Thus t h e main f e a t u r e s o f t h e T a - ^  2  c a n c e l l a t i o n curve  i n F i g . 13, which resembles t h a t o f t h e h y p o t h e t i c a l 1.2 Mev. gammar a y , have been e x p l a i n e d .  F i n a l l y , x>re must pass t o t h e case o f  Ka(B + C) w i t h i t s many l i n e s  (see Table I ) extending t o 2.4 Mev.  At 1 Mev. i n t h e Ra spectrum the preponderance o f Compton e l e c t r o n s comes from t h e h i g h e s t energy gamma r a y s .  So t h e poor  cancellation  o f the Comptons from l i n e s whose e n e r g i e s a r e j u s t above 1 Mev. i s  15 not n o t i c e d .  F o r t h i s reason t h e form o f the c a n c e l l a t i o n  curve  1$2 o f Ra resembles t h a t o f Ta  (see F i g . 1 3 ) , except o f course f o r  a s h i f t i n energy. E.  Loss o f Photopeak I n t e n s i t y The use o f t h e a n t i c o i n c i d e n c e method r e s u l t s i n a s m a l l  l o s s o f photopeak i n t e n s i t y .  Almost  a l l o f t h i s l o s s i s due t o  chance c o i n c i d e n c e s , i . e . a p h o t o e l e c t r o n i s f o c u s s e d but not recorded because source counter.  o f an a c c i d e n t a l l y simultaneous event i n the With a gamma-ray source o f | mc. s t r e n g t h , t h e  photopeak l o s s was measured t o be 5$.  Ah almost n e g l i g i b l e  loss  o f p h o t o e l e c t r o n s r e s u l t s from events i n t h e source counter which are t r u l y c o i n c i d e n t w i t h the p h o t o e l e c t r o n s , i . e . Compton events from gamma-rays which a r e i n cascade w i t h t h e gamma-ray c a u s i n g the p h o t o e l e c t r o n . 0.1$,  T h i s l o s s i s estimated t o be much l e s s than  as was mentioned  i n Section IIB.  I t might be thought t h a t another cause o f photopeak i n t e n s i t y l o s s would be the somewhat l a r g e r s o u r c e - t o - r a d i a t o r d i s t a n c e n e c e s s i t a t e d by t h e presence o f t h e source c r y s t a l . d i s t a n c e i s about 2.5 mm.  i n s t e a d o f say 1.5 mm.  source h o l d e r without the c r y s t a l . does not lower  This  i n the conventional  However, t h e l a r g e r d i s t a n c e  t h e p h o t o e l e c t r o n i n t e n s i t y s i n c e both t h e source and  r a d i a t o r are not p o i n t s but are extended.  Furthermore,  the a n g u l a r  d i s t r i b u t i o n o f t h e p h o t o e l e c t r o n l e a n s so f a r f o r w a r d " ^ at t h e range o f e n e r g i e s i n q u e s t i o n t h a t the s o u r c e - t o - r a d i a t o r d i s t a n c e tends t o l o s e i t s importance,  i . e . i f the source touched t h e  r a d i a t o r , many e l e c t r o n t r a j e c t o r i e s would be at t o o l a r g e an angle t o the spectrometer a x i s t o be accepted by t h e b a f f l e s .  The  l o s s 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  o f Appendix 1.  \  1200  2400  4800  Hp (Gauss-cm.) FIG.  14 PHOTOELECTRON SPECTRA FROM GAMMA RAYS S a ( B t C ) .  9600  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 i s shown in the lower curve of Fig. 14.  The upper curve i s 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/cm thickness. 2  In the earlier work-^, a lead f o i l  of 40 mg/cm thickness was used. 2  The energies of the gamma rays detected are listed i n , Table 1, together with the results of Mann and Ozeroff ^, of 1  Latyschev  and co-workers, and of E l l i s  found at0.391, 0.857 and 1.00 Mev.  .  New lines have been  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 i s found between the separate investigators on the majority of lines. No photoelectrons were found for the line l i s t e d at 2.41 Mev.  This line i s calculated from the Compton endpoint of  2.15 Mev. B.  Ta  l g 2  The gamma ray spectrum of a 1 mc. source of T a ^ 1  Compton suppression i s shown in the lower curve of Fig. 15.  2  with The upper  curve i s 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  .292 .350 .391 .456 .507 .607 .766 .737 .^57 .933 1.00  Ellis  Latyschev 16  Mann 15  .291 .352  .4260  .426 .450 • 496 .607  .4930 .6067 .766 .933 1.12  1.37  1.233 1.379 1.414 1.761  2.15 2.41  7  .2937 .3499  1.10 1.22  1.55  1  2.193  .606 .766  .731  .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  1.12 1.22 1.40  1.77 2.21 2.40  1200  1697  2400  3396  H P FIG. 15  PHOTOELECTRON SPECTRA FROM GAMMA RAYS T a l 8 2 .  4800  (Gauss-cm.)  6787  1 {Jo  Tantalum  was i n c l u d e d i n the present i n v e s t i g a t i o n  because o f c o n s i d e r a b l e disagreement i n r e p o r t e d energy measurements above 300 Kev.  O'Meara  has r e p o r t e d f o u r t e e n l i n e s i n  the r e g i o n from 300 Kev. t o 1.1 investigators. l i n e s at 1.23,  1 9 , 2 0  1.24  '  2 1  *  Mev.  which were not found by o t h e r  In t h i s study, the t h r e e w e l l k n o w n  and 1.13  Mev.  have been i d e n t i f i e d .  a d d i t i o n , a new l i n e at 1.01  Mev.  has been found.  1 9  >  2 0  j  2 1  In  None o f the  l i n e s r e p o r t e d by 0'Meara are p r e s e n t . The smooth prominance 1.0  Mev.  i n the spectrum at a p p r o x i m a t e l y  r e s u l t s from the sudden l o s s o f Compton s u p p r e s s i o n  d i s c u s s e d i n S e c t i o n I I , D.  T h i s o c c u r s where the Compton d i s -  t r i b u t i o n has t h e steepest slope and g i v e s r i s e t o a c h a r a c t e r i s t i c hump.  T h i s hump i s q u i t e d i f f e r e n t from the narrow,  form t y p i c a l to a photopeak. 1.01  Mev.  U n f o r t u n a t e l y , t h e new peak at  i s superimposed on t h i s prominence,  make c e r t a i n t h a t t h e 1.01 r a d i a t o r was  triangular  Mev.  (see F i g . 15).  To  peak was not s p u r i o u s , a l e a d  s u b s t i t u t e d f o r the uranium r a d i a t o r .  The peak i n  q u e s t i o n s h i f t e d by e x a c t l y the d i f f e r e n c e i n K - s h e l l b i n d i n g energy between l e a d and C.  uranium.  Co . 6 0  The p h o t o e l e c t r o n spectrum o f a 0.5  me.  source o f  Go ^ 0  taken w i t h and without Compton s u p p r e s s i o n i s shown i n F i g . 16. The same s o u r c e - h o l d e r geometry was used i n each spectrum. the two w e l l - k n o w n > 3 l i n e s at 1.17 22  2  Mev.  and 1.33  No evidence appeared f o r any weak gamma t r a n s i t i o n s . hump occurs i n t h e C o ^  Only  Mev. were found. A smooth  spectrum as i t d i d i n the case o f T a ^ ^  which has a s i m i l a r h i g h energy spectrum.  2  It i s interesting that  PHOTOELECTRON SPECTRA FROM GAMMA RAYS Sbl24.  GAUSS--.CM-.  i s  yy  ^698  2400  3396 1=3  FIG. 16  PHOTOELECTRON SPECTRA FROM GAMMA RAYS Cc-60.  4300 c m .  19 t h e r e i s no hump i n t h e upper p a r t o f the Ra(B + C) spectrum. Presumably t h i s spectrum i s t h e sum o f the^ s p e c t r a from the many Ra gamma-rays.  Each o f t h e s e s i n g l e s p e c t r a may i t s e l f have a  hump, but t h e composite spectrum may w e l l be smooth. D.  Sb  1 2 4  . F i g . 17 shows the spectrum o f p h o t o e l e c t r o n s e j e c t e d  from a uranium r a d i a t o r by the gamma r a y s from 1 mc. of S b ^ . 1 2  Only t h e spectrum w i t h Compton e l e c t r o n s suppressed i s shown, the o r d i n a r y spectrum b e i n g omitted f o r the sake o f c l a r i t y .  The  percentage s u p p r e s s i o n i s s i m i l a r t o t h a t o f the p r e v i o u s work, except t h a t the gamma r a y s from Sb"'' ^ are so d i s t r i b u t e d i n energy 2  t h a t smooth humps appear at both 2400 gauss-cm. and 5800 gauss-cm. A r a d i a t o r o f t h i c k n e s s 40 m /cm was used t o take t h e spectrum 2  above 4000 gauss-cm. and 24 m /cm was used 2  below.  The gamma r a y e n e r g i e s so o b t a i n e d are given i n T a b l e 2 t o g e t h e r w i t h the v a l u e s o f some other workers. have been found at 0.472 Mev.' and 0.843 Mev.  New gamma r a y s There i s some  evidence f o r the e x i s t e n c e o f a gamma r a y a t 0.609 Mev., because of t h e l a r g e width of the photopeak from the 0.600 t r a n s i t i o n . The remaining energy v a l u e s are i n good agreement previous i n v e s t i g a t o r s .  w i t h those o f  20  TABLE GAMMA RAYS OF S b  Present Investigation  Kern e t a l ^ " 2  2 1 2  ^ IN Mev.  Cook e t a l ^ 2  .472  -  -  .600  .603  .608  .609  -  -  .650  .650  .654  •714  .714  Iowa S t a t e  .598 .645  .732 .817 .843  -  1.71  1.708  1.708  I.67  2.04  2.056  2.04  2.07  2.072  -  -  4 0  530 KEV.  400  KURIE PLOT OF THE  T a " * BETA  qROUP.  21 IV SOME ASSOCIATED STUDIES IN T a ^ 1  A.  The Beta Spectra of T a  2  l g 2  Comparison of observed beta spectra with theory i s usually made with the aid of a Kurie plot. fN(p}/Fp J ^ 2  The function  i s plotted against the energy E, where N(p)  i s the intensity of electrons of momentum p, and F i s a factor to correct for the Coulomb f i e l d of the nucleus. relation  From the Fermi  27  [Nlpi/Fp ]* 2  =  E f f l a x  -E  ,  the plot should be a straight line intercepting the axis at E _ . 182 m  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. i s shown in Fig. 18. A single beta group with an endpoint at 530 ± 3 Kev. can be seen. This endpoint i s 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 d i f f i c u l t . In this plot, the approximation to the Coulomb factor 28 F which i s valid for large Z was used. It i s given by F = l/p •where  p  i s in units of  mc 0  -  0.355  t>ISCRiMtWAT0R  €*c?  A  M  SAC?  Rossi PAIR - — - OUTPUT tsj? t/kcj  INPUT,  COIMCI0€MC£  MIXffR .  22  B.  182 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 i t s e l f .  This was done  by using large anthracene crystals with photomultipliers and accepting only the largest pulses, which were presumably caused by the high energy gammas. Approximately 10 microcuries of Ta the two anthracene crystals.  was placed between  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 i n Fig. 19. coincidence mixer had a resolving time of 0.136 usee.  The  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 i n the following way:  coincidences were  observed from the two gamma rays of a Co ® source in a similar An 4 22 23 geometry. The C o gamma rays at 1.17 and 1.33 Mev. ' ' are in D  00  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  in cascade.  are  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 improvement 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 i s obtained from ,Np — N deviation  29  - N  T  <X_,  , we may write for the standard  of N„ T  where  c  C  '  i s the standard deviation of N  standard deviation of H .  Since  ~t^T  and C£  T  e t c  *  f  o  r r a n d o m  i s the events,  we have N  y  T  +• N  2N„ -lc  NP Taking the usual figure of merit 30 T  1  Np ^ 7  N  „_ ^ ~rr we have IMp *  p  as the condition that the anticoincidence device represents an  24 improvement over c o n v e n t i o n a l o p e r a t i o n .  Or  ^ 2Wg  + N  Nj/  ^  y' 2N  4  C  P  N  Nj  (i)  p  P  Let us d e f i n e the r a t i o s o f the new t o the o l d counting r a t e s f o r p h o t o e l e c t r o n s and Comptons r e s p e c t i v e l y by  . P  C  =  V  ,  (2a)  -  ^1  •  (2b)  The c o n d i t i o n (1) then becomes  N  C  ^  pTi  r—py  T y p i c a l e x p e r i m e n t a l values of respectively.  p  (3)  and  c  Using these v a l u e s i n ( 3 ) ,  are 0.95 and  0.12  the method r e p r e s e n t s  an improvement i n counting f o r a l l cases where  1J[P<;3C" N  C  This  ^  i s always t h e case i n p r a c t i c e s i n c e such l a r g e peaks do not occur.  I t f o l l o w s t h a t the a n t i c o i n c i d e n c e method i s an  improvement i n a l l s i t u a t i o n s .  APPENDIX I I INTERNAL REFLECTION IN LUCITE  The  c r i t i c a l angle 0,  for total internal  reflection  of l i g h t at an i n t e r f a c e i s g i v e n by  0 where  ji  ^  i s the index of For l u c i t e  ji  sin"  1  1/jb  j  refraction. -  1.50  .  Hence the c r i t i c a l angle  0  f  '  = 42°.  Thus the s h o u l d e r s o f the l u c i t e l i g h t pipe (see F i g . 6)  which are at an angle o f 45°,  most s c i n t i l l a t i o n s from the c r y s t a l the p h o t o m u l t i p l i e r .  are adequate t o r e f l e c t t o the photocathbde  of  P A R T  2  ON THE DOUBLE BETA DECAY OF S n  1 2 Z f  26 I INTRODUCTION The  emission  of s i n g l e e l e c t r o n s from r a d i o a c t i v e n u c l e i 12  has been the subject of many r e s e a r c h e s years.  I t i s now  '  d u r i n g the past  fifty  w e l l e s t a b l i s h e d t h a t i n s i n g l e beta-decay, the  atomic number of the nucleus  ZJ  changes from  upon whether a negative e l e c t r o n (negatron) ( p o s i t r o n ) has been emitt ed31.  Z —  to  1 depending  or a p o s i t i v e e l e c t r o n  If,however, the process i n v o l v e s  only the emission o f a s i n g l e e l e c t r o n , i t can e a s i l y be shown t h a t the laws of c o n s e r v a t i o n of energy and angular momentum are  violated,  To a v o i d t h i s impasse, P a u l i proposed h i s n e u t r i n o h y p o t h e s i s which d e s c r i b e s any s i n g l e beta-decay process as b e i n g r e a l l y the taneous emission  of two  simul-  p a r t i c l e s , the e l e c t r o n and a p a r t i c l e  n e g l i g e a b l e mass and no  charge.  as the c o n v e r s i o n of one  Then negatron  of  decay i s d e s c r i b e d  of the neutrons i n the n u c l e u s  into a  proton a c c o r d i n g t o N where  N  and  P  — P  +-  r e p r e s e n t neutron  r e p r e s e n t s the emitted negatron with no  p  charge and  small mass.  +- \)  ,  and proton r e s p e c t i v e l y ,  l)  and  r e p r e s e n t s the  ^  "neutrino"  On the other hand p o s i t r o n decay i s  d e s c r i b e d as the c o n v e r s i o n o f a n u c l e a r proton i n t o a  neutron  according to P where  ^  — -  N  ^  r e p r e s e n t s the p o s i t r o n and  n e u t r i n o " a l s o with no assigned t o both  \)  charge or mass.  and  \)  +  \)  ,  -J r e p r e s e n t s the  "anti-  By v i r t u e o f the p r o p e r t i e s  they are e s s e n t i a l l y  undetectable.  In the o r d i n a r y form of the n e u t r i n o t h e o r y , 2 7  neutrinos  ATOMIC  MASS  i  i Z  THE  i Z+f  i z + z  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 neutrino.  A particle i n such a state i s called  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 i n 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 i s energetically possible i s 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.  I f the nuclear mass difference be greater  than this lower limit, then the excess mass i s 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 i s possible i f the mass of the whole parent atom i s 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 Weizsacherand others which predicts atomic masses with reasonable accuracy. Nucleus 2 w i l l 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 3, i t would have to change from £  to  To decay  which would  correspond to the simultaneous emission of two negatrons.  Such a  double beta process has not been definitely abserved, although i t i s apparently energetically possible in several cases. The calculated h a l f - l i f e for double beta-decay depends upon a l l the factors which affect the h a l f - l i f e of the single betadecay process.  These are the energy available (mass difference)  }  the spins of the i n i t i a l and f i n a l 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 f i n a l 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 h a l f - l i f e of the double beta-decay process i s 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 , i t s activity would be very weak.  Hence i t i s not  surprising that previously published work ^"^ describing searches 3  for this process quote conflicting results and that the existance of the process i t s e l f i s 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 i s to be expected since N  — *  P  -t- ^  +• 0  N  — *  P  4- ^  f J  2N —=*> 2P -+-  ' }  2*-f-2\)  .  Thus, t h e double beta-decay  p r o c e s s r e s u l t s i n t h e simultaneous  emission o f f o u r p a r t i c l e s ,  (two negatrons  and two n e u t r i n o s ) .  Goeppert-Mayer-^ has c a l c u l a t e d t h e h a l f - l i f e  of t h i s process t o  be o f the order o f 1 0 ^ y e a r s . 2  32 The  o t h e r n e u t r i n o t h e o r y , t h e Majorana fornr  no d i s t i n c t i o n between n e u t r i n o and a n t i n e u t r i n o . the e q u i v a l e n t o f O f * N  Now c o n s i d e r the s i n g l e P t f ^ v) .  Thus  makes v) i s  process  1  T h i s corresponds t o the simultaneous neutrino.  f  emission o f a negatron and a  But on t h e o r d i n a r y t h e o r y , an a n t i - n e u t r i n o "0 f  g u s t a vacant n e g a t i v e energy 1  f  is  s t a t e o f a n e u t r i n o , so t h a t t h e  emission o f a n e u t r i n o i s e x a c t l y t h e same as the a b s o r p t i o n o f an a n t i n e u t r i n o .  T h e r e f o r e t h e process c o u l d be w r i t t e n -J* ~h  N  ^  P  ,  and a c c o r d i n g t o Majorana t h i s i s the same as  x) -+- N  — ^  P -h  In the Majorana p i c t u r e , the f i r s t neutron decays e m i t t i n g a v i r t u a l n e u t r i n o which i s absorbed  by the second decaying  neutron.  Thus 2N  2P  +-  2(?  No n e u t r i n o s are emitted and t h e process i n v o l v e s t h e emission o f two  p a r t i c l e s only (two n e g a t r o n s ) .  As might be expected t h e  c a l c u l a t e d p r o b a b i l i t y o f t h i s event i s g r e a t e r than f o r t h e f o u r particle theory.  F u r r y - ^ has c a l c u l a t e d t h e h a l f - l i f e  p a r t i c l e p r o c e s s t o be o f the o r d e r o f 10^ A  lO **" 2  year a c t i v i t y  o f the two  years.  ( f o u r p a r t i c l e concept) i s  i n c a p a b l e of d e t e c t i o n u s i n g o r d i n a r y t e c h n i q u e s , but 1 0 ^ y e a r s may be j u s t w i t h i n r e a c h .  Thus double beta decay o f f e r s a means o f d e c i d i n g between t h e o r d i n a r y and the Majorana form o f t h e n e u t r i n o I t v e r y probably of  \)  and o f  theory.  o f f e r s the only means s i n c e t h e d i r e c t d e t e c t i o n appears t o be e q u a l l y  impossible.  Another consequence of double beta decay would be that the e l e c t r i c  charge o f t h e e l e c t r o n would be shown equal  that of the proton.  K  36  Remark by Oppenheimer E. L. Firemen P r i n c e t o n T h e s i s .  31  II  )  SOME PREVIOUS EXPERIMENTS  A.  The T r i p l e t  Sn »- - * S b 50 51 12/  1 2 4  -  52  Te  1 2 Z f  S e v e r a l e a r l i e r workers s e l e c t e d ^ g S n ^ as a source. 1 2  Double b e t a decay has been r e p o r t e d by Fireman36 S n 4 with a h a l f - l i f e o f 0.4 x 1 0 1 2  1 d  years.  a  s  observed i n  The p r o c e s s  alleg-  e d l y observed was 5  C  S n ^  ...»  5 2 T e  124  ^  2  ^ ~ -  .  Fireman used two t h i n window Geiger counters on each s i d e o f a t h i n f l a t t i n source.  Only c o i n c i d e n t events were r e c o r d e d i n  order t o i d e n t i f y t h e double beta p r o c e s s and i n order t o lower the background from cosmic r a y s and l o c a l  contamination.  The background r a t e was f u r t h e r reduced by the use o f l e a d s h i e l d i n g and by banks o f Geiger counters connected coincidence. hour.  i n anti-  The background r a t e r e p o r t e d was 14 counts p e r  With the t i n i n p l a c e a s l i g h t l y l a r g e r r a t e was o b t a i n e d .  Fireman i n t e r p r e t e d t h i s d i f f e r e n c e t o be caused by double  beta  decay, and c a l c u l a t e d the h a l f l i f e from the mass o f t h e source i?  and the s o l i d angles subtended  by t h e counters.  showed t h e "beta p a r t i c l e s " had a maximum energy L i b b y and K a l k s t e i n ^ ? performed on Sn  ^ and obtained a n e g a t i v e r e s u l t .  Absorption  curves  of 1.5 Mev.  a similar  experiment  They were able t o p l a c e  a lower l i m i t o f 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  - ^Cd  1 1 0  1 1 0  R e c e n t l y Winter-^ i n v e s t i g a t e d t h e r e a c t i o n , 110  n  4 6  Pa  ^ „ ,110 ^ ^gCd  n  r> ~.  2 V*  i n a c l o u d chamber and obtained n e g a t i v e r e s u l t s . of 0.6 x 1 0  C.  1 8  , A lower  limit  years was p l a c e d on t h e h a l f - l i f e of the decay.  The T r i p l e t  5  ° ~ • 53 '  T e l 3  1  2  ~  1 3 0  54  X e l 3  °  An i n v e s t i g a t i o n of the process , Te Reynolds^. presence  1 3 0  —=*• X e  1 3  2 ^  ° -+  )  was made by Inghram and  A mass spectrometer was used t o search f o r t h e  o f Xe"^® i n t e l l u r i d e o r e s .  and was a t t r i b u t e d t o the double  An excess o f X e ^ was found 1  beta decay o f T e - ^ . 1  From a  f a i r l y r e l i a b l e estimate o f the age o f t h e ore, a h a l f - l i f e o f 21 1.4 x 10 " y e a r s was found. D.  The T r i p l e t  9 2  U  -  2 3 8  ^Np  2 3 8  -  ?u ^ 2  9l  A n e g a t i v e r e s u l t has been o b t a i n e d by Seaborg e t a l ^ i n an i n v e s t i g a t i o n o f t h e process  92 The a v a i l a b l e energy  }  * 94'  u  i s known t o be 1.1 Mev. from the decay schemes  of neighbouring i s o t o p e s . 90 day P u  2 3 8  i n pure U  of the P u  2 3 8  f o l l o w e d by a search f o r the 5.51 Mev. alpha p a r t i c l e s  from P u ^ . 2 3  2 3 8  .  A s e a r c h was made f o r the presence o f T h i s was done by chemical s e p a r a t i o n  No such a c t i v i t y was found, and a lower l i m i t o f  IS 6 x 10 years was s e t on t h e h a l f - l i f e .  33  Since double b e t a decay was r e p o r t e d i n Sn- ^, t h i s 14  i s o t o p e was chosen f o r our i n v e s t i g a t i o n . has measured t h e S n  1 2  ^ - Te  1 2  R e c e n t l y Duckworth^  ^ mass d i f f e r e n c e to.be 1.5  +  2  0.4  by an a c c u r a t e mass s p e c t r o m e t e r . Our experiment was q u i t e d i f f e r e n t i n n a t u r e from the experiments d e s c r i b e d above, and we f e e l , should l e a d t o 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 b e t a decay, should i t e x i s t .  Mev.  COINCIDENCE  OUTPUT CONTROLS GATE  PM.  WAX RM  k  Lectd  PULSE  COINCIDENCE  ADDER  MIXER  >  r  r  GATE  PULSE T O > KICKSORTER  34 III EXPERIMENTAL PROCEDURE  A.  The  P r i n c i p l e o f the I f double  from the h a l f - l i f e two  Experiment  beta decay i s observable at a l l ,  i t follows  c o n s i d e r a t i o n s t h a t i t w i l l very probably be a  p a r t i c l e process.  For t h i s reason, the experiment  was  designed t o make f u l l e r use o f the p r o p e r t i e s o f the two process than was  made i n p r e v i o u s work.  A d i s t i n g u i s h i n g f e a t u r e of the two i s t h a t the sum  particle  energy  T h i s i s not the case when n e u t r i n o s are e m i t t e d .  T h i s unique  f e a t u r e was  used i n an experiment  o f the e n e r g i e s of c o i n c i d e n t events i n S n ^ ^ 2  recorded on a p u l s e amplitude spectrum  a  a v a i l a b l e f o r the p r o c e s s . 'i  T h i s i s because the two b e t a p a r t i c l e s r e c e i v e a l l the  the sum  process  of t h e e n e r g i e s o f the two b e t a p a r t i c l e s has  constant v a l u e e q u a l to t h e t o t a l energy  c a r r i e d away.  particle  of the double  beta energies..  The  was  a n a l y s e r or " k i c k s o r t e r " .  beta p r o c e s s thus d i s p l a y e d should  of a sharp peak a t an energy  corresponding t o the sum  i n which  The  energy  consist  o f the  two  background spectrum, however, s h o u l d 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. p l a c e d between two The  source was  was  e n r i c h e d t o 95%  A source c o n s i s t i n g of a 200  tin foil  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  on l o a n from the Oak Sn ^ 1 2  was  box.  Ridge N a t i o n a l L a b o r a t o r i e s and  by e l e c t r o m a g n e t i c s e p a r a t i o n .  C o i n c i d e n t beta p a r t i c l e s were d e t e c t e d i n t h i c k anthracene  crystals  (1 x 1 x J) which f a c e d the t i n f o i l .  The p u l s e amplitudes  from  the s c i n t i l l a t i o n counters were used as a measure o f the b e t a energies. ted  The sura o f the e n e r g i e s of the two betas was r e p r e s e n -  by the sum o f the amplitudes  obtained i n the p u l s e adder.  o f c o i n c i d e n t p u l s e s which was  The p u l s e r e p r e s e n t i n g t h i s sum was  then d i s p l a y e d on an 18 channel k i c k s o r t e r o f Chalk R i v e r d e s i g n . A gate c o n t r o l l e d by the c o i n c i d e n c e mixer allowed the output  from  the pulse adder t o reach the k i c k s o r t e r o n l y i n the case o f a coincident C.  event.  C a l i b r a t i o n o f the K i c k s o r t e r In  order t o c a l i b r a t e t h e energy  the beta spectrum The endpoint was  of Sb^ ^ 2  s c a l e on t h e k i c k s o r t e r ,  was observed i n each c r y s t a l s e p a r a t e l y .  o f the most e n e r g e t i c b e t a groups o f Sb"*" ^ at 2.4 Mev. ^ 2  used as a c a l i b r a t i o n p o i n t .  2  Using the same p o i n t , t h e o v e r a l l  g a i n s o f the s c i n t i l l a t i o n counters were made equal by a d j u s t i n g t h e i r high v o l t a g e s u p p l i e s . D. ^ The  Optimum Source  Thickness  The t h i c k n e s s of m a t e r i a l between the two c r y s t a l s seemed t o have an e f f e c t on t h e amount o f s c a t t e r i n g from to  c r y s t a l and hence on t h e c o i n c i d e n c e r a t e .  crystal  T h e r e f o r e i t was  found n e c e s s a r y t o s u b s t i t u t e a dummy f o i l f o r the t i n source while t a k i n g the background r a t e i n order t o keep the background i n t e n s i t y equal t o t h a t from the t i n .  Both the t i n and dummy  f o i l were made the same t h i c k n e s s , 100 m^./cm . 2  This particular  weight was chosen f o r the f o l l o w i n g reasons: (i)  a t h i c k source reduces  from c r y s t a l t o c r y s t a l , and  scattering of stray radiation  1  5  j  |)  Ji  K> O  o I  o  o ry  «-  9  29  V*  *>  K> i  o  1  +  1  8-  l  «»  i.  ?3  o —<r r  IN*  to  \s  6 a>  *  00  o  0  «  o  $  •  —y  *  K»  •  4 0^  ro  +  *  *  1  o  1  o v. o  k  i  I  5j  o  •o «/>  do IS.  o w  JD  •1 00 6 AO  •> o  ^9  V>  v* N  3  #> ~~  3:  *-  ?  o o % 9  i  ^ Ki  o—  9  5f  *  ?> *p —  I*  &*  e $  S •9  •  s  5  <P  -  fO  ^*  v.  — *• —  <0 6^  N.  ">  o o  i  S  <P *•>  ©  k 5) ><• m— k  p  r»  o  >4  •«  *> oo  c  r> o  ^>  I/  o  4  \0  O  •Si  3? ro  v>  O • o  «vj  «N <• «M  <*  k  ?  O  «•<  «  ?k  ON  o»  »*>  «  o  o  o o  .<  o  1 +  t V-» -a.  o  o  6 i  «M  ! 1  o  *-> •*> —-  V* rv.  QO  o  o  eg  u CSJ  •*»  V>  oo  o  C  ro _c  «0  2?  U op  (ii)  M u l t i p l e s c a t t e r i n g i n a t h i c k source tends t o remove  any angular c o r r e l a t i o n which might occur i n double  beta decay  and so a f f e c t t h e measured c o i n c i d e n c e r a t e . A compromise between the l o s s o f t r a n s m i s s i o n f o r betas w i t h a t h i c k source and t h e f a v o u r a b l e e f f e c t s mentioned above was made a t a t h i c k n e s s o f 100 m^,/cm  2  I t i s t o be expected t h a t t h e source t h i c k n e s s w i l l not have much e f f e c t  on -the width  because the r a t e o f energy  of the k i c k s o r t e r peak.  Thisi s  l o s s i n t h e r e g i o n from 0.3 Mev. t o  3 Mev. and t h e t o t a l path l e n g t h o f t h e two betas are both n e a r l y constant.  Thus the t o t a l energy  l o s t i n the f o i l tends t o be  constant, and the peak remains sharp f o r t h i c k E.  foils.  O b t a i n i n g t h e Data Dummy f o i l s ' made of s i l v e r and o f n a t u r a l t i n were  found t o be s l i g h t l y a c t i v e and were d i s c a r d e d i n f a v o u r o f aluminum.  The t i n and dummy were i n t e r c h a n g e d p e r i o d i c a l l y during  the run i n order t o compensate f o r any i n s t r u m e n t a l d r i f t . were mounted on a s l i d e w i t h an e x t e r n a l c o n t r o l . the source and dummy c o u l d be i n t e r c h a n g e d without  They  In t h i s way disturbing the  The counters were s h i e l d e d by 16 cm. o f l e a d from above,  equipment.  and by 8 cm. of l e a d i n other d i r e c t i o n s .  The t o t a l number o f  c o i n c i d e n c e s were r e c o r d e d on a s c a l e r d u r i n g each r u n . The d a t a shown i n t h e p o r t i o n o f Table I I I c a l l e d " k i c k s o r t e r r e a d i n g s " were o b t a i n e d d u r i n g a t o t a l counting p e r i o d o f 264 hours.  The r i g h t  s i d e of the t a b l e shows the average  per hour as c a l c u l a t e d f o r each channel.  counts  The d i f f e r e n c e between  the t i n i n t e n s i t y and t h e background i s shown on t h e extreme r i g h t .  TO 34-3 "l  o  BACKGROUND  *  Sn'  24  T  |/1  T T T  I  " V  o ^ C * -1  - •  T  o 1, 1  i i  1  *  1  O '4a  10  1-5 SUM OF THE  F i g . 22.  20 BETA ENERGIES -  2-5  30  Mev.  Coincidence S p e c t r a Obtained  on K i c k s o r t e r .  37 The s p e c t r a from t i n and from background are p l o t t e d i n F i g . 22. background p o i n t s .  Standard d e v i a t i o n s are shown f o r the  the s t a n d a r d d e v i a t i o n s 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 . in  channel 18 ( e n e r g i e s o f 3 Mev.  radiation.  so o b t a i n e d  The l a r g e number o f counts  and l a r g e r ) are due t o cosmic  The c o u n t i n g r a t e of channel IS was l a r g e enough t o  p r o v i d e an e x p e r i m e n t a l check on t h e s t a b i l i t y o f the k i c k s o r t e r and of the p u l s e a m p l i f i e r s .  The r a t e i n channel 18 i s shown at  the bottom of Table H i t o g e t h e r w i t h t h e t o t a l c o i n c i d e n c e r a t e o b t a i n e d on the e x t e r n a l s c a l e r . was  satisfactorily  F.  The Shape of the Background  I t i s seen t h a t the arrangement -.•  stable.  The f i r s t two  Spectrum  channels r e c e i v e d no counts because the  c o i n c i d e n c e mixer d i s c r i m i n a t o r s were set t o accept o n l y 0.3 events.  Mev.  T h i s was done s i n c e the acceptance o f lower energy  p a r t i c l e s i n c r e a s e d the background  i n a l l channels.  The  effect  of d i s c r i m i n a t o r s e t t i n g on the background per channel can be seen from t h e s e measurements taken i n the 2 Mev. at d i s c r i m i n a t o r s e t t i n g s of 0.1, 0.2 2, 1 and J counts per hour.  channel:  and 0.3  Mev.  the  backgrounds  were approximately  A compromise between h a v i n g a low  background and l o s i n g p a r t o f t h e double b e t a spectrum was by s e t t i n g the d i s c r i m i n a t o r s at 0.3  taken  Mev.  Another f a c t o r t o which the background r a t e proved s e n s i t i v e was t h e s e p a r a t i o n o f the anthracene c r y s t a l s . t o t a l background  at 3 mm.  per hour, and at 1 cm. was  c r y s t a l s e p a r a t i o n was 155 44  ±  1  counts per hour.  The 10 counts So the 1  cm.  s e p a r a t i o n was p r e f e r a b l e n o t w i t h s t a n d i n g the s l i g h t l o s s i n s o l i d angle.  In an attempt t o lower t h e background r a t e s t i l l f u r t h e r an a n t i c o i n c i d e n c e device was d e v i s e d .  I t consisted of a s c i n t i l -  l a t i o n counter u s i n g a s o l u t i o n of t e r p h e n y l i n t o l u e n e scintillator.  as a  T h i s s o l u t i o n was c o n t a i n e d i n a l a r g e s i l v e r e d  v e s s e l p l a c e d above and around t h e two anthracene c r y s t a l s .  This  had t h e e f f e c t o f removing one f o u r t h o f the counts from t h e l#th channel and l e a v i n g the important t h i s reason  c e n t e r channels untouched.  For  i t was d i s c a r d e d as an unnecessary c o m p l i c a t i o n .  The  f i n a l arrangement had a c r y s t a l s e p a r a t i o n o f 1 cm., and no a n t i coincidence  device. The  poor e f f i c i e n c y of the a n t i c o i n c i d e n c e device was  rather surprising. it  However, at t h e d i s c a r d e d 3 mm.  separation  s u c c e s s f u l l y c a n c e l l e d 30% of t h e t o t a l background.  So the  e x p l a n a t i o n o f t h e poor c a n c e l l a t i o n must l i e i n a poor geometry for triple  coincidences.  Probably  the anticoincidence device i s  very s e n s i t i v e t o p e n e t r a t i n g showers and t h e geometry f o r showers i s best when the c r y s t a l s a r e close,. The  low r a t e o f 0.5.per hour per channel made experimen-  t i n g with t h e geometry t e d i o u s .  At l e a s t two days were  necessary  to t e s t t h e background w i t h each new arrangement. G.  Comparison with P r e v i o u s Work A f a i r comparison of background i n t h i s and,previous  work can be made i f we c o n s i d e r backgrounds at t h e peak p o s i t i o n . I t has been shown ( p a r t D) t h a t a b s o r p t i o n i n t h e f o i l would not widen the peak.  So t h e peak width may be caused by t h e energy  r e s o l u t i o n of the s c i n t i l l a t i o n  counters  only.  I f this resolution  i s 10$, then the peak would occupy one, o r at t h e most two, channels. Since t h e background i s 0.5 counts p e r hour p e r channel, t h e  39 " e f f e c t i v e background!' i s no more than 1.0 favourably with that of F i r e m a n ^  (14  per hour.  T h i s compared  per hour) and t h a t o f Libby37  (80 p e r hour;}. The second advantage  of t h i s experiment was t h a t i t was  capable of unambiguous i n t e r p r e t a t i o n .  I t i s d i f f i c u l t t o imagine  a r a d i o a c t i v e contamination which would have g i v e n r i s e t o a peak. On the o t h e r hand, contamination e f f e c t s i n c o n v e n t i o n a l c o u n t i n g experiments are i n d i s t i n g u i s h a b l e from t r u e e f f e c t s .  The  conflic-  t i n g r e s u l t s of Fireman and L i b b y r e p r e s e n t a case i n p o i n t .  1  1  1  1  + 0-2  '  c)  + 0*1  —  HOUR  c <  00  1  PER COUNTS  c  - 0*1  1 -L O 11 1  c  (  c  f J  >  - c  c  c  —  1  - 0-2  0-5  (>  1  IO  1 l'5  SUM OF THE BETA F i g . 23.  1  1  20  2-5  ENERGIES  D i f f e r e n c e Between Coincidence  c)+07_  30  - Mev.  Spectra Shown i n F i g . 22.  II RESULTS AND  CONCLUSIONS  F i g . 23 shows the c o i n c i d e n c e spectrum d e r i v e d from F i g . 22 by s u b t r a c t i n g the background. are shown.  No t r a c e of a peak was  The  standard d e v i a t i o n s  found.  In order t o p l a c e 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 n e g a t i v e r e s u l t , the s m a l l e s t e f f e c t d e t e c t a b l e i n F i g . 23 must be e s t i m a t e d . of the peak width i s important.. S e c t i o n I I I G t o be one, peak of 0.2  To do t h i s a knowledge  The width was  or at the most two,  estimated i n  channels.  Thus a  counts per hour would j u s t be d e t e c t a b l e 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  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 c o u n t i n g i s t h e r e f o r e c l o s e t o 0.2, not noted by F i r e m a n ^ .  The  0  crystal  a consideration  c o r r e c t h a l f l i f e observed by  Fireman  i s 0.2 x 10^° y e a r s r a t h e r than 0.4 x 10"^° y e a r s . The l o s s e s from a b s o r p t i o n i n the source may  be taken  i n t o account by a t r a n s m i s s i o n f a c t o r , F, which i s e s t i m a t e d t o 43 be 0.4  at 1.0 Mev.  and 0.7  at 3.0 Mev.  This transmission factor  i n c l u d e s an estimate of the c o u n t i n g l o s s e s i n c u r r e d by b i a s s i n g the c o i n c i d e n c e mixers at 0.3  Mev.  Thus a d e t e c t a b l e d i s i n t e g r a t i o n r a t e i s dN dt . =  _ *  0.2 ' 0.2F  I / 3,760  ™ counts per hour  ,  counts per. y e a r .  The number of Sn- - ^ atoms present i n the source 1  be c a l c u l a t e d by N  =  Ijn£  -  2  may  where m (the mass used)  =  200  L (Loschmidt's number)  md 6 x 10 3 2  j  f (the f r a c t i o n o f enrichment i n Sn *-) - 0.95, 122  and  Mji(the molar weight o f t i n ) =  So  -  N  r  6  x  0.92  10 3 124  .2 x  2  x  x 10  124  gms.  0.95  atoms.  2 1  Then t h e lower l i m i t o f the h a l f - l i f e i s g i v e n by T  -  1  log — dN  2  0.693  N  x  f 0.3 x 1 0  10  2 1  x  0.92  S,760 1 7  t o 0.7 x 1 0 ? 1  years.  The n e g a t i v e r e s u l t s cannot be c o n s t r u e d as showing the Majorana t h e o r y wrong, f o r the t r a n s i t i o n s t u d i e d might f o r b i d d e n by the change of s p i n or p a r i t y The work i s i n agreement and J.S. L a w s o n . 3 8  of E.L. 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