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Axial alignment in a ring-collection beta-ray spectrometer Earle, Eric Davis 1960

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AXIAL ALIGNMENT IN A RING-COLLECTION BETA-RAY SPECTROMETER by ERIC DAVIS EARLE .Sc.,  Memorial U n i v e r s i t y of Newfoundland, 19  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of PHYSICS  We accept t h i s t h e s i s as conforming to the required  THE  standard  UNIVERSITY OF BRITISH COLUMBIA August, 1960  In presenting the  this  r e q u i r e m e n t s f o r an  thesis in partial advanced degree a t  of B r i t i s h Columbia, I agree that it  freely  agree that for  available  the  f o r r e f e r e n c e and  permission f o r extensive  s c h o l a r l y p u r p o s e s may  D e p a r t m e n t o r by  be  gain  s h a l l not  Department  be  a l l o w e d . w i t h o u t my  of  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a .  shall  study.  I  Columbia  the  of  University  copying of  his representatives.  copying or p u b l i c a t i o n of t h i s  the  Library  g r a n t e d by  that  fulfilment  make  further this  Head o f  thesis my  I t i s understood  thesis for written  financial  permission.  ABSTRACT  A t h i n - l e a s beta-ray c o l l e c t i o n was m o d i f i e d .  spectrometer u s i n g r i n g - f o c u s These m o d i f i c a t i o n s c o n s i s t e d o f ;  1) a c e n t e r i n g mechanism e n a b l i n g the s o u r c e - d e t e c t o r to be a l i g n e d with the magnetic a x i s ;  axis  2) an e x t e n s i o n of the  vacuum chamber p l a c i n g the d e t e c t o r f u r t h e r from the magnet coils.  The l a t t e r c o n s i d e r a b l y decreased  i n g requirements f o r the d e t e c t o r .  the magnetic s h i e l d -  A misalignment of 0.25  mm.  fox- p a r a l l e l axes and of 0°09' f o r i n t e r s e c t i n g axes produced n o t i c e a b l y poorer .70%,  performance.  Using a g a t h e r i n g power of  a r e s o l v i n g power of .94% was o b t a i n e d f o r the 661.6 Kev.  K-conversion  peak of Cs  .  i  iii  TABLE OF CONTENTS  PAGE INTRODUCTION A. Nuclear Spectroscopy  1  B. B a s i c Ideas on Beta- and Gamma-Decay  3  C. E a r l y Spectrometers Characteristics II  III  and t h e i r  THE DEVELOPMENT OF THE THIN-LENS SPECTROMETER  13  21  PRESENT INVESTIGATION A. Instrumentation  27  B. Experimental T e s t s  34  C. C o n c l u s i o n s  41  BIBLIOGRAPHY  45  iv  TABLE OF ILLUSTRATIONS  Figure  F o l l o w i n g Page  lb') y P T  i c a l  Decay Schemes  4  2.  T y p i c a l Beta-Ray Spectrum  3.  H e l i c a l Spectrometer E l e c t r o n T r a j e c t o r i e s  16  4.  Unmodified Thin-Lens Spectrometer  16  5.  D e t e c t o r Assembly  22  6.  M o d i f i e d Thin-Lens Spectrometer  23  7.  Spectrometer Assembly  27  8.  The Source Assembly  27  9.  The D e t e c t o r Assembly  28  10.  Sample Entrance B a f f l e  30  11.  Chamber Support  31  12.  V a r i a t i o n o f Peak Shape w i t h Chamber  13.  of Mann and Payne  4  Position  38  Some Sample C o n v e r s i o n Peaks  40  V  ACKNOWLEDGEMENTS  The work described i n t h i s t h e s i s was supported by a Fund-in-Aid-of-Research t o Dr. K.C. Mann by the N a t i o n a l Research C o u n c i l of Canada. The author wishes t o thank Dr. K.C. Mann f o r h i s able guidance and a s s i s t a n c e throughout the course of t h i s work. Also the author thanks the N a t i o n a l Research C o u n c i l of Canada f o r awarding him a Bursary and Studentship f o r the p e r i o d June 1958 t o August 1960. He wishes t o express h i s a p p r e c i a t i o n t o H.R. Schneider and F.A. Payne f o r h e l p f u l suggestions and c r i t i c i s m s and t o F.J.  Morgan f o r h i s a s s i s t a n c e i n the experimental work.  I. INTRODUCTION  A. NUCLEAR SPECTROSCOPY  I t has been e x p e r i m e n t a l l y e v i d e n t f o r some time that matter  i s composed of atoms.  These atoms are minute p a r t i c l e s  and c o n s i s t of a combination of a number of o t h e r , more fundamental p a r t i c l e s .  The number and arrangement of these funda-  mental p a r t i c l e s determine the p r o p e r t i e s of any  particular  atom and, g e n e r a l l y , make i t d i s t i n g u i s h a b l e from other atoms w i t h a d i f f e r e n t number and/or particles.  arrangement of  fundamental  B a s i c a l l y , atoms c o n s i s t of a heavy, p o s i t i v e l y  charged c o r e , c a l l e d the nucleus, around which n e g a t i v e l y charged p a r t i c l e s , c a l l e d e l e c t r o n s , " o r b i t " .  Nuclear p h y s i c s  i s the study of the c h a r a c t e r i s t i c s of the nucleus.  The  purpose of n u c l e a r s p e c t r o s c o p y i s t o e s t a b l i s h some of these characteristics. To a f i r s t  approximation, the nucleus i s composed of  k i n d s of p a r t i c l e s c a l l e d nucleons, a p o s i t i v e l y  charged  p a r t i c l e c a l l e d the p r o t o n and a n e u t r a l p a r t i c l e ,  the neutron.  The number of p r o t o n s , Z, the number of neutrons, N, motions  two  their  and i n t e r a c t i o n s determine the n u c l e a r c h a r a c t e r i s t i c s .  N u c l e i may  be s t a b l e or u n s t a b l e .  The u n s t a b l e ones are  termed r a d i o a c t i v e and decay t o s t a t e s of lower energy through the e m i s s i o n of r a d i a t i o n of some k i n d . nucleus may  usually  A particular  e x i s t i n any one of s e v e r a l n u c l e o n i c c o n f i g u r a t i o n s ,  l  -2-  each w i t h i t s c h a r a c t e r i s t i c energy.  The lowest energy l e v e l  i s c a l l e d the ground s t a t e and any nucleus i n a higher energy s t a t e may decay t o t h i s ground s t a t e by the emission of energy i n the form of electromagnetic r a d i a t i o n (photons or gammarays).  A r a d i o a c t i v e nucleus may a l s o decay by p a r t i c l e  emission, where energy i s removed k i n e t i c a l l y .  Generally,  during p a r t i c l e emission, the number of neutrons or the number of protons i n the o r i g i n a l or parent nucleus changes, thereby producing a new or daughter nucleus.  In the low energy region  the predominant modes of r a d i o a c t i v e decay are gamma-ray emission and b e t a - p a r t i c l e emission.  The b e t a - p a r t i c l e s may  be n e g a t i v e l y charged (the negatron, experimentally i d e n t i c a l w i t h the atomic e l e c t r o n ) or p o s i t i v e l y charged (the p o s i t r o n ) . 1  The f u n c t i o n of the nuclear spectrometer  i s t o study the  energy of the b e t a - p a r t i c l e s and gamma-rays emitted during r a d i o a c t i v e decay i n the low energy r e g i o n and t o use the experimental data obtained t o give information concerning the angular momentum ( s p i n ) , the energy l e v e l s and the p a r i t y * of the nuclear s t a t e s involved i n the decay.  I t i s hoped that  t h i s data w i t h data c o l l e c t e d i n other branches of nuclear physics w i l l enable p h y s i c i s t s t o construct a theory  capable  of e x p l a i n i n g and p r e d i c t i n g nuclear phenomena.  * P a r i t y a r i s e s from the wave mechanical c o n s i d e r a t i o n s of the r e f l e c t i o n p r o p e r t i e s of the s p a c i a l part of s o l u t i o n s of the wave equation.  -3-  B. BASIC IDEAS ON BETA- AND GAMMA-DECAY  A b r i e f summary of present n u c l e a r t h e o r y , i n p a r t i c u l a r beta-decay  and " i n t e r n a l c o n v e r s i o n " theory, c o n s i s t e n t w i t h  the e x p e r i m e n t a l evidence c o l l e c t e d t o date i s g i v e n below. The types o f decay of p a r t i c u l a r i n t e r e s t a r e : 1) B e t a - p a r t i c l e decay; negatron or p o s i t r o n .  an u n s t a b l e nucleus emits a  The parent nucleus c o n t a i n i n g Z protons  and N neutrons decays t o a daughter protons and N "f 1 neutrons.  nucleus c o n t a i n i n g Z + 1  The nucleus may a l s o r e a c h a  lower energy s t a t e by the capture of an o r b i t a l  electron,  l e a d i n g t o the f o r m a t i o n of the same nucleus as i s reached by p o s i t r o n decay.  T h i s i s c a l l e d o r b i t a l e l e c t r o n capture.  2) Gamma-ray e m i s s i o n ;  an u n s t a b l e or e x c i t e d nucleus  spontaneously emits e l e c t r o m a g n e t i c r a d i a t i o n and drops t o a lower energy s t a t e o f the same nucleus.  The energy  (hV) of  the gamma-ray equals the energy d i f f e r e n c e o f the s t a t e s involved.  (h - Plank's u n i v e r s a l c o n s t a n t .  of the e m i t t e d gamma-ray.)  The lower energy s t a t e may o r may  not be t h e ground s t a t e of the nucleus. i s not, subsequent reached.  V - the frequency  In the event that i t  decays w i l l occur u n t i l t h i s s t a t e i s  An a l t e r n a t i v e method of d e - e x c i t a t i o n may occur  when the e x c i t a t i o n energy  i s t r a n s f e r r e d t o an o r b i t a l  electron.  The o r b i t a l e l e c t r o n escapes from the atom w i t h an energy e q u a l to hy  - E  b  where  i s the b i n d i n g energy o f the atomic  e l e c t r o n t o the nucleus.  T h i s l a s t mode of decay i s c a l l e d  -4-  "Internal  Conversion".  In g e n e r a l , a decay scheme c o n s i s t s of a combination these modes of decay and o f t e n may  be q u i t e complex.  decay schemes are shown i n F i g u r e 1.  of  Two  and ft<£ r e p r e s e n t  Here  b e t a - p a r t i c l e emission and y r e p r e s e n t s e l e c t r o m a g n e t i c r a d i a t i o n which may  or may  not be accompanied by i n t e r n a l  con-  version. Beta-decay. N u c l e i w i t h the same mass number, i . e . A = N + Z, c a l l e d isobars.  are  Members of an i s o b a r i c group d i f f e r i n Z  (and N) and i n t h e i r a c t u a l n u c l e a r masses.  A l l beta-decays  occur between members of the same i s o b a r i c group; e m i s s i o n the n u c l e a r charge  i n negatron  i s i n c r e a s e d by one p o s i t i v e u n i t ;  i n p o s i t r o n e m i s s i o n or o r b i t a l e l e c t r o n capture the n u c l e a r charge  i s decreased by one p o s i t i v e u n i t .  To d e c i d e i f some  mode of p a r t i c l e decay i s e n e r g e t i c a l l y p o s s i b l e one must c o n s i d e r the masses of parent, daughter  and e m i t t e d  particle.  Let "a" r e p r e s e n t a parent nucleus, "b" and " c " r e p r e s e n t the product of i t s decay. M(a) >  M(b)  + M(c) where M(x)  the x p a r t i c l e . accounted  Then decay w i l l occur o n l y i f r e p r e s e n t s the n u c l e a r mass of  The excess mass m = M(a)  f o r by the e x t r a k i n e t i c energy  - 13(b) - M(c) i s of the two product 2  p a r t i c l e s , u s i n g E i n s t e i n ' s energy e q u a t i o n E = m c . C o n s i d e r i n g the atomic mass M ^,(Z, A) as d i s t i n c t a  the n u c l e a r mass M(Z,  A) we  from  see t h a t the above types of decay  are e n e r g e t i c a l l y p o s s i b l e i f :  PARENT Z N  DAUGHTER  protons neutrons  Figs,  Z f l protons N - l neutrons  l a & lb.  Typical  PARENT  DAUGHTER  Decay Schemes  c _ =» O k. >  Momentum  F i g . 2.  Typical  Beta-Ray Spectrum  -5-  H  A T  .(Z,  A)  >  M  A T  .(Z,  A)  >  M  at  ( Z , A)  >  M M  ( Z -h 1, A )  A T  f o r negatron decay  a t . ( Z - 1, A ) + 2 MQ M  A  T  >  (Z  -  f o r p o s i t r o n decay  1, A )  for orbital e l e c t r o n capture  where M  Q  i s the r e s t mass of the e l e c t r o n .  O r b i t a l e l e c t r o n c a p t u r e produces the same daughter nucleus as p o s i t r o n e m i s s i o n .  I t u s u a l l y o c c u r s i n decays  where p o s i t r o n e m i s s i o n i s present and, because the mass r e quirements a r e not so severe, i t sometimes o c c u r s where p o s i t r o n emission i s impossible. For any group of i s o b a r i c n u c l e i the h i g h e r mass members always tend t o decay t o those o f lower mass.  For i s o b a r i c  groups o f odd A t h e r e e x i s t s o n l y one s t a b l e member which i s the end product t o which a l l other i s o b a r i c members decay. However, f o r even A n u c l e i there may e x i s t two o r more s t a b l e n u c l e i i n the same i s o b a r i c group.  T h i s i s because the mass  of even Z, even N n u c l e i i s o f t e n l e s s than the mass of e i t h e r of i t s two odd-odd neighbours and s o , i f a lower mass s t a t e e x i s t s , the even-even n u c l e i can o n l y reach i t by double b e t a decay.  Ingrahm and Reynolds showed t h a t the h a l f  f o r double beta-decay of T e if  1 3 0  i s 1.2 x 1 0  t h i s p r o c e s s o c c u r s , i t i s very  2 1  life  period  y e a r s , so that  infrequent.  The decay scheme ( F i g . 1) suggests t h a t the b e t a - p a r t i c l e s are e m i t t e d w i t h d i s c r e t e e n e r g i e s .  One would expect, i f  number of b e t a - p a r t i c l e s e m i t t e d per u n i t time were p l o t t e d  -6-  as a f u n c t i o n of energy  (oz* momentum), t h a t o n l y at c e r t a i n  e n e r g i e s would b e t a - p a r t i c l e s be observed and these would correspond t o the energy d i f f e r e n c e s between the i n i t i a l and f i n a l s t a t e s .  Such i s not t h e case, however.  Experimental  evidence shows t h a t the beta-energy spectrum i s a continuous d i s t r i b u t i o n , up t o some end p o i n t energy E  m a x  ).  ( r e f e r r e d t o as  F r e q u e n t l y , s p e c t r a l " l i n e s " or peaks  ( F i g . 2) are  superimposed on t h e continuum.  These peaks a r e due t o t h e  i n t e r n a l conversion electrons.  The continuous energy d i s -  t r i b u t i o n , on the other hand, poses a dilemma, not e a s i l y r e s o l v e d i f one accepts the assumption that n u c l e a r energy s t a t e s a r e f i x e d , s i n c e f i x e d energy s t a t e s would suggest a "line" of  s t r u c t u r e f o r the primary beta-decay as w e l l .  The law  c o n s e r v a t i o n of energy appears t o be v i o l a t e d . From a c o n s i d e r a t i o n of the angular momenta i n v o l v e d  i n t h e decay, i . e . the s p i n s of the i n i t i a l  and f i n a l  states  and o f the e l e c t r o n , i t appears that the law of c o n s e r v a t i o n of  angular momentum i s a l s o v i o l a t e d .  shown t h a t " s t a t i s t i c s "  Finally,  i t may be  a r e not conserved i f o n l y the b e t a -  p a r t i c l e i s i n v o l v e d i n the decay. These three d i f f i c u l t i e s ,  i . e . the energy  angular momentum and s t a t i s t i c s ,  continuum,  may a l l be overcome i f one  accepts P a u l i ' s s u g g e s t i o n t h a t the beta-decay p r o c e s s i n v o l v e s t h e simultaneous e m i s s i o n o f two p a r t i c l e s — e l e c t r o n and the n e u t r i n o .  the  The n e u t r i n o i s p o s t u l a t e d t o  be a new fundamental p a r t i c l e w i t h no charge, v e r y s m a l l mass  -7-  (probably z e r o ) , Dirac  s p i n equal t o ^ f t  statistics.  and which obeys Fermi-  The e x i s t e n c e o f the n e u t r i n o now seems  t o be confirmed, e x p e r i m e n t a l l y . In t h i s concept  t h e energy o f the decay i s shared  between the two p a r t i c l e s .  A c c o r d i n g t o the theory o f the  process worked out by Fermi, the b e t a - p a r t i c l e ' s  energy  d i s t r i b u t i o n may be expressed by P(E)dE a F ( Z , E ) p ( E 2  where  P(E)dE  - E) dE  (1)  2  m a x  i s the f r a c t i o n of d i s i n t e g r a t i o n s which emit b e t a - p a r t i c l e s w i t h energy  between  E and E + dE Ejjjg^jj.  i s the maximum energy  observed  i n the  spectrum E  i s the energy o f the b e t a - p a r t i c l e  p  i s the momentum of t h e b e t a - p a r t i c l e  F(Z,E)  i s a c o m p l i c a t e d f u n c t i o n which d e s c r i b e s the e f f e c t of the Coulomb f i e l d o f the nucleus on the e m i t t e d  From E q u a t i o n  (1) we see t h a t  /£iPJ. a ( E ^ ^ - E) where P  N(p)  beta-particles.  2F  i s the number o f b e t a - p a r t i c l e s e m i t t e d w i t h momentum p.  Hence, i f we p l o t /  V  a P F  s  a function  o f E we g e t a s t r a i g h t  2  l i n e i n t e r s e c t i n g the energy  a x i s a t E^,,.  Fermi p l o t .  I f other independent  the spectrum  then t h e end p o i n t  This i s c a l l e d a  beta-groups  are present i n  e n e r g i e s of these groups may  j  be o b t a i n e d by s u b t r a c t i n g  successive contributions  from t h e  -8-  composite  Fermi  Equation  plot.  (1) i s based on the assumption  change ( A I) i s _fl,0 is  t h a t the s p i n  and t h a t there i s no p a r i t y change.  the most probable mode of beta-decay  "allowed" t r a n s i t i o n .  This  and i s c a l l e d an  A l l other t r a n s i t i o n s are c a l l e d  " f o r b i d d e n " , the degree ox f o r b i d d e n n e s s depending on the value of A I and the presence Equation  o r absence of p a r i t y change.  (1) g i v e s a s t r a i g h t l i n e p l o t o n l y d u r i n g allowed  transitions. For f o r b i d d e n t r a n s i t i o n s , i n v o l v i n g higher s p i n changes and p o s s i b l e p a r i t y changes, the " c o n s t a n t " i n e q u a t i o n (1) becomes energy  dependent and so g i v e s a n o n - l i n e a r Fermi  plot.  C e r t a i n c o r r e c t i o n terms have been worked out and by a p p l y i n g these, i t i s p o s s i b l e t o determine of  the degree o f f o r b i d d e n n e s s  the decay i n q u e s t i o n . If  P(E)dE r e p r e s e n t s the p r o b a b i l i t y of beta-emission i n  the energy  interval  (E,E  dE) then the p r o b a b i l i t y , A , t h a t  the nucleus w i l l decay b y the e m i s s i o n of an e l e c t r o n i n a p a r t i c u l a r b e t a group i s ;  o T h i s A may be s a i d t o equal af where " a " i s a constant and " f " i s some f u n c t i o n of Z and E. bility, is  A , has dimensions  the mean l i f e  (T)  T h i s t o t a l decay proba.  disintegrations/time.  of the e x c i t e d p a r t i c l e .  shown t h a t t h e mean l i f e , f  ,  a  n  d  the half l i f e ,  Hence 1/^ I t can be T i , are r e -  -9-  l a t e d by !  ^  Ti  1  1  r- —= _ =af— . In 2 \  The q u a n t i t y  f T ^ i s c a l l e d the comparative h a l f - l i f e of the  transition.  The l o g a r i t h m  of £ T | has been found convenient  t o work w i t h i n the comparison o f beta-decay groups and i s a u s e f u l way of i n d i c a t i n g the degree of f o r b i d d e n n e s s and hence s p i n and p a r i t y changes of the decay.  ,  Gamma-decay. As has been s t a t e d , a nucleus i n an e x c i t e d s t a t e may decay spontaneously t o a lower s t a t e of the same nucleus by the e m i s s i o n of a gamma-ray.  The energy o f t h i s gamma-ray i s  g i v e n by:  where  Eg, E^ a r e the e n e r g i e s of the upper and lower energy s t a t e s .  The nucleus may a l s o decay t o t h i s lower energy s t a t e by g i v i n g t h i s energy t o an e l e c t r o n i n the K,L, same atom.  The e l e c t r o n i s then e j e c t e d w i t h energy hV - Eg,  hV - Ej^, ... where Ej*, E^, ... a r e the b i n d i n g the o r b i t a l  - s h e l l of the  energies of  electrons.  T h i s l a s t d e - e x c i t a t i o n process i s c a l l e d i n t e r n a l conv e r s i o n and the e j e c t e d e l e c t r o n s , c o n v e r s i o n e l e c t r o n s .  -10-  These c o n v e r s i o n e l e c t r o n s are emitted at d i s c r e t e and w i l l  appear i n a beta-energy  conversion l i n e s  (Fig. 2).  spectrum as sharp peaks or  Those e l e c t r o n s i n the  s h e l l w i l l have the g r e a t e s t i n t e r a c t i o n nucleus.  Thus the Reconversion  i n t e n s e than the L l i n e , The  line,  and so  total probability,  of  \  =  +  will  of gamma-emission, A y , The  be broken down i n t o  + *e " \  the  u s u a l l y , w i l l be more  e  *  with  A, t h a t an e x c i t e d nucleus  of K c o n v e r s i o n , L c o n v e r s i o n , e t c .  The  probability  of i n t e r n a l c o n v e r s i o n , A .  i n t e r n a l c o n v e r s i o n may  innermost  on.  decay depends on the p r o b a b i l i t y the p r o b a b i l i t y  energies  *K  and  probability  probabilities  Thus  +  A  L  +  •' *  r a t i o of the number of decays by i n t e r n a l c o n v e r s i o n to  the number of decays by gamma-emission i s c a l l e d the conv e r s i o n c o e f f i c i e n t and  A  e T  where  a^,  a^,  A  K  i s g i v e n by  A  7  L  7  ... are K, L,  ... c o n v e r s i o n  coefficients.  To determine these c o e f f i c i e n t s e x p e r i m e n t a l l y , i t i s necessary  to compare the r e l a t i v e i n t e n s i t i e s of the  different  modes of decay. i  While the i n t e n s i t y of i n t e r n a l l y converted gamma-rays can be measured by the use of a beta-ray spectrometer, i n t e n s i t y of gamma-rays cannot be measured d i r e c t l y .  the However,  i f the gamma-rays are allowed to s t r i k e a f o i l ,  of h i g h Z  m a t e r i a l , p l a c e d near the source, they w i l l undergo a  process  c a l l e d e x t e r n a l c o n v e r s i o n or a p h o t o e l e c t r i c process.  In  t h i s process the energy of the gamma-ray i s t r a n s f e r r e d to an o r b i t a l e l e c t r o n i n the f o i l is  (or t a r g e t ) and the e l e c t r o n  then e j e c t e d w i t h an energy equal to t h a t of the gamma-ray  l e s s the b i n d i n g energy of the o r b i t a l e l e c t r o n .  Electrons  e j e c t e d i n t h i s manner are c a l l e d p h o t o - e l e c t r o n s and may analysed i n the spectrometer.  The  foil,  as a source of photo-  e l e c t r o n s , becomes the source as seen by the One  might thus expect  be  spectrometer.  t h a t a comparison of  intensity  measurements on i n t e r n a l l y and e x t e r n a l l y converted e l e c t r o n s would g i v e s u f f i c i e n t  i n f o r m a t i o n to determine e x p e r i m e n t a l l y  the c o n v e r s i o n c o e f f i c i e n t s .  As theory p r e d i c t s t h a t these  c o e f f i c i e n t s are f u n c t i o n s of c e r t a i n n u c l e a r the experimental  characteristics,  e v a l u a t i o n of these c o e f f i c i e n t s would g i v e  v a l u a b l e i n f o r m a t i o n on the s p i n s and p a r i t i e s of the nuclear energy s t a t e s . U n f o r t u n a t e l y , because of the l a c k of d e t a i l e d knowledge of  the 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 of the t a r g e t i n the  energy r e g i o n where the i n t e r n a l c o n v e r s i o n process  low  pre-  dominates and because of the v a r i a t i o n of the angle of e m i s s i o n of the p h o t o - e l e c t r o n s w i t h energy, one  cannot use  the e x t e r n a l c o n v e r s i o n spectrum f o r r e l i a b l e comparison w i t h the i n t e r n a l c o n v e r s i o n spectrum.  A l l one  can do i s compare  p h o t o - e l e c t r o n i n t e n s i t i e s of gamma-rays whose e n e r g i e s are  -12-  not too  different.  However, the spectrometer of K, L, M, transition.  ...  may  be used f o r a comparison  i n t e r n a l c o n v e r s i o n i n t e n s i t i e s f o r any  Values of these r a t i o s , f[K, £[K,  Z v a l u e s , have been t a b u l a t e d .  one  . .., f o r v a r i o u s  A comparison of measured  t h e o r e t i c a l values of these r a t i o s may  and  g i v e i n f o r m a t i o n con-  c e r n i n g the n u c l e a r s t a t e s i n v o l v e d . As i l l u s t r a t e d i n F i g u r e l a , n u c l e a r s t a t e s may  decay by  the emission of a gamma-ray or c o n v e r s i o n e l e c t r o n . many energy s t a t e s are i n v o l v e d and the nucleus may m u l t i p l e b e t a - and gamma-rays ( F i g . l b ) .  Frequently, emit  In the m a j o r i t y of  c a s e s , e x c i t e d n u c l e a r s t a t e s decay to lower s t a t e s very q u i c k l y , f o r a l l p r a c t i c a l purposes i n s t a n t a n e o u s l y . l a t i v e l y few  Re-  n u c l e a r s t a t e s have l i f e t i m e s g r e a t e r than lO"" ^ 1  3 4 sees.  Such s t a t e s are c a l l e d i s o m e r i c s t a t e s '  and  this  d e s i g n a t i o n merely means t h a t the l i f e t i m e can be measured with techniques  now  available.  These "cascade"  decays are  r e f e r r e d to as gamma-gamma-coincident decays. A n a l y s i s of these cascade decays, of beta-gamma-decays and of the angular r a y s or between two  c o r r e l a t i o n between the b e t a - and gammagamma-rays i n cascade are u s e f u l i n de-  t e r m i n i n g s p i n and p a r i t y changes and sometimes these analyses are performed w i t h the a i d of a  spectrometer.  -13-  C. EARLY SPECTROMETERS AND THEIR  CHARACTERISTICS  As we have shown, measurements on n u c l e a r t r a n s i t i o n s are  important i n the development of a' c o n s i s t e n t n u c l e a r  theory.  These measurements may be o b t a i n e d by the use o f  v a r i o u s instruments. for  In p a r t i c u l a r , spectrometers are used  measurements on i n t e r n a l and e x 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 ,  primary b e t a - p a r t i c l e s and f o r c o i n c i d e n c e and angular c o r r e l a t i o n work.  These processes can o n l y be s t u d i e d p r o p e r l y i f  r e a s o n a b l y a c c u r a t e energy and i n t e n s i t y measurements o f the c o n v e r s i o n e l e c t r o n l i n e s and of the primary beta-groups can be o b t a i n e d .  T h i s i s the f u n c t i o n of the b e t a - r a y spectrometer.  Beta-ray spectrometers may employ e l e c t r o s t a t i c or magnetic focussing.  The e l e c t r o s t a t i c spectrometer i s energy  w h i l e t h e , more g e n e r a l l y used, magnetic  spectrometer i s  momentum s e l e c t i v e .  :.  The e l e c t r o n t r a j e c t o r i e s i n the magnetic are  selective  spectrometers  determined by the momentum of the e l e c t r o n and the magnetic  f i e l d such that B ev =  where  B  i s the component o f the magnetic f i e l d  normal t o  the p a r t i c l e ' s d i r e c t i o n of motion e,m,v  a r e the e l e c t r o n ' s charge, r e l a t i v i s t i c mass and velocity  f  i s the r a d i u s of c u r v a t u r e o f the e l e c t r o n ' s path.  -14-  The in  magnetic plotting  electron  stiffness, an e l e c t r o n  momentum  spectrum  two  be  d i s c u s s e d i n more  taining  of  ideal  electron  ground, and  economy,  designed  form,  power  accurate  combinations for analysis  o f 300,000  path  while  mainly  i n the h e l i c a l  first  deflection  designed  to  back-  flexibility etc.  A  minimize are  or high  spectrometers  only  limited  and o f t e n  t h e low energy  These  the ob-  detector  (e.g. alignment),  energy  can  eco-  i n t h e low energy  region,  t h e same r a d i u s o f  gauss. may  be  divided  into  spectrometers  in a direction spectrometers  were  groups,  magnetic to the  the lines  electron's  of force  are  path.  determinations of beta-particle field  two  the  normal  of the electron's  i n a magnetic  i s  and c o o l i n g ,  producing  In the f l a t  are mainly  size,  are  power.  power  limitations  used  o f 1000  i n the d i r e c t i o n  The  be  spectrometers  of force  to the  In p r a c t i c e ,  source  electrostatic  volts/cm.  as a f i e l d  and h e l i c a l .  lines  later.  consumption  i n either  and p r a c t i c a l l y  flat  detail  of these  nomically  Magnetic  abscissa  characteristics  i t sresolving  have been  F o r example,  curvature  and  adjustments  regions.  field  as the  i s proportional  important  aberrations,  of spectrometers  different  and  used  t r a n s m i s s i o n and r e s o l v i n g  optical  field  variety  a  most  t r a n s m i s s i o n (% c o l l e c t e d )  will  by  , i s normally  (mv).  A spectrometer's its  Bp  carried  energy  by  out by von  their  Baeyer  5 and from  Hahn  by  the "direct  a radioactive  source  deflection were  method".  allowed  to pass  Beta-rays through  emitted a  narrow  slit  and then, a f t e r t r a v e l l i n g an a r b i t r a r y d i s t a n c e through  a magnetic  f i e l d , were r e c o r d e d on a photographic p l a t e .  Only crude measurements of i n t e n s i t y were p o s s i b l e s i n c e attempt was  no  made to f o c u s the b e t a - r a y s .  The f i r s t magnetic  f o c u s s i n g d e v i c e , the s e m i - c i r c u l a r  f o c u s s i n g spectrometer, soon f o l l o w e d , a f t e r a s u g g e s t i o n by Danysz .  I t i s based on the geometric f a c t that i f two  c i r c l e s w i t h the same r a d i u s are drav/n w i t h t h e i r c e n t r e s s e p a r a t e d by a s m a l l d i s t a n c e w i t h r e s p e c t to the r a d i u s then they i n t e r s e c t at approximately d i a m e t r i c a l l y o p p o s i t e p o i n t s . The c h i e f disadvantage of the s e m i - c i r c u l a r  focussing  p r i n c i p l e i s t h a t t h e r e i s o n l y one-dimensional i.e.  i n the plane of the c i r c l e s .  In 1946  a d e v i c e was  developed which combined many of the advantages d i m e n s i o n a l f o c u s s i n g w i t h those of the h e l i c a l or l e n s f o c u s s i n g .  T h i s was  focussing,  of the  one-  two-dimensional  the double  focussing  spectrometer^. Another f l a t spectrometer developed was  the t h i r d order  f o c u s s i n g spectrometer which c o r r e c t e d f o r the s p h e r i c a l a b e r r a t i o n c h a r a c t e r i s t i c s of the homogeneous magnetic  field 8 9  used i n s e m i - c i r c u l a r f o c u s s i n g by shaping the magnetic  field  S t i l l o t h e r s i n c l u d e those a r r a n g i n g a f o c u s s i n g "prism" f i e l d where the source and d e t e c t o r are o u t s i d e the magnetic  field  or  pole  a s e c t o r f i e l d w i t h inhomogeneous f i e l d s and shaped  pieces The h e l i c a l or l e n s - t y p e spectrometer was  first  suggested  -16-  by K a p i t z a i n 1924 ( r e f e r r e d t o by T r i c k e r i n r e f e r e n c e 12), the e l e c t r o n f o c u s s i n g p r o p e r t i e s of s h o r t and long c o i l s having been known f o r some time.  Busch*^ was the f i r s t t o  p o i n t out the c l o s e analogy between l i g h t and e l e c t r o n o p t i c s i f one r e p l a c e s the o p t i c a l l e n s by a magnetic  "lens".  I f e l e c t r o n s a r e e m i t t e d from a source, p l a c e d on the a x i s of an a x i a l symmetric  field,  a t some angle (other than  0° or 90°) w i t h r e s p e c t t o t h i s a x i s , they w i l l f o l l o w  helical  t r a j e c t o r i e s and r e t u r n t o the a x i s at some p o i n t P. ( F i g . 3) Of course, the angle o f e m i s s i o n cannot be so great as to c a r r y the e l e c t r o n out of the i n f l u e n c e of the magnetic Due t o s p h e r i c a l a b e r r a t i o n the maximum convergence  field.  of these  t r a j e c t o r i e s o c c u r s , not on the a x i s , at P, but a t some r i n g of p o i n t s c o n c e n t r i c w i t h the a x i s , i . e . at the " r i n g f o c u s " , F . T h i s i s t y p i c a l of a l l l e n s - t y p e spectrometers whether the field  i s homogeneous ( s o l e n o i d a l spectrometers) or inhomo-  geneous (long and t h i n l e n s s p e c t r o m e t e r s ) . The f i r s t  attempts t o use a magnetic  lens f o r beta-ray  12 s p e c t r o s c o p y were made by T r i c k e r field,  who used a long uniform 13  i . e . a s o l e n o i d a l spectrometer, and Klemperer  who  used a s h o r t f i e l d .  These e a r l y instruments c o u l d not compare  w i t h the performance  o f the f l a t spectrometers because no  s e r i o u s e f f o r t s were made t o improve  t h e i r performance.  The  p o t e n t i a l i t i e s o f these h e l i c a l instruments v/ere not f u l l y r e a l i z e d u n t i l the e a r l y f o r t i e s when Witcher s o l e n o i d a l spectrometer and Deutsch e t a l .  '  developed the , the s h o r t  F i g . 3.  H e l i c a l Spectrometer E l e c t r o n  F i g . 4.  Unmodified Thin-Lens  Trajectories  Spectrometer  lens the  spectrometer. predominant  followed This  the  i s not  The  role  i n the  successful meant  to  best  performance.  easy  to  construct with  in  an  important  do  most  herently The  The  calculated  power  large  obtainable  by  of  sensitivity  the Thus  two  normal  process the  forming.  The  adjacent  that  of  thin  lens  of  gives  the  correlation  and  magnetic  work  and  inexpensive  to  operate  However,  to  than  i t has  in-  field  result  i s an  spectrometer  better  technique  fields,  image.  in  electrons  second image  focussing reversing  of  Another  the  than  lens  thin of  the  first.  example  has  disadvantages  they  i s employed  theoretically the  of  is  first  axial  and  performance  the  mirror  advantages  However,  the  the  may  trajectories  where  the  and  the  fields.  Still  which  had  adjustment  outside  aberration  advantages  spectrometers  electron  easy  a r i n g f o c u s and  final  chief  method.  helical  This  lens  a  spherical  lens  power  spectrometer  having  less  so  requirements.  to  the  spectrometer  to  by  lone; l e n s  cantly  way  other  field,  " i n t e r m e d i a t e image" through  which  d e t e c t o r out  electric  rigorously,  field  data  i t is relatively  solenoidal  magnetic  of  aberration.  characteristics  have  the  Also  played  i n i t s performance  feature i n angular  spherical  low  the  and  spectrometers.  relatively  pass  is flexible  source  Deutsch  s h o r t or  magnetic  a uniform  the  the  other  of  be  that  of  less  large  by  accumulation  requires  mentioned.  may  used  and  be  in  It  detector shielding.  construct  lens  introduction  imply  the  field,  short  is  signifispectrometer. thin  lens  -18-  spectrometer have a l r e a d y been mentioned  and i t i s t h i s type  of spectrometer which i s i n use i n t h i s l a b o r a t o r y . diagram  i s shown i n F i g u r e 4.  the p r e v i o u s d i s c u s s i o n .  A simple  Its operation i s clear  from  The gamma-baffle i s t o p r o t e c t the  counter from d i r e c t r a d i a t i o n and the o t h e r b a f f l e s a r e f o r electron selection.  Only those e l e c t r o n s which pass  the entrance and e x i t b a f f l e s a r e counted. the e l e c t r o n depends on the magnetic  through  S i n c e the path of  f i e l d and e l e c t r o n  momentum, one may, by keeping the r a d i u s of c u r v a t u r e constant and v a r y i n g B, determine the r e l a t i v e i n t e n s i t y  distribution  of the momentum of the e l e c t r o n s being e m i t t e d by the source and c o l l e c t e d by the d e t e c t o r .  Some important spectrometer  parameters.  It i s convenient i n the d i s c u s s i o n of a spectrometer's performance  and f o r comparison w i t h other spectrometers t o  d e f i n e the two parameters  a l r e a d y mentioned, t r a n s m i s s i o n and  r e s o l v i n g power, and s e v e r a l o t h e r s i n p r e c i s e  mathematical  terms. Transmission:  The t r a n s m i s s i o n , T, i s a measure of the  c o l l e c t i n g power of the spectrometer and i s expressed as a percentage.  T i s the percentage of e l e c t r o n s e m i t t e d by the  source t h a t r e a c h the d e t e c t o r and are counted when t h e instrument i s a d j u s t e d t o focus these e l e c t r o n s , i . e .  the  f r a c t i o n of the s o l i d angle a t the source "seen" by the detector.  R e l a t e d t o T i s the g a t h e r i n g power, u?, d e f i n e d  -19-  as the r a t i o of the s o l i d - a c c e p t a n c e s o l i d angle.  angle, -TL , t o the  I t i s d e f i n e d b y the entrance  baffles.  of course, T ^ u>  = Resolution:  total  The  r e s o l u t i o n , R,  i s a measure of  s e l e c t i v e power of the d e t e c t o r system. e l e c t r o n s are emitted by the source,  the  I f monoenergic  as i n the case of con-  v e r s i o n e l e c t r o n s , they w i l l not appear i n the spectrum as l i n e s but r a t h e r as peaks of f i n i t e width. by scattex*ing, f i n i t e source  T h i s i s caused  and b a f f l e s i z e and  s p h e r i c a l a b e r r a t i o n of the f o c u s s i n g f i e l d ,  the  inherent  a l l of which  p r o h i b i t a " p o i n t " (or r i n g of p o i n t s ) focus which i s r e q u i r e d f o r t r u e spectrum l i n e s . percentage by the  The  r e s o l u t i o n , R,  i s d e f i n e d as a  equation: R - A(B/° ) B f  where  Bp  i s the magnetic s t i f f n e s s of the  focussed  electrons A ( B p ) i s the peak w i d t h at h a l f i n t e n s i t y . Dispersion:  D i s p e r s i o n , D,  as the name i m p l i e s , i s a  measure of the a b i l i t y of the instrument  to separate  energies.  to be of any  Thus we  see f o r an instrument  adjacent value  the d i s p e r s i o n or l i n e s e p a r a t i o n must be g r e a t e r than the l i n e or peak width.  It i s defined „  D =  dx d(Bf  )  as  -20-  where  x  i s the c o - o r d i n a t e of the focus.  A c o n s i d e r a t i o n of the two parameters,  transmission  and r e s o l u t i o n , shows that they a r e , t o a c e r t a i n extent, mutually by  conflicting.  I f one improves the t r a n s m i s s i o n  i n c r e a s i n g the source  s i z e or by opening the entrance  s l o t , the r e s o l v i n g power decreases.  The r a t i o of t r a n s -  T m i s s i o n t o r e s o l u t i o n , — , i s a good measure of the q u a l i t y R of a spectrometer and i s used e x t e n s i v e l y i n the comparison of spectrometers. Merit".  I t i s r e f e r r e d t o as "the F i g u r e of  -21-  I I . THE DEVELOPMENT OF THE THIN-LENS SPECTROMETER  The f i r s t major c o n t r i b u t i o n s t o the theory and s t r u c t i o n of t h i n - l e n s spectrometers were made by et a l .  .  Deutsch  They c a l c u l a t e d the e l e c t r o n t r a j e c t o r i e s u s i n g  the procedure of B u s c h * 1  and analysed the s p h e r i c a l  r a t i o n e f f e c t t h e o r e t i c a l l y and e x p e r i m e n t a l l y . and d e t e c t o r on the a x i s of the magnetic parameters  con-  aber-  With source  f i e l d they v a r i e d  such as source s i z e , emergent a n g l e s , e t c .  They  used a b a f f l i n g system t o d e f i n e the e l e c t r o n path, thus d e t e r m i n i n g the r e s o l u t i o n and t r a n s m i s s i o n of the ter.  spectrome-  Other b a f f l e s were used t o s t o p d i r e c t gamma-rays from  r e a c h i n g the counter and t o reduce counts due t o secondary e l e c t r o n s s c a t t e r e d from b a f f l e s and from the vacuum chamber walls.  A l s o by u s i n g a s p i r a l b a f f l e they were a b l e t o  d i s t i n g u i s h between p o s i t r o n s and negatrons.  I t i s important  to note that c o l l e c t i o n by means of a g e i g e r counter was made on the a x i s .  T h i s l i m i t e d the p r a c t i c a l t r a n s m i s s i o n t o s m a l l  v a l u e s , s i n c e o n l y i n these circumstances was  the a x i a l  "image" s m a l l enough t o be handled c o n v e n i e n t l y . observed t h a t good alignment of the magnetic d e t e c t o r a x i s was  They a l s o  a x i s and s o u r c e -  necessary f o r optimum f o c u s s i n g and thus f o r  best f i g u r e of m e r i t . As can be seen from F i g u r e 3, a c a l c u l a t i o n of the e l e c t r o n t r a j e c t o r i e s shows t h a t the envelope of monoe n e r g e t i c e l e c t r o n s e m i t t e d by the entrance b a f f l e has,  after  -22-  p a s s i n g through the f i e l d ,  i t s p o i n t of maximum convergence  not on the a x i s but on a r i n g of p o i n t s c i r c u m s c r i b i n g the axis.  This fact led several w o r k e r s  an annular  slit  at the r i n g f o c u s .  of M e r i t by a f a c t o r of 2.  Due  1 7  *  1 8  *  1 9  *  2 0  to  introduce  T h i s improved the  t o the divergence  e l e c t r o n envelope past the r i n g f o c u s , an a x i a l  of  Figure the  detector  must be f a i r l y l a r g e t o c o l l e c t a l l the e l e c t r o n s and  hence  i s s u b j e c t to l a r g e background noise or i f i t i s made s m a l l e r t o operate mission.  at lower background i t causes a l o s s i n t r a n s A r e a l i z a t i o n of t h i s f a c t l e d J.A.L. Thompson  (unpublished)  of t h i s l a b o r a t o r y t o i n v e s t i g a t e the  of e l e c t r o n s at the r i n g  collection  focus.  Thompson's d e t e c t o r c o n s i s t e d of a r i n g of anthracene s c i n t i l l a t i o n c r y s t a l s "cemented" w i t h h i g h v i s c o s i t y cone o i l i n t o a groove on the open l i p of a l u c i t e cone". oil,  The  l i g h t cone was  o p t i c a l l y coupled,  sili-  "light-  u s i n g the same  t o a p h o t o m u l t i p l i e r tube. V a r i o u s d e t e c t o r systems were t r i e d w i t h a view to  t a i n i n g the best s i g n a l - t o - n o i s e r a t i o p o s s i b l e .  The  ob-  system  21 d e s c r i b e d by K.C. being one  Mann and F.A.  of the most r e c e n t .  Payne  and shown i n F i g u r e 5  I t i s t h i s d e t e c t o r system,  w i t h some minor a l t e r a t i o n s , which i s i n use r a t o r y at the present  time.  The  present  in this  labo-  d e t e c t o r system w i l l  be d e s c r i b e d i n d e t a i l i n a l a t e r s e c t i o n . 21 While the d e t e c t o r used by Mann and Payne  was  s i d e r a b l y f u r t h e r away from the magnet c o i l s than the  consource  F i g . 5.  Detector Assembly of Mann and Payne  -23-  it s t i l l  l a y w i t h i n a r e s i d u a l megnetic f i e l d ,  which a f f e c t e d  the performance of the photo-tube at high magnet c u r r e n t s . The  photo-tube was  s h i e l d e d from these f i e l d s by p l a c i n g  e n t i r e d e t e c t o r system ( l u c i t e and  a l l ) i n a Conetic s h i e l d  and by p l a c i n g a Mu-metal s h i e l d around the photo-tube It was  found that t h i s arrangement gave s u f f i c i e n t  to leave the photo-tube output u n a f f e c t e d field  itself.  protection  by the magnetic  f o r e l e c t r o n momenta l e s s than 4,000 gauss-cm.  above t h i s momentum i t was  the  found t h a t the g r e a t e r  f i e l d began to reduce the p h o t o m u l t i p l i e r output  However,  focussing pulse.  F i g u r e 6 shows the r e l a t i v e p o s i t i o n s of source, magnet and  detector,  of the entrance and e x i t b a f f l e s and  of  the  2 1  source c e n t e r i n g c o n t r o l s used by Mann and entrance b a f f l i n g system was holder which was  Payne  .  The  mounted r i g i d l y to the source  mounted on a c e n t e r i n g mechanism capable of  moving the source t o any d e s i r e d p o s i t i o n i n a plane perpend i c u l a r to the magnetic f i e l d  axis.  T h i s was  found t o  be  a b s o l u t e l y necessary, p a r t i c u l a r l y when the b a f f l e s were chosen f o r optimum r e s o l u t i o n , s i n c e otherwise the r i n g focus of the e l e c t r o n beam was w i t h the annular e x i t s l o t . spectrometer was  i t was  necessarily  concentric  B e f o r e c e n t e r i n g the source,  a l i g n e d i n the center of the magnet as  as p o s s i b l e by v i s u a l o b s e r v a t i o n . ing  not  circular  Even a f t e r source  u n l i k e l y that the s o u r c e - d e t e c t o r  well  center-  and magnetic axes  were c o i n c i d e n t , s i n c e the c e n t e r i n g mechanism d i d not the r e q u i r e d number of degrees of freedom.  the  give  F i g . 6.  M o d i f i e d Thin-Lens  Spectrometer  -24-  They i n v e s t i g a t e d the performance o f t h i s  instrument  u s i n g a constant s o u r c e - d e t e c t o r d i s t a n c e o f 59.7 cm.; entrance b a f f l e s g i v i n g g a t h e r i n g powers of 0.7, 1.1 t o 1.6%, the tangent  o f the mean angle of each t r a j e c t o r y  b e i n g .4, .385 o r .352; 5.15  r i n g f o c u s d e t e c t i o n by means o f a  cm.mean r a d i u s c i r c l e of anthracene  c r y s t a l s and a Cs backing.  envelope  scintillation  source mounted on a t h i n aluminum  For each g a t h e r i n g power they o b t a i n e d an optimum  source t o m a g n e t - c o i l - c e n t e r  d i s t a n c e , S, by i n s t a l l i n g a 137  l a r g e e x i t s l o t and o b s e r v i n g the p r o f i l e of t h e Cs v e r s i o n peak f o r d i f f e r e n t  S positions.  con-  The optimum v a l u e  of S was c o n s i d e r e d t o be the one of the s e t which gave a peak p r o f i l e having maximum t r a n s m i s s i o n and best (The two o c c u r r e d s i m u l t a n e o u s l y . )  resolution.  They then matched the  annular e x i t s l o t w i t h the annular entrance s l o t by r e d u c i n g the e x i t s l o t width u n t i l  the t r a n s m i s s i o n s t a r t e d t o drop.  Each r e d u c t i o n i n e x i t s l o t v/idth b e f o r e t h e t r a n s m i s s i o n s t a r t e d t o d e c l i n e improved the r e s o l u t i o n .  Any f u r t h e r  r e d u c t i o n d i d not improve the r e s o l u t i o n but d i d c u t down the t r a n s m i s s i o n .  The b a f f l e s were matched when maximum  t r a n s m i s s i o n and minimum r e s o l u t i o n were o b t a i n e d . A comparison of the performance of some h e l i c a l spectromet e r s was t a b u l a t e d and t h a t t a b l e i s r e p r i n t e d here:  -25-  TABLE I Comparison of some high-performance h e l i c a l Type  Iron  (%)  1  R(%)  spectrometers. (JfL) x  100  R Solenoidal Intermediate Long l e n s Solenoidal Long l e n s Intermediate Intermediate Long lens Thin lens  image  image image  No No Yes Yes Yes Yes Yes No No  x 100 r e p r e s e n t s  0.4 1.6 2.4 1.2 1.3 4 5.5 9 1.37  2 4.5 6.3 3 2.7 8 10 11 1.6  500 280 262 250 208 200 180 122 118  a rough f i g u r e of m e r i t .  R The m o d i f i e d  spectrometer d e s c r i b e d above has  certain  limitations. 1) Magnetic S h i e l d i n g .  The  source-detector  distance i s  s m a l l enough t o cause the d e t e c t o r assembly t o l i e i n a residual focussing f i e l d .  At s u f f i c i e n t l y h i g h magnet c u r r e n t s ,  t h i s f i e l d a d v e r s e l y a f f e c t s the p h o t o m u l t i p l i e r C a l c u l a t i o n s show t h a t an i n c r e a s e of 20 cm.  output.  i n the magnet-  d e t e c t o r d i s t a n c e would p r a c t i c a l l y e l i m i n a t e the  shielding  problem. 2) C e n t e r i n g and Alignment. source  I t has been found t h a t  c e n t e r i n g i s very c r i t i c a l .  There i s evidence  that  c e n t e r i n g by source movement o n l y , produces an "optimum" f o r any r e l a t i v e p o s i t i o n of s o u r c e - d e t e c t o r However, i f these two may  and magnetic axes.  axes are not c o i n c i d e n t , t h i s "optimum"  not be the best a t t a i n a b l e .  This condition conceivably  -26-  c o u l d be improved i f a method of b r i n g i n g the two axes i n t o c o i n c i d e n c e were  adopted.  3 ) Source P o s i t i o n .  In the m o d i f i e d spectrometer the  source i s p l a c e d 11.6 cms. i n s i d e the end of the vacuum chamber.  The w a l l s of the chamber and the source c e n t e r i n g  mechanism behind the source p r o h i b i t one from m o d i f y i n g the end p l a t e f o r angular c o r r e l a t i o n v/ork.  A s i m p l e r source  holder which p l a c e s the source a t the end of the chamber would permit the p o s s i b l e use of the spectrometer f o r angular c o r r e l a t i o n work. 4 ) A minor inconvenience i s the n e c e s s i t y of d i s t u r b i n g the source t o r e a c h the d e t e c t o r .  III.  PRESENT INVESTIGATION  A.  The ter  present  INSTRUMENTATION  i n v e s t i g a t i o n was  s i m i l a r to the one  d i f f e r e n c e s i n the two  used by Mann and Payne. spectrometers  1) The magnetic f i e l d i n s t e a d of f o u r .  2) The  are:  (The innermost but one winding had,  sometime  Thus the f i e l d s t r e n g t h i s  s m a l l e r f o r a g i v e n magnet c u r r e n t .  i n l e n g t h and of the same diameter as  o r i g i n a l chamber.  3) The  major  vacuum chamber has been extended by a c y l i n d e r of  b r a s s 38 cms.  the d e t e c t o r  The  i s formed by three s e t s of windings  b e f o r e , s h o r t e d to the case.) correspondingly  c a r r i e d out on a spectrome-  the  T h i s e x t e n s i o n , shown i n F i g u r e 7, houses  assembly. end of the chamber which housed the source  holds the d e t e c t o r and  v i c e versa.  A l s o the source  has been c o n s t r u c t e d so that the source  lies  now  assembly  i n the plane  of  the end of the chamber ( F i g . 8). 4) A c e n t e r i n g mechanism has been i n t r o d u c e d so t h a t accurate a x i a l alignment can be made. The  source  h o l d e r , the entrance  b a f f l e and  t h e i r means  of attachment to the chamber are shown i n F i g u r e 8. entrance  b a f f l e i s attached  The  i n a f i x e d p o s i t i o n to a p l a t e  which i n t u r n i s f i x e d to the end of the chamber.  The  source  i s attached to t h i s p l a t e so t h a t i t can be p o s i t i o n e d on  the  F i g . 7.  Spectrometer  Assembly  i  Low EnergyBaffle  -Source  Entrance Baffle ~  Entrance Baffle  n  Supports  Chamber  Fig.  8.  The  Source Assembly  Source Holder  Vac "Cap  uum  c e n t r a l a x i s of the entrance or entrance  b a f f l e may  baffle.  In t h i s way,  be r e p l a c e d without  the assembly alignment.  baffle.  A l s o t h i s assembly allows the  In a d d i t i o n , F i g u r e 8 shows a "low  e l e c t r o n s which might otherwise  The  pass o u t s i d e the  low  energy  focussed.  i s shown i n F i g u r e 9. slot  i n the f a c e of the C o n e t i c s h i e l d , impinge upon anthracene  p l a c e d immediately behind.  The  focus which has  assembly  The e l e c t r o n s , a f t e r t r a v e l l i n g through  through the e x i t b a f f l e and  crystals  energy"  entrance  end of the chamber c o n t a i n i n g the d e t e c t o r  the annular  source  angular  T h i s b a f f l e p r o t e c t s the d e t e c t o r from any  b a f f l e and be  source  materially disturbing  to be removed e a s i l y and exposes i t f o r p o s s i b l e c o r r e l a t i o n work.  the  pass crystals  e x i t b a f f l e d e f i n e s the  a mean r a d i u s of 5.15  cms.  The  scintillation  are embedded i n the f a c e of a l u c i t e cone with  a i d of a s i l i c o n e g e l and  the cone i s coupled  ring  the  t o the photo-  s e n s i t i v e f a c e of a p h o t o m u l t i p l i e r tube by s i l i c o n e o i l . T h i s cone, and hence the c r y s t a l s , b a k e l i t e r i n g and  a screw.  base of the cone and  i s kept i n p l a c e by a  The b a k e l i t e r i n g surrounds the  i s cemented to the photo-tube f a c e .  screw passes through the f a c e of the C o n e t i c s h i e l d screws through the e x i t b a f f l e .  The  and  screw, by pushing a g a i n s t  the f a c e of the l u c i t e cone, f o r c e s the cone a g a i n s t photo-tube and f o r c e s the photo-tube i n t o the base of Mu-metal s h i e l d .  Since the Mu-metal s h i e l d  r e s p e c t to the C o n e t i c s h i e l d  The  is fixed  the the with  and s i n c e the e x i t b a f f l e i s  Chamber Extension"  Fig. 9 .  The Detector Assembly  Bakeiite Support  Aluminium | Foil  -29-  attached cap,  to the f a c e of the C o n e t i c s h i e l d by a m i l d  the whole assembly i s f i x e d w i t h r e s p e c t  shield.  The c r y s t a l s  the e x i t annular on the i n s i d e  steel  t o the C o n e t i c  t h e r e f o r e are always immediately behind  slot.  F i n a l l y , by means of r i n g supports  of the chamber and on the o u t s i d e of the  Conetic s h i e l d ,  the C o n e t i c s h i e l d  i s placed  p o s i t i o n w i t h r e s p e c t t o the chamber.  i n a fixed  This p o s i t i o n i s  approximately i n the center of the chamber and i s always 1  reproducible. The  C o n e t i c s h i e l d was obtained  Company, Chicago, 111.  from P e r f e c t i o n Mica  Inside t h i s s h i e l d and sttrrounding  the photo-tube i s a Mu-metal s h i e l d used to d i m i n i s h the e f f e c t of low f i e l d s which penetrate  the C o n e t i c  cores.  The  p h o t o m u l t i p l i e r tube used i s a Dumont 6364 which has a 5 " diameter p h o t o s e n s i t i v e cathode.  I t was s e l e c t e d from the  three tubes a v a i l a b l e because i t had the best s i g n a l - t o noise r a t i o .  Normalizing  c  the s i g n a l - t o - n o i s e r a t i o of the  photo-tube used t o 1, the corresponding  signal-to-noise  r a t i o s of the two remaining photo-tubes were found t o be .74 and  .55.  F i n a l l y , the s i d e s of the l u c i t e cone are s h o r t  and were machined i n the form of a s e c t i o n of a l o g a r i t h m i c spiral.  The s i d e s and f r o n t  face  (except  f o r the c r y s t a l  area) are covered w i t h aluminum f o i l so as t o minimize photon l o s s through the s i d e s and end of the cone. Each b a f f l i n g system c o n s i s t s of tv/o b a f f l e s , and  an outer.  The inner b a f f l e  i s attached  an inner  by means of  " s p i d e r " l e g s (as shown i n F i g . 10) to the outer which i s attached vacuum chamber.  t o a support  baffle  f i x e d w i t h r e s p e c t t o the  These s p i d e r l e g s and b a f f l e s were c a r e -  f u l l y machined so that they f i t t e d together  e x a c t l y and so  that d i f f e r e n t permutations of b a f f l e s c o u l d be made without r e q u i r i n g adjustments t o the system.  Entrance  b a f f l e s were  machined t o g i v e g a t h e r i n g powers o f 1.5%, 1.1% and 0.7% at each of three mean admission angles  angles.  are .400, .388 and .353.  The tangents of these  Sets of e x i t b a f f l e s were  machined t o p r o v i d e v a r i a t i o n of the e x i t s l o t width by .25mm. steps from 2.25 mm.  to 4  mm.  With t h i s design one may remove the source  h o l d e r or  d e t e c t o r assembly s e p a r a t e l y , so as t o change b a f f l e s , c r y s t a l s , e t c . , and r e p l a c e the source  source,  holder or d e t e c t o r  assembly w i t h a minimum amount of readjustment.  Because of  the a d d i t i o n a l chamber l e n g t h and the removal of the bulky source  c e n t e r i n g mechanism, the s o u r c e - d e t e c t o r  d i s t a n c e has  been i n c r e a s e d from 69.7 cms. as used by Mann and Payne t o 100 cms. The  aim o f the new c e n t e r i n g technique  i s to obtain  c o i n c i d e n c e of the magnetic a x i s and the s o u r c e - d e t e c t o r  axis.  The magnetic a x i s i s f i x e d by the p o s i t i o n of the magnet  coils.  The  source-detector  chamber.  Therefore,  a x i s i s f i x e d w i t h r e s p e c t t o the vacuum t o o b t a i n a x i a l alignment and c o i n c i -  dence, i t i s necessary  t o be able t o c o n t r o l the o r i e n t a t i o n  of the chamber a x i s with r e s p e c t t o the magnetic a x i s , i . e . t o  F i g . 10.  Sample Entrance B a f f l e  -31-  ;  be able t o move the chamber l a t e r a l l y t o any two dimensional g r i d p o s i t i o n and t o r o t a t e i t with r e s p e c t to the f i x e d magnetic a x i s .  The range of motion i s , of course,  by the s i z e of the magnet c o i l opening.  limited  F i n a l l y , s i n c e the  source-magnet d i s t a n c e i s a parameter which cannot be constant, one must a l s o be able t o move the chamber l o n g i t u d i n a l l y  along  i t s own a x i s . To achieve t h i s , the e n t i r e chamber i s supported on two i d e n t i c a l stands which were c o n s t r u c t e d t o permit the r e q u i r e d freedom of motion.  An i l l u s t r a t i o n of the stands used i s  shown i n F i g u r e 11. trometer  The two stands are p l a c e d on the spec-  t a b l e i n l i n e v/ith the magnet c o i l opening.  The  vacuum chamber r e s t s on the c o r r u g a t e d r o l l e r s which permit motion of the chamber p e r p e n d i c u l a r t o the magnet v a r i a t i o n of S ) .  (i.e. a  As can be seen from F i g u r e 11, one can move  the chamber h o r i z o n t a l l y by t u r n i n g d i a l A, or v e r t i c a l l y by t u r n i n g d i a l 3. The  1  circumference of the c e n t e r i n g d i a l s  i s divided  i n t o 10 d i v i s i o n s so that the r e l a t i v e motion of the chamber may be observed  and so that the chamber may be r e t u r n e d t o  any s e t p o s i t i o n .  The motion of the chamber i s determined  by the motion of the screw threads attached t o these  dials.  Since these screws have 25 threads t o the i n c h , a r o t a t i o n of one t e n t h of a r e v o l u t i o n on the d i a l moves the chamber approximately The  .1 mm.  a x i s of the magnet i s p l a c e d p a r a l l e l t o the h o r i -  A  F i g . 11  Chamber Support  -32-  z o n t a l component o f the earth's magnetic f i e l d so that favourable  defocussing  ponent o f the e a r t h ' s  e f f e c t s due field  through two planes  to the h o r i z o n t a l com-  are minimized.  ponent i s minimized by p a s s i n g  contained  above and below the spectrometer.  1 meter. coils.  The  the planes  The  v e r t i c a l com-  a d i r e c t c u r r e n t of 1.1  compensating c o i l s ,  b y 1.5 meters and  i n two  They are 1.1  c o n t a i n i n g them are separated  line.  magnet c u r r e n t  The  current  two  field  i n d i r e c t i o n to t h a t of  v e r t i c a l component of the e a r t h ' s  d.c.  i s obtained  from a 110  i s r e g u l a t e d as f o l l o w s .  volt  The  current  a bank of 6 A S 7 t r i o d e s  a standard  v o l t a g e developed  0. 0 3 - n - r e s i s t o r .  The  two  provides  6AS7's. By  Any  i n current  against  e r r o r s i g n a l between these  a compensating v o l t a g e  Regulation  and  across  r e s i s t o r i s balanced by a c o n t r o l c h a s s i s  the output o f a potentiometer.  the  field.  passes through the magnet c o l l s ,  the standard  to the g r i d s of  to 1 part i n 10  is  the achieved.  v a r y i n g the potentiometer s o t t i n g , the magnet c u r r e n t  be v a r i e d from 0 to 10 amperes (H The  by  amperes i n these c o i l s c r e a t e s at  i n magnitude but o p p o s i t e  d.c.  meters  vacuum chamber l i e s midway between these  A c u r r e n t of 1.1  The  amperes  horizontal  the mid-point of the e l e c t r o n envelope, a magnetic equal  un-  p h o t o m u l t i p l i e r H.T.  can  - 0 to 5,000 gauss-cm.).  i s obtained  by t a p p i n g  d e s i r e d v o l t a g e from a bank of v o l t a g e r e f e r e n c e across the output o f a r e g u l a t e d H.T.  supply.  the  tubes p l a c e d  Photomultiplier  p u l s e s pass through a cathode f o l l o w e r c i r c u i t mounted on  the  jovacuum chamber (see F i g . 9) i n t o a commercial and b i a s d i s c r i m i n a t o r  whose c o n s t a n t h e i g h t  p u l s e s a r e counted by a s c a l a r .  i.  amplifier output  -34-  B. EXPERIMENTAL TESTS  The c h i e f purpose  of the present i n v e s t i g a t i o n was  to  study the e f f e c t of the c e n t e r i n g mechanism on the performance of the spectrometer.  As has been s t a t e d , i t i s  b e l i e v e d that f o r best performance  the s o u r c e - d e t e c t o r a x i s  ( f i x e d w i t h r e s p e c t to the vacuum chamber) should c o i n c i d e w i t h the a x i s of the magnetic  field.  Since the  magnetic  a x i s i s f i x e d , the vacuum chamber must have s u f f i c i e n t of motion  to permit t h i s a x i a l alignment.  freedom  I t i s necessary  that the supports a l l o w one to move the chamber a x i s anywhere w i t h i n a s m a l l s o l i d cone which has i t s apex at the magnet center and i t s a x i s along the magnetic  axis.  A l s o the supports  must a l l o w one t o move the chamber a x i s onto the magnet c e n t e r . It has been shown that the chamber supports permit t h i s  motion.  To c a l i b r a t e the instrument, the p r o f i l e and h e i g h t of 1 "37  the 661.6  Kev.  K-conversion peak of Cs  v a r i o u s chamber p o s i t i o n s .  was  examined at  The optimum p o s i t i o n f o r any s e t  of entrance and e x i t b a f f l e s i s that p o s i t i o n g i v i n g minimum l "37 peak h a l f - w i d t h and maximum peak h e i g h t . were s t u d i e d . 1.6 mm.  One  Two  Cs  had a source diameter of 2.4  sources mm.,  the other  These were the tv/o sources used by Mann and Payne i n  the c a l i b r a t i o n of t h e i r spectrometer thus p e r m i t t i n g us to compare the performance  of the two  instruments.  A v a r i e t y of methods of moving the chamber were c o n s i d e r e d w i t h a view to o b t a i n i n g a procedure whereby a x i a l  alignment  c o u l d be r e a d i l y obtained.  I t i s reasonable t o assume that  the magnetic a x i s i s approximately normal t o the magnet and  that the s o u r c e - d e t e c t o r  a x i s i s approximately  v/ith the c y l i n d r i c a l a x i s of the vacuum chamber. v i s u a l alignment o f the chamber p e r p e n d i c u l a r and  coil  coincident Hence  to the magnet  v i s u a l c e n t e r i n g o f the chamber i n the hole o f the magnet  c o i l should  be a f i r s t  approximation t o a x i a l alignment.  Of  course, a f t e r v i s u a l alignment the two axes are not l i k e l y to c o i n c i d e e x a c t l y or even to l i e i n the same plane. Assuming that the axes do not l i e i n the same plane a f t e r v i s u a l alignment and c o n s i d e r i n g  the o p t i c a l analogy o f a  simple converging l e n s , an improved f o c u s s i n g c o n d i t i o n a r i s e when the magnetic a x i s and the s o u r c e - d e t e c t o r i n t e r s e c t a t the magnet  (lens) center.  should  axis  Presumably, t h i s  " i n t e r s e c t i o n of axes" c o n d i t i o n may be reached from an a r b i trarily  l o c a t e d source p o s i t i o n by r o t a t i n g the chamber, and  hence the s o u r c e - d e t e c t o r  a x i s , about the p o s i t i o n of the source  u n t i l the best peak i s obtained.  T h i s i s not the i d e a l  sing condition since neither "object"nor on the " o p t i c " a x i s .  "image" a r e l o c a t e d  A f t e r o b t a i n i n g the optimum by r o t a t i o n  about the source, i d e a l a x i a l alignment should if  focus-  the chamber i s r o t a t e d about the magnet  then be reached  center.  A p o s s i b l e a l t e r n a t i v e procedure would be t o use a t r a n s l a t i o n a l motion o f the chamber to o b t a i n an i n t e r s e c t i o n of the two axes a t the magnet c e n t e r ,  as evidenced by maximum  transmission  Then, the f i n a l r o t a t i o n  and r e s o l v i n g power.  about t h i s i n t e r s e c t i o n p o i n t should put the axes i n c o i n c i dence. It i s probable that a l l motions are i n t e r c o n n e c t e d , e.g. the optimum reached on a h o r i z o n t a l r o t a t i o n may depend somewhat on the v e r t i c a l s e t t i n g .  To take t h i s p o s s i b i l i t y  into  account, repeat runs s h o u l d form p a r t of the s e t t i n g up procedure. A f t e r c o n s i d e r a b l e e x p e r i m e n t a t i o n v/ith s e v e r a l  combi-  n a t i o n s of entrance and e x i t b a f f l e s , a procedure was decided upon which appeared  t o g i v e the optimum p o s i t i o n .  T h i s pro-  cedure may be d e s c r i b e d by l i s t i n g a sequence o f s t e p s t o follow: 1) S e l e c t and i n s t a l l the d e s i r e d entrance 2) I n s t a l l some e x i t b a f f l e .  baffle.  P r e f e r a b l y , the e x i t  width s h o u l d be l a r g e r than the s l o t width expected  slot  from  previous experience. 3) A l i g n the chamber v i s u a l l y i n the c e n t e r o f the magnet opening and p e r p e n d i c u l a r to the plane o f the magnet. 4) Obtain an optimum source-magnet d i s t a n c e by moving the chamber l o n g i t u d i n a l l y u n t i l i t i s s e t a t the p o s i t i o n 1*37  g i v i n g the b e s t Cs  conversion electron  profile.  5 ) Rotate the chamber s u c c e s s i v e l y i n a h o r i z o n t a l and v e r t i c a l plane about p r o f i l e i s found.  the source p o s i t i o n u n t i l an optimum  Repeat t h i s procedure as many times as i s  necessary t o o b t a i n the same d i a l r e a d i n g s on two s u c c e s s i v e runs.  -37-  6) Rotate the chamber s u c c e s s i v e l y i n a h o r i z o n t a l and v e r t i c a l plane about  the magnet c e n t e r u n t i l the b e s t p r o f i l e  i s obtained. 7) Cut down the s i z e of the e x i t s l o t u n t i l a match has been obtained. without l o s s  The e x i t b a f f l e g i v i n g  the best  resolution  i n t r a n s m i s s i o n i s c o n s i d e r e d to be the matching  b a f f l e f o r any p a r t i c u l a r entrance b a f f l e . While t h i s procedure was f a i r l y r e l i a b l e , we found i t necessary t o check back and i*epeat a f t e r c e r t a i n other s t e p s had been completed.  The optimum S p o s i t i o n  after  visual  alignment was sometimes found t o be s l i g h t l y d i f f e r e n t the optimum a f t e r r o t a t i o n a l s l o t match had been made.  than  alignment or a f t e r a b e t t e r e x i t  A l s o the chamber p o s i t i o n  thought  to be the optimum w i t h one e x i t b a f f l e was not always the optimum found a f t e r another b a f f l e w i t h s m a l l e r e x i t s l o t width had been i n s t a l l e d . The n e c e s s i t y t o check the S p o s i t i o n  i s to be expected  s i n c e i t i s o b t a i n e d i n the b e g i n n i n g when the chamber i s p o o r l y a l i g n e d and the e x i t s l o t width i s too l a r g e . a repeat on the r o t a t i o n a l  Ideally,  alignment s h o u l d not be necessary.  It would be necessary i f the p o s i t i o n  of the e x i t b a f f l e v/ith  r e s p e c t to the chamber i s changed i n the process o f changing the b a f f l e .  I t would a l s o be necessary i f the optimum  found w i t h the l a r g e e x i t s l o t width i s o n l y After  position  approximate.  some e x p e r i e n c e i t v/as concluded that the r o t a t i o n a l  procedure s h o u l d be r e p e a t e d because the optimum  position  i  1 Q  o b t a i n e d w i t h a l a r g e e x i t s l o t width i s g e n e r a l l y not the true optimum as determined w i t h matching We about  a l s o found  baffles.  that, f o r the m a j o r i t y of runs, r o t a t i o n  the magnet center a f t e r r o t a t i o n about  not g i v e a n o t i c e a b l y b e t t e r p r o f i l e .  the source d i d  Thus we  concluded  t h a t , because of the f i n i t e s i z e of the source and because of  the s m a l l source-magnet d i s t a n c e , (approximately 18  the magnetic  cms.)  a x i s probably passed through the source area  even a f t e r v i s u a l alignment o n l y . the chamber about  the source was  axes s a t i s f a c t o r i l y .  T h e r e f o r e , r o t a t i o n of s u f f i c i e n t to a l i g n  Conceivably w i t h a s m a l l e r source or  g r e a t e r source-magnet d i s t a n c e (necessary i f entrance g i v i n g a s m a l l e r emergence angle were used) r o t a t i o n the magnet center would be Initially,  we  the  baffles about  necessary.  thought 'that a t r a n s l a t i o n a l  motion,  a f t e r v i s u a l alignment would g i v e the same r e s u l t s as r o t a t i o n about  the source.  However, when t h i s method was  tried,  the  r e s u l t s v/ere f a r l e s s s a t i s f a c t o r y and so the method v/as abandoned. Changes i a chamber p o s i t i o n r e q u i r e d to produce c a n t l y poorer performance expected.  were much s m a l l e r than  0.25  mm.  produced  I t shows that a misalignment  has a n o t i c e a b l e e f f e c t on the peak p r o f i l e and  a misalignment was  originally  F i g u r e 12 shows the change i n peak p r o f i l e  by a h o r i z o n t a l t r a n s l a t i o n .  signifi-  of 0.5 mm.  is intolerable.  of  that  T h i s s e t of curves  taken with a poor b a f f l e match, before the chamber had  been p r o p e r l y centered and w h i l e u s i n g the l e s s s e n s i t i v e 2.4 mm.  diameter source.  A f t e r good alignment w i t h the  s m a l l e r source, a r o t a t i o n a l motion  away from the optimum  caused by moving the i n n e r support by 1.0 mm. support by 2.5 mm.  was  and the outer  found to have a n o t i c e a b l e e f f e c t .  T h i s source r o t a t i o n i s e q u i v a l e n t to changing  the angle  between the s o u r c e - d e t e c t o r a x i s and the magnetic 0°09'.  a x i s by  U n f o r t u n a t e l y , t h i s extreme s e n s i t i v i t y caused  c o n s i d e r a b l e t r o u b l e f o r , not a n t i c i p a t i n g such  us  sensitivity,  the chamber supports l a c k e d the r i g i d i t y r e q u i r e d f o r good stability.  B a c k l a s h i n the screw threads a l s o c o n t r i b u t e d  to our experimental d i f f i c u l t i e s . One  other p o i n t s h o u l d be mentioned.  We  had hoped, i n  the d e s i g n of the source and d e t e c t o r assemblies, that e x i t b a f f l e , c r y s t a l s , , e t c . and the entrance b a f f l e  the  and  source c o u l d be changed without r e q u i r i n g f u r t h e r c e n t e r i n g . The d e t e c t o r system may  be removed and r e p l a c e d without  d i s t u r b i n g the c e n t e r i n g .  However, i t seems that any change  i n entrance b a f f l e s and c e r t a i n l y any change i n source r e quires recentering.  T h i s i s probably because the source  assembly i s much c l o s e r to the c e n t e r of the magnetic  field  than the d e t e c t o r assembly and t h e r e f o r e i t s p o s i t i o n i s much more c r i t i c a l . Using an entrance b a f f l e g i v i n g a g a t h e r i n g power of .70% at a mean e m i s s i o n angle of arc-tangent 1.50  i t was  found 137  that the optimum S p o s i t i o n was  IS cms.  With the l a r g e Cs  -40-  source, a. r e s o l v i n g power or h a l f peak-height width of 1.0S _+ .01% was o b t a i n e d .  Mann and Payne, w i t h t h e i r spectrome-  t e r , o b t a i n e d 1.15% w i t h s i m i l a r entrance s l o t c h a r a c t e r i s t i c s , i The  l a r g e source was then removed and the s m a l l e r one i n s t a l l e d .  Before f u r t h e r 1.00%.  centering  After further  the r e s o l u t i o n was observed to be  centering  the optimum p o s i t i o n gave a  r e s o l v i n g power o f .94 +_ .01%.  F i g u r e 13 shows a sample of  the curves g i v i n g  these r e s u l t s .  —  A  8 0  - 7 0  / \  — 6 0  1 08  <* CD •I-  counted  \  optimum 2 0  large  / - 2  t.  o a  with  - 1 0  s o u r c e  \  peak obtained after changing to small source  —*-  P A R T I C L E  MOMENTA  Fig.  Some Sample Conversion Peaks  13.  1  I 0 0 % \  9 4 % \  0  |  tides  j  \  c  % \  - 4 0  - 3 0  /  /m  |  1 1  — 3 0  / 1  — 5 0  —  A  A  - 4 0  j  \  optimum small  with  source  C.  CONCLUSION  I t h a s been m e n t i o n e d center  g e n e r a l l y was n o t n e c e s s a r y .  the present can  One m i g h t s u s p e c t  that  c e n t e r i n g m e c h a n i s m g i v e s t h e same a l i g n m e n t as  be o b t a i n e d  by Mann and P a y n e .  t h e y move t h e s o u r c e  the source  The d i f f e r e n c e i s t h a t  t o o b t a i n an o p t i m u m p o s i t i o n w h i l e o u r  r o t a t i o n about t h e source Since  c h a t r o t a t i o n a b o u t t h e magnet  i s e q u i v a l e n t t o moving t h e d e t e c t o r .  i s much c l o s e r t o t h e magnet t h a n t h e d e -  t e c t o r , t h e p r o b a b i l i t y of t h e s o u r c e axis after visual probability  l y i n g on t h e m a g n e t i c  alignment i s considerably greater  that the detector  likely  that r o t a t i o n  source  gives axial  does.  than the  For t h i s reason i t i s  the source-detector  a x i s about t h e  alignment whereas p o s i t i o n i n g o f t h e source  does n o t . A c o m p a r i s o n o f t h e r o u g h f i g u r e s o f m e r i t , ^ ~ x 100, o b ~  \  R  t a i n c d b y Mann a n d P a y n e a t d i f f e r e n t g a t h e r i n g p o w e r s , i s made i n T a b l e I I .  Also  t h e v a l u e s we o b t a i n e d  with a gathering  power of .70% a r e shown i n p a r e n t h e s i s . TABLE I I (%)  ( i ^ ) x 100 Large source  1.6  1.1  .7  108  34  64  Small source 117  (64)  89  64 (74)  Mann a n d P a y n e f o u n d t h a t t h e r o u g h f i g u r e o f m e r i t became  -42-  p o o r e r as t h e e n t r a n c e s l o t w i d t h was this  change i n p e r f o r m a n c e  t o be c a u s e d by two  1) M i s a l i g n m e n t o f a x e s . ring  decreased-  This resulted  We  believe  factors.  in a  non-circular  f o c u s at the d e t e c t o r which d i d not c o i n c i d e e x a c t l y w i t h  the e x i t s l o t . performance s e r i o u s as  T h i s l a c k of c o i n c i d e n c e would  at a l l g a t h e r i n g powers. t h e g a t h e r i n g power was  s m a l l e r the entrance s l o t ,  I t would  cause  become more  decreased because  and h e n c e t h e g r e a t e r t h e a d v e r s e e f f e c t ox  circular  image on  to the entrance s l o t width. be p o o r e r  a  The  s o u r c e d i a m e t e r was  As a r e s u l t  than f o r a p o i n t s o u r c e  i n c r e a s e as t h e e n t r a n c e b a f f l e s l o t  would due  that in fact  will  i s decreased.  r e s u l t s o f Mann and P a y n e show c h a t a t l a r g e g a t h e r i n g  T h i s i s t o be e x p e c t e d f r o m  e v e r , a t a g a t h e r i n g power o f improved  comparable  i s e x p e c t e d and  powers the s m a l l s o u r c e g i v e s b e t t e r performance source.  non-  a performance  the d e v i a t i o n from the i d e a l p o i n t source performance  The  electron  resolution.  2) F i n i t e s o u r c e s i z e .  would  the  the s m a l l e r the w i d t h of the  envelope  the  poorer  than the  the above argument.  ,70% t h e p e r f o r m a n c e  was  How-  not  by r e p l a c i n g t h e l a r g e s o u r c e b y t h e s m a l l one.  i n d i c a t e t h a t a t t h i s g a t h e r i n g power t h e a d v e r s e  This effects  t o m i s a l i g n m e n t w e r e much more s e r i o u s t h a n t h o s e due  s o u r c e s i z e a n d s o any s i z e was  lost.  with better  Our  advantage  large  to  e x p e c t e d by s m a l l e r s o u r c e  r e s u l t s seem t o c o n f i r m t h i s b e l i e f f o r ,  a x i a l a l i g n m e n t , we  did obtain better  performance  w i t h the s m a l l e r source.  )  A more d e t a i l e d study of the e f f e c t s of the diameter of the soiirce on the spectrometer performance  i s necessary b e f o r e  any q u a n t i t a t i v e answers can'be g i v e n on the r e l a t i v e portance of these two  im-  effects.  The a c t i v i t y of the s m a l l source was  measured by 137  H.R.  Schneider of t h i s l a b o r a t o r y by comparison  source which he c a r e f u l l y c a l i b r a t e d .  I t was  w i t h a Cs  found that  the  t r a n s m i s s i o n o b t a i n e d by Mann and Payne w i t h a g a t h e r i n g power of 1.1% was  . f>G'7o.  S i m i l a r l y , we  found t h a t w i t h a g a t h e r i n g  power of .70% our t r a n s m i s s i o n was  .24%.  T h i s reiDresents a  l o s s i n t h e o r e t i c a l t r a n s m i s s i o n of 50% and 55% This r e s u l t  is difficult  spectrometer performance expected.  As yet we  and i t remains  respectively.  to understand and. i t r e s u l t s i n a c o n s i d e r a b l y poorer than s h o u l d be  have not found an e x p l a n a t i o n f o r t h i s  to check t h i s e f f e c t , to e x p l a i n i t and, i f  p o s s i b l e , to c o r r e c t i t . Two Firstly,  other recommendations f o r improvement might be made. the supports v/ere o r i g i n a l l y c o n s t r u c t e d without  the  r i g i d i t y necessary and w i t h a l a r g e amount of b a c k l a s h i n the p o s i t i o n i n g screws.  The most s e r i o u s f a u l t of the supports  i s t h e i r l a c k of r i g i d i t y which meant that d u r i n g the exp e r i m e n t a t i o n i t v/as v e r y d i f f i c u l t s e l e c t e d as the optimum, a f t e r  to r e t u r n to the p o s i t i o n  i t had been passed.  An  im-  proved support d e s i g n might i n v o l v e a j a c k s u p p o r t i n g the chamber from below r a t h e r than from the s i d e s and  having  s u f f i c i e n t r i g i d i t y to p r o h i b i t even minute motions under the  -44-  f r i c t i o n a l f o r c e s i n v o l v e d i n moving the chamber.  Finally,  i f screw threads are used f o r moving the chamber they s h o u l d be designed f o r a minimum amount of b a c k l a s h . The other recommendation coils.  i s r e l a t e d to the compensating  Deutsch et a l . * ' found that 1  "At low e n e r g i e s s t r a y magnetic f i e l d s may be a s e r i o u s source of t r o u b l e . A component of magnetic f i e l d perp e n d i c u l a r to the a x i s of o n l y 0.01 gauss w i l l d i s p l a c e the image by about 0.1 cm. w i t h e l e c t r o n s of about 0.1 Mev. energy". We  found that w i t h the d e s i r e d c u r r e n t of 1.1  compensating  amperes i n the  c o i l s the v e r t i c a l component of extraneous mag-  n e t i c f i e l d s v a r i e d from 0 gauss along 75% of the e l e c t r o n ' s t r a j e c t o r y to approximately .05 gauss at the extreme ends of the t r a j e c t o r y . magnetic f i e l d  C a l c u l a t i o n s show that t h i s extraneous has n e g l i g i b l e e f f e c t on the e l e c t r o n  tra-  j e c t o r i e s i n the energy i n t e r v a l used i n t h i s experiment (^>  .6 Mev.).  However, f o r work i n the low "energy r e g i o n s  t h i s magnetic f i e l d would z-esult i n poorer performance.  New  compensating c o i l s to c o r r e c t f o r the e a r t h ' s f i e l d over the e n t i r e e l e c t r o n envelope are now  b e i n g c o n s t r u c t e d and so  the adverse e f f e c t s due to the e a r t h ' s f i e l d e x p e r i e n c e d at low e n e r g i e s w i l l be e l i m i n a t e d .  BIBLIOGRAPHY  1. M. G o l d h a b e r 2. 3. 4. 5. S. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.  a n d G. S c h a r f f - G o l d h a b e r , P h y s . Rev. 7 3 , 1472 (1948JT M. G o l d h a b e r a n d A.W. S u n y a r , P h y s . R e v . 8 3 , 9 0 3 ( 1 9 5 1 ) . E. Segre and A.C. Helmholz, Rev. Mod. Phys. 21, 271 (1949). M. G o l d h a b e r a n d R.I). H i l l , R e v . Mod. P h y s . 179 ( 1 9 5 2 ) . v. Baeyer and Hahn, Phys. Z. 11, 488 (1910). J . D a n y s z , L e R a d i u m 9, 1 ( 1 9 T 5 ) a n d 1 0 , 4 ( 1 9 1 3 ) . IC. Siegbahn and N. Svartholm, Nature To7, 872 (1946). F.M. B e i d u k a n d E . J . K o n o p i n s h i , R . S . T T " 1 9 , 594 ( 1 9 4 S ) . L.M. Langer and C.S. Cook, R.S.I. 19, 257~(1948). H.O.W. R i c h a r d s o n , P r o c . P h y s . S o c . 5 9 , 7 9 1 ( 1 9 4 7 ) . v. H. Busch, Ann. Physik 81, 974 (19"2~3). R.A.R. T r i c k e r , P r o c . CambT P h v s . S o c . 22, 4 5 4 ( 1 9 2 4 ) . O. Klemperer, P h i l . Mag. 20, 545 (1935). C M . W i t c h e r , Phj's. Rev. T7(J, 3 2 ( 1 9 4 1 ) . M. Deutsch, Phys. Rev. 59, 684A (1941). M. D e u t s c h , L.G. E l l i o t T a n d R.D. E v a n s , R . S . I . 1 5 , 178 (1944). S. F r a n k e l , P h y s . R e v . 7 3 , 804A ( 1 9 4 8 ) . J.W.M. DuMond, R.S.I. 2(57 160 (1949). J.M. K e l l e r , E. K o e n i g s b " e r g , A. P a s k i n , R . S . I . 2 1 , 7 1 3 (1950). W.W. P r a t t , F . I . B o l e y , R.T. N i c h o l s , R . S . I . _22, 9 2 ( 1 9 5 1 ) . K.C. Mann, F.A. Payne, R.S.I. 30, 408 (1959). I . K a p l a n , N u c l e a r P h y s i c s , AdcTTson W e s l e y . K. Siegbahn, B e t a - and Gamma-Ray Spectroscopy, North-Holland P u b l i s h i n g Company. T.R. Gerholm, Handbuch der P h y s i k , V o l . XXXIII.  

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