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The preparation and operation of lithium drift germanium detectors Thompson, Albert Charles 1966

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THE PREPARATION AND OPERATION OF LITHIUM DRIFT GERMANIUM DETECTORS  by  ALBERT CHARLES THOMPSON B.Sc,  University  o f B r i t i s h Columbia, 1964  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 required  thesis  as conforming  t o the  standard  THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1966  In p r e s e n t i n g requirements Columbia, for  by  that  the  and s t u d y .  copying  of  this  Library I  further  thesis  t h e Head o f my D e p a r t m e n t  understood cial  gain  that shall  thesis  an a d v a n c e d d e g r e e  I agree  reference  tensive  for  this  copying not  or  or  for  in p a r t i a l at  agree  publication  Columbia  that  scholarly  be a l l o w e d w i t h o u t  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  University  s h a l l make i t  by h i s  Department  the  fulfilment of  freely  the  British available  permission  for  ex-  p u r p o s e s may be  representatives. of  of  this  thesis  my w r i t t e n  It for  granted is  finan-  permission.  ABSTRACT Lithium d r i f t e d use  germanium d e t e c t o r s have been prepared f o r  as h i g h r e s o l u t i o n gamma r a y spectrometers.  The f a b r i c a t i o n procedure  and t h e problems which can a r i s e d u r i n g p r e p a r a t i o n are d i s c u s s e d Using the techniques c h a r a c t e r i s t i c s were  d e s c r i b e d , germanium d e t e c t o r s h a v i n g t h e f o l l o w i n g prepared.  A c t i v e Volume 1.0  in detail.  cm  3  0.5 cm  T o t a l R e s o l u t i o n a t 661 keV 5.0 KeV 4.0 KeV  2.0  cm  3  4.0 KeV  1.7  cm  3  2.9 KeV  - iii  -  TABLE OF CONTENTS  Chapter  1 - INTRODUCTION  1  Chapter  2 - THEORY OF DETECTOR OPERATION  3  A.  I n t e r a c t i o n o f Gamma Rays w i t h a Semiconductor Crystal i ) I n t e r a c t i o n with c r y s t a l e l e c t r o n s i i ) Conversion  o f e l e c t r o n energy t o i o n i z e d . •  charge •  Chapter  3  B.  Use o f Semiconductor C r y s t a l as a D e t e c t o r  C.  Necessary  D.  L i t h i u m Ion D r i f t i n g  P r o p e r t i e s o f Germanium f o r D e t e c t o r s  V 8 10 12  E. , S u r f a c e . Problems  15  F.  18  R e s o l u t i o n o f Germanium D e t e c t o r s  3 - EXPERIMENTAL EQUIPMENT  22  A.  L i t h i u m E v a p o r a t i o n Equipment  22  B.  D r i f t i n g Units  22  C.  Power Supply  23  D.  D r i f t Current C o n t r o l l e r  24  E.  D e t e c t o r Holders  23"  F.  Detector Preamplifier  26  Chapter  4 - DETECTOR FABRICATION PROCEDURE  27  Chapter  5 - PRESENTATION OF RESULTS  32  - iv -  A.  S i l i c o n Detector F a b r i c a t i o n  32  B.  Germanium D e t e c t o r P r o d u c t i o n  33  C.  Detector Operation  36  Chapter 6 - CONCLUSIONS  40  Appendix  42  Bibliography  43  -  V -  LIST OF TABLES Following page 20  Table 2-1  Energy R e s o l u t i o n s f o r Fano F a c t o r s o f 0.075 and 0.16  T a b l e 3-1  V a l u e s o f R e s i s t a n c e Used f o r V a r i o u s  T a b l e 5-1  Gamma Ray E f f i c i e n c y as F u n c t i o n o f Energy  38  T a b l e 5-2  L e a s t Squares S t r a i g h t Line F i t o f Gamma Ray S p e c t r a  38  Drift  Currents  24  - vi -  LIST OF FIGURES F i g u r e 2-1:  following  V a r i a t i o n of T h e o r e t i c a l Absorption  C o e f f i c i e n t s with  —2Si£  Energy  6  F i g u r e 2-2:  The  8  F i g u r e 2-3:  Dependence o f I n t r i n s i c C a r r i e r D e n s i t y  F i g u r e 2-4:  Dependence o f E l e c t r o n and  F i g u r e 2-5:  Impurity  F i g u r e 2-6: F i g u r e 2-7:  Growth o f t h e Compensated Region Normal E l e c t r i c F i e l d D i s t r i b u t i o n  F i g u r e 2-8:  D r i f t e d Region Thickness  Energy Loss Process  Concentration  on Temperature  Hole M o b i l i t i e s on Temperature  versus  Distance  versus  from Surface  11 11 13 13 14  Time f o r V a r i o u s  Temperatures  15  F i g u r e 2-9:  R e s o l u t i o n versus  F i g u r e 3-1:  a)  Evaporation  Assembly  20  b)  Evaporation  Boat  22  a)  Germanium D r i f t U n i t  b)  Three Complete D r i f t  F i g u r e 3-2:  Energy f o r V a r i o u s  Fano F a c t o r s  20  23 Units  F i g u r e 3-3:  Power Supply C i r c u i t  23  F i g u r e 3-4:  UBC  D r i f t Current  24  F i g u r e 3-5:  a)  V e r t i c a l Detector  b)  H o r i z o n t a l Detector  Controller Holder  25  Holder  F i g u r e 3-6:  Detector  Holder  25  F i g u r e 3-7:  Low  F i g u r e 4-1:  L i t h i u m D i f f u s i o n Heating  F i g u r e 4-2:  Arrangement f o r Measuring D e t e c t o r  F i g u r e 5-1:  Cs  Noise F i e l d - E f f e c t T r a n s i s t o r P r e a m p l i f i e r Cycle  26 28  Leakage Current  31  137 Gamma Ray  Spectrum w i t h S i l i c o n D e t e c t o r  33  - viiFollowing page 137 Figure 5-2:  Cs  Spectrum with Detector #2  35  Figure 5-3: Figure 5-4:  137 Cs Spectrum with Detector #4 Typical Leakage Current versus Bias Voltage ,  35 36  Figure 5-5:  Peak Shape f o r Various Bias Voltages  36  57 Figure 5-6:  Co  Spectrum  Figure 5-7:  Cs  Figure 5-8:  Expanded Cs  37  134 Spectrum 134  37 Spectrum  37  154 Figure 5-9:  Eu  Spectrum  37  Figure 5-10: RdTh Spectrum  37  Figure 5-11: Resolution Squared versus Gamma Ray Energy  38  - viii -  ACKNOWLEDGEMENTS I would l i k e t o thank Dr. G. Jones, my r e s e a r c h s u p e r v i s o r , f o r t h e h e l p f u l a d v i c e and a s s i s t a n c e which he g e n e r o u s l y my r e s e a r c h and t h e p r e p a r a t i o n o f t h i s The k i n d h e l p o f F.S. Laboratory  working w i t h him,  thesis.  Goulding  i s s i n c e r e l y appreciated.  gave me i n  o f t h e Lawrence R a d i a t i o n  The month spent  i nhis laboratory  B. J a r r e t t , and W. Hansen was o f g r e a t h e l p i n  l e a r n i n g t h e techniques  of detector preparation.  I am indebted t o my f e l l o w graduate s t u d e n t s ;  D. Dalby,  P. Tamminga, and, R.Bradbeer f o r t h e i r a s s i s t a n c e i n p r e p a r i n g d e t e c t o r s . I would a l s o l i k e t o thank I . Fowler o f C h a l k R i v e r L a b o r a t o r i e s f o r p r o v i d i n g an e x c e l l e n t i n g o t o f germanium.  -1-  CHAPTER 1  INTRODUCTION  It i s f r e q u e n t l y important accuracy  t o measure gamma r a y e n e r g i e s w i t h  and with good e f f i c i e n c y i n many f i e l d s  high  of nuclear physics.  L i t h i u m d r i f t semiconductor d e t e c t o r s have r e c e n t l y made i t p o s s i b l e t o measure gamma r a y e n e r g i e s w i t h h i g h r e s o l u t i o n s i m u l t a n e o u s l y range o f energy.  P r e v i o u s l y , i t was necessary  t o use a v e r y  over a wide  low e f f i c i e n c y  magnetic spectrometer i f h i g h energy r e s o l u t i o n was d e s i r e d o r t o use a u moderate resolution (>8%) N a l s c i n t i l l a t i o n necessary.  d e t e c t o r i f good e f f i c i e n c y was  L i t h i u m d r i f t germanium d e t e c t o r s combine a r e s o l u t i o n c l o s e t o  t h a t o f magnetic spectrometers  witH"the h i g h e f f i c i e n c y and wide energy  range c h a r a c t e r i s t i c o f Nal s c i n t i l l a t i o n e f f i c i e n c y o f germanium d e t e c t o r s  detectors.  C u r r e n t l y the detection  i s about t e n times l e s s than  scintillation  d e t e c t o r s but t h i s d e f e c t w i l l be overcome as t h e a c t i v e volume o f germanium d e t e c t o r s i s  increased.  With germanium d e t e c t o r s i t i s p o s s i b l e t o do a g r e a t range o f experiments which were not f e a s i b l e b e f o r e because o f l i m i t a t i o n s i n t h e r e s o l u t i o n or e f f i c i e n c y o f previous  detectors.  In a d d i t i o n , many p r e v i o u s  experiments can be redone with an o r d e r o f magnitude i n c r e a s e i n accuracy. Two f i e l d s  which have b e n e f i t t e d p a r t i c u l a r l y by t h e use o f germanium  d e t e c t o r s a r e l i f e t i m e measurements o f e x c i t e d n u c l e a r s t a t e s  (Alexander  and A l l e n 1965; Alexander, L i t h e r l a n d , and Broude 1965) and the p r e c i s e a n a l y s i s o f X-Rays from mu-mesic atoms (Bardin e t a l . 1966a, B a r d i n e t a l .  -1-  CHAPTER 1  INTRODUCTION  It i s f r e q u e n t l y important accuracy  t o measure gamma r a y e n e r g i e s w i t h  and w i t h good e f f i c i e n c y i n many f i e l d s  o f nuclear  high  physics.  L i t h i u m d r i f t semiconductor d e t e c t o r s have r e c e n t l y made i t p o s s i b l e t o measure gamma r a y e n e r g i e s w i t h high r e s o l u t i o n s i m u l t a n e o u s l y range o f energy.  P r e v i o u s l y , i t was n e c e s s a r y  t o use a v e r y  over a wide  low e f f i c i e n c y  magnetic spectrometer i f h i g h energy r e s o l u t i o n was d e s i r e d o r t o use a moderate r e s o l t i o n necessary.  (>8%) N a l s c i n t i l l a t i o n  d e t e c t o r i f good e f f i c i e n c y was  L i t h i u m d r i f t germanium d e t e c t o r s combine a r e s o l u t i o n c l o s e t o  t h a t o f magnetic spectrometers  with" t h e h i g h e f f i c i e n c y and wide energy  range c h a r a c t e r i s t i c o f N a l s c i n t i l l a t i o n  detectors.  Currently the detection  e f f i c i e n c y o f germanium d e t e c t o r s i s about t e n times l e s s than  scintillation  d e t e c t o r s b u t t h i s d e f e c t w i l l be overcome as t h e a c t i v e volume o f germanium d e t e c t o r s i s i n c r e a s e d . With germanium d e t e c t o r s i t i s p o s s i b l e t o do a g r e a t range o f experiments which were not f e a s i b l e b e f o r e because o f l i m i t a t i o n s i n t h e r e s o l u t i o n or e f f i c i e n c y o f previous  detectors.  In a d d i t i o n , many p r e v i o u s  experiments can be redone w i t h an o r d e r o f magnitude i n c r e a s e i n accuracy. Two f i e l d s  which have b e n e f i t t e d p a r t i c u l a r l y by t h e use o f germanium  d e t e c t o r s a r e l i f e t i m e measurements o f e x c i t e d n u c l e a r s t a t e s  (Alexander  and A l l e n 1965; Alexander, L i t h e r l a n d , and Broude 1965) and the p r e c i s e a n a l y s i s o f X-Rays from mu-mesic atoms (Bardin e t a l . 1966a, B a r d i n e t a l .  -21966b).  D i r e c t gamma r a y s p e c t r a measurements  B o l o t i n 1966), neutron Wall  (Freedman, Wagner, P o r t e r , and  a c t i v a t i o n a n a l y s i s (Hughes, Kennett, P r e s t w i c h , and  1966), and gamma r a y c o i n c i d e n c e s t u d i e s w i l l  a l l be made more a c c u r a t e  u s i n g germanium d e t e c t o r s . This t h e s i s presents  the work done at t h e U n i v e r s i t y o f B r i t i s h  Columbia on t h e p r e p a r a t i o n o f l i t h i u m d r i f t  germanium d e t e c t o r s .  Chapter 2, t h e t h e o r y o f the o p e r a t i o n o f l i t h i u m d r i f t presented  and i n Chapter 3 the apparatus which was b u i l t  detectors i s given.  In  detectors i s to fabricate  In Chapter 4 t h e f a b r i c a t i o n procedure i s g i v e n and i n  Chapter 5 t h e experimental  r e s u l t s are presented.  F i n a l l y , i n Chapter 6  some c o n c l u s i o n s a r e drawn on the p r e p a r a t i o n and o p e r a t i o n o f l i t h i u m drift  detectors.  -3-  CHAPTER 2  THEORY OiF DETECTOR OPERATION  A.  Interaction  The the  o f Gamma Rays w i t h a_ Semiconductor C r y s t a l  detection  o f gamma rays w i t h semiconductor d e t e c t o r s  i n t e r a c t i o n o f gamma rays w i t h t h e e l e c t r o n s  Detection  i s based on  i n a semiconductor c r y s t a l .  o f - a gamma r a y can be c o n s i d e r e d as p r o c e e d i n g i n two s t e p s .  F i r s t , t h e gamma r a y i n t e r a c t s d i r e c t l y w i t h one o f t h e e l e c t r o n s . r e s u l t i n g energetic  electrons  then i n t e r a c t w i t h t h e o t h e r e l e c t r o n s  c r y s t a l p r o d u c i n g many f r e e e l e c t r o n s v a l e n c e band.  The e l e c t r o n s  The i n the  i n the c o n d u c t i o n band and h o l e s i n  and h o l e s a r e c o l l e c t e d and t h e i r t o t a l  charge  measured t o g i v e t h e energy o f t h e incoming gamma r a y . i ) Interaction with C r y s t a l  Electrons  There a r e t h r e e ways i n which an incoming gamma r a y can l o s e energy to the electrons  i na crystal.  The f i r s t  i s p h o t o e l e c t r i c absorption i n  which a l l t h e gamma r a y energy i s used t o e x c i t e an e l e c t r o n from an atom i n the  crystal.  The k i n e t i c energy o f t h e e l e c t r o n , E , i s then:  E  where:  e  =  " b  E  E  Y  C2-1)  '  E^  =  energy o f t h e gamma r a y  E^  =  binding  energy o f t h e e l e c t r o n  Momentum i s conserved by the r e c o i l o f t h e r e s i d u a l i o n . for photoelectric  absorption  are most s t r o n g l y  coupled t o the n u c l e u s .  section f o r K s h e l l electrons  i s therefore  The p r o b a b i l i t y  largest f o r K s h e l l electrons The p h o t o e l e c t r i c  was g i v e n by H a l l  (1936) a s :  absorption  which cross  =  4 /2 !_I  137  assuming:  hv  < <  m c Q  mc 0  (2-2)  hv  (non-relativistic)  hv >>binding where:  Z t  energy o f K s h e l l  electron  0^  =  K s h e l l p h o t o e l e c t r i c absorption cross  Z  =  atomic number o f atom  section  mass o f e l e c t r o n  m o  energy o f incoming gamma r a y  hv constant  =  6.6  -25 x 10  2 cm  f o r germanium: =  3.37 x 10 7/2 (hv)  -27 (2-3)  cm  where hv: i s g i v e n i n MeV. T h i s process i s dominant  f o r low energy gamma r a y s ( l e s s than 200 KeV) i n  germanium d e t e c t o r s . The second way an e l e c t r o n may r e c e i v e energy from a gamma r a y i s by Compton s c a t t e r i n g .  The p r o c e s s can be c o n s i d e r e d as an e l a s t i c  collision  between t h e gamma r a y and an e l e c t r o n i n which t h e energy i s shared and t h e the energy o f the emergent photon i s l e s s than t h a t o f the i n c i d e n t  photon.  The  energy o f t h e incoming  gamma r a y i s so l a r g e compared t o t h e b i n d i n g  energy o f t h e c r y s t a l e l e c t r o n s t h a t t h e e l e c t r o n s behave as i f they were unbound.  The energy o f t h e s c a t t e r e d gamma~ray i s g i v e n by:  hu'  The  hu  =  c r o s s s e c t i o n f o r Compton s c a t t e r i n g was g i v e n by K l e i n and N i s h i n a  (1929) a s :  a  c  =  2TTT  J2a(l+a) \ l+2a  1 i r-, o • > _ l o g ( l 2 a ) +  +  where:  1+a  2  =  log(l+2a)  l+3a  T I T 2 S  .  (2-5)  ) 2  c l a s s i c a l electron radius  ,-13  ( 2 . 8 1 8 x 10  cm)  h m zl v  hv  =  energy o f incoming  m  =  r e s t mass o f e l e c t r o n  o  Compton s c a t t e r i n g i s t h e dominant p r o c e s s  gamma r a y  i n germanium d e t e c t o r s f o r gamma  r a y s i n t h e energy range 200 KeV t o 6 MeV. The  t h i r d e x c i t a t i o n process  i s p a i r production.  gamma r a y i n t e r a c t s w i t h t h e Coulomb f i e l d o f a n u c l e u s positron-electron pair. and  and c r e a t e s a  In t h e c e n t e r o f mass c o o r d i n a t e frame the p o s i t r o n  e l e c t r o n conserve momentum by going o f f i n o p p o s i t e  Conservation  In t h i s p r o c e s s a  directions.  o f energy then r e q u i r e s t h a t each has k i n e t i c  K.E.  e  +'-.=  2  m c o  energy:  (2-6)  The. t o t a l p a i r p r o d u c t i o n c r o s s s e c t i o n , a  , was g i v e n by Bethe and Bacher  (1936) a s :  %  28  Z  2h  In  m c o  co  where:  =  5.80 x 1 0 "  cm  2 8  (2-7)  218 27  2  charge o f nucleus m o  =  2 m c « o  assuming:  mass o f e l e c t r o n h v «  137  m c o  2 -1/3 Z '  The p o s i t r o n and e l e c t r o n a r e q u i c k l y slowed down by c o l l i s i o n s w i t h o t h e r e l e c t r o n s and t h e p o s i t r o n e v e n t u a l l y a n n i h i l a t e s w i t h an e l e c t r o n i n the c r y s t a l t o produce two photons o f energy m c  2  o  a n n i h i l a t i o n photons  escapes from t h e c r y s t a l t h e t o t a l 2  in  t h e c r y s t a l w i l l be hv - m c Q  the  I f one o f t h e  energy r e l e a s e d  1 .  T h i s p r o c e s s thus y i e l d s a peak  (called  s i n g l e escape peak) i n t h e d e t e c t e d energy spectrum 0.51 MeV below the  full  energy peak.  S i m i l a r i t y , i f both a n n i h i l a t i o n photons escape, the 2  energy g i v e n t o t h e c r y s t a l i s h v ~ ^ in  (0.510 MeV).  m  c 0  p r o d u c i n g a double escape peak  t h e energy spectrum 1.02 MeV below t h e f u l l  energy peak.  The p a i r  p r o d u c t i o n p r o c e s s i s p o s s i b l e o n l y f o r gamma rays above 1.02 MeV and i s t h e dominant  p r o c e s s above s i x MeV i n germanium. The v a r i a t i o n o f the t h e o r e t i c a l a b s o r p t i o n c o e f f i c i e n t s w i t h gamma  ray  energy i n germanium f o r p h o t o e l e c t r i c a b s o r p t i o n , Compton s c a t t e r i n g ,  and p a i r p r o d u c t i o n i s shown i n F i q u r e 2-1. The t h r e e p r o c e s s e s d e s c r i b e d f o r t h e i n t e r a c t i o n o f a gamma r a y 1. but t h e o t h e r photon i s c o m p l e t e l y absorbed  (say by a p h o t o e l e c t r i c p r o c e s s )  10  Gamma-ray energy Figure 2-1  (MeV)  Variation of Theoretical Absorption Coefficients with Energy.  -7w i t h t h e c r y s t a l e l e c t r o n s are not e x c l u s i v e s i n c e the degraded  photon  from  a Compton p r o c e s s o r an a n n i h i l a t i o n photon can f u r t h e r i n t e r a c t by another Compton o r p h o t o e l e c t r i c p r o c e s s .  More o f t h e gamma r a y energy  i s given to  the c r y s t a l e l e c t r o n s by these m u l t i p l e p r o c e s s e s and' t h e r e f o r e t h e number of  counts i n t h e f u l l  background.  energy peak i s i n c r e a s e d r e l a t i v e t o t h e Compton  The p r o b a b i l i t y f o r o c c u r r e n c e o f these m u l t i p l e p r o c e s s e s  i n c r e a s e s w i t h c r y s t a l volume and t h e r e f o r e l a r g e d e t e c t o r volumes a r e v e r y desirable. i i ) C o n v e r s i o n o f E l e c t r o n Energy The  t o I o n i z e d Charge  e n e r g e t i c e l e c t r o n s produced  by a gamma r a y l o s e energy by  i n e l a s t i c c o l l i s i o n s w i t h t h e bound e l e c t r o n s . The p r i n c i p a l way f r e e charges  a r e produced  i s by t h e e n e r g e t i c  e l e c t r o n s e x c i t i n g e l e c t r o n s from t h e v a l e n c e band t o t h e c o n d u c t i o n band. F o r every e l e c t r o n e x c i t e d a h o l e i s produced energy  lost  i n t h e v a l e n c e band and t h e  i s a t l e a s t t h e band gap energy o f 0.67 eV f o r germanium.  Another way the e n e r g e t i c e l e c t r o n s can l o s e energy with t h e c r y s t a l  lattice.  i s by i n t e r a c t i n g  The i n t e r a c t i o n e x c i t e s t h e l a t t i c e i n t o an  o p t i c a l mode o f ' v i b r a t i o n w i t h t h e energy exchange b e i n g q u a n t i z e d and  -3 c h a r a c t e r i z e d by the Raman frequency o f t h e l a t t i c e  (5 x 10  eV f o r  germanium)« T h i r d l y , l a r g e numbers o f v e r y low energy remaining a f t e r t h e f i r s t produce the  secondary  e l e c t r o n s and h o l e s  two p r o c e s s e s and not having s u f f i c i e n t  energy t o  i o n i z a t i o n , l o s e t h e i r energy by thermal c o l l i s i o n s  with  lattice. Goulding  (1965b) gave a d i a g r a m a t i c r e p r e s e n t a t i o n o f t h e energy  2.. F. 5. G o u l d i n g , ul'RL-16231, 85 (1965a)  -8l o s s process  which i s g i v e n  i n Figure 2-2.  B. Use o f a Semiconductor C r y s t a l as a D e t e c t o r  A l a r g e number' o f e l e c t r o n - h o l e p a i r s a r e produced when a gamma r a y i n t e r a c t s w i t h a semiconductor c r y s t a l . the c r y s t a l , t h e e l e c t r o n s and h o l e s electrodes.  I f an e l e c t r i c f i e l d  a r e separated  i s applied to  and move t o o p p o s i t e  The charge which i s c o l l e c t e d i n t h e e x t e r n a l c i r c u i t w i l l  thus  be p r o p o r t i o n a l t o t h e energy o f t h e i n t e r a c t i n g gamma r a y .  H  h-®n Ge with E l e c t r o d e s Applied  Unfortunately, very d i f f i c u l t the  i n normal germanium o r s i l i c o n s i n g l e c r y s t a l s i t i s  t o measure t h e charge f l u c u a t i o n s due t o gamma rays because'  leakage c u r r e n t  i s very" h i g h .  cool the c r y s t a l t o very  I t would, t h e r e f o r e , be n e c e s s a r y t o  low temperatures where t h e i m p u r i t y  i n a c t i v e and t h e leakage c u r r e n t  c a r r i e r s become  low i n order t o use normal germanium as a  detector. ^ Another w a y t o reduce t h e leakage c u r r e n t :  n-p  junction.  The c a r r i e r d e p l e t e d  used as a d e t e c t o r .  3.  biased  r e g i o n at t h e j u n c t i o n c o u l d then be  T h i s type o f d e t e c t o r  Such o p e r a t i o n has,  i s t o use a r e v e r s e  (usually s i l i c o n ) i s widely  i n f a c t , been employed.  The r e s o l u t i o n  however, was not as good as t h a t p o s s i b l e by t h e f o l l o w i n g  used  obtained, technique.  <  ELECTRON OR I [OLE ENERGY E  Probability r  Probability ( 1 -  3 ^ 0 OPTICAL PHONON LOSS  ELECTRON OR HOLE (l-p)(E-e)  ELECTRON P(E-  e<)  )/2  These now become parents f o r f u t u r e g e n e r a t i o n i f t h e i r energy i s adequate f o r p r o d u c t i o n of secondaries  e = Raman phonon energy f o r l a t t i c e e = Band gap o f m a t e r i a l p = Assumes a random value i n the range 0 t o 1 K  3  Figure 2 - 2  The Energy Loss  Process  -9for  t h e d e t e c t i o n o f charged p a r t i c l e s b u t , because o f i t s s m a l l a c t i v e  volume, i t . i s n o t n o r m a l l y  used f o r gamma r a y s .  Before  t h e development o f  l i t h i u m d r i f t d e t e c t o r s , however, Donovan, M i l l e r , and Foreman a high r e s i s t i v i t y  (1960) used  d i f f u s e d j u n c t i o n c o u n t e r f o r t h e d e t e c t i o n o f 120 KeV  gamma r a y s . L i t h i u m d r i f t e d d e t e c t o r s a l s o use a r e v e r s e b i a s e d j u n c t i o n but i n between t h e p and n r e g i o n s  i s an i m p u r i t y compensated r e g i o n .  MW/VVW— 1  V The i m p u r i t y compensated r e g i o n can be v e r y l a r g e . drift  d e t e c t o r s were small s i l i c o n  d e v i c e s which gave a f u l l  Early lithium energy peak  i n t e n s i t y which was o n l y one p e r cent o f t h e Compton edge i n t e n s i t y f o r 662 Kev gamma rays  (Mayer, B a i l e y , and Dunlap 1960).  Germanium i s p r e f e r a b l e t o  s i l i c o n because o f i t s g r e a t e r atomic number and t h e r e f o r e l a r g e r c r o s s s e c t i o n f o r gamma r a y s . germanium  Freck  and W a k e f i e l d  (1962) r e p o r t e d o p e r a t i o n o f a  d e t e c t o r 1.5 mm deep a t a b i a s o f 12 v o l t s .  have p r o g r e s s i v e l y enlarged  S i n c e then many groups  t h e a c t i v e volume o f d e t e c t o r s .  Malm and Fowler  (1966) have r e c e n t l y r e p o r t e d o p e r a t i o n o f a germanium l i t h i u m d r i f t c o a x i a l 3 d e t e c t o r with an a c t i v e volume o f 54 cm a t 600 v o l t s b i a s .  -10C. Necessary P r o p e r t i e s ' o f Germanium f o r D e t e c t o r s  To make good'detectors p u r i t y and  few  the germanium c r y s t a l used must have v e r y  c r y s t a l ' f a u l t s because a long c a r r i e r l i f e t i m e i s d e s i r e d  good compensation o f i m p u r i t i e s by The  lithium i s  c a r r i e r l i f e t i m e should be  high and  necessary.  long because i t i s d e s i r e d to have  most o f the'charge produced by the gamma r a y c o l l e c t e d by the a p p l i e d e l e c t r i c f i e l d and  s u p p l i e d t o the a m p l i f i e r r a t h e r than l o s t by  processes  w i t h i n the c r y s t a l .  energy band gap take p l a c e .  Traps p r o v i d e  through which recombination  Traps are due  recombination  intermediate and  generation processes  t o i m p u r i t y c e n t e r s or c r y s t a l  ( v a c a n c i e s , d i s l o c a t i o n s , e t c . ) and may  l e v e l s i n the can  imperfections  exhibit preferential  trapping  p r o p e r t i e s f o r e i t h e r e l e c t r o n s or h o l e s thus i n h i b i t i n g c o l l e c t i o n c a r r i e r s i n the d e t e c t o r . e f f e c t s and  Goulding  causes o f t r a p s .  the number o f t r a p s and  (1965c) g i v e s a d i s c u s s i o n o f  He p o i n t s out t h a t heat treatments  oxygen can form complex' ions w i t h +  ion.  p r e c i p i t a t e and crystal.  The  because, i m p u r i t i e s  g r e a t e r than one p a r t i n 10  r a t e o f l i t h i u m (Goulding  q u a l i t y o f the i n i t i a l  1965d).  L i t h i u m can a l s o are p r e s e n t  c o n d u c t i v i t y must be  i n the  low  detector.  leakage c u r r e n t s t o be r e a l i z e d ,  c o n d u c t i v i t y o f the m a t e r i a l must be governed by the d e n s i t y o f e x c i t e d c a r r i e r s r a t h e r than by  9  germanium c r y s t a l , t h e r e f o r e , determines  the q u a l i t y o f the r e s u l t i n g  In o r d e r f o r s u f f i c i e n t l y  like  +  lose i t s e l e c t r i c a l a c t i v i t y i f vacancies  t o a great extent  increase  l i t h i u m ( l i k e L i O ) which are l e s s mobile  Oxygen c o n c e n t r a t i o n s  g r e a t l y reduce the d r i f t  the  t h e r e f o r e should be kept to a minimum.  Very h i g h p u r i t y germanium i s n e c e s s a r y  than the L i  of  i m p u r i t y atoms.  i n t r i n s i c and not  the  thermally  In o t h e r words, the  e x t r i n s i c at the o p e r a t i n g  temperature.  In i n t r i n s i c m a t e r i a l the t h e r m a l l y generated  current density, J ^ , i s given  by: J  where:  i  ^V^h  =  +  V  () 2_8  q  =  charge p e r e l e c t r o n (1.6 x 10  n^  =  intrinsic  E  =  electric  y^  =  hole m o b i l i t y  y  =  electron mobility  g  the dependence o f n . ,  y^, and y  carrier  -19  coul)  concentration  field  on temperature, T,was g i v e n by Conwell  g  (1958) as:  n.(T)  -  1.76  x 10  y (T)  -  1.05  x 10  9  •  I"  y  (T)  =  4.9  x 10  7  •  T" "  T  =  a b s o l u t e temperature  h  where: The of  • T  1 6  -  3 / 2  2  e-  ,  3  1  3  4 5 5 0 / T  on temperature i s shown i n F i q u r e  It i s necessary  to operate  (2-9)  3  cm /volt-sec  (2-10)  cm /volt-sec  (2-11)  2  6 6  2  (°K)  dependence o f n^ on temperature i s shown i n F i q u r e 2-3 y^ and y  cm"  and  the dependence  2-4.  germanium d e t e c t o r s a t l i q u i d  nitrogen  o temperature  (77 K) to keep the thermal  c o n t r i b u t e very l i t t l e  to the t o t a l  d e n s i t y g i v e n by e q u a t i o n  n.(77)  generation  noise.  c u r r e n t very low  At 77°K the i n t r i n s i c  and  carrier  (2-9) i s :  =  2.5  x 10  - 7  cm"  3  However, the h i g h e s t p u r i t y germanium o b t a i n a b l e commercially  has  thus  about  TEMPERATURE  F i g u r e 2-3  (°K)  Dependence o f I n t r i n s i c C a r r i e r D e n s i t y on Temperature  I O  1  1  .50  100  F i q u r e 2-4  J  1  1510 ZOO TEMPERATURE (°K)  1 2.50  i _ 300  Dependence o f E l e c t r o n and-Hole M o b i l i t i e s on Temperature  3SO  10  1 3  i m p u r i t i e s per cm  o f a l l one  3  o f the i m p u r i t i e s w i l l be i m p u r i t i e s , j.- p» w i l l n  m  type  i o n i z e d and  (acceptor or donor).  At 77°K most  t h e r e f o r e the c a r r i e r d e n s i t y due  be:  n. (77) imp v  =  10  cm  1 3  -3  There i s a very great d i f f e r e n c e between n ( 7 7 ) and n p ( 7 7 ) . i  n  m  i n t r o d u c i n g i m p u r i t i e s o f the o p p o s i t e of acceptor  and  The  i n t o the c r y s t a l .  donor i m p u r i t i e s then the  c o n c e n t r a t i o n w i l l be the i n t r i n s i c c o n d u c t i v i t y w i l l be  type  I f there i s  carrier  c a r r i e r c o n c e n t r a t i o n and  the  intrinsic.  m a t e r i a l s i n c e they are decreased  from the c h a r g e d " i m p u r i t i e s m o b i l i t i e s are decreased  ( A d l e r , Smith, and  i m p u r i t y s c a t t e r i n g i s reduced. i o n s by doing  final  by s c a t t e r i n g o f c a r r i e r s L o n g i n i 1964).  the charge c o l l e c t i o n time w i l l be  i m p u r i t y s c a t t e r i n g . • I f the a c c e p t o r  and  the  increased  by  T h i s i o n p a i r i n g can be done w i t h l i t h i u m  compensation at a lower temperature where the L i  mobile enough to move -very f a r by  Ion  Since  donor i m p u r i t i e s are p a i r e d  i s j u s t mobile enough to be a t t r a c t e d by the a c c e p t o r  Lithium  i s done by  e l e c t r o n and h o l e m o b i l i t i e s , however, w i l l not be the same as  the i n t r i n s i c  D.  To  i m  reduce , j_ p'(77) compensation o f the i m p u r i t i e s i n the c r y s t a l  an exact b a l a n c e  to  i m p u r i t y but  +  ion  not  drifting.  Drifting  To reduce the number o f i o n i z e d ' i m p u r i t i e s i n germanium the o f l i t h i u m i o n d r i f t has been developed. t h a t the major i m p u r i t y i s " t h e ' a c c e p t o r  A germanium c r y s t a l gallium.  i s r e f i n e d so  L i t h i u m , which i s a donor  i m p u r i t y , i s used to compensate the g a l l i u m i m p u r i t y and impurity ion concentration.  technique  thus reduce the  L i t h i u m i s used because i t i s a v e r y mobile  net  -13donor i m p u r i t y w i t h a low i o n i z a t i o n The  lithium  energy  (0.0093 eV) i n germanium.  i s u s u a l l y vacuum evaporated onto one f a c e o f the c r y s t a l  which i s then heated t o 450°C f o r f i v e minutes t o a l l o w t h e l i t h i u m t o diffuse  i n t o the c r y s t a l .  The d i f f u s i o n ' o f  i s d i s c u s s e d v e r y completely the  lithium  i m p u r i t i e s into" semiconductors  by Warner and Fordemwalt  c o n c e n t r a t i o n has t h e d i s t r i b u t i o n  (1965).  After  diffusion  shown i n F i q u r e 2-5 and t h e  c o n c e n t r a t i o n i s g i v e n by:  N.  N «erfc o  (2-12) 2/DT o  donor c o n c e n t r a t i o n a t depth x from s u r f a c e  where:  lithium  surface  concentration  t  duration of d i f f u s i o n  D  diffusion  S i n c e the germanium c r y s t a l  constant  i s s l i g h t l y p-type and a n-type  l a y e r has been formed on one s u r f a c e , t h e r e i s a p-n j u n c t i o n a t X d N  and  =  acceptor  N  concentration i n c r y s t a l  from:  o  «erfc  (2-13) o o  When t h i s p-n j u n c t i o n i s r e v e r s e b i a s e d the p o s i t i v e l y i o n s move i n t o  where:  a  x i s therefore obtained o  lithium  q  lithium  charged  the p s i d e o f the j u n c t i o n where they compensate the  n e g a t i v e l y charged a c c e p t o r  ions by charge n e u t r a l i z a t i o n .  F i q u r e 2-6 shows  the growth o f t h e compensated r e g i o n a f t e r a s h o r t p e r i o d o f d r i f t i n g .  N  After  Diffusion  L i t h i u m donors  Bulk a c c e p t o r s i  N  After  d  Drift  x = t d  F i g u r e 2-6  Growth of the Compensated Region  -14The compensation i s v e r y exact  s i n c e , f o r example, i f a p i l e up o f donors  occurs i n t h e compensated r e g i o n t h e ' e l e c t r i c f i e l d as  t o d i s s i p a t e ' t h e excess c o n c e n t r a t i o n .  gradient i s m o d i f i e d so  F i q u r e 2-7 shows the normal  e l e c t r i c f i e l d d i s t r i b u t i o n and t h a t r e s u l t i n g from an excess o f i o n s . F o l l o w i n g t h e arguments o f G o u l d i n g the compensated  (1965e) t h e r a t e o f growth o f  ( i n t r i n s i c ) r e g i o n can be e s t i m a t e d assuming t h a t t h e c u r r e n t  o f l i t h i u m ions i s due e n t i r e l y t o t h e e l e c t r i c  field,  that the d i f f u s i o n  c u r r e n t i s n e g l i g i b l e , and t h a t the l i t h i u m i o n s have a l r e a d y d r i f t e d a d i s t a n c e W.  In the i n t r i n s i c r e g i o n N  = N, and the e l e c t r i c f i e l d  a where V i s t h e a p p l i e d v o l t a g e . J , L  i s V/W  d  The c u r r e n t o f l i t h i u m ions p e r square cm,  i s then g i v e n by:  J  where:  L  =  y -N .V  y^  =  m o b i l i t y o f l i t h i u m i o n s . i n t h e semiconductor a t  L  a  ( 2  .  1 4 )  the d r i f t temperature  The number o f a c c e p t o r s p e r u n i t a r e a which can be compensated i n time dt is therefore  J «dt and, s i n c e the a c c e p t o r T  c o n c e n t r a t i o n i n the compensated  Li material i s  N  a  l a y e r i n time  , the i n c r e a s e , AW, ' ' dt  =  a  rate,  dW dt  W  o f the i n t r i n s i c  i s g i v e n by:  N -dW  The d r i f t  i n the t h i c k n e s s  V . y - N -dt L  a  (2-15)  , is:  dW dt  =  V W ' L y  (2-16)  F i g u r e 2-7  Normal E l e c t r i c F i e l d  Distribution  -15T h e r e f o r e by i n t e g r a t i o n :  W  2  =  2  V*  y • t  Lt W  =  /2n  • V-  t  (2-17)  Li Note t h a t the d r i f t  r a t e i s independent o f the r e s i s t i v i t y o f the  m a t e r i a l but t h a t i t i s i n c r e a s e d by r a i s i n g the temperature  starting  ( s i n c e the L i  i o n m o b i l i t y i n c r e a s e s w i t h temperature) or by i n c r e a s i n g the a p p l i e d v o l t a g e . The  a p p l i e d v o l t a g e i s l i m i t e d by s u r f a c e breakdown and  600  volts.  The  drift  becomes i n t r i n s i c and  temperature has  is typically  100  to  an upper l i m i t above which the m a t e r i a l  the c r y s t a l ceases to behave as a diode.  Drift  temperatures w e l l below the i n t r i n s i c temperature are p r e f e r r e d t o a v o i d compensation by the  l i t h i u m o f the t h e r m a l l y generated c a r r i e r s .  This  overcompensation can be reduced by d r i f t i n g at lower temperatures at end  o f the p r i m a r y d r i f t .  time f o r v a r i o u s d r i f t  E. S u r f a c e  The critical  The  the  t h i c k n e s s o f the d r i f t e d r e g i o n v e r s u s  temperatures i s shown i n F i q u r e  drift  2-8.  Problems  s u r f a c e s o f the c r y s t a l c o n t a i n i n g the exposed j u n c t i o n are  regions during d r i f t .  Problems can develop at t h i s stage which  s e r i o u s l y a f f e c t the c h a r a c t e r i s t i c s o f the f i n a l Before  d r i f t i n g the edges are etched  a clean surface.  detector.  to remove the s u r f a c e and  P r i o r t o e t c h i n g the c r y s t a l  f a c e s are taped w i t h an  expose etch  4 r e s i s t a n t tape to prevent the c r y s t a l  e t c h i n g o f the l i t h i u m l a y e r .  During  i s kept i n a dry i n e r t atmosphere to reduce the  4. Scotch brand  #471  can  drifting  collection  F i q u r e 2-8  D r i f t e d Region Thickness versus Time f o r V a r i o u s  Temperatures  -16o f i m p u r i t i e s on t h e j u n c t i o n . can be  I f i m p u r i t i e s , such as water, do c o l l e c t  a l t e r the e l e c t r i c a l p r o p e r t i e s shorted  impurities  they  s u f f i c i e n t l y such t h a t t h e j u n c t i o n can  and stop the l i t h i u m d r i f t i n g . can be removed by r e - e t c h i n g  Under such c o n d i t i o n s t h e  the edges a f t e r which d r i f t i n g  will  n o r m a l l y resume. Sometimes the c r y s t a l stops d r i f t i n g and cannot be r e - s t a r t e d by edge e t c h i n g .  S t a i n i n g the edges by r e v e r s e  b i a s i n g i n a copper s u l p h a t e  s o l u t i o n t o r e v e a l the j u n c t i o n w i l l then o f t e n show t h a t somewhere t h e j u n c t i o n curves a b r u p t l y  Li  ( u s u a l l y at a c o r n e r ) .  Surface  ain  This  i n d i c a t e s t h a t the j u n c t i o n has h i t a bad spot i n t h e c r y s t a l .  the bad spot has been sawn o f f and t h e edges r e - e t c h e d d r i f t i n g be  After  can o f t e n  resumed. When d r i f t i n g  junction surface  condition  conductance s u r f a c e surface  i s completed and the c r y s t a l i s ready f o r mounting, the is still  i s needed t o keep t h e t o t a l  leakage c u r r e n t  low.  The  s t a t e s which are formed can have a profound i n f l u e n c e on t h e f i n a l  c h a r a c t e r i s t i c s o f the d e v i c e . result  v e r y important because a s t a b l e , low  F o r a p-n j u n c t i o n the s u r f a c e  i n an i n v e r s i o n l a y e r extending across  t h i s i s to g r e a t l y increase  the j u n c t i o n .  t h e j u n c t i o n area and t h e r e f o r e  s t a t e s can  The r e s u l t o f the c a p a c i t a n c e  and  reverse  leakage c u r r e n t  Armantrout drift  (1966) has  germanium'detectors.  are p r e s e n t  increase. s t u d i e d the e f f e c t o f s u r f a c e s t a t e s on l i t h i u m  The  s u r f a c e - s t a t e s s e n s i t i v e t o ambient  i n or on the o u t s i d e o f the oxide  l a y e r which forms on  germanium when i t . i s exposed t o .the atmosphere.  The  and behave as an a c c e p t o r  For example, oxygen may  site.  the  e f f e c t o f these  i s to a l t e r the energy band s t r u c t u r e o f the c r y s t a l by l e v e l s i n the s u r f a c e l a y e r .  conditions  states  i n t r o d u c i n g energy  gain a negative  charge  To s a t i s f y charge n e u t r a l i t y a h o l e i s  formed i n the germanium near the s u r f a c e thus c r e a t i n g a p-type i n v e r s i o n layer.  S i m i l a r l y , o t h e r absorbed atoms can r e s u l t  i n a n-type or p-type  inversion layer. Llacer  (1964) has proposed a model which p r o v i d e s  o f the r o l e o f s u r f a c e s t a t e s i n determining c h a r a c t e r i s t i c s of a lithium d r i f t s t a t e s may and  detector.  a good  explanation  the leakage and v o l t a g e breakdown He  suggests t h a t the  surface  cause an i n v e r s i o n l a y e r which extends a c r o s s the i n t r i n s i c  overlaps  region  the o p p o s i t e j u n c t i o n .  Undrifted Material  High f i e l d s are p r e s e n t opposite  junction.  where the i n v e r s i o n l a y e r o v e r l a p s  the  Zener breakdown can occur at low b i a s and then  the  s u r f a c e becomes a conducting The  s u r f a c e breakdown may  channel r e s u l t i n g i n h i g h  occur  leakage c u r r e n t .  i n i t i a l l y as l a r g e p u l s e s which are  similar  -18to p u l s e s  from r a d i a t i o n .  I t i s , t h e r e f o r e , d e s i r a b l e t o have as l i g h t an  i n v e r s i o n l a y e r on t h e s u r f a c e as p o s s i b l e s i n c e the h e a v i e r t h e i n v e r s i o n l a y e r the longer the surface Llacer  channels.  (1966) has p o i n t e d - o u t  with high e l e c t r i c  t h a t i f ' d e t e c t o r s c o u l d be c o n s t r u c t e d  f i e l d s normal t o t h e j u n c t i o n at' the i - p j u n c t i o n then  the r e s i s t a n c e o f t h e s u r f a c e channel would be i n c r e a s e d .  At present,  h o w e v e r i ' t h e b e s t s u r f a c e treatment found i s a methyl a l c o h o l r i n s e a f t e r a r  short etch.  The d e t e c t o r i s then immediately p u t i n a good vacuum t o m a i n t a i n  the c o n d i t i o n o f t h e s u r f a c e .  F. R e s o l u t i o n o f Germanium  The  Detectors  resolution of lithium d r i f t  germanium d e t e c t o r s i s an o r d e r o f  magnitude b e t t e r than t h a t o f Nal s c i n t i l l a t i o n d e t e c t o r s .  In f a c t , t h e  o v e r a l l r e s o l u t i o n i s l i m i t e d not o n l y by t h e q u a l i t y o f t h e d e t e c t o r but a l s o by t h e n o i s e and s t a b i l i t y c h a r a c t e r i s t i c s o f t h e e l e c t r o n i c equipment. The  u l t i m a t e r e s o l u t i o n o f germanium d e t e c t o r s can be estimated by  c o n s i d e r i n g t h e charge p r o d u c t i o n p r o c e s s e s  which o c c u r .  l o s t by a gamma r a y i n a d e t e c t o r was converted  I f a l l t h e energy  i n t o i o n i z a t i o n the s i g n a l s  produced b y monochromatic r a d i a t i o n would show n e g l i b l e f l u c u a t i o n . i f the energy was d i s s i p a t e d by thermal processes  However,  then normal s t a t i s i c a l  f l u c t u a t i o n s i n t h e number o f e l e c t r o n - h o l e . p a i r s produced would be expected. In t h i s case, t h e RMS f l u c t u a t i o n , <n>, o f the number o f p a i r s would be g i v e n by:  < n  where:  '  _  E  =  E 7  energy l o s t  (2-18)  i n the d e t e c t o r by t h e gamma r a y  -19E  =  average energy r e q u i r e d  t o produce an  electron-hole  pair = The RMS f l u c t u a t i o n  2.94 eV p e r p a i r f o r germanium  i n energy. AE . would be: ' rms' 6 7  AE  The a c t u a l  <n>-e  statisical  those c h a r a c t e r i z i n g Fano  =  rms  =  /elf  fluctuation  f2-191 • >  i n germanium d e t e c t o r s i s between  pure i o n i z a t i o n and normal s t a t i s i c a l  fluctuation.  (194T1) i n t r o d u c e d the Fano f a c t o r as a convenient way o f e x p r e s s i n g t h i s  situation.  I t i s defined as:  n  2  •f-  where:  n  o  n  (2-20)  =  observed RMS  fluctuation  =  normal s t a t i s i c a l  fluctuation  The f u l l width a t h a l f maximum, A E p ^ ^ , o f an energy peak i s the convenient measure o f r e s o l u t i o n  f o r gamma r a y s p e c t r a .  The A E p ^ ^ f o r a  Gaussian d i s t r i b u t i o n i s r e l a t e d t o the RMS f l u c t u a t i o n by:  1/2 AE  FWHM  =  /  8  ^  r  T  " C e  r  =  /8 In 2 • e( F E ) e  =  2.355 • (e E F )  1  1/2  /  (2-21)  2  R e c e n t l y , Mann (1966) has measured t h e Fano f a c t o r f o r l i t h i u m d e t e c t o r s as a f u n c t i o n  o f the a p p l i e d  electric  field.  drift  A dependence o f the  -20Fano f a c t o r on the f i e l d was observed. F  For one d e t e c t o r he measured  =  0.16  - 0.01 and by e x t r a p o l a t i n g t o i n f i n i t e e l e c t r i c f i e l d he  that  0.05  < F < 0.10.  to  inferred  F i g u r e 2-9 shows the expected d e t e c t o r c o n t r i b u t i o n  the t o t a l r e s o l u t i o n f o r the v a l u e s  F = 0.075 and  F = 0.16.  Table  2-1  g i v e s the p o s s i b l e r e s o l u t i o n and the percentage r e s o l u t i o n f o r both F v a l u e s at  various energies.  Use o f very h i g h e l e c t r i c f i e l d s  i s l i m i t e d by  d e t e r i o r a t i o n o f r e s o l u t i o n due t o t h e i n c r e a s e i n leakage c u r r e n t higher e l e c t r i c f i e l d s .  Leakage c u r r e n t s above one nanoampere  with  a f f e c t the  resolution noticably. Besides  the d e t e c t o r , the e l e c t r o n i c s used f o r a m p l i f y i n g the  c o l l e c t e d charge r e p r e s e n t s resolution.  At present  o f O-^S'KeV p l u s "06 capacitance.  a significant  the b e s t  l i m i t a t i o n i n o b t a i n i n g good  low n o i s e a m p l i f i e r s have a n o i s e f i q u r e  KeV p e r pF where the c a p a c i t a n c e  i s the t o t a l  input  The d e t e c t o r a c t s as a c a p a c i t o r w i t h c a p a c i t y g i v e n by:  (2-22) 4TT  where:  for  W  K  =  d i e l e c t r i c constant  of material  A  =  area o f the d e t e c t o r  W  =  depletion layer thickness  germanium:  Det Goulding  1.37  (1965f) has p r e s e n t e d  f  pF  (2-23)  a very good d i s c u s s i o n o f n o i s e i n a r e c e n t  paper. The s t a b i l i t y o f the e l e c t r o n i c system a l s o becomes important  since  0  1  2  3  4  5  ENERGY F i q u r e 2-9  R e s o l u t i o n versus  Energy f o r V a r i o u s  6  (MeV) Fano F a c t o r s  7  8  9  1  0  Fano F a c t o r = 0.16  Fano F a c t o r = 0 .075 Energy o f Gamma Ray •  , R e s o l u t i o n (KeV) FWHM .  % Resolution' .  R e s o l u t i o n (KeV)  Resolution.  250 KeV  0.54  0.22  0.79  0.32  500 KeV  0.76  0.15  1.11  0.22  750 KeV  0.93  0.12  1.36  0.18  1.00 MeV  1.08  0.11  1.58  0.16  1.50 MeV  1.32  0.088  1.93  0.13  2.00 MeV  1.53  0.076  2.23  0.11  2.50 MeV  1.71  0.068  2.49  0.10  3.00 MeV  1.87  0.062  2.73  0.091  4.00 MeV  2.16  0.054  3.15  0.073  5.00 MeV  2.41  0.048  3.52  0,070  • 6.00 MeV  2.64  0.044  3.86  0,064  7.00 MeV  2^86  0.041  4.17  0,060  8.00 MeV  3.05  0.038  4.46  0.056  9.00  3.24  0.036  4.73  0.053  10.00  3.41  0.034  4.98  0,050  T a b l e 2-1  •  R e s o l u t i o n f o r Fano F a c t o r s o f 0.075 and 0.16  -21-  gain s h i f t s o f 0.1  % seriously  a f f e c t the r e s o l u t i o n .  The use o f g a i n  s t a b i l i z a t i o n over the whole system can h e l p reduce the g a i n s h i f t s o f the electronic  equipment.  -22-  CHAPTER 3  EXPERIMENTAL EQUIPMENT  The a lithium  experimental  evaporation  crystals, a drift  A.  system. heated Vhe  system, d r i f t  evaporation  The  deflector  d e t e c t o r s was  and d i f f u s i o n  and power s u p p l i e s f o r d r i f t i n g  and two  styles  of detector holder.  o f l i t h i u m was  done i n a CVE-15 vacuum Lithium  e v a p o r a t i o n boat o f the shape shown i n F i q u r e  above the boat d i r e c t e d  s h i e l d allowed the l i t h i u m  most o f the l i t h i u m  a stainless  vacuum system c l e a n d u r i n g l i t h i u m  steel  ;  Equipment  Around the boat was  stainless  f o r producing  e v a p o r a t i o n assembly i s shown i n F i q u r e 3 - l a .  i n a tantalum  crystal.  units  current c o n t r o l l e r ,  Lithium Evaporation  The  equipment developed  was 3-lb.  downwards onto the  s t e e l s h i e l d to keep most o f the  evaporation.  A h o l e i n the bottom o f the  to be t r a n s m i t t e d to the c r y s t a l .  A movable  f l a g between the boat,and the c r y s t a l prevented  contaminants  r e l e a s e d d u r i n g the i n i t i a l h e a t i n g o f the boat from d e p o s i t i n g on  the  crystal. The thick  c r y s t a l was  mounted on a f o u r i n c h diameter by one  g r a p h i t e b l o c k which was  e v a p o r a t i o n , the l i t h i u m was  used as the d i f f u s i o n h e a t e r .  allowed t o d i f f u s e  the b l o c k to.450°C u s i n g a c u r r e n t o f 150  quarter inch After  i n t o the germanium by  heating  amperes at 4 v o l t s .  B. D r i f t i n g U n i t s  The  d r i f t i n g of lithium  through the germanium c r y s t a l s was  performed  F i g u r e 3-1  (a)  Evaporation  Assembly  Figure 3-1  (b)  Evaporation Boat  -23w i t h the c r y s t a l mounted on a temperature  controlled plate.  The d r i f t  unit  d e s i g n i s based on a system d e s c r i b e d by G o u l d i n g and Hansen (1964). A p i c t u r e of  one o f the d r i f t  u n i t s i s shown i n F i q u r e 3-2a.  The c r y s t a l was p l a c e d  on a chrome p l a t e d copper b l o c k which was mounted, w i t h f i v e o t h e r u n i t s , on a r e f r i g e r a t e d  (-10°C) copper p l a t e t o p r o v i d e a heat  watts from the p l a t e on which the c r y s t a l was p l a c e d .  leak o f twenty  The d r i f t  heated w i t h a 120 ohm 11 watt power r e s i s t o r mounted underneath plate.  unit  was  the upper  The h i g h v o l t a g e d r i f t b i a s was a p p l i e d to the c r y s t a l by a s p r i n g  c o n t a c t made o f phosphor bronze and i n s u l a t e d from" the r e s t o f the assembly. The d r i f t  u n i t was e l e c t r i c a l l y i n s u l a t e d from t h e r e f r i g e r a t e d p l a t e w i t h  a t h i n sheet o f mylar  ( s i l i c o n e thermal grease on both s i d e s ) so t h a t the  d r i f t - c u r r e n t passed through' the- d r i f t ' c u r r e n t " c o n t r o l t o ground. The -temperature thermometer.  o f the upper' plate-was' monitored w i t h a r e s i s t a n c e  Each' d r i f t  u n i t was covered w i t h an i n v e r t e d 1000 ml g l a s s  beaker which' was f i l l e d w i t h n i t r o g e n d u r i n g ' d r i f t " t o i n h i b i t -  of  the c r y s t a l .  In F i q u r e 3-2b t h r e e o f the complete  drift  contamination  u n i t s are shown.  T h i s type o f d r i f t " u n i t has p r o v e n " v e r y ' c o n v e n i e n t and r e l i a b l e f o r :  the d r i f t i n g o f c r y s t a l s .  C. Power Supply  The power supply.used t o p r o v i d e ' t h e h i g h : v o l t a g e b i a s f o r d r i f t i n g -  i s s i m i l a r t o t h a t used by Hansen and J a r r e t t power s u p p l y i s shown i n F i q u r e 3-3. volts.  (1964).  The c i r c u i t  f o r the  I t can supply up to 100 ma a t 1000  A l a r g e v a r i a b l e r e s i s t o r i s p l a c e d i n s e r i e s w i t h the output t o  l i m i t the power output.  The l i m i t  i s s e t w i t h the c r y s t a l d r i f t i n g by  a d j u s t i n g the v a r i a b l e r e s i s t o r u n t i l r e c t i f i e d voltage.  the output v o l t a g e i s h a l f o f the  Figure  3-2  (b)  1000 V o l t - 100 ma Power Supply  F i g u r e 3-3  Power Supply C i r c u i t  (from Hansen and J a r r e t t  (1964)).  -24D. D r i f t Current C o n t r o l l e r  A c u r r e n t c o n t r o l l e r was designed through a c r y s t a l t o a p r e s e t v a l u e .  to maintain'the  The d r i f t  drift  current  c u r r e n t was c o n t r o l l e d by  s u i t a b l y h e a t i n g o r c o o l i n g t h e p l a t e on-which t h e c r y s t a l was. mounted. The  circuit  i s based o h ' t h a t o f r  Hansen and J a r r e t t  (1964) and i s  shown i n F i q u r e 3-4. The the d r i f t  c o n t r o l l e r measured t h e v o l t a g e across  c u r r e n t and v a r i e d t h e power s u p - l i e d t o a h e a t e r  underneath the d r i f t drift  p l a t e so as t o m a i n t a i n  resistor  t h i s voltage constant.  The  c u r r e n t l e v e l was s e t by changing t h e r e s i s t o r ' i n s e r i e s w i t h the  current.  The lower the r e s i s t a n c e the h i g h e r the the d r i f t  the c o n t r o l l e r maintains -7.5  a r e s i s t o r i n s e r i e s with  volts.  Table  current  since  t h e v o l t a g e a c r o s s t h e r e s i s t o r a t the v a l u e  3^1 g i v e s t h e v a l u e o f t h e - r e s i s t o r f o r the v a r i o u s  drift  currents. A d i f f e r e n t i a l a m p l i f i e r compared t h e v o l t a g e a c r o s s t h e r e s i s t o r t o a set level.  F o l l o w i n g t h e a m p l i f i e r , the c i r c u i t r y d i f f e r s  from t h a t d e s c r i b e d by Hansen and J a r r e t t output  o f t h e a m p l i f i e r c o n t r o l l e d t h e frequency  oscillator. (SCR)  of a unijunction transistor  The o s c i l l a t o r i n t u r n t r i g g e r e d a s i l i c o n c o n t r o l l e d  frequency  o f the o s c i l l a t o r when t h e d r i f t  rectifier  c u r r e n t was  balanced  l e s s than s i x t y c y c l e s p e r second so t h a t t h e SCR was o n l y on d u r i n g  part o f the r e c t i f y i n g c y c l e . heater was  In the UBC system, the  which was i n s e r i e s w i t h the r e s i s t o r under the d r i f t p l a t e . The  was  (1964).  significantly  The o s c i l l a t o r c o n t r o l l e d t h e power t o the  s i n c e t h e h i g h e r t h e o s c i l l a t o r frequency  on and s u p p l y i n g power t o the h e a t e r .  drift  the more o f t e n the r e c t i f i e r  The maximum temperature o f t h e  p l a t e c o u l d be s e t by a d j u s t i n g t h e maximum o s c i l l a t o r frequency  with  110 V A.C. +10 V D.C.  51.5K <  < ^>3.9K  -\AAyV 200K  ? >  2N2925  DRIFT CURRENT INPUT •  N I2N2925  V  AAAAf-  W \ / V  ^7—vAAA>-  •AAAA^-^*-  ^Sv\AAA-|  -\A/\/\r^ -AAAA/-^-*  •&9—'XAAA  -AAAA/—  -S^A/\/\/v4  CURRENT DEMAND  15K  £5.6K  J4.7K  < •f  SWITCH . 1.5K  GROUINID  Figure 3-4  U.B.C. C u r r e n t C o n t r o l l e r  |  Drift Current  Resistances Used (ohms)  Total Resistance (ohms)  (mA) 5  .100- + 150 + 1100  10  100- + 390 + 220  710  15  100* + 390  490  20  100* + 150 +47+82  379  25  100* + 150 + 47  297  30  100* + 150  250  40 50  ' 100* + 27 + 27 + 39' 100* + 2 7 + 2 7  • • 1350  193 152  60  • 100* + 27  127  70  100* +12  112  80  100*  100  * - 5 watt resistor Table 3-1  Values of resistance used for various d r i f t currents.  -25the v a r i a b l e r e s i s t o r I f the d r i f t  (see F i q u r e 3-4). c u r r e n t was too low the d i f f e r e n t i a l a m p l i f i e r output  would t r i g g e r t h e o s c i l l a t o r so t h a t the r e c t i f i e r and  therefore the d r i f t  drift  p l a t e would be h e a t i n g .  was on most o f the time  On the o t h e r hand, i f the  c u r r e n t was too h i g h the o s c i l l a t o r would be o f f and the d r i f t  would t h e r e f o r e c o o l .  I f the d r i f t  c u r r e n t was at the s e l e c t e d value the  o s c i l l a t o r would t r i g g e r j u s t enough t o m a i n t a i n The  plate  the temperature.  advantages o f t h i s c o n t r o l l e r were t h a t t h e h e a t i n g element was  a p a s s i v e element, a power r e s i s t o r , t h e maximum temperature was a d j u s t a b l e , and  the c o n t r o l was continuous  E. D e t e c t o r  The  on t o o f f .  Holders  d e t e c t o r h o l d e r s used f o r mounting the f i n i s h e d d e t e c t o r s were  o f two types The  from f u l l  s i m i l a r t o those  d e s c r i b e d by Miner  (1965).  f i r s t type, F i g u r e 3-5a, was used f o r d e t e c t o r t e s t i n g and f o r  counting with r a d i o a c t i v e sources.  I t was convenient  f o rtesting  drifted  c r y s t a l s as i t was s m a l l thus making i t easy t o pump, c o o l , and keep c l e a n . It was a l s o convenient  when used w i t h r a d i o a c t i v e sources  which c o u l d be  mounted e a s i l y under the h o l d e r . The  second h o l d e r , F i q u r e 3-5b and F i q u r e 3-6,was more s u i t e d t o  s t u d i e s w i t h the UBC Van de G r a a f f a c c e l e r a t o r as the d e t e c t o r was mounted horizontally. Both h o l d e r s use a Linde CR-10 l i q u i d n i t r o g e n dewar which  normally  r e t a i n s c o o l a n t f o r about a week. A one l i t e r p e r second V a c l o n pump, V a r i a n Model No. 913-0008, was used t o m a i n t a i n  a pressure  Electrical  o f around 2 x 10 ^ t o r r i n the h o l d e r .  connections  t o the p r e a m p l i f i e r were made by i n s e r t i n g  F i g u r e 3*5  (a)  V e r t i c a l Detector Holder  F i g u r e 3-5  (b)  H o r i z o n t a l Detector Holder  Fiqure 3-6  Detector Holder  -26a l e a d through field-effect  an i n s u l a t i n g Kovar connector on t h e d e t e c t o r h o l d e r .  When the  t r a n s i s t o r a m p l i f i e r was used a f o u r p i n Kovar connector was  used t o make e l e c t r i c a l  connections.  F. D e t e c t o r P r e a m p l i f i e r  The p r e a m p l i f i e r used  i n i t i a l l y was-the O r t e c Model 203-101XL, which  used vacuum tubes as the a c t i v e elements;  T h i s was l a t e r . r e p l a c e d by a  preamplifier u t i l i z i n g a cooled f i e l d - e f f e c t t r a n s i s t o r -  The  circuit, illustrated  was  used because o f i t s improved n o i s e  (Goulding 1966).  i n Fiqure-3-7,- was c o n s t r u c t e d by D. Dalby and characteristics.  With t h e FET preamp the s i g n a l from-the d e t e c t o r went t o a 2N3823 FET which was mounted i n s i d e fromthe cold finger.  ,r  the h o l d e r on a ' s t a i n l e s s s t e e l screw two cm  The d e t e c t o r w a s i n s u l a t e d e l e c t r i c a l l y ;  from the c o l d  f i n g e r by u s i n g a t h i n sheet o f n y l o n between two sheets o f indium :  f i l m s o f h i g h vacuum grease between s h e e t s .  with  ^- 24 2N3823  s.. 4  *  12V  Bottom  are  1 % precision  resistors  -  24V Fiqure 3 - 7 Field-Effect Preamplifier  Low  Noise  Transistor  CHAPTER 4  DETECTOR FABRICATION PROCEDURE  The  f o l l o w i n g procedure was used t o p r e p a r e t h e l i t h i u m d r i f t  detectors. The  s t a r y i n g m a t e r i a l was.a z o n e - l e v e l l e d ; g a l l i u m doped germanium  i n g o t o f 5.5 ohm-cm r e s i s t i v i t y and s i x t y microsecond c a r r i e r The  i n g o t was about 15 cm i n l e n g t h w i t h a c r o s s  lifetime.^'^  s e c t i o n as shown:  actual  size  For s l i c i n g , the i n g o t was mounted on a g r a p h i t e b l o c k w i t h Apiezon b l a c k wax and c u t on a , M i c r q - M e c h " p r e c i s i o n diamond saw t o the d e s i r e d thickness. cutting.  The g r a p h i t e b l o c k p r o v i d e d  a shock mount f o r t h e c r y s t a l  during  The maximum cut d e p t h p e r saw b l a d e pass was p r e v e n t e d from  exceeding s i x mm t o reduce c r y s t a l damage-" d u r i n g The  c r y s t a l was cut t o a t h i c k n e s s  lapped on both f a c e s with 800 g r i t damage from sawing.  cutting.  o f from s i x t o t e n mm and then  alumina g r i n d i n g powder t o remove c r y s t a l  The c r y s t a l was then c l e a n e d  w i t h TCE ( t r i c h l o r o e t h y l e n e )  and methyl a l c o h o l u s i n g Johnson c o t t o n Q-Dabs f o r w i p i n g t h e c r y s t a l .  5. from S y l v a n i a E l e c t r i c P r o d u c t s , Towanda, Perin. 6. e x c e l l e n t m a t e r i a l (20 ohm-cm and 250 m i c r s e c l i f e t i m e ) was o b t a i n e d Socie'te' Ge'ne'rale Me"tallurique de Hoboken, B r u s s e l s , Belgium  from  -28The  c r y s t a l was then p a i n t e d on the edges and one f a c e with Aquadag  graphite i n water). edges and a l s o h e l p e d bottom.  T h i s treatment  inhibited  t o produce good thermal  (colloidal  l i t h i u m d i f f u s i n g i n t o the c o n t a c t w i t h t h e h e a t e r on t h e  When t h e Aquadag was d r y t h e c r y s t a l was ready f o r t h e e v a p o r a t i o n  o f l i t h i u m onto t h e u n p a i n t e d  face.  A p i e c e o f l i t h i u m metal about 1.5 cm x 0.5 cm x 0.3 cm was c u t , washed w i t h TCE, blown d r y w i t h n i t r o g e n , and" p l a c e d ' i n t h e tantalum e v a p o r a t i o n b o a t ' i n the vacuum'system.  T h e - s t a i n l e s s " s t e e l s h i e l d was p u t  around t h e boat and t h e c r y s t a l was p l a c e d on t h e g r a p h i t e h e a t e r under t h e hole i n the s h i e l d . The The  system was then evacuated t o a p r e s s u r e  of'around  2 x 10 ^ t o r r .  c u r r e n t supply t o the e v a p o r a t i o n boat was-turned"on' and r a i s e d  u n t i l ' 1 4 0 amperes boat.  slowly  (7 amperes on the p a n e l meter) was f l o w i n g through t h e r  When the l i t h i u m d e p o s i t e d on ;the f l a g t u r n e d  from b l a c k t o grey t h e  f l a g was moved a s i d e and l i t h i u m was d e p o s i t e d onto t h e c r y s t a l  f o r one  minute. The  e v a p o r a t i o n b e l l j a r was c l o s e d o f f " f r o m the'pumping system and  the h e a t e r power o f 600 watts turned'on. the h e a t e r read was  When'the thermocouple a t t a c h e d t o  200°C, n i t r o g e n was l e t i n t o ' t h e b e l l  jar.  The h e a t e r power,  a d j u s t e d t o g i v e a temperature o f 450^C f o r ' s e v e n minutes and then  turned o f f .  F i g u r e 4-1 shows a t y p i c a l temperature versus  procedure gave a d i f f u s e d  lithium  time curve.  This  l a y e r about.0.5 mm deep.  When t h e c r y s t a l ' h a d c o o l e d , t h e excess l i t h i u m was washed o f f with d i s t i l l e d water and t h e Aquadag was removed.  The c r y s t a l was g i v e n a f i f t e e n  second etch*which was quenched w i t h - d i s t i l l e d water. l i t h i u m s u r f a c e was measured-with a - f o u T p o i n t probe'.  The r e s i s t i v i t y o f t h e I f the r e s i s t i v i t y  o f t h e . s u r f a c e was above 0.2 ohm-cm"the l i t h i u m - h a d not d i f f u s e d  correctly  50  0-i  1  b F i q u r e 4-1  1  1  |  I  i  1  1  1  5  10  TIME L i t h i u m d i f f u s i o n Heating  1  T-I  Cycle  (minutes)  1  1  1  !-]  15  "  1  r  20  -29and  the  l i t h i u m evaporation  procedure was  I f the s u r f a c e r e s i s t i v i t y was a minute i n an e t c h s o l u t i o n o f 3:1 HNO^  repeated.  acceptable  etched  for  HNO^:-HF/'-with 'a sm'al' l.--amoimt.of r e d fuming ,  added to speed t h e - s t a r t • of'the' e t c h .  d e i o n i z e d d i s t i l l e d water and  the c r y s t a l was /v  :  The^-eteh-was quenched w i t h  then the c r y s t a l - w a s  r i n s e d w i t h methyl a l c o h o l  and b1own • d r y wi t h n i t rogen. Indium g a l l i u m e u t e c t i c w a s - a p p l i e d - t o the - c r y s t a l f a c e s and  the  c r y s t a l was-placed l i t h i u m side-down-on-a d r i f t " u n i t . ' A f t e r the c o v e r put  on,  the d r i f t  u n i t was  f i l l e d with n i t r o g e n ; heated to 30°C, and  v o l t s reverse bias applied. to around 500  v o l t s and  A f t e r one  h o u r . t h e ' d r i f t v o l t a g e was  the" current"demand switch  adjusted  was  100  increased  ( u s u a l l y 15  ma)  t o give" a d r i f t " temperature.around 35°C. During d r i f t a drift  the c u r r e n t demand" w a s g r a d u a l l y i n c r e a s e d to -  temperature above' 30°C.  I f the diode j u n c t i o n broke down  ( c h a r a c t e r i z e d "by an adrupt" f a l l s u r f a c e treatment as' f o l l o w s .  the c r y s t a l g i v e n a one  methyl a l c o h o l .  removed and  the f a c e s were taped w i t h #471  f o l l o w e d as f o r a new  minute etch a f t e r which i t was  r e p l a c e d on the d r i f t  tape  quenched  with  u n i t and  the same procedure  would not  the" d r i f t e d r e g i o n was  drift  i t was  removed from the  made v i s i b l e - b y r e v e r s e b i a s i n g the  i n a weak copper" sulphate: s o l u t i o n (20 grams "per l i t e r o f w a t e r ) . d r i f t e d r e g i o n " does not  area was  removing  crystal.  I f the c r y s t a l s t i l l  brown colour".  given  Scotch  A f t e r blowing the c r y s t a l dry with n i t r o g e n and  the tape the c r y s t a l was  unit" and  in" temperature^) - i t was  A f t e r removing •• t h e indium" g a l l i u m e u t e c t i c  with" a Q-Bud wetted w i t h TGE, and  maintain  drift crystal  The  s t a i n ' where" as * the - u n d r i f t e d ' r e g i o n i s s t a i n e d  I f the j u n c t i o n was  T  a  d i s t o r t e d at some p o i n t , the d i s t o r t e d  sawn o f f w i t h the diamond-saw.  The  ;  sawn s u r f a c e was  lapped,  the  a  -30f a c e s taped, and  the c r y s t a l etched  then r e p l a c e d on the d r i f t  f o r one minute, quenched i n methyl a l c o h o l , unit. 7  I f the c r y s t a l s t i l l  would not d r i f t  i t was  given a " f r o s t  T h i s c o n s i s t e d o f p l a c i n g the c r y s t a l i n a c l a m p - ( i n s u l a t e d c o u l d be a p p l i e d ) , immersing i t completely  test".  so r e v e r s e  bias  i n l i q u i d n i t r o g e n f o r seven  seconds, blowing on i t to form' a" l a y e r o f frost", then r e v e r s e b i a s i n g to a c u r r e n t of' s i x t y ma;"  If" the f r o s t ~ : m e ' l t e d " p r e f e r e n t i a l l y i n one  i n d i c a t e d t h a t t h e ' c u r r e n t was p r e d o m i n a n t l y . " T h i s area was  area i t  flowing across t h e " c r y s t a l i n that then'removed by" sawing and  area  the same procedure  f o l l o w e d as i n the p r e v i o u s p a r a g r a p h t o put the c r y s t a l back on the -  drift  unit. 'When the width o f the d r i f t e d "region"was estimated to  be about 1 mm  from the u n d r i f t e d f a c e the c r y s t a l was  I f the c r y s t a l was was  performed and  drifting.  using Fiqure  removed and  not the d e s i r e d depth' the etch treatment d e s c i b e d the c r y s t a l r e p l a c e d on the d r i f t  I f , however, the c r y s t a l had  unit for  2-8  stained. above  continued  d r i f t e d the d e s i r e d depth i t was  g i v e n a s u r f a c e treatment' and  r e p l a c e d on the d r i f t e r f o r twelve hours at  low  500  c u r r e n t demand (5 ma)  t h e s e - c o n d i t i o n s was  and  about -5°C.  volts.  The  temperature c h a r a c t e r i z i n g  T h i s p o s t d r i f f w a s done' to l e t the l i t h i u m  c o n c e n t r a t i o n ' a d j u s t t o the reduced g e n e r a t i o n  c u r r e n t and  to l e t the l i t h i u m  ions p a i r with''the' a c c e p t o r i m p u r i t i e s . To  t e s t the d r i f t e d c r y s t a l " t h e gate-and d r a i n leads o f the FET i n  the h o l d e r were s h o r t e d  :  so t h a t the-leakage  i s t i c o f the c r y s t a l c o u l d be measured. etch  7.  :  The  c u r r e n t versus v o l t a g e c r y s t a l was  (faces taped) a f t e r which the c r y s t a l was  E. Kashy and  M.  character-  g i v e n a one  minute  q u i c k l y moved to a beaker o f  Rickey,Rev. S c i . I n s t r . 35, 1364  (1964).  -31methyl a l c o h o l f o r f i f t e e n seconds and 'then-blown.dry. mounted  I t was immediately  i n a h o l d e r w i t h the" l i t h i u m s i d e upwards and a s m a l l amount o f  e u t e c t i c placed"on~ the l i t h i u m " side- where--the a m p l i f i e r c o n t a c t p r e s s e d on ;  the c r y s t a l " .  The holder" was; evacuated" and,-when the p r e s s u r e was 2.x 10 ^ v  :  t o r r , l i q u i d nitrogen"was put i n -the~ co!d" f i n g e r - o f t h e h o l d e r . ;  ;  The- l e k a g e ' c u r r e n t  versus "^voltage" c h a r a c t e r i s t i c " was measured u s i n g  the' arrangement-shown" i n Fiqure"'4-1.' U s u a l l y s e v e r a l , treatments o f t h e s o r t d e s c r i b e d above were r e q u i r e d b e f o r e " t h e ^leakage current"was below t e n nanoamps' a t t h e - o p e r a t i n g - v o l t a g e " ( a b o u t "  75 v o l t s p e r mm).  When t h i s was  a c h i e v e d "the" g a t e - d r a i n s h o r t i n g ' w i r e was removed and t h e h o l d e r c o o l e d and t h e V a c l o n pump v a l v e opened-'and the~roughing -  re^evacuated,  valve closed.  The a m p l i f i e r was turned on, d e t e c t o r " b i a s voltage' a p p l i e d , and t h e d e t e c t o r 57 was t e s t e d f o r r e s o l u t i o n u s i n g Co  137 and Cs  r a d i o a c t i v e sources.  The  f a b r i c a t i o n procedure was now compTeted"-and' t h e ' d e t e c t or" ready. f o r o p e r a t i o n .  P r e c i s i o n High  VTVM (100 Meg  Voltage 100  Supply  Meg  Input  Resistance) 1 v o l t = 20 nA  L i face of Detector  F i g u r e 4-2  Arrangement f o r measuring  d e t e c t o r leakage c u r r e n t .  - 32 -  CHAPTER 5 - PRESENTATION OF RESULTS  A.  S i l i c o n Detector F a b r i c a t i o n P r i o r t o making germanium d e t e c t o r s s e v e r a l s i l i c o n l i t h i u m  drift  d e t e c t o r s were produced.  I t was  found r e l a t i v e l y easy t o f a b r i c a t e  s i l i c o n d e t e c t o r s so they served the purpose o f t e s t i n g the d i f f u s i o n  and  d r i f t i n g apparatus f o r f a u l t s b e f o r e t r y i n g t o prepare germanium d e t e c t o r s . The procedure f o l l o w e d f o r the f a b r i c a t i o n of s i l i c o n was  s i m i l a r t o t h a t o f Lothrop and Smith ( 1 9 6 5 ) .  detectors  The procedure d i f f e r e d i n  s e v e r a l r e s p e c t s from t h a t used f o r the p r e p a r a t i o n " o f : germanium d e t e c t o r s . The d i f f u s i o n temperature was The l i t h i u m d r i f t i n g was  350°C and the d i f f u s i o n time was two  done on a p l a t e as d e s c r i b e d by Lothrop and Smith  (1965) except t h a t the h e a t i n g was described  i n Chapter 4.  c u r r e n t was  minutes.  performed u s i n g the d r i f t  controller  T y p i c a l d r i f t temperatures were 110°C and t h e  four milliamperes  same as used by Lothrop and  at 500 v o l t s .  Smith.  The s u r f a c e treatment was  drift the  A t y p i c a l c r o s s s e c t i o n o f the compensated  r e g i o n i s shown i n t h e f o l l o w i n g diagram.  / — L i  +  diffused layer  Depletion  region  - 33 The s i l i c o n detectors produced by t h i s means had an active volume of about 0.7 cm . (12 mm. i n diameter by 2 mm. deep). The d r i f t i n g rate of lithium i n s i l i c o n i s approximately h a l f Of; that i n germanium so s i l i c o n detectors are rarely made with a compensated depth greater than 5 mm. 137 A Cs  gamma ray spectrum taken with a s i l i c o n detector  at 77°K with a FET preamplifier i s shown i n Figure 5-1.  The counting rate  in the f u l l energy (661 keV) channel r e l a t i v e t o that of a channel i n the Compton background i s 4%.  The resolution of the peak is 4.8 KeV FWHM  ( f u l l width at h a l f maximum). Because of t h e i r low gamma ray detection e f f i c i e n c y , devices are r a r e l y used as gamma spectrometers.  silicon  For detection of charged  p a r t i c l e s , on the other hand, they are widely used because of t h e i r easy f a b r i c a t i o n , stable c h a r a c t e r i s t i c s , and, most important, t h e i r high resolution performance, even when operated at room temperature. B.  Germanium Detector Production The f i n a l quality of a detector i s very dependent on the  c h a r a c t e r i s t i c s of the germanium ingot from which i t i s fabricated.  Except  for the l a s t one described, a l l detectors mentioned in t h i s thesis were produced from the same ingot which was obtained from Sylvania E l e c t r i c Products. I n i t i a l measurements on a detector made from one end of t h i s ingot gave moderately good r e s u l t s .  Two more detectors were fabricated,  with d i f f i c u l t y , from the same end of the ingot but, despite very careful techniques, the remaining two thirds of the ingot produced no useable detect  o  > -Jr. CO  •r  CC  Ijjo '-!-!• ~! ,  -j.  ;  T.  „L-t 'L "J.  -in!- -ir ••rf-ir^-i-  O CD  OQO  20.0QQ  Figure  5-1  Cs  30..oc;n  T  55.000  CHRNNEL  NUMBER  Gamma R a y S p e c t r u m w i t h  yo..ooo  (X10  Silicon  ,  IJ5. oon  1  Detector  fjO.OGQ  55.OVQ  - 34 -  D u r i n g t h e l i t h i u m d r i f t stage many e t c h i n g and sawing o p e r a t i o n s r e q u i r e d and, although still  a d r i f t depth o f 5 mm. was o b t a i n e d ,  d i d n o t make u s a b l e  the c r y s t a l s  d e t e c t o r s because o f h i g h leakage c u r r e n t .  Repeating s u r f a c e t r e a t m e n t s ,  sawing o f f v a r i o u s a r e a s , and r e d i f f u s i n g  l i t h i u m d i d n o t reduce t h e leakage c u r r e n t t o s e v e r a l months o f n e g a t i v e  a useable  level.  r e s u l t s a new ingot was o b t a i n e d  After  from Hoboken  o f Belgium which produced a very good d e t e c t o r w i t h t h e f i r s t One  slice.  c h a r a c t e r i s t i c o f t h e ingot which would seem t o be important  initial resistivity.  were  i s the  The S y l v a n i a ingot had a r e s i s t i v i t y o f 5 ohm-cm  w h i l e t h e Hoboken i n g o t had a r e s i s t i v i t y o f 20 ohm-cm. The  first  i n g o t had an i n s t r i n s i c  germanium d e t e c t o r produced from t h e S y l v a n i a r e s o l u t i o n o f 4.0 KeV f o r 661 KeV gamma r a y s at  an o p e r a t i n g v o l t a g e o f 175 v o l t s .  The d e p l e t i o n depth was about 4 mm. 3  g i v i n g an a c t i v e volume o f approximately  1 cm .  Unfortunately  this  d e t e c t o r was damaged when t h e vacuum pump f o r t h e d e t e c t o r h o l d e r and  became contaminated.  failed  At t h e time i t was n o t f e a s i b l e t o s t o r e t h e  d e t e c t o r a t l i q u i d n i t r o g e n temperatures w h i l e t h e pump was r e p a i r e d so the d e t e c t o r was The  destroyed. second d e t e c t o r had a d e p l e t i o n depth o f 5 mm. and 3  an a c t i v e volume o f about 0.5 cm . was  The energy r e s o l u t i o n a t 661 KeV  4.0 KeV ( i n t r i n i s i c r e s o l u t i o n 1.9 KeV) a t a b i a s v o l t a g e o f 200  volts. The  third  d e t e c t o r a l s o had a d e p l e t i o n depth o f 5 mm. 3  but t h e area was l a r g e r g i v i n g an a c t i v e volume o f 2.0 cm .  At a b i a s  v o l t a g e o f 225 v o l t s i t gave an energy r e s o l u t i o n o f 5.1 KeV (4.5 KeV intrinsic  r e s o l u t i o n ) f o r 661 KeV gamma r a y s .  - 35 -  The fourth and best detector, which was prepared  from the  3 Hoboken ingot, had an active volume of 1.7 cm 5 mm.  and a depletion depth of  The energy resolution obtained with t h i s detector i s summarized  in the following table.  Gamma Ray Energy  Total Resolution (FET preamp)  I n t r i n s i c Resolution  122 KeV  2.5 KeV  1.3 Kev  136 KeV  2. 6 KeV  1.5 KeV  661 KeV  2.9 KeV  2.0  KeV  4.1 KeV  3.5 KeV  2614 KeV  5.2 KeV  4.9 KeV  1332  KeV  137 Figures 5-2 #2 and #4.  and 5-3 show similar Cs  spectra taken with detectors  The higher resolution of detector #4 compared to the other  detectors i s c l e a r l y seen by comparing the two figures.  The improved  resolution of the fourth detector probably resulted from the use of a better germanium ingot.  o o  CD W H  o o  T L J O 0  0  c->  —Ico"  -J- -J.  it-.,  CO  .-Jo  V  CD  Hi^-r-r-iio o o  i - i  500.000  '  1  575;D00  '\—  650.0OD CHRNNEE  F i c r u r e 5-2  ,  _  "  725.000  I  800.000  NUMBER  13? Cs S p e c t r u m w i t h D e t e c t o r II- 2  r  875.000  j — i ^ - r r - H  950.000  -ii-H  |  —  10£5.00  —  o in"  >  C D  H vO VD  en o  O •:-J-  (JO 7 0  _JP7" CO •  .. v >  —>o D o  u_ 01 51 ..  no2:  O CJ  32U.000  Figure  " i Mpy.000  r:  1  1  521J.000 62A1.000 724.000 CHRNNEL NUMBER  82U.0OO  92'J.OOO  1021.000  - 36 -  C.  Detector  Operation When u s i n g germanium d e t e c t o r s i t i s important  the detector  t o optimize  resolution. The  bias voltage.  d e t e c t o r r e s o l u t i o n i s dependent on t h e d e t e c t o r  F i g u r e 5-4 shows a t y p i c a l leakage c u r r e n t  b i a s v o l t a g e graph.. The best o p e r a t i n g v o l t a g e  versus  isa little  the v o l t a g e where t h e leakage c u r r e n t i n c r e a s e s r a p i d l y .  below  F i g u r e 5-5 0_'.v-  137 g i v e s t h e shape o f a 661 KeV (Cs voltages obtained  ))gamma r a y peak f o r v a r i o u s  w i t h d e t e c t o r #2.  Below 200 v o l t s t h e r e s o l u t i o n  i n c r e a s e d as t h e v o l t a g e was i n c r e a s e d because t h e charge e f f i c i e n c y i n c r e a s e s with e l e c t r i c f i e l d . with a t a i l for  charge  asymmetric  collection  Above 200 v o l t s , on t h e other hand, t h e  peak broadens r a p i d l y due t o a r a p i d The  collection  The peaks a r e very  on t h e low energy s i d e due t o incomplete  some o f the gamma r a y s .  bias  peak i s symmetric but much wider.  i n c r e a s e i n t h e leakage c u r r e n t . The optimum b i a s v o l t a g e i s ,  t h e r e f o r e , r e l a t i v e l y e a s i l y found by i n c r e a s i n g t h e b i a s v o l t a g e the  asymmetry i n t h e spectrum peaks d i s a p p e a r s  symmetric but does not widen.  and t h e peak becomes  With good .detectors t h e leakage c u r r e n t  remains low a t h i g h e l e c t r i c f i e l d s and t h e r e f o r e t h e o p e r a t i n g i s not as c r i t i c a l  until  voltage  ( e l e c t r i c f i e l d s a r e t y p i c a l l y 100 v o l t s per mm.).  F o r h i g h energy gamma r a y s ( g r e a t e r than 3 MeV),  however, c a r e f u l  o p t i m i z a t i o n o f the b i a s v o l t a g e can i n c r e a s e t h e r e s o l u t i o n  considerably.  10  -6  to  ft e ^  lCf'  EH W K O  10 M  10  10  -9  -10 100  F i g u r e 5-4  Typical  200  300 400 • BIAS VOLTAGE ( v o l t s )  500  Leakage Current versus Bias Voltage  600  700  CHANNEL NUMBER  Figure  5-5  Peak  Shape  for  Various  Bias  Voltages  - 37 -  The replacement of the Ortec 101XL  preamplifier with  the cooled FET preamplifier reduced the e l e c t r o n i c noise contribution from 3.2 KeV to 2.1 KeV.  l.O^tsec  A single d i f f e r e n t i a t i o n time constant of  was found to y i e l d the lowest noise. The gamma ray spectra shown in Figures 5-6 to 5-10  were obtained using detector #4 with an FET preamp and i l l u s t r a t e 57 the high resolution achievable with germanium detectors.  The Co  spectra of Figure 5-6 shows the resolution obtained at low energy. The separation of the two peaks i s 14 KeV and the resolution i s 2.5 KeV.  At low energy most of the peak width is due to electronic  noise.  134 154 The Cs spectra of figures 5-7 and 5-8 and the Eu  spectrum of Figure 5-9 v i v i d l y i l l u s t r a t e the value of high r e s o l u t i o n in the separation and measurement of complex gamma ray spectra. Figure 5-8 i l l u s t r a t e s the region from channel 318 to 332 of Figure 5-7 with the background subtracted. KeV  apart, are c l e a r l y resolved.  The two gamma rays, which are 6.7  In Figure 5-9 the many gamma rays of  the Eu^*^ are c l e a r l y shown. The f u l l energy peak (2614 KeV), the f i r s t escape KeV), and the double escape peak (1592 KeV) shown i n Figure 5-10.  (2103  of the RdTh spectrum are  The number of counts i n the double escape peak  (with background subtracted) i s 30 times the number in the f u l l energy peak.  For gamma ray energies above 2 MeV the double escape peak i s the  dominant peak and the f u l l energy and single escape peaks give a  o o  o o  • —. °  , 3  0.00  00  , '  6.DO  CHANNEL  Figure  5-6  Co  Spectrum  1 9 . 0 0  NUMBER  1 12.00  [XIO ] A  1 15.00  o o  1.367 KeV + 122 KeV  IT.' ' .1-  W  > w  o  co  > 0>  O O CD  > o  W CO VO  o  > > >  CD  0) <D  0)  WWW o I N .4-  -Jo?"  e  e  J.  •  fAOMA ^DVO O  J  -l-  J-  j.1-  V-. "  -!A-  cc LLJ CO  O  r5ii.  CO  •  * -l-  -H-i r-H- -J- - i -:-:-:--;-!-f?-?-w-j- -:-.>-i-:-r -ir-;;- i-<;-H-;-;J  o o  CD  i  i  184.000  r  304.000  :  M24.000  544.000  CHANNEL  1>4 Cs Spectrum 7  F i g u r e 5-7  J  n  1 NUMBER  664.000  784.000  T  •-:-*r'K-;-:-!ffi-;-  904.000  10211,000  f  NUMBER OF COUNTS (x 10 )  i  -.000  NUMu i . ooc  OF  COUNTS (LOG) 2.000 • 3.000  y.oco  5.000  H* "  -3^5  ro VJl  i  c  h-' VJl  -p-  CD  KeV  <  a fog-  o 3  7 8 2 KeV  ' i  i  •967  KeV  • 1 0 9 2 KeV • 1 1 1 8 KeV  VM CJ>  00 CD <  1416 KeV  o  O O  3  H  ru -p-  CD <  NUMBER - .  GOO  OF  COUNTS JUL)  (LOG) "3  0  J.OOi  000  fi  **1 £  KeV double escape  "1592  o  1  I— o  1  &  P" CO CD  O c+ 3  KeV s i n g l e escape  2103  •J ZD CT:  '  ' CD  O  ,40-  o CD  -2614  full  KeV • energy  - 38 -  convenient  check o f t h e energy c a l i b r a t i o n o f t h e a n a l y z e r s i n c e t h e  energy d i f f e r e n c e between t h e peaks i s 511 KeV. In F i g u r e 5-11 t h e square o f t h e r e s o l u t i o n i s p l o t t e d as a f u n c t i o n o f gamma r a y energy f o r d e t e c t o r #4. o f t h e Fano f a c t o r o b t a i n e d  from t h i s data  h i g h e r than t h a t measured by Mann (1966).  A crude  estimate  i s 0.5 which i s c o n s i d e r a b l y The l a r g e Fano f a c t o r  o b t a i n e d here i s a t l e a s t p a r t i a l l y a t t r i b u t a b l e t o a r t i f i c i a l broadening o f t h e peaks due t o e l e c t r o n i c i n s t a b i l i t y and d r i f t i n g since the counting  r a t e s were f a i r l y h i g h and gain  stabilization  was n o t used. P r e l i m i n a r y e f f i c i e n c y measurements were made w i t h d e t e c t o r #4 a t d i f f e r e n t e n e r g i e s . Table it  5-1.  falls  The r e s u l t s a r e t a b u l a t e d i n  Fow low energy gamma rays t h e e f f i c i e n c y i s h i g h but  o f f r a p i d l y so t h a t above 2.5 MeV t h e f u l l energy peak  e f f i c i e n c y i s l e s s than 0.1%. The double escape peak e f f i c i e n c y , however, f o r gamma r a y e n e r g i e s at  a value  o f 0.3% f o r t h i s The  above 2 MeV remains almost a constant  detector.  l i n e a r i t y o f t h e a m p l i f i e r ( O r t e c 201 Multi-Mode)  and t h e p u l s e h e i g h t a n a l y s e r ( N u c l e a r Data ND-160) were measured 134 u s i n g Cs  154 and Eu  s p e c t r a ( f i g u r e s 5-7 and 5-9).  were c a l c u l a t e d and, u s i n g t h e t a b u l a t e d e n e r g i e s a least  f o r t h e gamma r a y s ,  squares s t r a i g h t l i n e f i t o f t h e gamma r a y energy as a f u n c t i o n  o f c h a n n e l p o s i t i o n was performed. 134 Table  The peak p o s i t i o n s  5-2.  I n Both t h e Cs  The r e s u l t s a r e presented i n 154  and Eu  s p e c t r a t h e d e v i a t i o n from  a s t r a i g h t l i n e was l e s s than t h e quoted accuracy  o f t h e gamma r a y  Table  5-1  E  f  E  Source 1)  2)  (MeV)  Strength  Distance  ~  #of nr's  through  Co 60 Co  1.172 1.332  4913^01  RdTh  2.614  6.6^Ci  7 cm  7.2 x 1 0  D.E.  1.592  6.65^Ci  7cm  7.2 x l O  6 0  Table 5-2a  ttCounted  Eff.  Detector 4913/iCi  '  14.5 cm 14.5 cm  1.2 x 1 0 1.2 x 10  7  7  6  6  4.0 x 1 0 4 3.6 x 10  .29%  6.2 x 1 0  .08%  4  2.2 x l O  2  3  .32%  .31%  L i n e a r i t y f o r Eu  Channel P o s i t i o n  Energy  Energy C a l c u l a t e d f r o m S t r a i g h t Line F i t  Energy D i f f e r e n c e •  86.9  245.0 KeV  243.6 KeV  - 1.4 KeV  161.4  345 0 KeV  345 5 KeV  + 0.5 KeV  210.0  412 0 KeV  412 0 KeV  0.0 KeV  234.0  445 0 KeV  445 2 KeV  + 0.2 KeV  481.1  7132 0 KeV  783 1 KeV  + 1.1 KeV  617.2  969 0 KeV  969 3 KeV  + 0.3 KeV  706. 8  10'32 0 KeV  1032 0 KeV  0.0 KeV  726.0  1118 0 KeV  1118 2 KeV  + 0.2 KeV  942.9  1416. 0 KeV  1415 1 KeV  - 0.9 KeV  Energy = 124 KeV  +  1.368 KeV/channel  T a b l e 5-2b  L i n e a r i t y o f Cs  Spectrum  Channel P o s i t i o n  Energy. .  Energy . C a l c u l a t e d  322.8  563.0 KeV  563.3 KeV  + 0.3  327.3  569.7 KeV  569.4 KeV  - 0.3  353.1  605.4 KeV  604.7 KeV  -  671.5  1039.0 KeV  1039.9 KeV .  + 0.9  766.2  1168.0 KeV  1169.3 KeV  + 1.3  910.5 .:  1368.0 KeV  1366.5 KeV  - 1.5  Energy = 122 KeV + 1.367  KeV/channel.  Energy. D i f f e r e n c e  0.7  - 39 -  e n e r g i e s and t h e a c c u r a c y o f t h e peak d e t e r m i n a t i o n .  From t h i s  data  the i n t e g r a l l i n e a r i t y o f t h e system was estimated t o be b e t t e r than 0.2% over the channel range 100 t o 1000  channels.  - 40 -  CHAPTER SIX  - CONCLUSIONS  The although  f a b r i c a t i o n o f germanium l i t h i u m d r i f t  b a s i c a l l y a simple  process,  r e q u i r e s c o n s i d e r a b l e care  c l e a n l i n e s s t o produce h i g h q u a l i t y gamma r a y The  detectors, and  spectrometers.  major problems i n f a b r i c a t i o n are the  initial  q u a l i t y o f t h e m a t e r i a l and the c o n t r o l o f the s u r f a c e s t a t e s .  If  t h e o r i g i n a l germanium i n g o t is^damaged or o f poor q u a l i t y c o n s i d e r a b l e e f f o r t can be wasted t r y i n g t o make d e t e c t o r s f r o m . i t .  Controlling  the s u r f a c e s t a t e o f t h e exposed compensated r e g i o n i s a p o o r l y understood t e c h n i q u e .  The  leakage c u r r e n t o f t h e d e t e c t o r i s  determined p r i m a r i l y by the s u r f a c e leakage. found t o c o n t r o l and m a i n t a i n p r o t e c t i v e Sip The  I f a b e t t e r wiy  was  n e u t r a l s u r f a c e s t a t e s , such as a  l a y e r , the applied e l e c t r i c  f i e l d c o u l d be  increased.  lise o f h i g h e r e l e c t r i c f i e l d s c o u l d i n c r e a s e the charge c o l l e c t i o n  e f f i c i e n c y , and  t h u s improve the energy r e s o l u t i o n o f t h e Development o f lower n o i s e and h i g h e r  e l e c t r o n i c s w i l l a l s o improve the t o t a l r e s o l u t i o n . o f FET  p r e a m p l i f i e r s and  reduce t h e present  developments i n parametric  system r e s o l u t i o n t o around 1 KeV  Optimization a m p l i f i e r s should  however, w i l l l i m i t t h e  at 1 MeV  ultimate  f o r germanium d e t e c t o r s  Other semiconductor m a t e r i a l s c h a r a c t e r i z e d by  s m a l l e r energy gaps, such as GaAs may present,  stability  e l e c t r o n i c noise.  S t a t i s t i c a l processes,  (see F i g u r e 2-9).  detector.  g i v e b e t t e r r e s o l u t i o n but,  at  such m a t e r i a l s o f s u f f i c i e n t p u r i t y are not a v a i l a b l e . The  e f f i c i e n c y o f germanium d e t e c t o r s i s l i m i t e d by  the  -.41 -  a c t i v e volume o f the d e t e c t o r .  D e t e c t o r e f f i c i e n c y should g r a d u a l l y improve  as h i g h e r q u a l i t y germanium and b e t t e r f a b r i c a t i o n t e c h n i q u e s c o a x i a l and two and t e c h n i q u e s  way  (such as  d r i f t i n g ) make l a r g e r a c t i v e volumes e a s i e r t o produce  f o r s t a c k i n g o f s e v e r a l d e t e c t o r s are  improved.  Because o f the s u p e r i o r r e s o l u t i o n and h i g h e f f i c i e n c y germanium l i t h i u m d r i f t as the  of  d e t e c t o r s t h e y are r a p i d l y r e p l a c i n g o t h e r d e t e c t o r s  major e x p e r i m e n t a l  instrument  f o r gamma r a y  spectrometry.  - 42 -  APPENDIX  A.  Recipe f o r e t c h : Mix t h e f o l l o w i n g i n g r e d i e n t s i n t h e f o l l o w i n g o r d e r : 7 l b s . of concentrated n i t r i c 2 l b s . o f 48% h y d r o f l u o r i c \ lb.  B.  o f r e d fuming n i t r i c  acid  (70%).  acid. acid.  Ga-In E u t e c t i c : Prepare with 12% by weight o f g a l l i u m .  C.  Etch r e s i s t a n t  tape:  Scotch #471, 3 M Company, ( l o c a l s u p p l i e r , B l a c k B r o s . ) .  - 43 -  BIBLIOGRAPHY Adler, R.B.,,. Smith.,..A.,C.._..and..l.onpjin.iR.L., .Introduction t o Semiconductor Physics, (John, Wiley 8 Sons-', Inc., New York, 1964), .Vol.1. Sec. 1, p.50. Alexande  , T.K. and A l l e n , K.W. , Can.. J . Phys. 43_, 1563 (1965).  Aleaander, T.K., :Ldtherland, A.E.- and Broude, C. ,. Can:. J.. Phys. 43, 2310 (1965). Armantrout:,. G. IEEE Trans. Nucl. S c i . , Vol. NS-I3, No.l 85 (1966). Bardin, T.T.,.. Barrett, R., Cohen, R.C. , Devons, S., H i t l i n , D., Macagno, E.R., Sabat, C.N.,. Rainwater, .J. , Runge, K. and-Wu, C S . , Columbia , University Progress Report (1966), Columbia University Preprint (1966). Bethe, H.A. and Bacher, ,R.F;, Rev.Mod.Phys., 8_, 82 (1936). Conwell, P r o c IRE 46, 1281 (1958). Donovan, P.F.•., Miller., G.L..• and Foreman, B.M., B u l l . Amer. Phys. Soc., Ser. I l l , .5, -355 .(I960). . Fano, U., Phys. Rev. 72, 26 (1947). Freck, D.V. and .Wakefield, J . , Nature, 193, 669 (1962). Freedman, M.S;Wa\gner, F. j r . , Porter, F.T. and Boloton, H.H., Phys. Rev. 146, 791 (1966). • Goulding, F.S. a) Nucl. Instr. and Meth. 43_, 26 (1966)'.' b) 'Nucl.-Instr.. and Meth. 43, 27 (1966). c) Nucl. Instr'.' and Meth. 43, 8 (1966). d) . Nucl. Instr. and Meth. 43_, 20 (1966). e) Nucl. Instr.'and Meth. 43", 19 (1966). f ) Nucl. Instr. and Meth. 43, 28 (1966).. Goulding, F.S., private communication  (1966).  Goulding, F.S. arid .Hansen, W.L.,. UCRL-11261, 2 (1964). H a l l , H. , Rev. Mod. Phys. 8_, 35.8 ('l936). :  Hansen, W.L. 'and Jarrett,, B.V. , UCRL-11589, Fig. 3 (1964).  '  - 44 Hughes, L.B. , .Kennett-', T.J. and Prestwich, W.V., 44, 919 (1966).  and Wall, B.J.  Kashy, E. and Rickey, .M., Rev. S c i . Instr. 35, 1364  Can. J . Phys.,  (1964).  K l e i n , 0 and Nishina,- Y., Z. Physik. 52_, 853 (1929). Llacer, J.,. IEEE Trans. Nucl. S c i . , Vol. NS-I1, No.3,. 221 (1964). Llacer, J . IEEE Trans. Nucl. S c i . , Vol. NS-13, No.l, 93 (1966). Lothrop, R.P. and Smith,.H.E., •UCRL-16190, (1965). Malm, H.L. and Fowler, I.L, IEEE Trans. Nucl. S c i . Vol... NS-13, No.l, 62 (1966). Mann, H.M.,  B u l l . Amer. Phys. S o c , 11, 127 (1966).  Mayer, J.W., Bailey, N.A. arid Dunlap, H.L. • E l e c t r o n i c s , Ser I I , 5_, 355 (I960). y  Eroc. of. Conf. on Nuclear  Miner, C.E., UCRLr11946, 21. (1965). Warner, R.M. and Fordenwalt, Integrated C i r c u i t s (McGraw-Hill Book Co., • New York, 1965), Vol.1, Chap. ,3, p.72.  

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