Manufacturing techniques, testing, and fabrication of lithium-drifted germanium particle detector

Ritter, R. C.

Ziock, K.

English

NASA Technical Reports Server

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DEPARTMENT OF PHYSICS 
UNIVERSITY OF VIRGINIA 
McCORMICK ROAD 
CHARLOTTESVILLE, VIRGINIA 
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https://ntrs.nasa.gov/search.jsp?R=19660010361 2020-03-16T22:42:31+00:00Z
S t a t u s  Report #5  
Per iod  Ending: November 30, 1965 
Grant N o .  : N s G - 4 5 5  
C o n t r a c t o r :  Un ive r s i ty  of V i r g i n i a  
C h a r l o t t e s v i l l e ,  V i r g i n i a  
Authors of Repor t :  IC. Ziock 
R .  C.  R i t t e r  
Department of Phys ics  
j 
' .a E. 1 i 
S t a t u s  Report 
NsG-455 
General Information: 
Per sonne 1 
Two f a c u l t y  m e m b e r s  - t h e  p r i n c i p a l  i n v e s t i g a t o r s  - worked 
part-time on t h e  project. The development of a large volume sol id  
state detector w a s  assigned t o  a graduate s tudent  as a Ph.D, t hes i s .  
One instrument maker and one e l e c t r o n i c s  t echn ic i an  were 
working ha l f t ime  on p r o j e c t s  connected w i t h  t h e  grant. 
n i c s  t echn ic i an  spen t  halftime during t h e  r epor t  period. 
One slectro- 
In t roduct ion  
1) The research program ou t l ined  in our  o r i g i n a l  proposal 
foresaw a multipronged attack on some of t h e  then  o u t s t a n d i w  pro- 
blem of detector development. Since t h a t  time t h e  central. importance 
of Li d r i f t e d  Ge detectors for  high r e s o l u t i o n  nuclear spectroscopy 
as w e l l  as for pion and muon physics has emerged dramatical ly;  and 
i n  view of t h e  encouraging success  reportee previS2sly w e  have 
concent ra ted  our B A A W A  ---c- A - d n a  .. - --- the r epor t  period on the development 
of large volume annular Li d r i f t e d  Ge detectors, 
Progress R e p o r t  
Solid State Properties of Importance t o  Detector Development: 
The o r i g i n a l l y  proposed i n v e s t i g a t i o n  of sol id  s ta te  pro- 
perties and effects t h a t  might be of importance t o  t h e  development 
1) A Research Proposal for Development of Particle Detectors. 
2)  Grant NsG-455, S t a t u s  Report #4, May 31, 1965 
Universi ty  of Virginia, December 17, 1962 
2 
of new particle detectors had been postponed i n  view of t h e  then 
pending expansion of t h e  l o w  temperature fac i l i t i es  of t h i s  depart- 
ment. Th i s  expansion is still under way and a new l o w  temperature 
labora tory  total l ing 2200 square f e e t  is being cons t ruc ted  for  
occupancy t h i s  spring. The department has also m a t e r i a l l y  added 
t o  its f a c u l t y  s t r e n g t h  i n  t h e  area of l o w  temperature physics.  
Under these circumstances w e  have decided t o  postpone a f u r t h e r  
experimental  s tudy of t he  i n t e r a c t i o n  tetTeen i o n i z i a g  r a d i a t i o n  
and superconducting junc t ions  u n t i l  our new laboratory w i t h  its 
g r e a t l y  improved faci l i t ies  w i l l  be ava i l ab le  i n  spring. In  t h e  
meantime, a theoretical s tudy of t h e  problem is being made by Dr. 
M. Zuckermann of t h i s  department. 
Large Volume Li Drifted G e  Detectors 
General Information: -. 
Since one of us ,  R. R i t t e r ,  first proposed t o  increase  
t h e  u s e f u l  volume of a L i  d r i f t ed  G e  detector by d r i f t i n g  conceiitrf- 
3a l ly  i n t o  t h e  o u t e r  w a l l  of a hollow G e  cy l inder ,  another group has 
pursued a similar idea and produced large i r r e g u l a r l y  shaped '*co-axial 
detectors. *' 
Compared w i t h  these de tec to r s ,  t he  t r u l y  eo-axial  detectors, 
henceforth called annular detectors, developed i n  t h e  meantime under 
t h i s  g r a n t ,  have some d i s t i n c t  advantages which w e  d i scuss  i n  detail 
i n  t h i s  r e p o r t  . 
A l s o ,  i n  t h e  meantime, serious material problems have 
3 
a r i s e n  at  Sy lvan ia ,  t h e  so le  U. S, s u p p l i e r  of detector grade ger- 
hianium. Seve ra l  u s e r s  s ta te  that  t h e i r  f a i l u r e  r a t e  i n  t h e  manufacture 
of L i  d r i f t e d  G e  detectors, as a r e s u l t  of t h i s ,  has  r i s e n  t o  more 
t h a n  90%. I n  t w o  a t t empt s  ( w i t h  material from t w o  d i f f e r e n t  batches), 
w e  have made t w o  s u c c e s s f u l  annular  detectors. We t h i n k  t h a t  w e  now 
understand t h e  t r u e  n a t u r e  of t h e s e  defects i n  t h e  material, and w e  
w i l l  demcnstrate  below t h a t  t h e y  are of no d e t r i m e n t a l  i n f l u e n c e  i n  
t h e  mAnufacture of our annu la r  detectors, T h i s  g i v e s  hope t h a t  the 
t r u e  annu la r  c o n f i g d r a t i o n  w i l l  avoid m o s t  of t h e  p r e s e n t  material 
problem, and may permit t h e  use  of poorer  material which can be grown 
i n  l a r g e r  i n g o t s .  
During t h e  report  pe r iod ,  work at t h e  Chalk River  (Canada) 
Laboratory has led t o  t h e  i n t r o d u c t i o n  of s u c c e s s f u l  l i q u i d  coo l ing ,  
which allows be t t e r  c o n t r o l  and h igher  temperatux'e dur ing  t h e  d r i f t i n g ,  
s o  t h a t  t h e  d r i f t  ra te  is increased .  T h i s  method is be ing  i n s t a l l e d  
hcre. Coupled w i t h  t h e  annular  d r i f t i n g  it w i l l  be used i n  attempts 
3 t o  make detectors of up t o  150 c m  . 
Work Performed During t h e  Report Period: 
I n  t h i s  r e p o r t  per iod work i n  t h i s  laboratory has involved 
t h e  development of improved t echn iques ,  and t h e  f a b r i c a t i o n  and tes t -  
i n g  of t w o  annu la r  detectors, 
Development of New Techniques: 
a )  During ou r  work, w e  have made e x t e n s i v e  use  of t h e  
spark c u t t i n g  f ac i l i t i e s  of t h i s  department.  T o  our  knowledge t h i s  
4 
is t h e  first t i m e  t h a t  Ge d e t e c t o r  c r y s t a l s  have been spark c u t ,  
and t h e  r e s u l t s  have been very g r a t i f y i n g .  N o t  on ly  did t h e  expected' 
r educ t ion  i n  damage of t h e  c u t  edge occur  As demonstrated by g rea t ly  
reduced bubbl ing i n  t h e  f o l l o w i n g  btcih; w e  a l s o  found t h a t  the s p a r k  
c u t  ends of t h e  c y l i n d e r  clearly show t h e  depth  of t h e  L i  d i f f u s i o n .  
(see below) 
b) A computer program was w r i t t e n  which provides  t h e  
necessary information to cmple te ly  predict and c o n t r o l  t h e  d i f f u s i o n  
process. The r a d i a n t  heating method, first developed here for db- 
tector work, has been improved and f u l l y  instrumented as a n  ad junc t  
t o  t h e  programmed d i f f u s i o n .  Spark-cut t ing of t h e  ends shows t h e  
exac t  depth of t h e  d i f f u s i o n  (See Figure 1). From t h e s e  tests, 
w e  b e l i e v e  t h a t  t h e  programmed d i f f u s i o n  procedure is capable  of 
depths c o n t r o l l e d  to + 0.1 mm. As t h e  amount and depth of d i f f u s e d  
l i t h i u m  is. expected t o  have 91 s t r o n g  bear ing  on d r i f t  depth l i m i t a -  
tiom far veyy large detectors, we have g iven  t h i s  probiem carisider- 
able emphasis. 
- 
c) The dzkhgbr of using gd i ium-meta l  c o n t a c t s ,  as w e  had 
beeri doing, was po ih ted  out t a  us by Chasman ' I .  
gone back t a  indium contac ts4  Mew c o a l i n g  blocks w e r e  devised  fo r  
d r i f t i n g ,  and ake shown i n  Pigdre 2. 
d) 
Since then ,  w e  have 
Contrary t o  opin ions  g iven  to u s  by s e v e r a l  o t h e r  
workers i n  t h i s  f i e l d ,  d r i f t i n g  i n  a vacuum seems t o  be i n f e r i o r  t o  
d r i f t i n g  i n  a d ry  n i t rogen  atmosphere. Figure 3 shows one test of 
2) L. Chasman, ' . p r iva t e  communication 
. 
Fig.  1. Spark-cut ends of a n n u l i  a f te r  d i f f u s i o n ,  be fo re  d r i f t i n g .  
Fig.  2. Blocks f o r  coo l ing  a n n u l i  du r ing  d r i f t i n g ,  
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t h i s .  A t  cons tan t  c u r r e n t  and temperature ,  t he  i n t r o d u c t i o n  of 
n i t rogen  m a k e s  t h e  device  a much better diode. Of course,  t h e  
inc reased  vo l t age  reduces t h e  d r i f t  t i m e  - a n  added b e n e f i t .  
p l a n  t o  d r i f t  i n  n i t rogen  (or a l i q u i d )  henceforth.  
We 
e)  During t h e  summer, w e  d id  a large amount of theoretical 
work on t h e  response of so l id  s t a t e  detectors. 
a v a i l a b l e  work involves  approximations which w i l l  be untenable  for 
r ea l ly  large detectors. 
Much of t h e  p rev ious ly  
T h i s  t h e o r y ,  which invoives  some very unusual  d i f f e r e n t i a l  
equa t ibns ,  is now about half  confpietedr 
f )  A i i q t d d  temperature-contpoi l ing d r i f t  appa ra tus  is 
This more-saphisticate$ method shoultl ha lve  t h e  nea r ly  f i n i s h e d .  
d r i f t  t i m e  f o r  a given volume of our detectors, o r  it should j p e r m i t  
us t o  make l a r g e r  d e t e c t o r s .  
F a b r i c a t i o n  and Tes t ing  of Annular Detectors: 
a )  The y i e i i i  of two success fu l  detectors i n  t w o  a t t empt s  
compares w j t h  three out  of n ine  a t t empt s  for  p l a n a r  detectors. 
t h i s  may reflect our  improved techniques ,  but t h e  l o w  edge-to-volume 
ra t io  of t h e  annulus ,  p l u s  t h e  apparent  immunity t o  defects i n  t h e  
r a w  material would lead one t o  expect a h igher  y i e ld .  
P a r t l y  
b) About f o u r  and one-half weeks d r i f t i n g  t i m e  were re- 
q u i r e d  t o  reach ,-6 cm3 volume i n  each of our  a n n u l i .  
detector of t h e  same volume would r e q u i r e  18 weeks under our  d r i f t -  
i n g  cond i t ions .  E i t h e r  w e  can have t h e  advantage of a much faster 
d r i f t ,  or use a much c o o l e r  d r i f t  compared t o  p l a n a r  detectors. 
A p l a n a r  
6 
Figulre 4a shows an  ehd view of an annulus  of 4 mm e f f e c t i v e  t h i c k n e s s &  
Leakage impedance at l i qd id  N2 tempephture is dbodt 2000 c )  
megohms, compared w i t h  10-20 megohms for our  p l a n a r  detectors. 
d )  The r e s o l u t i o n  s tar ted a t  r A 3 0  K e V  and s t e a d i l y  i m -  
proved. A f t e r  two weeks i t  has reached 21 KeV. F igu re  5 shows a 
B i  207 spectrum e x h i b i t i n g  t h i s .  
well w i t h  t h a t  of commercial ly  a v a i l a b l e  L i  d r i f t e d  G e  detectors is 
not e q u a l  t o  t h e  best pub l i shed  r e su l t s  '). 
choice on our part ,  . s i n c e  w e  intended t o  test  the  detectors w i t h  
n u c l e a r  r e a c t i o n  s p e c t r a ,  which  are heav i ly  doppler-broadened, w e  
T h i s  r e s o l u t i o n ,  a l though it compares 
T h i s  reflects a deliberate 
decided t o  forego a c l e a n  up d r i f t  i n  order t o  expedite the t es t s .  
We are conf iden t  t h a t  a clean-up d r i f t  would have i n c r e a s e d  t h e  re- 
s o l u t i o n  t o  the  maximum theoret ical ly  o b t a i n a b l e  w i t h  a detector of 
t h i s  d e p l e t i o n  depth  (4 mm).  
e) Count rates 2 50,000 per second do not degrade our  
s p e c t r a .  F igu re  6 shows a gamma spectrum from 1-MeV deu te rons  on 
a F1' t a rge t .  Twenty-seven d i f f e r e n t  gamma l i n e s  have been i d e n t i -  
10 -11 $4 
f i e d  on t h i s  spectrum. Using f i v e  o t h e r  t a r g e t s  - L3 , IJ , Y 
A27, and n a t u r a l  S i  - w e  have observed 96 o ther  gamma l i n e s .  
of these l i n e s  are doppler-broadened t o  be from 30 K e V  t o  70 KeV 
FWHM. Hence, 20 K e V  r e s o l u t i o n  is not a l i m i t i n g  factor  i n  t h i s  
t y p e  of n u c l e a r  work. 
Most 
I 
f )  A 3-crystal p a i r  spec t rometer  has  been assembled which 
u s e s  a 3" x 1" N a I  detector on each s ide  of t h e  annulus .  
s p e c t r a  f r o m  t h e  above targets have been t a k e n  s o  t h a t  fu l l - ene rgy  
Using t h i s ,  
1 and escape peaks can be i d e n t i f i e d .  
3) G. T.  Ewan and A .  J. Tavendale, Can. J.  Phys. 42, 2286 (1964). -
Fig.  4 a. Copper-stained end of annulus a f t e r  p a r t i a l  d r i f t i n g ;  
4 mm d r i f t  depth.  
Fig.  4 b. Side  v iew of annulus .  
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Fig. 6 .  Spectrum of d 
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g )  Oscil lograph measurements i n d i c a t e  t h a t  t h e  p u l s e  shape 
v a r i a t i o n  is very  small. With  commercial medium-fast c rossover -p ick  
off e l e c t r o n i c  equipment, w e  f i n d  something less  t h a n  30 n sec t o t a l  
t i m e  d i s p e r s i o n  for  p u l s e s  vary ing  i n  energy f r o m  0.5 MeV t o  12 MeV. 
More q u a n t i t a t i v e  tests a k e  i n  progress .  
h )  A s  w a s  mehhtioned above, there were s e r i o u s  defects i n  
some of t h s  Paw materiai supi j l ied by Sylvania  which l e d  t o  t h e  f a i l u r e  
of appkoxiihtitely 90% of t h e  d e t e c t o r s  manufactured by some major u s e r s .  
The n a t u r e  of t h e  defects i n  t h e  raw material w a s  unknown except t h a t  
t h e y  occur red  af ter  t h e  fu rnaces  at t h e  manufac turers  p l a n t  had been 
r ea r r anged .  T h i s  l e d  a m e m b e r  of t h i s  department'.  t o  sugges t  t h a t  
t h e  defects w e r e  a x i a l  d i s l o c a t i o n s  t y p i c a l  of boa t - r e f ined  material, 
which shorted ou t  t h e  a x i a l  detectors cu t  at  r i g h t  a n g l e s  t o  t h e  
crystal  a x i s .  I n  ou r  annu la r  d e t e c t o r s  these d i s l o c a t i o n s  would be 
p a r a l l e l  t o  t h e  d e p l e t i o n  l a y e r  a n d  therefore should have no i n f l u e n c e  
on tiie detec tor  Ferformance. W e  a r e  checking t h i s  p o s s i b i l i t y  i n  
coope ra t ion  w i t h  t h e  manufacturer  and hope t h a t ,  i f  confirmed, o u r  
f i n d i n g s  w i l l  lead t o  t h e  manufacture of still  larger detectors. 
* 
SUMMARY 
N e w  t echn iques  f o r  t h e  manufacture of L i -d r i f t ed  G e  detec- 
t o r s  have been developed and employed t o  make large volume annu la r  
detectors. These  d e t e c t o r s  have t h e  fo l lowing  advantages over  
p l a n a r  d e t e c t o r s  and f i v e  sided "co-axial" detectors. 
* 
J. M i t c h e l l ,  p r i v a t e  communication. 
8 
a )  Due t o  t h e  fact  t h a t  t h e  d e p l e t i o n  r e g i o n  is everywhere 
p a r a l l e l  t o  t h e  a x i s  of t h e  r a w  c rys t a l ,  d i s l o c a t i o n s  in t roduced  by 
t h e  the rma l  stress dur ing  manufacture,  should have no effect  on 
detector performance. T h i s  is p a r t l y  borne o u t  by our  lower Sailiirc 
rate i n  t h e  manufacture of annu la r  detectors, and w i l l  be i n v e s t i g a t e 3  
f u r t h e r .  
b )  Compared t o  p l a n a r  d e t e c t o r s  a much l a r g e r  volume c m  
be ob ta ined  w i t h  ou r  annu la r  des ign ,  a l s o ,  t h e  larger vcrliime t o  
s u r f a c e  r a t i o  reduces edge e f f e c t s .  
c )  Compared t o  "co-axial" d e t e c t o r s  t h e  annu la r  detector  
has ,  due t o  its ro t a t iona l  symmetry, a more uniform s e n s i t i v i t y  aiid 
a more uniform, and g e n e r a l l y  bet ter ,  t i m e  r e s o l u t i o n .  


Theoretical and experimental investigation of particle detection and particle detectors to improve effectiveness, accuracy, and discrimination  Status report no. 5, period ending Nov. 30, 1965

Theoretical and experimental investigation of particle detection and particle detectors to improve effectiveness, accuracy, and discrimination Status report no. 5, period ending Nov. 30, 1965

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