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Technique for determination of depletion depth in Ge(Li) counters
NUCLEAR
INSTRUMENTS
AND METHODS
60 ( 1 9 6 8 ) 1 1 6 - 1 2 0 ; © N O R T H - H O L L A N D
PUBLISHING
CO.
T E C H N I Q U E FOR D E T E R M I N A T I O N OF D E P L E T I O N D E P T H IN Ge(Li) COUNTERS* J. B R O W N R I D G E and D. M c L O U G H L I N
Physics Department, State University of New York at Binghamton, Bingharnton, New York, U.S.A. Received 23 October 1967 A method for determining the extent of the compensated region of p-type germanium drifted with lithium is described. It is a nondestructive method which is applied while the crystal is in the drifting apparatus and does not interrupt the drifting process.
In the process of the development of germanium gamma-ray detectors 1,9)by drifting lithium ions through the crystal, it is important to be able to ascertain the depth through which the lithium has been drifted. Without extensive study the rate of drift is unpredictable and depends on the germanium material, the amount of lithium diffused at the start, and other environmental conditions 2' 3). The rate of drift changes in time for a given germanium crystal and may actually go to zero. It is desirable, therefore, to measure the depth of the lithium ions from time to time during the course of the drifting process, but without disturbing the process. Different techniques have been used for this purpose 2-10) but none are completely satisfactory. For example, many laboratories stain the germanium crystal under reverse bias with copper sulphate s) or barium titanate6). Probe measurements of surface resistivity have been used in other laboratories 7' 8), a few laboratories have used measurements of the capacitance of the systemg), and still others have used measurements of the thermoelectric power3). Most of these methods involve the interruption of the
drifting process and/or touching the crystal faces with possible sources of contamination. The inconvenience caused by the interruption of the drifting process is compounded by the need for lapping and etching to purge the surfaces of the germanium crystal of possible contamination. We have developed a simple method to measure the depletion depth of the crystal during the drifting process which does not require a probe contact of any sort. In this method we made use of the different photoconducting properties of the n-type region, the intrinsic region, and the p-type region of the germanium crystal. A collimated light pulse from a stroboscope is transmitted through a narrow slit to the side of the crystal. The amplitude of the resulting electrical pulse observed with an oscilloscope connected to the p-side of the diode, depends on the region of the crystal illuminated by the split system. This method is different from the one reported by Schuler 1°) which was limited to use with thin crystals. The drifting apparatus described by Hansen and * Work supported by the National Science Foundation under Grant G P 6213.
/~-VERNIEADJUSTI R NG SCREW
/
"
&
?
_
N.
F-
I L__ I
/
B,ASO,,AOE PROBE
"~~YSTAL
='-COLLIMATING STROBOSCOPE LIGHTSOURCE
INIIIIIITE
Fig. 1. Schematic of slit system for illuminating the germanium crystal with pulses of light.
116
DEPLETION
DEPTH
Jarrett 5) was modified by moving the diode clamp to the edge of the drifting plate to which was attached a movable slit of 1 mm as shown in fig. 1. The drifting apparatus and the slit system are contained in a home made drybox in a dry nitrogen atmosphere. The amplitude of the electrical pulses resulting from illumination of the p-type region of the diode are about the same as those coming from the general background. The discrimination between the amplitude of the pulses from the intrinsic region and those from the p-type region was enhanced by increasing the magnitude of the reverse bias within the limitations imposed by the current through the crystal. In fig. 2 we show a typical curve of the amplitude of the electrical pulses as a function of the position of the slit. The experimental measurements to obtain the curve of fig. 2 involved scanning the surface of the diode with the light pulse and recording the pulse amplitude and corresponding position of the slit. Initially the beam of light is in a position below the top of the drifting plate, and as it is advanced into the n-type region, the amplitude of the pulses increases above background. As the beam leaves the compensated material and enters the p-type region the amplitude of the pulses decreases as is illustrated in fig. 2. The difference in the position of the slit when these events occur corresponds to the width of the diffused and drifted region.
IN Ge(Li) C O U N T E R S
117
At the present with a 1 mm slit we can measure the width of the compensated region to within 0.3 mm provided the depleted region is more than 1 mm thick. The validity of this method was confirmed by application of the copper staining technique to the first few crystals under study. This phototechnique is very useful for determining the relative concentration of lithium ions throughout the crystal. For example, in the very early phase of the drifting procedure very large pulses, attributed to the n-type region formed after the diffusion of lithium, are observed. As the drifting proceeds from day to day, the amplitude of the pulses decrease but the region in which the pulses are produced increases in time. One, therefore, sees the increase in size of the compensated region as the drifting process goes on in time. The amplitudes of the pulses associated with the n-type region decrease continuously in time indicating a decrease in the lithium ion concentration as shown in fig. 3. As the drifting process continues, it is possible that the lithium ion concentration is reduced by precipitation of the lithium ion. Then the heretofore n-type region will exhibit properties generally attributed to behaviour of p-type regions. Fig. 4 graphically demonstrates this behavior as observed in the decrease in the amplitudes of the pulses originating as the former n-type region. It is well known 2) that the lithium ion is rediffused by
I0
9 (/)
I._1 o> (/) ILl (,9 ..-I :) 0-
8
7 6 5
o
ILl r'~ :::) F-
N-T~PE AND COMPENSATED MATERIAL
I
LL
4 5
{3_
2 3"
I
0
® I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
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8
9
10
POSITION OF SLIT (mm) Fig. 2. Typical response curve o f photoconductivity o f the germanium crystal pulses as a function o f slit position. The germanium crystal was 9 m m thick.
J. B R O W N R 1 D G E AND D. M c L O U G H L 1 N
118
I0 T
CURVE I~ " - ~ - ~ --~IMMEDIATELY DIFFUSION TEMP = 4 0 " C CURRENT = 4 0 r n A
9
IJ o > hi
8
"t t
7
~
/
6
d D
a_
_L
CURVE l r[:
•,-
5
~
/
LI_ 0 Ill a :D
o • - - - - ~ - ~ 24. HOURS AFTER DIFFUSION T = 5 4 " C I = 70mA
CURVE Tr
A'-..
4
el"
,6
°°
~
~l
..9-. ne~. ~..~
/i.°
",~
AFTER
CURVE:SZ
48
HOURS T=35"C I:lOBmA
T
=
HOUR~
- - t - 7 2
oo.. = 32"C
°
3
_1 ¢1
Z
2 I
I
2
3
4
5
POSITION
6
7
8
9
OF S L I T ( r a m )
Fig. 3. Development of depletion region with time. The large amplitudes in curve I are attributed to the initially large concentration of lithium ions in the n-type region. The background has been subtracted from this data. The crystal was 9 mm thick.
I0
I-.J
o >
---G---
° ~
CURVE
I
CURVE
1T' ---~-- ° °
T DAYS T = 32 ° C I =120mA DAYS II D A Y S
C U R V E TFr ~ ----B---- --
.I.
T = 54* C I = 120 mA
Ul J D 13I I_ O ILl a D I-
5
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L
4
5
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d
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e , e. o_&
\;
o
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-
=
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,~ I
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. 3
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POSITION
5
.
. 6
, - o - - - ,-Skz__-'-,c7~_• • . a . . _
.
,r
7
8
m__.___=4 9
I0
OF S L I T ( m m )
Fig. 4. Demonstration of loss of lithium ions at n-type region and precipitation of lithium from compensated region after continued drifting. These data were taken on same crystal of fig. 3.
Ge(Li) C O U N T E R S
D E P L E T I O N D E P T H IN
119
I0 CURVE r :
9
b o >
laJ
8
-~ IMMEDIATELY A F T E R ..L ADDITION OF MORE LITHIUM A F T E R DAYS OF DRIFTING o • ~ - A F T E R 5 HOURS OF D R I F T I N G A F T E R CURVE I WAS OBTAINED
CURVE~:
7
d
-'~ ..
CURVE Trr:
y..-,----i-...,,...
6
II
~ 1 DAY AFTER REDIFFUSION
12 DAYS FROM INITIAL DIFFUSION
5 1.1.
o I.U
4
. ,.~---~
/
C)
I--
/ Y I
3
~
_/2/
"--~.
,z~
,-....
T '"
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"",¢
i/
J
2 I °
I
0
2
,
,
,
,
3
4
5
6
P O S I T I O N OF S L I T
7
8
I0
9
(mm)
Fig. 5. Restoration of n-type region after red;flus;on or heat-cycling: Note that heat treatment contributes to a decrease in the size of the compensated region. This region recovered quickly with drifting. The compensated region is extended with further drifting. The crystal of fig. 3 was used for these data.
10 CURVE I :
x • ~ 13th DAY OF DRIFTING AFTER I N I T I A L DIFFUSION. 2 DAYS A F T E R REDIFFUSION
CURVE]E:
A .I 151h DAY A F T E R I N I T I A L D I F F U S I O N ; A F T E R 1 DAY OF CLEANUP
9
~' o >
B 7
DRIFT AT LOWER TEMPERATURE
W J
D
I.,1_
o
W r~
D t-
6 5 I
4
d
"
k
=/
3
-,n,..
2
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0
e "~'1~
° "'11~
e ~"X'~
I
I
I
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2
3
4
5
6
7
8
9
P O S I T I O N OF S L I T
•
11
• "-")~
I0
(rnm)
Fig. 6. Clean-up drift. Temperature of crystal is lowered and voltage drop across crystal was increased by external means in the second day of cleanup. Curve II was taken after this increase in voltage across the crystal. These data were taken for the same crystal of germanium used to obtain data reported in the previous figures.
120
Jo B R O W N R I D G E
A N D D. M c L O U G H L I N
quickly raising the t e m p e r a t u r e o f the crystal to 425°C and h o l d i n g it there for a b o u t three minutes. This " h e a t cycle" t r e a t m e n t causes p a r t o f the intrinsic region o f the surface to revert to u n c o m p e n s a t e d p-type. W e show in fig. 5 curves o f the a m p l i t u d e p h o t o c o n d u c t i n g pulses which show the increases in the Li + c o n c e n t r a t i o n in the n-type region a n d a r e d u c t i o n o f the lithium in the intrinsic region following the " h e a t cycle". In fig. 6 we show t h a t c o n t i n u a t i o n o f the drifting process will lead to a recovery o f g o o d characteristics. It m u s t be e m p h a s i z e d t h a t there is no significance to be a t t a c h e d to the differences o f the heights o f the various curves o f the figures. The a m p l i t u d e o f the pulses d e p e n d on the voltage d r o p across the crystal which in t u r n d e p e n d s on the t e m p e r a t u r e . The general shape o f a p a r t i c u l a r curve is the i m p o r t a n t m a t t e r to be studied. The technique using light pulses shows p r o m i s e as a way to measure depletion depths w i t h o u t i n t e r r u p t i n g the drifting process. Its results are quite accurate c o m p a r e d to c o p p e r staining. The m e a s u r e m e n t s can be
m a d e entirely within a nitrogen a t m o s p h e r e o f the d r y box. The m e a s u r e m e n t s described above, can be used to study the lithium c o n c e n t r a t i o n in the n-type region as a function o f drift time. W e are indebted to A. G a i g a l a s , S. R a b o y and C. S t a n n a r d for s u p p o r t , guidance and stimulation.
References 1) A. J. T a v e n d a l e a n d G. T. Ewan, Nucl. Instr. a n d Meth. 25 (1963) 185.
z) H. M. Mann, F. J. Janarek and H. W. Helenberg, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 336. a) R. Henck, L. Stab, G. Lopes de Silva, P. Siffert and A. Coche, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 245. 4) H. L. Malm and I. L. Fowler, Can. Res. Council Publ. CRGP- 1224. 5) W. L. Hansen and B. V. Jarrett, Lawrence Rad. Lab. Report UCRL 11589. 6) C. S. Fuller and J. C. Severiens, Phys. Rev. 96 (1954) 21. 7) L. B. Valdes, Proc. IRE 42 (1964) 420. 8) F. S. Goulding and W. L. Hansen, IEEE Trans. Nucl. Sci. NS-11 (1964) 286. 9) E. M. Pell, J. Appl. Phys. 31 (1960) 291. 10) W. A. SchiJller, Rev. Sci. Instr. 38 (1967) 539.
INSTRUMENTS
AND METHODS
60 ( 1 9 6 8 ) 1 1 6 - 1 2 0 ; © N O R T H - H O L L A N D
PUBLISHING
CO.
T E C H N I Q U E FOR D E T E R M I N A T I O N OF D E P L E T I O N D E P T H IN Ge(Li) COUNTERS* J. B R O W N R I D G E and D. M c L O U G H L I N
Physics Department, State University of New York at Binghamton, Bingharnton, New York, U.S.A. Received 23 October 1967 A method for determining the extent of the compensated region of p-type germanium drifted with lithium is described. It is a nondestructive method which is applied while the crystal is in the drifting apparatus and does not interrupt the drifting process.
In the process of the development of germanium gamma-ray detectors 1,9)by drifting lithium ions through the crystal, it is important to be able to ascertain the depth through which the lithium has been drifted. Without extensive study the rate of drift is unpredictable and depends on the germanium material, the amount of lithium diffused at the start, and other environmental conditions 2' 3). The rate of drift changes in time for a given germanium crystal and may actually go to zero. It is desirable, therefore, to measure the depth of the lithium ions from time to time during the course of the drifting process, but without disturbing the process. Different techniques have been used for this purpose 2-10) but none are completely satisfactory. For example, many laboratories stain the germanium crystal under reverse bias with copper sulphate s) or barium titanate6). Probe measurements of surface resistivity have been used in other laboratories 7' 8), a few laboratories have used measurements of the capacitance of the systemg), and still others have used measurements of the thermoelectric power3). Most of these methods involve the interruption of the
drifting process and/or touching the crystal faces with possible sources of contamination. The inconvenience caused by the interruption of the drifting process is compounded by the need for lapping and etching to purge the surfaces of the germanium crystal of possible contamination. We have developed a simple method to measure the depletion depth of the crystal during the drifting process which does not require a probe contact of any sort. In this method we made use of the different photoconducting properties of the n-type region, the intrinsic region, and the p-type region of the germanium crystal. A collimated light pulse from a stroboscope is transmitted through a narrow slit to the side of the crystal. The amplitude of the resulting electrical pulse observed with an oscilloscope connected to the p-side of the diode, depends on the region of the crystal illuminated by the split system. This method is different from the one reported by Schuler 1°) which was limited to use with thin crystals. The drifting apparatus described by Hansen and * Work supported by the National Science Foundation under Grant G P 6213.
/~-VERNIEADJUSTI R NG SCREW
/
"
&
?
_
N.
F-
I L__ I
/
B,ASO,,AOE PROBE
"~~YSTAL
='-COLLIMATING STROBOSCOPE LIGHTSOURCE
INIIIIIITE
Fig. 1. Schematic of slit system for illuminating the germanium crystal with pulses of light.
116
DEPLETION
DEPTH
Jarrett 5) was modified by moving the diode clamp to the edge of the drifting plate to which was attached a movable slit of 1 mm as shown in fig. 1. The drifting apparatus and the slit system are contained in a home made drybox in a dry nitrogen atmosphere. The amplitude of the electrical pulses resulting from illumination of the p-type region of the diode are about the same as those coming from the general background. The discrimination between the amplitude of the pulses from the intrinsic region and those from the p-type region was enhanced by increasing the magnitude of the reverse bias within the limitations imposed by the current through the crystal. In fig. 2 we show a typical curve of the amplitude of the electrical pulses as a function of the position of the slit. The experimental measurements to obtain the curve of fig. 2 involved scanning the surface of the diode with the light pulse and recording the pulse amplitude and corresponding position of the slit. Initially the beam of light is in a position below the top of the drifting plate, and as it is advanced into the n-type region, the amplitude of the pulses increases above background. As the beam leaves the compensated material and enters the p-type region the amplitude of the pulses decreases as is illustrated in fig. 2. The difference in the position of the slit when these events occur corresponds to the width of the diffused and drifted region.
IN Ge(Li) C O U N T E R S
117
At the present with a 1 mm slit we can measure the width of the compensated region to within 0.3 mm provided the depleted region is more than 1 mm thick. The validity of this method was confirmed by application of the copper staining technique to the first few crystals under study. This phototechnique is very useful for determining the relative concentration of lithium ions throughout the crystal. For example, in the very early phase of the drifting procedure very large pulses, attributed to the n-type region formed after the diffusion of lithium, are observed. As the drifting proceeds from day to day, the amplitude of the pulses decrease but the region in which the pulses are produced increases in time. One, therefore, sees the increase in size of the compensated region as the drifting process goes on in time. The amplitudes of the pulses associated with the n-type region decrease continuously in time indicating a decrease in the lithium ion concentration as shown in fig. 3. As the drifting process continues, it is possible that the lithium ion concentration is reduced by precipitation of the lithium ion. Then the heretofore n-type region will exhibit properties generally attributed to behaviour of p-type regions. Fig. 4 graphically demonstrates this behavior as observed in the decrease in the amplitudes of the pulses originating as the former n-type region. It is well known 2) that the lithium ion is rediffused by
I0
9 (/)
I._1 o> (/) ILl (,9 ..-I :) 0-
8
7 6 5
o
ILl r'~ :::) F-
N-T~PE AND COMPENSATED MATERIAL
I
LL
4 5
{3_
2 3"
I
0
® I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
?
8
9
10
POSITION OF SLIT (mm) Fig. 2. Typical response curve o f photoconductivity o f the germanium crystal pulses as a function o f slit position. The germanium crystal was 9 m m thick.
J. B R O W N R 1 D G E AND D. M c L O U G H L 1 N
118
I0 T
CURVE I~ " - ~ - ~ --~IMMEDIATELY DIFFUSION TEMP = 4 0 " C CURRENT = 4 0 r n A
9
IJ o > hi
8
"t t
7
~
/
6
d D
a_
_L
CURVE l r[:
•,-
5
~
/
LI_ 0 Ill a :D
o • - - - - ~ - ~ 24. HOURS AFTER DIFFUSION T = 5 4 " C I = 70mA
CURVE Tr
A'-..
4
el"
,6
°°
~
~l
..9-. ne~. ~..~
/i.°
",~
AFTER
CURVE:SZ
48
HOURS T=35"C I:lOBmA
T
=
HOUR~
- - t - 7 2
oo.. = 32"C
°
3
_1 ¢1
Z
2 I
I
2
3
4
5
POSITION
6
7
8
9
OF S L I T ( r a m )
Fig. 3. Development of depletion region with time. The large amplitudes in curve I are attributed to the initially large concentration of lithium ions in the n-type region. The background has been subtracted from this data. The crystal was 9 mm thick.
I0
I-.J
o >
---G---
° ~
CURVE
I
CURVE
1T' ---~-- ° °
T DAYS T = 32 ° C I =120mA DAYS II D A Y S
C U R V E TFr ~ ----B---- --
.I.
T = 54* C I = 120 mA
Ul J D 13I I_ O ILl a D I-
5
.oa" .-~.
L
4
5
.t
d
:E
e , e. o_&
\;
o
2 I
-
=
o
,~ I
~
__a-~
;.~g-
o
2
. 3
4
POSITION
5
.
. 6
, - o - - - ,-Skz__-'-,c7~_• • . a . . _
.
,r
7
8
m__.___=4 9
I0
OF S L I T ( m m )
Fig. 4. Demonstration of loss of lithium ions at n-type region and precipitation of lithium from compensated region after continued drifting. These data were taken on same crystal of fig. 3.
Ge(Li) C O U N T E R S
D E P L E T I O N D E P T H IN
119
I0 CURVE r :
9
b o >
laJ
8
-~ IMMEDIATELY A F T E R ..L ADDITION OF MORE LITHIUM A F T E R DAYS OF DRIFTING o • ~ - A F T E R 5 HOURS OF D R I F T I N G A F T E R CURVE I WAS OBTAINED
CURVE~:
7
d
-'~ ..
CURVE Trr:
y..-,----i-...,,...
6
II
~ 1 DAY AFTER REDIFFUSION
12 DAYS FROM INITIAL DIFFUSION
5 1.1.
o I.U
4
. ,.~---~
/
C)
I--
/ Y I
3
~
_/2/
"--~.
,z~
,-....
T '"
""-~-~..~"
"",¢
i/
J
2 I °
I
0
2
,
,
,
,
3
4
5
6
P O S I T I O N OF S L I T
7
8
I0
9
(mm)
Fig. 5. Restoration of n-type region after red;flus;on or heat-cycling: Note that heat treatment contributes to a decrease in the size of the compensated region. This region recovered quickly with drifting. The compensated region is extended with further drifting. The crystal of fig. 3 was used for these data.
10 CURVE I :
x • ~ 13th DAY OF DRIFTING AFTER I N I T I A L DIFFUSION. 2 DAYS A F T E R REDIFFUSION
CURVE]E:
A .I 151h DAY A F T E R I N I T I A L D I F F U S I O N ; A F T E R 1 DAY OF CLEANUP
9
~' o >
B 7
DRIFT AT LOWER TEMPERATURE
W J
D
I.,1_
o
W r~
D t-
6 5 I
4
d
"
k
=/
3
-,n,..
2
\ "'1(""
0
e "~'1~
° "'11~
e ~"X'~
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
P O S I T I O N OF S L I T
•
11
• "-")~
I0
(rnm)
Fig. 6. Clean-up drift. Temperature of crystal is lowered and voltage drop across crystal was increased by external means in the second day of cleanup. Curve II was taken after this increase in voltage across the crystal. These data were taken for the same crystal of germanium used to obtain data reported in the previous figures.
120
Jo B R O W N R I D G E
A N D D. M c L O U G H L I N
quickly raising the t e m p e r a t u r e o f the crystal to 425°C and h o l d i n g it there for a b o u t three minutes. This " h e a t cycle" t r e a t m e n t causes p a r t o f the intrinsic region o f the surface to revert to u n c o m p e n s a t e d p-type. W e show in fig. 5 curves o f the a m p l i t u d e p h o t o c o n d u c t i n g pulses which show the increases in the Li + c o n c e n t r a t i o n in the n-type region a n d a r e d u c t i o n o f the lithium in the intrinsic region following the " h e a t cycle". In fig. 6 we show t h a t c o n t i n u a t i o n o f the drifting process will lead to a recovery o f g o o d characteristics. It m u s t be e m p h a s i z e d t h a t there is no significance to be a t t a c h e d to the differences o f the heights o f the various curves o f the figures. The a m p l i t u d e o f the pulses d e p e n d on the voltage d r o p across the crystal which in t u r n d e p e n d s on the t e m p e r a t u r e . The general shape o f a p a r t i c u l a r curve is the i m p o r t a n t m a t t e r to be studied. The technique using light pulses shows p r o m i s e as a way to measure depletion depths w i t h o u t i n t e r r u p t i n g the drifting process. Its results are quite accurate c o m p a r e d to c o p p e r staining. The m e a s u r e m e n t s can be
m a d e entirely within a nitrogen a t m o s p h e r e o f the d r y box. The m e a s u r e m e n t s described above, can be used to study the lithium c o n c e n t r a t i o n in the n-type region as a function o f drift time. W e are indebted to A. G a i g a l a s , S. R a b o y and C. S t a n n a r d for s u p p o r t , guidance and stimulation.
References 1) A. J. T a v e n d a l e a n d G. T. Ewan, Nucl. Instr. a n d Meth. 25 (1963) 185.
z) H. M. Mann, F. J. Janarek and H. W. Helenberg, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 336. a) R. Henck, L. Stab, G. Lopes de Silva, P. Siffert and A. Coche, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 245. 4) H. L. Malm and I. L. Fowler, Can. Res. Council Publ. CRGP- 1224. 5) W. L. Hansen and B. V. Jarrett, Lawrence Rad. Lab. Report UCRL 11589. 6) C. S. Fuller and J. C. Severiens, Phys. Rev. 96 (1954) 21. 7) L. B. Valdes, Proc. IRE 42 (1964) 420. 8) F. S. Goulding and W. L. Hansen, IEEE Trans. Nucl. Sci. NS-11 (1964) 286. 9) E. M. Pell, J. Appl. Phys. 31 (1960) 291. 10) W. A. SchiJller, Rev. Sci. Instr. 38 (1967) 539.