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Possible effects of photomultiplier-afterpulses on scintillation counter measurements
N U C L E A R I N S T R U M E N T S AND METHODS 84 (1970) 297-300 ; © N O R T H - H O L L A N D P U B L I S H I N G CO.
POSSIBLE EFFECTS OF PHOTOMULTIPLIER-AFTERPULSES ON SCINTILLATION
COUNTER
MEASUREMENTS
R. STAUBERT*, E. BOHM, K. HEIN, K. SAUERLAND and J. TR(]MPER Institut fiir Reine und Angewandte Kernphysik, Universitiit Kiel, Germany
Received 13 April 1970 Pholomultiplier-afterpulses can lead to serious errors when measuring particle numbers with scintillator-photomultiplier systems employing pulse-height-to-time converters. The effectiveness of a dynode gating system restraining afterpulse effects is demonstrated through tests with a nanosecond light source on samples of several types of photomultiplier tubes. 1. Introduction
photomultiplier anode )
The m e a s u r e m e n t o f pulse a m p l i t u d e s in nuclear e x p e r i m e n t s 1 -3) is frequently d o n e utilizing a m p l i t u d e t o - t i m e c o n v e r s i o n 4' 5). This m e t h o d is especially useful for s c i n t i l l a t o r - p h o t o m u l t i p l i e r systems: a c a p a c i t o r is c h a r g e d by the p h o t o t u b e pulse and is then allowed to discharge either linearly 6-8) or exponentially9-13). The value o f charge or a m p l i t u d e can t h e n be determined f r o m the t i m e o f the discharge (see fig. 1). One essential r e q u i r e m e n t for error-free measuremertts by this m e t h o d is the u n d i s t u r b e d decay of the voltage across the c a p a c i t o r C which was charged. I n practice this c o n d i t i o n is n o t always satisfied. Possible m e c h a n i s m s for d i s t u r b i n g the charge situation on such a c a p a c i t o r C and c o r r e s p o n d i n g c o u n t e r m e a s u r e s are listed in table 1. The way in which noise pulses or afterpulses can cause o v e r e s t i m a t i o n o f the i n p u t a m p l i t u d e is a]so illustrated in fig. 1. In devising c o u n t e r m e a s u r e s to a v o i d such errors, two m e t h o d s o f g a t i n g have been d e v e l o p e d : a. a linear gate between p h o t o m u l t i p l i e r a n o d e and
amplifier + SchmH't - t rigcjer
in
(input impedance>> R)
U
u = Uo e x p ( - y ~ )
Uo
in
U~
~
~U
Us i i I
i i I I
1_, \
J J
I I
t
out
J Fig. 1. Principal schematic diagram for logarithmic amplitudeto-time conversion.
* Present address: NASA-Manned Spacecraft Center, Houston, Texas 77058, U.S.A.
TABLE 1
Possible mechanisms for interfering with the desired voltage decay of the charged capacitor. Types of interfering pulses
Counter measures
Subsequent events
z<~ d t gating
m
Comments T discharge time constant At average time interval between two events
screening gating fast electronics (e.g. triggered spark chamber) (finish amplitude measurement before spark chamber triggering) Photomultiplier noise low noise PM tube T small pulses gating Photomultiplier afterpulses gating
External noise pulses
297
ref. 12
refs. 13, 20-23
298
R. S T A U B E R T et al.
•
m°'t 'p"er I p. . . . p 2 I--| ~
.
,ogter IL_r l ......
5
I I I I
• h i g h pressure light
IL
source.
i I "
.
I preamp
" multiplier
I
|
=
|
" Io~
I/
~1 I
R3
1
-
Fig. 2. Block schematic diagram o f the test arrangement to study photomultiplier-afterpulses and the efficiency of a dynode gating
system. integrating capacitor t o, 18, 19) ; b. gating of the photomultiplier 13) [-see als02°-23)]. In practice, only method b is applicable, when the amplitude range exceeds two orders of magnitude. In this article we are concerned with a scintillatori
, i , i i i , • with • dynodegQl-e o w~thou+
1~4~ --
J
i
a
~
,
J calibration~
conver#er
/
v i0 ~
i|
"//'/°J
/i,/0 -~.
_
"o 2 2 tO ~
,/ o,
•~
,7 /
-
o/
I01
" 2
-
°//
J i
S
I
-
;
i
i
i
z
pulse ,en,,h
i
i
I
-
i
[~.lsec]
Fig. 3. Performance of a logarithmic converter system with pulses from a 54 A V P (VALVO) photomultiplier.
photomultiplier system designed to measure particle densities in extensive air showers using a logarithmic time-to-amplitude converter. Typical densities to be measured vary from 1 to 104. We report an investigation of the effect of photomultiplier afterpulses on this type of measurement utilizing the following tubes: 53 AVP, 54 AVP (Valvo), EM1 9583B, RCA 6810A. The effectiveness of a photomultiplier dynode gating system is demonstrated. 2. Method Fig. 2 shows a block schematic diagram of the test arrangement. Photomultiplier 1 is under investigation. A high pressure H2 discharge lamp, developed following Franke et al.Z4), serves as a source for nanosecond light pulses. The lamp characteristics are as follows: a pressure of 15 atm; stainless steel spherical electrodes of diameter 1 mm and gap 0.2 ram; electrical pulses with a fwhm of < 1 ns; pulse rate of ~1 kHz with a 5 kV high voltage supply; and a flash intensity distribution with a relative fwhm of 12% (measured with a 54 AVP with no absorber). Note that photomultiplier I sees the light pulses from this lamp through an absorber of continuously variable thickness. The measurement of the phototube pulses is done with a logarithmic amplitude-to-time converter developed for the Kiel air shower experimentt3). A capacitor C = 680 pF at the anode of the phototube is charged to voltage U0 by the tube current and discharges exponentially through a resistor R = 2.2 kO with a time constant z = R C = 1.5 #s. A high input impedance amplifier follows the capacitor voltage and
299
POSSIBLE EFFECTS OF P H O T O M U L T I P L I E R - A F T E R P U L S E S
Photomultiplier 1 is equipped with a gating system 14) which allows the gain of all tested photomultipliers to be reduced by about a factor of 30 by shorting dynode 2 and dynode 3. A special compensation circuit prevents the generation of noise pulses while changing the potentials on the two dynodes.
1.5
'-~ 1.0 without dynode gate
¢
~ 0.5
¢
+ ¢
~
+
¢
÷
÷
*÷ with dynode gate
00
I
I
5
10
15
mean pulse length [IJSeC] Fig. 4. Width of typical pulse length distributions obtained with a 54 AVP.
finally a Schmitt-trigger produces constant amplitude pu][ses of length t. The relationship between t and Uo is: t = d n U o / U s . U s is the Schmitt-trigger threshold. An error-free performance of the whole system should show a linear relation between t and the absorber thickness d: t oc - d , because the flash intensity reaching the: phototube I is proportional to exp ( - d / 2 ) [2 being the: light absorption coefficient], while U0 is proportional to I and t is proportional to l n U o / U s . So the actual task is to measure t as a function of d. In order to minimize the effect of fluctuations in flash intensity, we introduced a second photomultiplierconverter system as a reference channel, which reduces the: overall amplitude jitter (for d = 0) to 4% (relative fwhm). ]?or convenience the Schmitt-trigger output pulses are, converted with a linear time-to-amplitude converter and recorded with a 400-channel pulse height analyser.
3. System checkout The characteristics of absorber, photomultiplier and converter were carefully controlled in order to ensure the intended performance of the test arrangement. The converter system (log+lin) was tested with a mercury switch pulser. The relation between input and output was strictly logarithmic from 1 mV to 10 V. The amplitudes of the photomultiplier output pulses showed an exponential dependence on the absorber thickness. Thus the exponential reduction of flash intensities by the absorber and the linear amplification characteristic of the photomultiplier were assumed to be as desired over four orders of magnitude. 4. Results
With the exception of RCA 6810A, all photomultiplier tubes which were tested showed basically the same behavior. Typical curves measured with a 54 AVP are shown in fig. 3. The mean pulse length at the output of the log converter is plotted against the converter input amplitude which corresponds to the absorber thickness. For measurements without the dynode gate (open circles) a deviation from the calibration curve is found which increases with input amplitude. The pulse lengthening is caused by photomultiplier-afterpulses which fall somewhere on the exponential tail of the input pulse. When using the dynode gate the effect of afterpulses is considerably reduced. This is also demonstrated in fig. 4 which shows the width of the pulse length distributions as a function of the mean pulse length for measurements with and without the dynode gate. Table 2 lists typical results for four different
TABLE 2 Amplitude measurements with the logarithmic converter for different phototubes, with and without dynode gate. Multiplier
54 AVP (I) 54 AVP (II) EMI 9583 B RCA 6810 A
HV
1400 1500 880 1600
Pulse amplitude on photomultiplier anode (C = 680 pF)
V V V V
2 2 2 2
V V V V
Amplitude measured with log converter with without dynode gate 2.6V 2.8V 2.4V 2.0V
IO.OV lO.4V 5.2V 2.0V
Relative half width of pulse length distribution with without dynode gate 4.2% 3.1% 6.6% 4.8%
8.0% 6.4% 7.7% 4.9%
300
R. STAUBERT et al.
photomultipliers. The one R C A 6810A did not show any afterpulsing. The effectiveness of the d y n o d e gate was also measured as a f u n c t i o n of the time of delay between the light pulse and firing the dynode gate. It t u r n e d out to be sufficient to switch the gate a short time ( ~ 3 ps) before the end of the pulse. This is because the most serious afterpulses are those which hit the very tail of the main pulse, just before this one reaches the Schmitt-trigger threshold. A noise pulse A U hitting the main pulse at U leads to a n overestimation of the input amplitude Uo by a factor of (1 +AU/U).
5. Discussion In agreement with D e L a n e y 16) and White~7), two sorts of afterpulses have been observed: a. "Fast afterpulses" with sharp time distributions within 1 #s after the main pulses and with amplitudes p r o p o r t i o n a l to the m a i n amplitudes (about 1" 50) were f o u n d by direct oscilloscopic observation. It is generally believed that these pulses are caused by positive ions which are formed between cathode and first dynode and are accelerated back to the cathode. b. "Slow afterpulses" with a b r o a d time d i s t r i b u t i o n exceeding a delay of 15 ps with respect to the main pulses were studied with the converter system described above. These pulses seem not to be single big pulses but rather a kind of increased noise. Intensity a n d amplitudes are correlated with the a m p l i t u d e of the main pulse. D e L a n e y and White discuss several mechanisms for p r o d u c t i o n of afterpulses. It ~,~eems possible that more t h a n one of these are working simultaneously. It has been shown that photomultiplier-afterpulses can lead to serious m e a s u r e m e n t errors when evaluating p h o t o m u l t i p l i e r pulses by means of logarithmic converters. The effectiveness of a dynode gating system
restraining afterpulse effects has been demonstrated. We would like to acknowledge the assistance of H. S a l z m a n n in designing the flash lamp a n d K. H. Liebst in preparing the electronics.
References 1) D. Maeder, Nucl. instr. 2 (1958) 299. 2) R. L. Chase, IRE Trans. Nucl. Sci. NS-9, no. 3 (1962) 275. 3) H. Guilion, Nucl, Instr. and Meth. 43 (1966) 240. 4) D. H. Wilkinson, Proc. Cambridge Phil. Mag. 46 (1950) 508. ,5) G. W. Hutchinson and G. G. Scarrot, Phil. Mag. 42 (1951) 792. 6) C. Bonsignori, D. Malosti and U. Pellegrini, Nucl. Instr. and Meth. 20 (1963) 362. 7) H. T. Pizer, Nucl. Instr. and Meth. 20 (1963) 358. s) A. Albrigi-Quaranta, Nucl. Instr. and Meth. 20 (1963) 355. 9) j. R. Green, Rev, Sci. Instr. 29 (1958) 10. 10) K. Suga, G. Clark and I. Escobar, Rev. Sci. Instr. 32 (1961) 1187. 11) R. Kajikawa, F. Makino, M. Matsuoka and Y. Tanaka, Japan J. Appl. Phys. 3 (1964) 724. 12) R. Humphreys, Rev. Sci. Instr. 38 (1967) 1123. 13) E. B6hm, U. J. Roose, R. Staubert and J. Trtimper, Nucl. Instr. and Meth. 40 (1966) 67. 14) U. J. Roose, Nucl. Instr. and Meth. 36 (1965) 333. 15) G. A. Marton, H. M. Smith and R. Wassermann, IEEE Trans. Nucl. Sci. NS-14, no. 1 (1967) 443. 16) C. F. G. De Laney and J. A. Haarwood, Soc. Proc. Roy. Dublin, Soc. Ser. A3, no. 5 (1967) 57. 17) G. White, Nucl. Instr. and Meth. 55 (1967) 157. is) K. B. Keller, Rev. Sci. Instr. 35 (1964) 1360. l,a) H. J. Schuster, Nucl. Instr. and Meth. 58 (1968) 179. 2o) K. B. Keller and B. M. K. Nefkens, Rev. Sci. Instr. 35 (1964) 1359. 21) D. Poenaru and E. Ivanov, Rev. Roum. Phys. 11 (1966) 369. 22) F. de Martini, Rev. Sci. Instr. 38 (1967) 866. 23) R. A. Schrack, H, T. Heaton and R. B. Schwartz, Nucl. Instr. and Meth. 77 (1970) 175. 24) H. G. Franke and H. Schmeing, Nucl. Instr. and Meth. 52 (1967) 171.
POSSIBLE EFFECTS OF PHOTOMULTIPLIER-AFTERPULSES ON SCINTILLATION
COUNTER
MEASUREMENTS
R. STAUBERT*, E. BOHM, K. HEIN, K. SAUERLAND and J. TR(]MPER Institut fiir Reine und Angewandte Kernphysik, Universitiit Kiel, Germany
Received 13 April 1970 Pholomultiplier-afterpulses can lead to serious errors when measuring particle numbers with scintillator-photomultiplier systems employing pulse-height-to-time converters. The effectiveness of a dynode gating system restraining afterpulse effects is demonstrated through tests with a nanosecond light source on samples of several types of photomultiplier tubes. 1. Introduction
photomultiplier anode )
The m e a s u r e m e n t o f pulse a m p l i t u d e s in nuclear e x p e r i m e n t s 1 -3) is frequently d o n e utilizing a m p l i t u d e t o - t i m e c o n v e r s i o n 4' 5). This m e t h o d is especially useful for s c i n t i l l a t o r - p h o t o m u l t i p l i e r systems: a c a p a c i t o r is c h a r g e d by the p h o t o t u b e pulse and is then allowed to discharge either linearly 6-8) or exponentially9-13). The value o f charge or a m p l i t u d e can t h e n be determined f r o m the t i m e o f the discharge (see fig. 1). One essential r e q u i r e m e n t for error-free measuremertts by this m e t h o d is the u n d i s t u r b e d decay of the voltage across the c a p a c i t o r C which was charged. I n practice this c o n d i t i o n is n o t always satisfied. Possible m e c h a n i s m s for d i s t u r b i n g the charge situation on such a c a p a c i t o r C and c o r r e s p o n d i n g c o u n t e r m e a s u r e s are listed in table 1. The way in which noise pulses or afterpulses can cause o v e r e s t i m a t i o n o f the i n p u t a m p l i t u d e is a]so illustrated in fig. 1. In devising c o u n t e r m e a s u r e s to a v o i d such errors, two m e t h o d s o f g a t i n g have been d e v e l o p e d : a. a linear gate between p h o t o m u l t i p l i e r a n o d e and
amplifier + SchmH't - t rigcjer
in
(input impedance>> R)
U
u = Uo e x p ( - y ~ )
Uo
in
U~
~
~U
Us i i I
i i I I
1_, \
J J
I I
t
out
J Fig. 1. Principal schematic diagram for logarithmic amplitudeto-time conversion.
* Present address: NASA-Manned Spacecraft Center, Houston, Texas 77058, U.S.A.
TABLE 1
Possible mechanisms for interfering with the desired voltage decay of the charged capacitor. Types of interfering pulses
Counter measures
Subsequent events
z<~ d t gating
m
Comments T discharge time constant At average time interval between two events
screening gating fast electronics (e.g. triggered spark chamber) (finish amplitude measurement before spark chamber triggering) Photomultiplier noise low noise PM tube T small pulses gating Photomultiplier afterpulses gating
External noise pulses
297
ref. 12
refs. 13, 20-23
298
R. S T A U B E R T et al.
•
m°'t 'p"er I p. . . . p 2 I--| ~
.
,ogter IL_r l ......
5
I I I I
• h i g h pressure light
IL
source.
i I "
.
I preamp
" multiplier
I
|
=
|
" Io~
I/
~1 I
R3
1
-
Fig. 2. Block schematic diagram o f the test arrangement to study photomultiplier-afterpulses and the efficiency of a dynode gating
system. integrating capacitor t o, 18, 19) ; b. gating of the photomultiplier 13) [-see als02°-23)]. In practice, only method b is applicable, when the amplitude range exceeds two orders of magnitude. In this article we are concerned with a scintillatori
, i , i i i , • with • dynodegQl-e o w~thou+
1~4~ --
J
i
a
~
,
J calibration~
conver#er
/
v i0 ~
i|
"//'/°J
/i,/0 -~.
_
"o 2 2 tO ~
,/ o,
•~
,7 /
-
o/
I01
" 2
-
°//
J i
S
I
-
;
i
i
i
z
pulse ,en,,h
i
i
I
-
i
[~.lsec]
Fig. 3. Performance of a logarithmic converter system with pulses from a 54 A V P (VALVO) photomultiplier.
photomultiplier system designed to measure particle densities in extensive air showers using a logarithmic time-to-amplitude converter. Typical densities to be measured vary from 1 to 104. We report an investigation of the effect of photomultiplier afterpulses on this type of measurement utilizing the following tubes: 53 AVP, 54 AVP (Valvo), EM1 9583B, RCA 6810A. The effectiveness of a photomultiplier dynode gating system is demonstrated. 2. Method Fig. 2 shows a block schematic diagram of the test arrangement. Photomultiplier 1 is under investigation. A high pressure H2 discharge lamp, developed following Franke et al.Z4), serves as a source for nanosecond light pulses. The lamp characteristics are as follows: a pressure of 15 atm; stainless steel spherical electrodes of diameter 1 mm and gap 0.2 ram; electrical pulses with a fwhm of < 1 ns; pulse rate of ~1 kHz with a 5 kV high voltage supply; and a flash intensity distribution with a relative fwhm of 12% (measured with a 54 AVP with no absorber). Note that photomultiplier I sees the light pulses from this lamp through an absorber of continuously variable thickness. The measurement of the phototube pulses is done with a logarithmic amplitude-to-time converter developed for the Kiel air shower experimentt3). A capacitor C = 680 pF at the anode of the phototube is charged to voltage U0 by the tube current and discharges exponentially through a resistor R = 2.2 kO with a time constant z = R C = 1.5 #s. A high input impedance amplifier follows the capacitor voltage and
299
POSSIBLE EFFECTS OF P H O T O M U L T I P L I E R - A F T E R P U L S E S
Photomultiplier 1 is equipped with a gating system 14) which allows the gain of all tested photomultipliers to be reduced by about a factor of 30 by shorting dynode 2 and dynode 3. A special compensation circuit prevents the generation of noise pulses while changing the potentials on the two dynodes.
1.5
'-~ 1.0 without dynode gate
¢
~ 0.5
¢
+ ¢
~
+
¢
÷
÷
*÷ with dynode gate
00
I
I
5
10
15
mean pulse length [IJSeC] Fig. 4. Width of typical pulse length distributions obtained with a 54 AVP.
finally a Schmitt-trigger produces constant amplitude pu][ses of length t. The relationship between t and Uo is: t = d n U o / U s . U s is the Schmitt-trigger threshold. An error-free performance of the whole system should show a linear relation between t and the absorber thickness d: t oc - d , because the flash intensity reaching the: phototube I is proportional to exp ( - d / 2 ) [2 being the: light absorption coefficient], while U0 is proportional to I and t is proportional to l n U o / U s . So the actual task is to measure t as a function of d. In order to minimize the effect of fluctuations in flash intensity, we introduced a second photomultiplierconverter system as a reference channel, which reduces the: overall amplitude jitter (for d = 0) to 4% (relative fwhm). ]?or convenience the Schmitt-trigger output pulses are, converted with a linear time-to-amplitude converter and recorded with a 400-channel pulse height analyser.
3. System checkout The characteristics of absorber, photomultiplier and converter were carefully controlled in order to ensure the intended performance of the test arrangement. The converter system (log+lin) was tested with a mercury switch pulser. The relation between input and output was strictly logarithmic from 1 mV to 10 V. The amplitudes of the photomultiplier output pulses showed an exponential dependence on the absorber thickness. Thus the exponential reduction of flash intensities by the absorber and the linear amplification characteristic of the photomultiplier were assumed to be as desired over four orders of magnitude. 4. Results
With the exception of RCA 6810A, all photomultiplier tubes which were tested showed basically the same behavior. Typical curves measured with a 54 AVP are shown in fig. 3. The mean pulse length at the output of the log converter is plotted against the converter input amplitude which corresponds to the absorber thickness. For measurements without the dynode gate (open circles) a deviation from the calibration curve is found which increases with input amplitude. The pulse lengthening is caused by photomultiplier-afterpulses which fall somewhere on the exponential tail of the input pulse. When using the dynode gate the effect of afterpulses is considerably reduced. This is also demonstrated in fig. 4 which shows the width of the pulse length distributions as a function of the mean pulse length for measurements with and without the dynode gate. Table 2 lists typical results for four different
TABLE 2 Amplitude measurements with the logarithmic converter for different phototubes, with and without dynode gate. Multiplier
54 AVP (I) 54 AVP (II) EMI 9583 B RCA 6810 A
HV
1400 1500 880 1600
Pulse amplitude on photomultiplier anode (C = 680 pF)
V V V V
2 2 2 2
V V V V
Amplitude measured with log converter with without dynode gate 2.6V 2.8V 2.4V 2.0V
IO.OV lO.4V 5.2V 2.0V
Relative half width of pulse length distribution with without dynode gate 4.2% 3.1% 6.6% 4.8%
8.0% 6.4% 7.7% 4.9%
300
R. STAUBERT et al.
photomultipliers. The one R C A 6810A did not show any afterpulsing. The effectiveness of the d y n o d e gate was also measured as a f u n c t i o n of the time of delay between the light pulse and firing the dynode gate. It t u r n e d out to be sufficient to switch the gate a short time ( ~ 3 ps) before the end of the pulse. This is because the most serious afterpulses are those which hit the very tail of the main pulse, just before this one reaches the Schmitt-trigger threshold. A noise pulse A U hitting the main pulse at U leads to a n overestimation of the input amplitude Uo by a factor of (1 +AU/U).
5. Discussion In agreement with D e L a n e y 16) and White~7), two sorts of afterpulses have been observed: a. "Fast afterpulses" with sharp time distributions within 1 #s after the main pulses and with amplitudes p r o p o r t i o n a l to the m a i n amplitudes (about 1" 50) were f o u n d by direct oscilloscopic observation. It is generally believed that these pulses are caused by positive ions which are formed between cathode and first dynode and are accelerated back to the cathode. b. "Slow afterpulses" with a b r o a d time d i s t r i b u t i o n exceeding a delay of 15 ps with respect to the main pulses were studied with the converter system described above. These pulses seem not to be single big pulses but rather a kind of increased noise. Intensity a n d amplitudes are correlated with the a m p l i t u d e of the main pulse. D e L a n e y and White discuss several mechanisms for p r o d u c t i o n of afterpulses. It ~,~eems possible that more t h a n one of these are working simultaneously. It has been shown that photomultiplier-afterpulses can lead to serious m e a s u r e m e n t errors when evaluating p h o t o m u l t i p l i e r pulses by means of logarithmic converters. The effectiveness of a dynode gating system
restraining afterpulse effects has been demonstrated. We would like to acknowledge the assistance of H. S a l z m a n n in designing the flash lamp a n d K. H. Liebst in preparing the electronics.
References 1) D. Maeder, Nucl. instr. 2 (1958) 299. 2) R. L. Chase, IRE Trans. Nucl. Sci. NS-9, no. 3 (1962) 275. 3) H. Guilion, Nucl, Instr. and Meth. 43 (1966) 240. 4) D. H. Wilkinson, Proc. Cambridge Phil. Mag. 46 (1950) 508. ,5) G. W. Hutchinson and G. G. Scarrot, Phil. Mag. 42 (1951) 792. 6) C. Bonsignori, D. Malosti and U. Pellegrini, Nucl. Instr. and Meth. 20 (1963) 362. 7) H. T. Pizer, Nucl. Instr. and Meth. 20 (1963) 358. s) A. Albrigi-Quaranta, Nucl. Instr. and Meth. 20 (1963) 355. 9) j. R. Green, Rev, Sci. Instr. 29 (1958) 10. 10) K. Suga, G. Clark and I. Escobar, Rev. Sci. Instr. 32 (1961) 1187. 11) R. Kajikawa, F. Makino, M. Matsuoka and Y. Tanaka, Japan J. Appl. Phys. 3 (1964) 724. 12) R. Humphreys, Rev. Sci. Instr. 38 (1967) 1123. 13) E. B6hm, U. J. Roose, R. Staubert and J. Trtimper, Nucl. Instr. and Meth. 40 (1966) 67. 14) U. J. Roose, Nucl. Instr. and Meth. 36 (1965) 333. 15) G. A. Marton, H. M. Smith and R. Wassermann, IEEE Trans. Nucl. Sci. NS-14, no. 1 (1967) 443. 16) C. F. G. De Laney and J. A. Haarwood, Soc. Proc. Roy. Dublin, Soc. Ser. A3, no. 5 (1967) 57. 17) G. White, Nucl. Instr. and Meth. 55 (1967) 157. is) K. B. Keller, Rev. Sci. Instr. 35 (1964) 1360. l,a) H. J. Schuster, Nucl. Instr. and Meth. 58 (1968) 179. 2o) K. B. Keller and B. M. K. Nefkens, Rev. Sci. Instr. 35 (1964) 1359. 21) D. Poenaru and E. Ivanov, Rev. Roum. Phys. 11 (1966) 369. 22) F. de Martini, Rev. Sci. Instr. 38 (1967) 866. 23) R. A. Schrack, H, T. Heaton and R. B. Schwartz, Nucl. Instr. and Meth. 77 (1970) 175. 24) H. G. Franke and H. Schmeing, Nucl. Instr. and Meth. 52 (1967) 171.