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Gain stability in high-current photomultipliers at high variable counting rates
NUCLEAR INSTRUMENTS AND METHODS 53 (I967) 352-354;
©
NORTH-HOLLAND P U B L I S H I N G CO.
GAIN S T A B I L I T Y I N H I G H - C U R R E N T P H O T O M U L T I P L I E R S AT H I G H VARIABLE C O U N T I N G RATES A. K. GUPTA and N. NATH Department of Physics, Banaras Hindu University, Varanasi, India
Received 25 April 1967 Variations in the gain of 56AVP photomultiplier tubes is studied by changing the counting rates. It is observed that at high counting rates the gain of a photomultiplier tube appreciably increases with increasing counting rate. The energy resolution of the photomultiplier tube is found to deteriorate at high counting rates.
It is found that this gain shift is nearly eliminated by keeping the voltages stabilized for the last two stages of the photomultiplier chain. This also helps in maintaining the energy resolution.
• It is well known t - s ) that Changes in overall gain of a photomultiplier tube occur when the counting rates are changed. Earlier workers have observed this effect using different photomultipliers. In the case of a fast high gain photomultiplier the gain shift observed is significantly large and can cause considerable difficulty in the study of fast decaying radioactive sources or in nuclear reaction studies where total counting rates fluctuate within wide limits. Michealis et a l : ) have studied the energy resolution of the scintillation set-up using RCA6810 photomultiplier tubes. They report a deterioration in resolution as the counting rate is increased. In the present work we have attempted to investigate further the apparent cause of these undesirable effects. We have successfully overcome these limitations through stabilized regulation of dynode potentials by introducing zener semi-conductor diodes or voltage regulating tubes in the photomultiplier chain. Measurements: We have studied this effect mainly for the 56 AVP photomultiplier tube using a NaI(TI) crystal and a strong source of 137Cs. The photomultiplier chain corresponds closely to the suggested "Philips Distribution Type B". The linear output was taken from the 8 th dynode. Standard electronic equipments were used under stable conditions. The counting rate was varied by moving the source backward or forward in front of the scintillator-photomultiplier assembly. Fig. 1 shows the effect of the counting rate on the overall gain of the 56 AVP photomultiplier tube for a photomultiplier chain current of 0.8 mA. It is seen that at low total integral counting rates the gain shift is small. At much higher counting rates, however, the magnitude of the gain shift becomes too large even for a relatively smaller change in the counting rate. When the resistances in the photomultiplier chain
were decreased by a factor of seven while maintaining the dynode potentials very close to the values to obtain the results shown in fig. 1, the gain shift characteristics indicated some improvement as shown in fig. 2. The chain current for this case was 5.8 mA. The laTCs peak 56
52
Chain current
= 0.8mA
48 44
t-
40
.o o 36 ,,t O
o. p-
32
28
24 A 20--'-- ~" 0
B I
2
,
1
4
J
r
6 Time (hr)
L
I
8
I
P
10
~
I
12
Fig. 1. Variation in photomultiplier gain following counting rate change for a chain current of 0.8 mA. A = 1 700 counts/s D = 48 500 counts/s B -- 11 700 counts/s E = 56 400 counts/s C = 33 000 counts/s
352
GAIN STABILITY IN HIGH-CURRENT PHOTOMULTIPLIERS 48 Choin c u r r e n t - 5 . B m A
v
.~_ 44
[
03 0
3e
A F---B:
C, 32
0
I
2
1
4
,
!
~
!
6 8 Time (hr)
i
I
10
12
Fig. 2. Variation in photomultiplier gain following counting rate change for a chain current of 5.8 rnA. A = 1 700 counts/s C = 34 000 counts/s B = 12 700 counts/s D = 56 400 counts/s shifted its position by about 45% for a change in integral counting rate from 1700 counts/see to 56000 counts/see, while at low chain current (0.8 mA) the corresponding gain shift was about 180%. It evidently points out that the .gain shift observed is intimately associated with the relative change in the chain current. However, it may not always be possible to provide very high chain currents to minimise the effect of gain shift with counting rate. It may be mentioned here that these observations have been made using high-value condensers across the last four dynode-ehain resistors. The effect observed here will be much reduced in magnitude for lower gain photomultiplier tubes and it may also vary with the design of the particular tube used.
353
The voltage differences between various dynodes were measured with the help of a VTVM at two extreme counting rates. Table 1 gives the data. It is seen that the voltage across the last stage of the P M chain gets reduced appreciably at high counting rate. In contrast, the voltages across the lower stages are observed to increase, the total voltage across the whole chain remaining constant. In our measurements, the linear pulse output was taken from the 8 th dynode. Since the dynode potentials go up in all the lower stages, we. observe an increase in pulse amplitude with increasing counting rate. At higher chain current (5.8 mA), the voltage differences between various dynodes do vary but the magnitude of the change with the same change in counting rate gets reduced as shown in table 1. The data in table 1 should only be considered as a qualitative indication of the actual dynamic situation inside the photomultiplier tube. Using appropriate voltage regulator tubes across the last stage instead of the chain resistor helped in reducing considerably the gain shift with counting rate (fig. 3a). Slight increase in the photomultiplier gain was still observed at the extreme high counting rate (67000 counts/see). When another voltage regulator tube was used across the last but one stage, the photomultiplier gain was observed to remain substantially constantupto the highest counting rates that we tried (fig. 3b). The energy resolution for the full energy gamma-ray peak was also found to deteriorate at high counting rates. It went down from a figure of 14% to 21% for the two extreme counting rates used. The introduction of
TABLE 1 Voltages across different dynodes. Low current chain Dynode no. ht-Dx4 D14-D13 Dla-Dx2 Dxa-D11 Dlx-Dlo Dj0-Da Dg-Ds Ds~D7 D7-De De-D5 Ds-D4 D4-D3 Da-D2 Dz-D1 Dl-cathode
1 700 counts/s 281 V 226 230 164 165 150 120 92 87 85 80 80 80 95 282
56 000 counts/s 161 V 233 242 176 175 159 128 99 93 91 86 86 86 102 300
High current chain 1 700 counts/s 300 V 232 242 174 168 170
123 95 88 88 80 76 78 89 283
56 000 counts/s 251 V 232 245 178 172 174 126 97 90 90 82 78 80 91 290
When VR tubes are connected across the last two stages of PM chain 17 000 counts/s 300 V 255 246 177 171 173 125 97 89
56 000 counts/s 300 V 255 246 177 171 173 125 97 89
89
89
81 77 79 90 285
81 77 79 90 285
354
A. K. G U P T A
°
D
4~
A
V
a
_C." ~
B
(a):
42
0-
~
3a
H E
r 34/ O
--
,
F = I
2
G
= 2---o-------0----~ I
I 4
A
.
,[
:
.
. (b)
Y I
6 Time
I
l
8 (hr)
I
, 10
I
i
12
Fig. 3. Variation in photomultiplier gain following counting rate change when voltage regulating tubes are connected across (a) last stage of photomultiplier chain; (b) last two stages of the photomultiplier chain. A = 2 000 counts/s E = 1 600 counts/s (a) B = 16 000 counts/s (b) F = 13 000 counts/s C = 35 000 counts/s G = 33 000 counts/s D = 67 000 counts/s H = 65 000 counts/s voltage regulating tubes in the last two stages also helped in maintaining the energy resolution at a b o u t 14%. Concluding remarks: At high counting rates, the n u m b e r o f electrons participating in the multiplication process in the last two stages of a high gain photomultiplier tube like 56 A V P becomes extremely large. It is quite probable that a substantial n u m b e r o f them may flow t h r o u g h the external chain resistor and get collected at the positive terminal of the ht. Thus there will be some effective current that will flow through a
AND
N. N A T H
particular chain resistor. Variation in the counting rates m a y change this effective current. By keeping the potentials stabilized for the last two stages which contributed significantly to the observed gain shift, one can obtain stable conditions for the operation o f the photomultiplier tube. The improvements in the behaviour o f a p h o t o multiplier as reported here can also be obtained by using zener diodes of appropriate values instead o f voltage regulating tubes, if compactness is desired. Further, the zener diodes have the advantage that they can be used in a low-current chain also. In case a negative high voltage supply is available for the photomultiplier chain, one may think o f introducing a separate positive stabilized power supply for the last two stages to achieve somewhat similar results. One o f us (A.K.G.) is thankful to the University Grants Commission, India, for the award o f a research fellowship.
References 1) R. L. Caldwell and S. E. Turner, Nucleonics 12 (1954) 47. 2) p. R. Bell, R. C. Davis and W. Bernstein, Rev. Sci. Instr. 26 (1955) 726. a) R. D. Connor and M. K. Husain, Nucl. Instr. and Meth. 6 (1960) 337. 4) H. Jung, Ph. Panussi and J. J~necke, Nucl. Instr. and Meth. 9 (1960) 121. 5) W. Michaelis, H. Schmidt and C. Weitkamp, Nucl. Instr. and Meth. 21 (1963) 65.
©
NORTH-HOLLAND P U B L I S H I N G CO.
GAIN S T A B I L I T Y I N H I G H - C U R R E N T P H O T O M U L T I P L I E R S AT H I G H VARIABLE C O U N T I N G RATES A. K. GUPTA and N. NATH Department of Physics, Banaras Hindu University, Varanasi, India
Received 25 April 1967 Variations in the gain of 56AVP photomultiplier tubes is studied by changing the counting rates. It is observed that at high counting rates the gain of a photomultiplier tube appreciably increases with increasing counting rate. The energy resolution of the photomultiplier tube is found to deteriorate at high counting rates.
It is found that this gain shift is nearly eliminated by keeping the voltages stabilized for the last two stages of the photomultiplier chain. This also helps in maintaining the energy resolution.
• It is well known t - s ) that Changes in overall gain of a photomultiplier tube occur when the counting rates are changed. Earlier workers have observed this effect using different photomultipliers. In the case of a fast high gain photomultiplier the gain shift observed is significantly large and can cause considerable difficulty in the study of fast decaying radioactive sources or in nuclear reaction studies where total counting rates fluctuate within wide limits. Michealis et a l : ) have studied the energy resolution of the scintillation set-up using RCA6810 photomultiplier tubes. They report a deterioration in resolution as the counting rate is increased. In the present work we have attempted to investigate further the apparent cause of these undesirable effects. We have successfully overcome these limitations through stabilized regulation of dynode potentials by introducing zener semi-conductor diodes or voltage regulating tubes in the photomultiplier chain. Measurements: We have studied this effect mainly for the 56 AVP photomultiplier tube using a NaI(TI) crystal and a strong source of 137Cs. The photomultiplier chain corresponds closely to the suggested "Philips Distribution Type B". The linear output was taken from the 8 th dynode. Standard electronic equipments were used under stable conditions. The counting rate was varied by moving the source backward or forward in front of the scintillator-photomultiplier assembly. Fig. 1 shows the effect of the counting rate on the overall gain of the 56 AVP photomultiplier tube for a photomultiplier chain current of 0.8 mA. It is seen that at low total integral counting rates the gain shift is small. At much higher counting rates, however, the magnitude of the gain shift becomes too large even for a relatively smaller change in the counting rate. When the resistances in the photomultiplier chain
were decreased by a factor of seven while maintaining the dynode potentials very close to the values to obtain the results shown in fig. 1, the gain shift characteristics indicated some improvement as shown in fig. 2. The chain current for this case was 5.8 mA. The laTCs peak 56
52
Chain current
= 0.8mA
48 44
t-
40
.o o 36 ,,t O
o. p-
32
28
24 A 20--'-- ~" 0
B I
2
,
1
4
J
r
6 Time (hr)
L
I
8
I
P
10
~
I
12
Fig. 1. Variation in photomultiplier gain following counting rate change for a chain current of 0.8 mA. A = 1 700 counts/s D = 48 500 counts/s B -- 11 700 counts/s E = 56 400 counts/s C = 33 000 counts/s
352
GAIN STABILITY IN HIGH-CURRENT PHOTOMULTIPLIERS 48 Choin c u r r e n t - 5 . B m A
v
.~_ 44
[
03 0
3e
A F---B:
C, 32
0
I
2
1
4
,
!
~
!
6 8 Time (hr)
i
I
10
12
Fig. 2. Variation in photomultiplier gain following counting rate change for a chain current of 5.8 rnA. A = 1 700 counts/s C = 34 000 counts/s B = 12 700 counts/s D = 56 400 counts/s shifted its position by about 45% for a change in integral counting rate from 1700 counts/see to 56000 counts/see, while at low chain current (0.8 mA) the corresponding gain shift was about 180%. It evidently points out that the .gain shift observed is intimately associated with the relative change in the chain current. However, it may not always be possible to provide very high chain currents to minimise the effect of gain shift with counting rate. It may be mentioned here that these observations have been made using high-value condensers across the last four dynode-ehain resistors. The effect observed here will be much reduced in magnitude for lower gain photomultiplier tubes and it may also vary with the design of the particular tube used.
353
The voltage differences between various dynodes were measured with the help of a VTVM at two extreme counting rates. Table 1 gives the data. It is seen that the voltage across the last stage of the P M chain gets reduced appreciably at high counting rate. In contrast, the voltages across the lower stages are observed to increase, the total voltage across the whole chain remaining constant. In our measurements, the linear pulse output was taken from the 8 th dynode. Since the dynode potentials go up in all the lower stages, we. observe an increase in pulse amplitude with increasing counting rate. At higher chain current (5.8 mA), the voltage differences between various dynodes do vary but the magnitude of the change with the same change in counting rate gets reduced as shown in table 1. The data in table 1 should only be considered as a qualitative indication of the actual dynamic situation inside the photomultiplier tube. Using appropriate voltage regulator tubes across the last stage instead of the chain resistor helped in reducing considerably the gain shift with counting rate (fig. 3a). Slight increase in the photomultiplier gain was still observed at the extreme high counting rate (67000 counts/see). When another voltage regulator tube was used across the last but one stage, the photomultiplier gain was observed to remain substantially constantupto the highest counting rates that we tried (fig. 3b). The energy resolution for the full energy gamma-ray peak was also found to deteriorate at high counting rates. It went down from a figure of 14% to 21% for the two extreme counting rates used. The introduction of
TABLE 1 Voltages across different dynodes. Low current chain Dynode no. ht-Dx4 D14-D13 Dla-Dx2 Dxa-D11 Dlx-Dlo Dj0-Da Dg-Ds Ds~D7 D7-De De-D5 Ds-D4 D4-D3 Da-D2 Dz-D1 Dl-cathode
1 700 counts/s 281 V 226 230 164 165 150 120 92 87 85 80 80 80 95 282
56 000 counts/s 161 V 233 242 176 175 159 128 99 93 91 86 86 86 102 300
High current chain 1 700 counts/s 300 V 232 242 174 168 170
123 95 88 88 80 76 78 89 283
56 000 counts/s 251 V 232 245 178 172 174 126 97 90 90 82 78 80 91 290
When VR tubes are connected across the last two stages of PM chain 17 000 counts/s 300 V 255 246 177 171 173 125 97 89
56 000 counts/s 300 V 255 246 177 171 173 125 97 89
89
89
81 77 79 90 285
81 77 79 90 285
354
A. K. G U P T A
°
D
4~
A
V
a
_C." ~
B
(a):
42
0-
~
3a
H E
r 34/ O
--
,
F = I
2
G
= 2---o-------0----~ I
I 4
A
.
,[
:
.
. (b)
Y I
6 Time
I
l
8 (hr)
I
, 10
I
i
12
Fig. 3. Variation in photomultiplier gain following counting rate change when voltage regulating tubes are connected across (a) last stage of photomultiplier chain; (b) last two stages of the photomultiplier chain. A = 2 000 counts/s E = 1 600 counts/s (a) B = 16 000 counts/s (b) F = 13 000 counts/s C = 35 000 counts/s G = 33 000 counts/s D = 67 000 counts/s H = 65 000 counts/s voltage regulating tubes in the last two stages also helped in maintaining the energy resolution at a b o u t 14%. Concluding remarks: At high counting rates, the n u m b e r o f electrons participating in the multiplication process in the last two stages of a high gain photomultiplier tube like 56 A V P becomes extremely large. It is quite probable that a substantial n u m b e r o f them may flow t h r o u g h the external chain resistor and get collected at the positive terminal of the ht. Thus there will be some effective current that will flow through a
AND
N. N A T H
particular chain resistor. Variation in the counting rates m a y change this effective current. By keeping the potentials stabilized for the last two stages which contributed significantly to the observed gain shift, one can obtain stable conditions for the operation o f the photomultiplier tube. The improvements in the behaviour o f a p h o t o multiplier as reported here can also be obtained by using zener diodes of appropriate values instead o f voltage regulating tubes, if compactness is desired. Further, the zener diodes have the advantage that they can be used in a low-current chain also. In case a negative high voltage supply is available for the photomultiplier chain, one may think o f introducing a separate positive stabilized power supply for the last two stages to achieve somewhat similar results. One o f us (A.K.G.) is thankful to the University Grants Commission, India, for the award o f a research fellowship.
References 1) R. L. Caldwell and S. E. Turner, Nucleonics 12 (1954) 47. 2) p. R. Bell, R. C. Davis and W. Bernstein, Rev. Sci. Instr. 26 (1955) 726. a) R. D. Connor and M. K. Husain, Nucl. Instr. and Meth. 6 (1960) 337. 4) H. Jung, Ph. Panussi and J. J~necke, Nucl. Instr. and Meth. 9 (1960) 121. 5) W. Michaelis, H. Schmidt and C. Weitkamp, Nucl. Instr. and Meth. 21 (1963) 65.