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A high-stability discriminator using a common diode as discriminating element. Application to a multiple channel pulse height analyzer
NUCLEAR
INSTRUMENTS
AND METHODS
60 (I968)
88-92;
©
NORTH-HOLLAND
PUBLISHING
CO.
A H I G H - S T A B I L I T Y D I S C R I M I N A T O R U S I N G A C O M M O N D I O D E AS D I S C R I M I N A T I N G E L E M E N T . A P P L I C A T I O N T O A M U L T I P L E C H A N N E L P U L S E H E I G H T ANALYZER. S. S. KLEIN*, L. H U L S T M A N
and J. BLOK
Natuurkundig Laboratorium der Vrije Universiteit, Amsterdam, Nederland Received 21 November 1967 A discriminator is described in which the decisions depend on the change in differential resistance of a forward biased diode when the bias current is compensated by a current pulse. The sensitivity, in the version described, is about 0.5 V; the dynamic range (depending on the properties of the diode) at least 1 : 50. The stability is about 0.3 m V / ° C . The dead time is 1.4/ts; it may be shortened for suitable input pulses.
Several discriminators of this type have been combined into a multiple channel pulse height analyzer with an anticoincidence input for gating purposes. This apparatus was experimentally tested in an e-7 angular correlation experiment. Multiple channel systems are compared with multiple detector systems. Where possible the former should be used.
1. Introduction
error voltages are relatively low when a diode is used as a current biased parallel switch, it seems attractive to use such a switch in the feedback loop of a biased regenerative amplifier to determine its discriminating point.
Semiconductor pulse height discriminators have been often described ~-6). Circuits depending on the switching of transistors from the nonconducting to the conducting state 1'2) are rather insensitive compared with the maximum bias which is limited by the maxim u m permissible base-emitter reverse voltage. For this reason such discriminators are generally preceded by biased amplifiers. In both discriminator and biased amplifier temperature effects occur. These are not easy to compensate because the transistors are in very different situations and not necessarily at the same temperature. Therefore it would be attractive to use a single, inherently more stable discriminating element. The use of tunnel diodes has been advocated3-6). The published circuits still suffer from large relative temperature coefficients a' 4) or a low sensitivity 5' 6). The most sensitive circuits use a tunnel diode between the collectors of a differential amplifier short circuiting the output pulse until the diode switches. The gain in sensitivity thus goes with loss in stability from temperature effects in the transistors. As it may be shown 7) that
2. Basic circuit
The basic circuit is shown in fig. 1. It is a semiconductor version of the trigger described by KandiahS). As long as the diode remains forward biased, the multivibrator is stable, because the feedback is small. As soon as the diode becomes reverse biased, i.e. /pulse --/bias becomes positive, the feedback gain becomes larger than one. The multivibrator now performs an oscillation. The length of this cycle is determined by C ( R c + Rb). It should not terminate too long before /pulse becomes equal to /bias again, otherwise multiple output pulses will be generated. As in the quiescent state all parts of the circuit are in low-impedance states, the circuit should not be sensitive to high counting rates. The current pulse may be provided by applying a voltage pulse through a large resistance. If a higher sensitivity is necessary, it may be necessary to use a transistor or even an operational amplifier as voltage to current converter. Many pulses, however, are originally delivered as current pulses, e.g. at photomultiplier anodes. In the circuit described in the next section, the principles outlined above were put to the test. To ensure temperature stability of the currentthrough the discriminating diode it was capacitatively coupled to the remainder of the circuit. This may cause some inconvenience at high counting rates. (It is possible to eliminate at least one of the capacitative links, viz. from the diode to the base of one transistor, by using low leakage current silicon transistors.) To avoid all diffi* Present address: Technische Hogeschool, Eindhoven.
Fig. 1. Basic circuit of the parallel diode discriminator.
88
A HIGH-STABILITY
89
DISCRIMINATOR
TABLE 1 Properties o f s o m e discriminator circuits described in literature. Author
Discriminating element
Discrimination limits
Dynamic range
T e m p e r a t u r e coefficients
Recovery time
(/Ls) Lower
Upper
Absolute
Relative to upper limit
(V)
(V)
(mV/°C)
(%/°C)
(%/°C)
0.6 0.2 0.5 1 0.5 ~ 0.5 +
0.03 0.25 0.05 0.01 0.005 0.008
0.4 6 5 0.2 0.12 1.0
034 ~ 0.1 < 1 10 * 5.6 t 0.1
0.3
0.001
0.06
1.4§
Van H e e k 1, 5) H v a m a) Sold 4) M a n t a k a s ~) D i a m o n d 6) Righini 9)
Transistor Tunneldiode Tunneldiode Tunneldiode Tunneldiode Tunneldiode
0.14 0.003 0.01 0.5 0.4 0.4
2.14 0.080 1.0 I0 10 6.5
l : 15 1:27 1 : 100 1:20 l : 25 1 : 16
Present work
Diode
0.5
27
1 : 54
* ? + §
Relative to lower limit
0.5% t h r e s h o l d variation. Fixed dead time. T e m p e r a t u r e coefficient n o t constant. 10% threshold variation.
culties associated with the stability of voltage-to-current converters of the active type, we used only high stability resistance networks between pulse and bias voltage sources and discriminator current input. Nevertheless, the sensitivity of our discriminator compares favourably with the more stable tunnel diode discriminators. Its stability is certainly superior, as may be seen from table 1. No attempt was made to attain an especially short recovery time. In some preliminary experiments it proved easy, however, to shorten the pulse width if necessary.
3. Circuit description 3.1. DISCRIMINATOR AND PULSE SHAPER
The diagrams of the discriminator and the pulse
shaper are shown in fig. 2. The circuit was used for positive pulses. The feedback loop of the multivibrator formed by T 1 and T 2 is capacitatively coupled to the discriminating diode D2, which is forward biased by a small current through R a. As soon as this negative current is compensated by a positive current the multivibrator will start; at the end of the multivibrator pulse D 2 will become conducting again, and a single pulse will be delivered unless the current through R 4 is still sufficiently positive to restart it after 1.4 #s. This may be avoided by using short pulses or by inserting the capacitor C 1. The voltage pulse at the input is converted to a current pulse by R1. Da and R2, together with the adjustable bias voltage (indicated by BIAS in fig. 2) form a current-biased switch which prevents P to become positive until the current through OUT
4x AF118
-12V ( C1 = = 120
R3 47KL~J 12K[J
R,
2K7
03
OA
R2 1K-~
L,J10K I
100~ ~ t70
/10.F ~ ';
q Ag0 J 2K2L,_J150L. 10K[
I 1 ?11,,
]'4 T2
Fig. 2. Discrimininator a n d pulse shaper.
T3
4K7~ 0V
90
s.s. KLEIN et al.
AFll8 AF102
2xAF 118
t tlOK i 1Kt {L~ 2~nFl ~_~(~7
T5 T6
T7
AFI02
MM1711
-12V
2k2t t 100 +7Voutput 12'/[~--'~ lOlOnF
'0Ki-120Kt
Tg
T9
TI0
0v
Fig. 3. Anticoincidence and output circuits. R 1 and D 3 exceeds the bias current. Therefore the value of the bias voltage determines the pulse height necessary to reverse the current through D 1. A very small excess over this pulse height causes D 2 to change also from the conducting to the nonconducting state and the multivibrator starts. The advantage of this modification is mainly that the working point of T 1 is not changed when the bias current is altered. D 3 was inserted to diminish the influence of the multivibrator pulse at Q on the input pulse and makes it possible to feed several discriminator units with small bias differences from the same source. The univibrator formed by T 3 and T 4 delivers a uniform output pulse, the length of which is adjustable with R 5.
3.2. ANTICO1NCIDENCE AND OUTPUT CIRCUIT Fig. 3 shows the scheme of the remaining basic unit of a differential discriminator. T 5 and T6 form a switch. When no negative pulses are present at A or B, T 6 conducts and a positive pulse at C is transmitted to its anti coincidence
sigoa,
r--
CH
O+
output
.....
C
c A+U+O
,b r-
A÷U÷0
71
-~
"-
Fig.4. Combination of the units described a multiple channel.
in figs. 2 and 3 into
collector. This pulse starts the output pulse shaper (Tv and Ts). The output amplifier (Y 9 and Txo ) delivers a 7 V positive output and a 2 V negative output. When A or B receives a negative pulse, T 6 is Cut Off and a negligible negative pulse appears at its collector, its quiescent current being very small. A negative pulse at C gives a similar result. 4. Combination of the basic units into a multiple differential discriminator The use of the described units is shown in fig. 4. The C inputs are coupled with small capacitors to the discriminator units that define lower channel edges. The back edges of the negative output pulses provide the short positive pulses that trigger the output pulse shapers. The B inputs are connected to the upper level discriminators. A variable time constant integrator (R6, C2; fig. 3) may be used to lengthen the pulse from this discriminator until its dead time has elapsed. In this way the upper channel edges remain well defined at higher counting rates. The A inputs are dc coupled. They may be used as inputs for gating pulses, e.g. when experiments with short half life sources produced intermittently are performed. Another application is to use it as input for the pulses from an anti pile up gate. For this aim a delayed pulse of the desired length is sent to A after every input pulse. 5. Preliminary and experimental tests The properties of the discriminator have been measured using a mercury-wetted relay precision pulse generator for the stability tests and a Berkeley double pulse generator for the determination of the dead time. The sensitivity is about 0.5 V. The maximum bias corresponds to an input pulse height of about 27 V.
91
A HIGH-STABILITY DISCRIMINATOR
This value is determined mainly by the m a x i m u m permissible current through D 1 and may be increased by adding diodes or selecting a type with a higher maxim u m current. The dynamic range is, therefore, at least 1:50. The temperature dependence of the threshold is 0.3 ___0.2 mV per °C; the inaccuracy is caused mainly by the minimum variation of the input pulse height. The recovery time after a pulse had passed the threshold (defined as the interval after which the discriminator responds again to a pulse 10% above threshold) is 1.4/~s, almost independent of the overload factor for pulses shorter than half the recovery time. By diminishing the RC time of the coupling between the collector of T t and the base of T2 it was possible to shorten the recovery time to 0.7/ts. When a 33 pF capacitance was put in parallel to the emitter resistance of Tj it was possible to shorten it still further to 0.3/~s. We are, however, not sure if the excellent stability characteristics given are still valid in the latter case, as the loop gain of the multivibrator is made less stable by eliminatingthe feedback action of the emitter resistance ofT1. A quadruple channel as described in section 4 has been tested experimentally in an e-7 angular correlation experiment 7) in which it was required to separate the correlations corresponding to the parts of a composite c~ line (fig. 5). The block scheme of the experimental
setup is given in fig. 6. During these measurements a drift of 1 keV out of the 1000 keV selected by the biased amplifier could be easily observed during the coincidence runs by monitoring the counting rates in adjacent channels over the composite e line. Such drifts were observed only over periods of hours, and were probably temperature effects on the detector, as was deduced from the temperature coefficient I 0). The stability ot the entire setup was of the order of 2-3 keV over periods of 48 h. 6. Multiple channel vs multiple detector
In our experiments we profited from the possibility to do several angular correlation experiments at the same time by gating the coincident g a m m a pulses by the outputs of the multiple channel. This caused a gain in coincidence counting rate; a similar gain could also have been attained by using several detectors. Whenever the level scheme of the nuclides investigated makes it possible, one should choose the first method, however. Its advantages are: a. In the investigation of a composite spectral line some systematic errors may be avoided because at any time the same discriminator defines the coincident edges of adjacent channels; b. No differences between detectors are introduced; c. Mechanical simplicity: there are no unnecessary (~'328
C['/.92
Ct/,72
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I,.,-
z
2
o¢ J I.L
10 2
m
5
O n,," '"
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~E
"'_~....'" • . ._..=i:.'_2 ".2"'""- . . . . . . - .. ..... ".
Z
T I
200
I 250
I
I 300
" CHANNEL NUMBER Fig. 5. Spectrum of the c~ transitions to the second, third and fourth excited states of 2°ST1. The subscript of c¢ indicates the energy of the excited state. The four channels of the pulse height analyzer covered ~3e8, the upper and lower part of the composite ~492/c~47~ peak and the part of the background containing ~61~ (not shown).
92
s . s . KLEIN et al. Source
I + ~,Prearnp~
Linear
g
r'7 U Fa , C.,.c,deo0o ~
I
/
Biased
"~fast
A DC
I /
512 Channet
J
I_
detayed coinc
[-
25ns I
Anatyzer
Amp
I Stow Coincidence l~s
t 27
~1 V C[2 V a 3 V CK,.
(Zuadrup[e
Channet
=" 2°..26
E
%~
Mi,er
V
~'~i
2B
V C~
"oo,i,,,
Fig. 6. Block scheme of the ~-y angular correlation experiment, at, ~2, ~a and ~4 represent pulses from the respective outputs of the quadruple channel. The symbol V denotes a logic OR. constraints on the m o v e m e n t s ot the detectors; d. Simplified electronic circuitry: only one linear amplifier a n d one fast coincidence circuit are necessary; in f a v o u r a b l e cases the n u m b e r o f d i s c r i m i n a t o r s is halved. T o the c o n t r a r y , with d e t e c t o r arrays, a. the a m o u n t o f d a t a t h a t m a y be o b t a i n e d at the same time is n o t limited by the n u m b e r o f interesting lines in the level scheme u n d e r s t u d y ; b. with several detectors in each o f the channels, the a m o u n t of d a t a o b t a i n e d goes r o u g h l y as the p r o d u c t instead o f the s u m o f d e t e c t o r n u m b e r s ; c. the m e a s u r i n g t i m e m a y be s h o r t e n e d for t h e m o r e intense lines a n d lengthened for the less intense ones. W i t h suitable level schemes the m u l t i c h a n n e l system offers m o r e possibilities with less expenditure. Therefore, it should always be considered to enhance coincidence c o u n t i n g rates first by this m e t h o d . Also when d a t a collecting rates are to be o p t i m i z e d the c o m b i n a t i o n o f b o t h m e t h o d s should be considered.
7. Conclusion The d i s c r i m i n a t o r described is very stable a n d has a long d y n a m i c range. C o n t i n u e d s t u d y will, however, be necessary to extend its possibilities with respect to higher sensitivity a n d s h o r t e r recovery time. Several d i s c r i m i n a t o r units have been c o m b i n e d into a multiple
channel pulse height analyzer for experiments on c o m posite spectral lines with the aid o f a r o u t i n g system. The value o f such an i n s t r u m e n t was p r o v e d in an a-7 a n g u l a r c o r r e l a t i o n e x p e r i m e n t involving several a-lines. The systematic accuracy of the system is higher t h a n t h a t o f a c o m b i n a t i o n o f single channel analyzers. The a u t h o r s t h a n k mr. A. R a d d e r for his help in c o n s t r u c t i n g some circuits, mr. A. P o m p e r and miss M. G r u y t e r s for their assistance in the p r e p a r a t i o n o f the figures a n d miss E. H o e n for the t y p i n g of the manuscript. Dr. J. W. Broer a n d Mr. W. Spierdijk we t h a n k for some valuable suggestions.
References 1) K. H. van Heek, H. D. Schilling and L. Wallek, Nucl. Instr. and Meth. 29 (1964) 35. 2) H. Verweij, Nucl. Instr. and Meth. 10 (1961) 308. 3) T. Hvam and M. Smedsdal, Nucl. Instr. and Met h. 24 (1963) 55. 4) U. Sold and S. Brojdo, Nucl. Instr. and Meth. 26 (1964) 147. ~) Ch. Mantakas, Nucl. Instr. and Meth. 29 (1964) 177. 6) j. M. Diamond, Nucl. Instr. and Meth. 36 (1965) 293. 7) S. S. Klein, Thesis (Vrije Universiteit, Amsterdam, 1966). 8) K. Kandiah, Proc. IEE 101, pt I1 (1954) 239. u) B. Righini, Nucl. Instr. and Meth. 29 (1964) 89. 10) S. S. Klein, L. Hulstman and J. Blok, Nucl. Instr. and Meth. 54 (1967) 190.
INSTRUMENTS
AND METHODS
60 (I968)
88-92;
©
NORTH-HOLLAND
PUBLISHING
CO.
A H I G H - S T A B I L I T Y D I S C R I M I N A T O R U S I N G A C O M M O N D I O D E AS D I S C R I M I N A T I N G E L E M E N T . A P P L I C A T I O N T O A M U L T I P L E C H A N N E L P U L S E H E I G H T ANALYZER. S. S. KLEIN*, L. H U L S T M A N
and J. BLOK
Natuurkundig Laboratorium der Vrije Universiteit, Amsterdam, Nederland Received 21 November 1967 A discriminator is described in which the decisions depend on the change in differential resistance of a forward biased diode when the bias current is compensated by a current pulse. The sensitivity, in the version described, is about 0.5 V; the dynamic range (depending on the properties of the diode) at least 1 : 50. The stability is about 0.3 m V / ° C . The dead time is 1.4/ts; it may be shortened for suitable input pulses.
Several discriminators of this type have been combined into a multiple channel pulse height analyzer with an anticoincidence input for gating purposes. This apparatus was experimentally tested in an e-7 angular correlation experiment. Multiple channel systems are compared with multiple detector systems. Where possible the former should be used.
1. Introduction
error voltages are relatively low when a diode is used as a current biased parallel switch, it seems attractive to use such a switch in the feedback loop of a biased regenerative amplifier to determine its discriminating point.
Semiconductor pulse height discriminators have been often described ~-6). Circuits depending on the switching of transistors from the nonconducting to the conducting state 1'2) are rather insensitive compared with the maximum bias which is limited by the maxim u m permissible base-emitter reverse voltage. For this reason such discriminators are generally preceded by biased amplifiers. In both discriminator and biased amplifier temperature effects occur. These are not easy to compensate because the transistors are in very different situations and not necessarily at the same temperature. Therefore it would be attractive to use a single, inherently more stable discriminating element. The use of tunnel diodes has been advocated3-6). The published circuits still suffer from large relative temperature coefficients a' 4) or a low sensitivity 5' 6). The most sensitive circuits use a tunnel diode between the collectors of a differential amplifier short circuiting the output pulse until the diode switches. The gain in sensitivity thus goes with loss in stability from temperature effects in the transistors. As it may be shown 7) that
2. Basic circuit
The basic circuit is shown in fig. 1. It is a semiconductor version of the trigger described by KandiahS). As long as the diode remains forward biased, the multivibrator is stable, because the feedback is small. As soon as the diode becomes reverse biased, i.e. /pulse --/bias becomes positive, the feedback gain becomes larger than one. The multivibrator now performs an oscillation. The length of this cycle is determined by C ( R c + Rb). It should not terminate too long before /pulse becomes equal to /bias again, otherwise multiple output pulses will be generated. As in the quiescent state all parts of the circuit are in low-impedance states, the circuit should not be sensitive to high counting rates. The current pulse may be provided by applying a voltage pulse through a large resistance. If a higher sensitivity is necessary, it may be necessary to use a transistor or even an operational amplifier as voltage to current converter. Many pulses, however, are originally delivered as current pulses, e.g. at photomultiplier anodes. In the circuit described in the next section, the principles outlined above were put to the test. To ensure temperature stability of the currentthrough the discriminating diode it was capacitatively coupled to the remainder of the circuit. This may cause some inconvenience at high counting rates. (It is possible to eliminate at least one of the capacitative links, viz. from the diode to the base of one transistor, by using low leakage current silicon transistors.) To avoid all diffi* Present address: Technische Hogeschool, Eindhoven.
Fig. 1. Basic circuit of the parallel diode discriminator.
88
A HIGH-STABILITY
89
DISCRIMINATOR
TABLE 1 Properties o f s o m e discriminator circuits described in literature. Author
Discriminating element
Discrimination limits
Dynamic range
T e m p e r a t u r e coefficients
Recovery time
(/Ls) Lower
Upper
Absolute
Relative to upper limit
(V)
(V)
(mV/°C)
(%/°C)
(%/°C)
0.6 0.2 0.5 1 0.5 ~ 0.5 +
0.03 0.25 0.05 0.01 0.005 0.008
0.4 6 5 0.2 0.12 1.0
034 ~ 0.1 < 1 10 * 5.6 t 0.1
0.3
0.001
0.06
1.4§
Van H e e k 1, 5) H v a m a) Sold 4) M a n t a k a s ~) D i a m o n d 6) Righini 9)
Transistor Tunneldiode Tunneldiode Tunneldiode Tunneldiode Tunneldiode
0.14 0.003 0.01 0.5 0.4 0.4
2.14 0.080 1.0 I0 10 6.5
l : 15 1:27 1 : 100 1:20 l : 25 1 : 16
Present work
Diode
0.5
27
1 : 54
* ? + §
Relative to lower limit
0.5% t h r e s h o l d variation. Fixed dead time. T e m p e r a t u r e coefficient n o t constant. 10% threshold variation.
culties associated with the stability of voltage-to-current converters of the active type, we used only high stability resistance networks between pulse and bias voltage sources and discriminator current input. Nevertheless, the sensitivity of our discriminator compares favourably with the more stable tunnel diode discriminators. Its stability is certainly superior, as may be seen from table 1. No attempt was made to attain an especially short recovery time. In some preliminary experiments it proved easy, however, to shorten the pulse width if necessary.
3. Circuit description 3.1. DISCRIMINATOR AND PULSE SHAPER
The diagrams of the discriminator and the pulse
shaper are shown in fig. 2. The circuit was used for positive pulses. The feedback loop of the multivibrator formed by T 1 and T 2 is capacitatively coupled to the discriminating diode D2, which is forward biased by a small current through R a. As soon as this negative current is compensated by a positive current the multivibrator will start; at the end of the multivibrator pulse D 2 will become conducting again, and a single pulse will be delivered unless the current through R 4 is still sufficiently positive to restart it after 1.4 #s. This may be avoided by using short pulses or by inserting the capacitor C 1. The voltage pulse at the input is converted to a current pulse by R1. Da and R2, together with the adjustable bias voltage (indicated by BIAS in fig. 2) form a current-biased switch which prevents P to become positive until the current through OUT
4x AF118
-12V ( C1 = = 120
R3 47KL~J 12K[J
R,
2K7
03
OA
R2 1K-~
L,J10K I
100~ ~ t70
/10.F ~ ';
q Ag0 J 2K2L,_J150L. 10K[
I 1 ?11,,
]'4 T2
Fig. 2. Discrimininator a n d pulse shaper.
T3
4K7~ 0V
90
s.s. KLEIN et al.
AFll8 AF102
2xAF 118
t tlOK i 1Kt {L~ 2~nFl ~_~(~7
T5 T6
T7
AFI02
MM1711
-12V
2k2t t 100 +7Voutput 12'/[~--'~ lOlOnF
'0Ki-120Kt
Tg
T9
TI0
0v
Fig. 3. Anticoincidence and output circuits. R 1 and D 3 exceeds the bias current. Therefore the value of the bias voltage determines the pulse height necessary to reverse the current through D 1. A very small excess over this pulse height causes D 2 to change also from the conducting to the nonconducting state and the multivibrator starts. The advantage of this modification is mainly that the working point of T 1 is not changed when the bias current is altered. D 3 was inserted to diminish the influence of the multivibrator pulse at Q on the input pulse and makes it possible to feed several discriminator units with small bias differences from the same source. The univibrator formed by T 3 and T 4 delivers a uniform output pulse, the length of which is adjustable with R 5.
3.2. ANTICO1NCIDENCE AND OUTPUT CIRCUIT Fig. 3 shows the scheme of the remaining basic unit of a differential discriminator. T 5 and T6 form a switch. When no negative pulses are present at A or B, T 6 conducts and a positive pulse at C is transmitted to its anti coincidence
sigoa,
r--
CH
O+
output
.....
C
c A+U+O
,b r-
A÷U÷0
71
-~
"-
Fig.4. Combination of the units described a multiple channel.
in figs. 2 and 3 into
collector. This pulse starts the output pulse shaper (Tv and Ts). The output amplifier (Y 9 and Txo ) delivers a 7 V positive output and a 2 V negative output. When A or B receives a negative pulse, T 6 is Cut Off and a negligible negative pulse appears at its collector, its quiescent current being very small. A negative pulse at C gives a similar result. 4. Combination of the basic units into a multiple differential discriminator The use of the described units is shown in fig. 4. The C inputs are coupled with small capacitors to the discriminator units that define lower channel edges. The back edges of the negative output pulses provide the short positive pulses that trigger the output pulse shapers. The B inputs are connected to the upper level discriminators. A variable time constant integrator (R6, C2; fig. 3) may be used to lengthen the pulse from this discriminator until its dead time has elapsed. In this way the upper channel edges remain well defined at higher counting rates. The A inputs are dc coupled. They may be used as inputs for gating pulses, e.g. when experiments with short half life sources produced intermittently are performed. Another application is to use it as input for the pulses from an anti pile up gate. For this aim a delayed pulse of the desired length is sent to A after every input pulse. 5. Preliminary and experimental tests The properties of the discriminator have been measured using a mercury-wetted relay precision pulse generator for the stability tests and a Berkeley double pulse generator for the determination of the dead time. The sensitivity is about 0.5 V. The maximum bias corresponds to an input pulse height of about 27 V.
91
A HIGH-STABILITY DISCRIMINATOR
This value is determined mainly by the m a x i m u m permissible current through D 1 and may be increased by adding diodes or selecting a type with a higher maxim u m current. The dynamic range is, therefore, at least 1:50. The temperature dependence of the threshold is 0.3 ___0.2 mV per °C; the inaccuracy is caused mainly by the minimum variation of the input pulse height. The recovery time after a pulse had passed the threshold (defined as the interval after which the discriminator responds again to a pulse 10% above threshold) is 1.4/~s, almost independent of the overload factor for pulses shorter than half the recovery time. By diminishing the RC time of the coupling between the collector of T t and the base of T2 it was possible to shorten the recovery time to 0.7/ts. When a 33 pF capacitance was put in parallel to the emitter resistance of Tj it was possible to shorten it still further to 0.3/~s. We are, however, not sure if the excellent stability characteristics given are still valid in the latter case, as the loop gain of the multivibrator is made less stable by eliminatingthe feedback action of the emitter resistance ofT1. A quadruple channel as described in section 4 has been tested experimentally in an e-7 angular correlation experiment 7) in which it was required to separate the correlations corresponding to the parts of a composite c~ line (fig. 5). The block scheme of the experimental
setup is given in fig. 6. During these measurements a drift of 1 keV out of the 1000 keV selected by the biased amplifier could be easily observed during the coincidence runs by monitoring the counting rates in adjacent channels over the composite e line. Such drifts were observed only over periods of hours, and were probably temperature effects on the detector, as was deduced from the temperature coefficient I 0). The stability ot the entire setup was of the order of 2-3 keV over periods of 48 h. 6. Multiple channel vs multiple detector
In our experiments we profited from the possibility to do several angular correlation experiments at the same time by gating the coincident g a m m a pulses by the outputs of the multiple channel. This caused a gain in coincidence counting rate; a similar gain could also have been attained by using several detectors. Whenever the level scheme of the nuclides investigated makes it possible, one should choose the first method, however. Its advantages are: a. In the investigation of a composite spectral line some systematic errors may be avoided because at any time the same discriminator defines the coincident edges of adjacent channels; b. No differences between detectors are introduced; c. Mechanical simplicity: there are no unnecessary (~'328
C['/.92
Ct/,72
. .''"..
I,.,-
z
2
o¢ J I.L
10 2
m
5
O n,," '"
."...-
~E
"'_~....'" • . ._..=i:.'_2 ".2"'""- . . . . . . - .. ..... ".
Z
T I
200
I 250
I
I 300
" CHANNEL NUMBER Fig. 5. Spectrum of the c~ transitions to the second, third and fourth excited states of 2°ST1. The subscript of c¢ indicates the energy of the excited state. The four channels of the pulse height analyzer covered ~3e8, the upper and lower part of the composite ~492/c~47~ peak and the part of the background containing ~61~ (not shown).
92
s . s . KLEIN et al. Source
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/
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Fig. 6. Block scheme of the ~-y angular correlation experiment, at, ~2, ~a and ~4 represent pulses from the respective outputs of the quadruple channel. The symbol V denotes a logic OR. constraints on the m o v e m e n t s ot the detectors; d. Simplified electronic circuitry: only one linear amplifier a n d one fast coincidence circuit are necessary; in f a v o u r a b l e cases the n u m b e r o f d i s c r i m i n a t o r s is halved. T o the c o n t r a r y , with d e t e c t o r arrays, a. the a m o u n t o f d a t a t h a t m a y be o b t a i n e d at the same time is n o t limited by the n u m b e r o f interesting lines in the level scheme u n d e r s t u d y ; b. with several detectors in each o f the channels, the a m o u n t of d a t a o b t a i n e d goes r o u g h l y as the p r o d u c t instead o f the s u m o f d e t e c t o r n u m b e r s ; c. the m e a s u r i n g t i m e m a y be s h o r t e n e d for t h e m o r e intense lines a n d lengthened for the less intense ones. W i t h suitable level schemes the m u l t i c h a n n e l system offers m o r e possibilities with less expenditure. Therefore, it should always be considered to enhance coincidence c o u n t i n g rates first by this m e t h o d . Also when d a t a collecting rates are to be o p t i m i z e d the c o m b i n a t i o n o f b o t h m e t h o d s should be considered.
7. Conclusion The d i s c r i m i n a t o r described is very stable a n d has a long d y n a m i c range. C o n t i n u e d s t u d y will, however, be necessary to extend its possibilities with respect to higher sensitivity a n d s h o r t e r recovery time. Several d i s c r i m i n a t o r units have been c o m b i n e d into a multiple
channel pulse height analyzer for experiments on c o m posite spectral lines with the aid o f a r o u t i n g system. The value o f such an i n s t r u m e n t was p r o v e d in an a-7 a n g u l a r c o r r e l a t i o n e x p e r i m e n t involving several a-lines. The systematic accuracy of the system is higher t h a n t h a t o f a c o m b i n a t i o n o f single channel analyzers. The a u t h o r s t h a n k mr. A. R a d d e r for his help in c o n s t r u c t i n g some circuits, mr. A. P o m p e r and miss M. G r u y t e r s for their assistance in the p r e p a r a t i o n o f the figures a n d miss E. H o e n for the t y p i n g of the manuscript. Dr. J. W. Broer a n d Mr. W. Spierdijk we t h a n k for some valuable suggestions.
References 1) K. H. van Heek, H. D. Schilling and L. Wallek, Nucl. Instr. and Meth. 29 (1964) 35. 2) H. Verweij, Nucl. Instr. and Meth. 10 (1961) 308. 3) T. Hvam and M. Smedsdal, Nucl. Instr. and Met h. 24 (1963) 55. 4) U. Sold and S. Brojdo, Nucl. Instr. and Meth. 26 (1964) 147. ~) Ch. Mantakas, Nucl. Instr. and Meth. 29 (1964) 177. 6) j. M. Diamond, Nucl. Instr. and Meth. 36 (1965) 293. 7) S. S. Klein, Thesis (Vrije Universiteit, Amsterdam, 1966). 8) K. Kandiah, Proc. IEE 101, pt I1 (1954) 239. u) B. Righini, Nucl. Instr. and Meth. 29 (1964) 89. 10) S. S. Klein, L. Hulstman and J. Blok, Nucl. Instr. and Meth. 54 (1967) 190.