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A simple time-to-amplitude converter and a quadrupole coincidence unit based on integrated-circuits for positron annihilation studies
NUCLEAR INSTRUMENTS AND METHODS I08
(I973) 253-255; © NORTH-HOLLAND PUBLISHING CO.
A SIMPLE TIME-TO-AMPLITUDE CONVERTER AND A QUADRUPOLE COINCIDENCE UNIT BASED ON INTEGRATED-CIRCUITS FOR POSITRON ANNIHILATION STUDIES* J. B. WANG, S.Y. CHUANG and S.J. TAO The New England Institute, Ridgefield, Connecticut 06877, U.S.A.
and A. OGATA Institute of Plasma Physics, Nagoya University, Japan
Received 11 December 1972 A square pulse generator constructed from a fast integratedcircuit MC 1661L and a fast discriminator constructed from a tunnel diode form the basic building blocks of the delayed coincidence unit for the time-to-amplitude converter. The integrated-circuit MC 1661L is also used for the quadrupole coincidence unit. The delayed coincidence unit is used to measure
the mean lives of positron annihilation. The quadrupole coincidence unit is used to measure the angular correlation of the two-gamma annihilation of the positron--electron pair. The specific features of the circuits are simple, fast, stable and moderate in cost.
1. Introduction
the other fundamental block. Based u p o n these two units, one can easily assemble either a T A C system for measuring the mean lives o f positron annihilation or a t w o - g a m m a angular correlation coincidence system for determining the energy o f the annihilating p o s i t r o n electron pair, or both6). Certainly, they can also be used for other systems of a similar nature. The advantages of these circuits are (1) ease o f construction and moderate cost, (2) fast time response and stable performance, and (3) coaxial cables o f manageable length used for matching time-delays.
The determination o f positron annihilation mean lives requires a delayed coincidence circuit o f fast response and fine resolution. The time interval between the two-coincidence pulses is converted into pulse height, which is analyzed and recorded in a multichannel analyzer. There is also a gate pulse that selects the true events defined by the energy o f the gamma-rays, or the pulse heights o f the outputs o f the photomultipliers. This is the standard technique for constructing a so-called " T i m e - t o - A m p l i t u d e C o n v e r t e r " (TAC) system1). In general, two principles are used in the construction of a TAC, (1) start and stop and (2) pulse overlap. The circuits based u p o n the start and stop principle do not require well defined square pulses as inputs; it is, however, more difficult to obtain a g o o d linear response o f amplitude f r o m the time duration. On the other hand, the circuits based u p o n the overlap principle require fast and stable square pulse generators. Recently, integrated circuits have been used to construct T A C systems2). We have constructed and tested a simple square pulse generator using the M E C L I I I integrated circuits o f Motorola3). This circuit, based on the same principle suggested by Motorola4), is used as one o f the building blocks o f the TAC. A tunnel diode fast discriminator circuit, similar to the timing unit reported previously5), is used as * Work supported by USAEC Contract No. AT (11-1) 3373.
2. Description
2.1. SQUARE PULSE GENERATOR The integrated circuit M C 1661L is used to construct ..J--
1°0t •
Delay
r
II I
/-
150 IL
! !
,s
250 -52V
-2V
Fig. 1. Schematic of the square pulse generator. 253
254
J.B.
W A N G et al.
the square pulse generator. MC 1661L is a dual 4-input gate similar to MC I023L 7) but with faster risetime and response time. The risetime and response time are in the vicinity of 1 ns. The schematic of the square pulse generator is shown in fig. I. Only two of the inputs of each of the dual-gate are used, while the remaining two are clamped at the lower voltage state, - 2 V nominally. Normally, all the inputs of the first gate are at the lower voltage state. The A N D output will be at the lower voltage state and the N A N D output will be at the higher voltage state, - 0 . 8 V. Since the N A N D output is connected directly to one of the inputs in the second gate, normally the A N D output of the second gate, which is also the output of the square pulse generator, will be at the higher voltage state and the N A N D at the lower voltage state. When a positive pulse of the order to 500 mV and 2 ns duration is applied at the input, the state of the first gate changes. This precipitates a change of state of the second gate, resulting in a negative pulse at the A N D terminal of the output stage. Since the N A N D output, which is at the higher voltage state now, is fed back to the first stage, this transient state will remain even when the original input to the first gate is removed. The return to the normal state is triggered by the arrival of the A N D pulse of the first gate at the input of the second gate. Once this pulse is sensed, the whole unit returns to the normal state. Since the arrival of the pulse depends upon the length of the delay cable, one can control the width of the output pulse. As is shown in fig. 1, the output of the unit is a negative square pulse with an amplitude of about 0.8 V and a width equal to the time delay of the delay line. It is obvious from the discussion that a second pulse closely following the original input will not affect the unit, if the time separation between these two pulses is less than the time of the delay of the delay line. 2.2. FAST DISCRIMINATOR The circuit of the fast discriminator is similar to the tunnel diode part of the timing unit used by Garvey 8) and usS). The control range of the bias level provided by changing the current flow to the tunnel diode is not much, less than 100 mV. However, this can be augmented by using attenuators before the input to the tunnel-diode discriminator. When BNC attenuators with 50~ termination are available, the adjustment of a proper bias level for the discriminator is not a very difficult problem. Due to the intrinsic properties of the tunnel diode, this discriminator circuit is very fast; delay time less than 1 ns and pulse width less than 4 ns can be easily
achieved. If a change of the polarity of the output pulse is desired, a simple transformer can be used as the coupling element. This discriminator is quite adequate for the purpose of defining the energy range, or the range of the amplitude of the output of the scintillation counters, produced by the gamma-rays in positron annihilation experiments, or for experiments where the calibration of the discriminator level is infrequent. 2.3. TAC SYSTEM A block diagram of the TAC system is shown in fig. 2. Three square pulse generators are used, two for the "start" and " s t o p " pulses and one for the gate pulse. The coincidence unit is comprised of a half of MC 1661L. The output of the coincidence unit is integrated and shaped to produce a negative output pulse with risetime of ~ I/~s and decay time of ~ 3 #s, a pulse acceptable to most commercial multichannel analyzers. A simple integrator circuit is shown in fig. 3. If the width of the square pulse generators of the "start" and " s t o p " pulses is 40 ns and that of the gate pulse, 50 ns, the output of the TAC system as shown in figs. 2 and 3 has a linear range of about 32 ns. The machine resolution is less than 50 ps. The intrinsic time-walk of the TAC system is about 200 ps/V, just about the triggering level of the square pulse generator. There are several methods available to reduce the time-walk. The simplest one is to use a zero crossover timing unit such as the one used by us 5)
From
,
, MCA
Fig. 2. The block diagram of the time-to-amplitude converter. P-M, Photomultiplier; T, timing unit; G, square pulse generator; D, delay; C, coincidence unit; I, integrator and pulse shaper; MCA, rnultichannel analyser.
4 71~
I[
.O01.J_47K~;
IK
82K IK
i
18-~K " 01 ---tl
_[L
1OK:
'390 2N706
2N706
HEP739
Fig. 3. A simple integrator for TAC.
>
SIMPLE T I M E - T O - A M P L I T U D E CONVERTER
to precede the square pulse generator. Optionally, one can use a constant fraction timing unit 9'1°) to precede the square pulse generator or an additional coincidence unit where the time sequence of start-stop is reversed and the summed output of the two coincicence units is used as the output11). The gate pulse used to define the true events is controlled by discriminators. This construction can be very similar to the one described in the next section. Because fast discriminators are used here, the time match between the gate and the timing pulses is quite simple. The gating of the TAC reduces the pile-up effect considerably over the gating of the multichannel analyzer. 2.4. TWO-GAMMAANGULAR CORRELATION The construction of a coincidence system to record the angular distribution of the two-gamma rays emitted at the annihilation of the positron electron pair is quite straightforward. Normally, no fast electronics are required and a coincidence resolution of about 1 ps is used. Here, we describe a system based upon the discriminators and square pulse generators mentioned above. In addition to being simple and fast, this system has the advantage of having a high resolution. A block diagram of the angular correlation set-up is shown in fig. 4. For such a fast system, plastic scintillators instead of NaI(T1) scintillators are more suitable detectors. In addition, for the large size used in angular correlation work, for example, -21!"diameter and 6" long, plastic
~f
Fig. 4. A system to record the two-gamma angular correlation of the annihilating positron-electron pair. PM, Photomultiplier; A, attenuator; FD, fast discriminator; G, square pulse generator; D, delay; C, quadrupole coincidence unit; ST, Schmidt trigger.
255
scintillators are much cheaper than the NaI(TI) scintillators. While fast photomultipliers must be used instead of the slower ones conventionally used with NaI(TI), the combined cost of scintillators and photomultipliers favors the choice of plastic scintillators and fast photomultipliers. We have found that a reasonably fast counting rate can be obtained by using plastic scintillators of the size 1½" diameter and 6" long with a 22Na source of activity of only 1 mCi and a slot width of 0.8 mrad. Four discriminators are used, two for each scintillation counter. One of the two serves as the upper limit discriminator whose output is fed into the NO input of the quadrupole coincidence unit (cf. fig. 4). The other one serves as the lower limit discriminator, and its output is fed into the YES input of the quadrupole coincidence unit. The pulse width of the square pulse generator for YES input is 8 ns and that for NO input is 16 ns. The YES inputs are delayed 4 ns in respect to the NO inputs to insure the proper action of the coincidence unit. The resolution of the coincidence unit is ~ 12 ns. At such a resolution the true events to random background ratio is found to be about 50:1. The accurate determination of the tail part of the angular correlation curve becomes quite easy because of the low random background. The stability of the whole system has been found surprisingly good; it has remained stable for as long as two months. References 1) A. Ogata, S. J. Tao and J. H. Green, Nucl. Instr. and Meth. 60 (1968) 141. This paper covers many articles before 1968. '~) I. J. Taylor and T. H. Becker, Nucl. Instr. and Meth. 99 (1972) 387. a) Please see data sheets regarding MECL III integrated circuits by Motorola. 4) W. R. Blood, Jr., M E C L system design handbook (Motorola Semiconductor Products Inc., 1971). 5) A. Ogata and S. J. Tao, Nucl. Instr. and Meth. 69 (1969) 344. 6) For general information regarding positron annihilation, please see: V. I. Goldanski, At. Energy Rev. 6 (1968) 3; J. A. Merrigan, J. H. Green and S. J. Tao, Methods o f chemistry, vol. l, part IIID (eds. A. Weisberger and B. W. Rossiter; J. Wiley, New York, 1972) p. 502; and others. 7) A. Barna and E. L. Cisneros, Nucl. Instr. and Meth. 75 (1969) 261. s) j. Garvey, Nucl. Instr. and Meth. 29 (1964) 137. 9) R. R. Fullwood, Nucl. Instr. and Meth. 93 (1971) 235. 10) W. J. McDonald and D. A. Gedcke, Nucl. Instr. and Meth. 55 (1967) 1. 11) p. Thieberger, Nucl. Instr. and Meth. 44 (1966) 349.
(I973) 253-255; © NORTH-HOLLAND PUBLISHING CO.
A SIMPLE TIME-TO-AMPLITUDE CONVERTER AND A QUADRUPOLE COINCIDENCE UNIT BASED ON INTEGRATED-CIRCUITS FOR POSITRON ANNIHILATION STUDIES* J. B. WANG, S.Y. CHUANG and S.J. TAO The New England Institute, Ridgefield, Connecticut 06877, U.S.A.
and A. OGATA Institute of Plasma Physics, Nagoya University, Japan
Received 11 December 1972 A square pulse generator constructed from a fast integratedcircuit MC 1661L and a fast discriminator constructed from a tunnel diode form the basic building blocks of the delayed coincidence unit for the time-to-amplitude converter. The integrated-circuit MC 1661L is also used for the quadrupole coincidence unit. The delayed coincidence unit is used to measure
the mean lives of positron annihilation. The quadrupole coincidence unit is used to measure the angular correlation of the two-gamma annihilation of the positron--electron pair. The specific features of the circuits are simple, fast, stable and moderate in cost.
1. Introduction
the other fundamental block. Based u p o n these two units, one can easily assemble either a T A C system for measuring the mean lives o f positron annihilation or a t w o - g a m m a angular correlation coincidence system for determining the energy o f the annihilating p o s i t r o n electron pair, or both6). Certainly, they can also be used for other systems of a similar nature. The advantages of these circuits are (1) ease o f construction and moderate cost, (2) fast time response and stable performance, and (3) coaxial cables o f manageable length used for matching time-delays.
The determination o f positron annihilation mean lives requires a delayed coincidence circuit o f fast response and fine resolution. The time interval between the two-coincidence pulses is converted into pulse height, which is analyzed and recorded in a multichannel analyzer. There is also a gate pulse that selects the true events defined by the energy o f the gamma-rays, or the pulse heights o f the outputs o f the photomultipliers. This is the standard technique for constructing a so-called " T i m e - t o - A m p l i t u d e C o n v e r t e r " (TAC) system1). In general, two principles are used in the construction of a TAC, (1) start and stop and (2) pulse overlap. The circuits based u p o n the start and stop principle do not require well defined square pulses as inputs; it is, however, more difficult to obtain a g o o d linear response o f amplitude f r o m the time duration. On the other hand, the circuits based u p o n the overlap principle require fast and stable square pulse generators. Recently, integrated circuits have been used to construct T A C systems2). We have constructed and tested a simple square pulse generator using the M E C L I I I integrated circuits o f Motorola3). This circuit, based on the same principle suggested by Motorola4), is used as one o f the building blocks o f the TAC. A tunnel diode fast discriminator circuit, similar to the timing unit reported previously5), is used as * Work supported by USAEC Contract No. AT (11-1) 3373.
2. Description
2.1. SQUARE PULSE GENERATOR The integrated circuit M C 1661L is used to construct ..J--
1°0t •
Delay
r
II I
/-
150 IL
! !
,s
250 -52V
-2V
Fig. 1. Schematic of the square pulse generator. 253
254
J.B.
W A N G et al.
the square pulse generator. MC 1661L is a dual 4-input gate similar to MC I023L 7) but with faster risetime and response time. The risetime and response time are in the vicinity of 1 ns. The schematic of the square pulse generator is shown in fig. I. Only two of the inputs of each of the dual-gate are used, while the remaining two are clamped at the lower voltage state, - 2 V nominally. Normally, all the inputs of the first gate are at the lower voltage state. The A N D output will be at the lower voltage state and the N A N D output will be at the higher voltage state, - 0 . 8 V. Since the N A N D output is connected directly to one of the inputs in the second gate, normally the A N D output of the second gate, which is also the output of the square pulse generator, will be at the higher voltage state and the N A N D at the lower voltage state. When a positive pulse of the order to 500 mV and 2 ns duration is applied at the input, the state of the first gate changes. This precipitates a change of state of the second gate, resulting in a negative pulse at the A N D terminal of the output stage. Since the N A N D output, which is at the higher voltage state now, is fed back to the first stage, this transient state will remain even when the original input to the first gate is removed. The return to the normal state is triggered by the arrival of the A N D pulse of the first gate at the input of the second gate. Once this pulse is sensed, the whole unit returns to the normal state. Since the arrival of the pulse depends upon the length of the delay cable, one can control the width of the output pulse. As is shown in fig. 1, the output of the unit is a negative square pulse with an amplitude of about 0.8 V and a width equal to the time delay of the delay line. It is obvious from the discussion that a second pulse closely following the original input will not affect the unit, if the time separation between these two pulses is less than the time of the delay of the delay line. 2.2. FAST DISCRIMINATOR The circuit of the fast discriminator is similar to the tunnel diode part of the timing unit used by Garvey 8) and usS). The control range of the bias level provided by changing the current flow to the tunnel diode is not much, less than 100 mV. However, this can be augmented by using attenuators before the input to the tunnel-diode discriminator. When BNC attenuators with 50~ termination are available, the adjustment of a proper bias level for the discriminator is not a very difficult problem. Due to the intrinsic properties of the tunnel diode, this discriminator circuit is very fast; delay time less than 1 ns and pulse width less than 4 ns can be easily
achieved. If a change of the polarity of the output pulse is desired, a simple transformer can be used as the coupling element. This discriminator is quite adequate for the purpose of defining the energy range, or the range of the amplitude of the output of the scintillation counters, produced by the gamma-rays in positron annihilation experiments, or for experiments where the calibration of the discriminator level is infrequent. 2.3. TAC SYSTEM A block diagram of the TAC system is shown in fig. 2. Three square pulse generators are used, two for the "start" and " s t o p " pulses and one for the gate pulse. The coincidence unit is comprised of a half of MC 1661L. The output of the coincidence unit is integrated and shaped to produce a negative output pulse with risetime of ~ I/~s and decay time of ~ 3 #s, a pulse acceptable to most commercial multichannel analyzers. A simple integrator circuit is shown in fig. 3. If the width of the square pulse generators of the "start" and " s t o p " pulses is 40 ns and that of the gate pulse, 50 ns, the output of the TAC system as shown in figs. 2 and 3 has a linear range of about 32 ns. The machine resolution is less than 50 ps. The intrinsic time-walk of the TAC system is about 200 ps/V, just about the triggering level of the square pulse generator. There are several methods available to reduce the time-walk. The simplest one is to use a zero crossover timing unit such as the one used by us 5)
From
,
, MCA
Fig. 2. The block diagram of the time-to-amplitude converter. P-M, Photomultiplier; T, timing unit; G, square pulse generator; D, delay; C, coincidence unit; I, integrator and pulse shaper; MCA, rnultichannel analyser.
4 71~
I[
.O01.J_47K~;
IK
82K IK
i
18-~K " 01 ---tl
_[L
1OK:
'390 2N706
2N706
HEP739
Fig. 3. A simple integrator for TAC.
>
SIMPLE T I M E - T O - A M P L I T U D E CONVERTER
to precede the square pulse generator. Optionally, one can use a constant fraction timing unit 9'1°) to precede the square pulse generator or an additional coincidence unit where the time sequence of start-stop is reversed and the summed output of the two coincicence units is used as the output11). The gate pulse used to define the true events is controlled by discriminators. This construction can be very similar to the one described in the next section. Because fast discriminators are used here, the time match between the gate and the timing pulses is quite simple. The gating of the TAC reduces the pile-up effect considerably over the gating of the multichannel analyzer. 2.4. TWO-GAMMAANGULAR CORRELATION The construction of a coincidence system to record the angular distribution of the two-gamma rays emitted at the annihilation of the positron electron pair is quite straightforward. Normally, no fast electronics are required and a coincidence resolution of about 1 ps is used. Here, we describe a system based upon the discriminators and square pulse generators mentioned above. In addition to being simple and fast, this system has the advantage of having a high resolution. A block diagram of the angular correlation set-up is shown in fig. 4. For such a fast system, plastic scintillators instead of NaI(T1) scintillators are more suitable detectors. In addition, for the large size used in angular correlation work, for example, -21!"diameter and 6" long, plastic
~f
Fig. 4. A system to record the two-gamma angular correlation of the annihilating positron-electron pair. PM, Photomultiplier; A, attenuator; FD, fast discriminator; G, square pulse generator; D, delay; C, quadrupole coincidence unit; ST, Schmidt trigger.
255
scintillators are much cheaper than the NaI(TI) scintillators. While fast photomultipliers must be used instead of the slower ones conventionally used with NaI(TI), the combined cost of scintillators and photomultipliers favors the choice of plastic scintillators and fast photomultipliers. We have found that a reasonably fast counting rate can be obtained by using plastic scintillators of the size 1½" diameter and 6" long with a 22Na source of activity of only 1 mCi and a slot width of 0.8 mrad. Four discriminators are used, two for each scintillation counter. One of the two serves as the upper limit discriminator whose output is fed into the NO input of the quadrupole coincidence unit (cf. fig. 4). The other one serves as the lower limit discriminator, and its output is fed into the YES input of the quadrupole coincidence unit. The pulse width of the square pulse generator for YES input is 8 ns and that for NO input is 16 ns. The YES inputs are delayed 4 ns in respect to the NO inputs to insure the proper action of the coincidence unit. The resolution of the coincidence unit is ~ 12 ns. At such a resolution the true events to random background ratio is found to be about 50:1. The accurate determination of the tail part of the angular correlation curve becomes quite easy because of the low random background. The stability of the whole system has been found surprisingly good; it has remained stable for as long as two months. References 1) A. Ogata, S. J. Tao and J. H. Green, Nucl. Instr. and Meth. 60 (1968) 141. This paper covers many articles before 1968. '~) I. J. Taylor and T. H. Becker, Nucl. Instr. and Meth. 99 (1972) 387. a) Please see data sheets regarding MECL III integrated circuits by Motorola. 4) W. R. Blood, Jr., M E C L system design handbook (Motorola Semiconductor Products Inc., 1971). 5) A. Ogata and S. J. Tao, Nucl. Instr. and Meth. 69 (1969) 344. 6) For general information regarding positron annihilation, please see: V. I. Goldanski, At. Energy Rev. 6 (1968) 3; J. A. Merrigan, J. H. Green and S. J. Tao, Methods o f chemistry, vol. l, part IIID (eds. A. Weisberger and B. W. Rossiter; J. Wiley, New York, 1972) p. 502; and others. 7) A. Barna and E. L. Cisneros, Nucl. Instr. and Meth. 75 (1969) 261. s) j. Garvey, Nucl. Instr. and Meth. 29 (1964) 137. 9) R. R. Fullwood, Nucl. Instr. and Meth. 93 (1971) 235. 10) W. J. McDonald and D. A. Gedcke, Nucl. Instr. and Meth. 55 (1967) 1. 11) p. Thieberger, Nucl. Instr. and Meth. 44 (1966) 349.