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Pulse shape discrimination
NUCLEAR INSTRUMENTS AND METHODS 31 (I964) I I 2 - I 2 4 ;
© NORTH-HOLLAND PUBLISHING CO.
PULSE SHAPE DISCRIMINATION M.L. ROUSH, M.A. WILSON and W.F. HORNYAK Department of Physics and Astronomy, University of Maryland, College Park, Maryland
Received 5 June 1964
A method for separating the scintillation signals from particles with different specific ionization is described which is relatively insensitive to signal amplitude. It is based upon a measurement of the zero-crossing time for signals produced by RC differentiation of dynode voltage pulses. The circuit utilizes the signal from a single--photomultiplier dynode and operates satisfactorily at high counting rates. In the case of gamma-neutron discrimination in NE-213, excellent separation was observed over a range of amplitudes corresponding to recoil protons of 0.5 to 15 MeV energy. The acceptance efficiency for the recoil proton events drops to 5070 at 0.3 MeV. Good experimental results are also shown for gamma-alpha discrimination in a
1. Introduction Scintillation counters have been used for m a n y years for measuring the number, time of arrival and energy of nuclear radiations. Recently, information related to the specific ionization of the exciting particles has been obtained f r o m variations in signal pulse shape. Following the circuit first introduced by Brooks1), a considerable n u m b e r of other circuits have been developed for distinguishing particles of differing specific ionization2). These circuits have f o u n d a wide range of application to cases in which neutroninduced recoil protons in organic scintillators are to be detected in the presence of a g a m m a - r a y background. The present w o r k was undertaken to develop a circuit which would leave the photomultiplier anode signals undistorted for proper use with existing time-offlight equipment. Further, it was considered desirable to develop a system capable of operating at high count rates and functioning properly over a wide range of pulse amplitudes, with the ability to successfully accept small pulses, thus allowing high neutron acceptance efficiencies. The masking presence of noise pulses generated at the p h o t o c a t h o d e and early dynodes can present considerable difficulty in certain experimental situations. F o r example, when geometrical circumstances pievent proper optical coupling between scintillator and photomultiplier or when the n u m b e r of p h o t o n s generated in the scintillator per event is small, the desired signals m a y have an amplitude comparable to noise pulses. Consideration is given to discrimination against noise pulses, based u p o n their pulse shape 112
stilbene scintillator. In addition, the pulse-shape sensing circuitry was used to separate small dynode signals due to lowintensity scintillations in NaI from phototube noise pulses of comparable amplitude. Calculations were made to select those circuit parameters which provide the optimum experimental ability to separate events due to two kinds of particles. Graphical displays of this optimization are shown for gamma-neutron discrimination in stilbene, NE-213 and NE-150 scintillators. A detailed comparison is made between the present procedure and the theoretically optimum use of the shape differences. This method of shape discrimination appears to make very efficient use of the information available in the pulse shape. when the desired photo-electrons.
events
involve at least
several
2. Method for shape discrimination A l t h o u g h several circuits have been described in the literature 3-5) which use only dynode signals, thus allowing independent use of the anode signals for fast timing circuits, satisfactory results have been demonstrated for a rather limited range of pulse amplitudes. This limitation is caused by photomultiplier saturation for large pulses and failure of the diode pulse-stretcher for small pulses. The use of circuitry not requiring diodes allows the extension of operation to smaller pulse amplitudes. All the various methods of pulse-shape discrimination make use of information contained in the time dependence of the current pulse from a photomultiplier. In the present case this information is derived f r o m a double R C differentiated dynode current pulse by detecting the zero-crossing time of the resulting signal, a method similar in concept to the treatment by Forte 3) of anode current pulses. This zero-crossing time depends sensitively upon the decay time characteristics of light emission f r o m the scintillator and is independent of pulse amplitude. First, consider the dependence of d y n o d e pulse shape u p o n the decay time of the current pulse. It will be assumed that the n u m b e r of photo-electrons involved is sufficient to warrant treatment of a continuous current pulse following an average behavior. * Research supported in part by the U.S. Atomic Energy Commission.
113
PULSE SHAPE DISCRIMINATION
For pulses produced by only a few photo-electrons, large fluctuations from the average shape must be expected and will be considered later. Assuming the net current arriving at the dynode to be produced by scintillator light with two principal decay constants, we have
i(t) = A(E) exp (--t/=) + B(E) exp (--t/ff).
NE- 213 SCINTILLATOR RiCl= 1.0F"sec
I
Vl(t) recoil electrons ~ t._.~ il I )_.i___i_~Vi(tl
(1)
¢,~ ~
1~.,o
An expression is then obtained for the voltage pulse of the form A exp (--t/a) 1 1
Vl(t) --
O~
(a)
B exp (--tiff) --
R1 C1
1
1
ff
R(C1
o*.z
+
0'4 +(~.sec)~
0~.6
R2C2 = 0.01/~sec
T
(2)
v~t)
\\ The expected pulse shapes for an NE-213 scintillator have been calculated and are shown in fig. la for a dynode R C time constant of 1.0#sec.
~,.
--A exp (--t/a)
clll 1) R?c;
O~
+ ~
1
1
RIC1 ×
--A
1 -- R1C1)( 1
,e,,_ -v~(t) il~ \\/,,,-'protons TI~;IIC t roml
It is readily seen that the time at which the pulses reach their peak value depends upon the relative amount of slow component present. This time can be determined by differentiating the signal and measuring the time of zero crossing. To accomplish this the voltage pulse is applied to a simple R C differentiator, which yields,
v~(t) =
_~
(b)
o
oiz
o.,4
+(~,soc)--
Fig. 1. (a) The voltage pulse shapes developed at a dynode for recoil-electron and recoil-proton events within an NE-213 scintillator. (b) The voltage pulse shapes resulting from an RC differentiation of the dynode pulses. The insert displays the complete pulse for a recoil-proton event.
B exp (--tiff)
.,G) tl 1
1
+ ~--
ff
R(c~ - 1
-R1C1
"
R1 C1
@
-
C1
-
R2 C2
1 B/1 R2C2 -~x ) ( ~ - R I C ; ) ----
Figure lb shows the result of such a "differentiation" using R 1 C I = 1.0#sec and R2C2=0.01#sec. The differentiated pulses are amplified and limited with the result indicated in fig. 2. Signals resulting from very small dynode pulses will not be limited for their full length (if at all) and
!
A1
/7-1
R1-CI) @ (~
R1-CI
R2 C~
(1
R---~I) .(3,
the determination of zero-crossing time is hindered by the gradual slope. In the circuitry to be described, the signals are applied to a tunnel diode which is immediately driven into its high voltage state of approximately 1 V and remains in this state until its input current through a 60-ohm series resistor drops
114
M.L. ROUSH et al. NE-215 I.O/z sec
desired information and a discrimination level can be set to accept those events whose zero-crossing times are sufficiently long to produce signals above the trigger level. Subsequently, this discriminator output may be used to generate gating pulses. Figure 3 shows the distribution of pulse amplitudes at the discriminator input when an NE-213 scintillator is exposed to a combination of g a m m a rays and neutrons. A single-channel analyzer was used to select events producing dynode pulses of an amplitude corresponding to a recoil proton energy of 0.8 MeV. A great increase in sensitivity in the measurement of the zero-crossing times is accomplished by subtracting a constant time from all pulses. The pulses from the tunnel diode are applied to the first control grid of the constant current tube, (a 6BN6 operated in a manner similar to that of Green and Bell 6) and to a delay line of appropriate length. After a time delay A, the signals are applied to the second control grid and current will flow during the time both control grids are "pulsed
RlCl =
R~Cz= 0.0] p.sec
l __
~
Electrons
~-~
Protons
Regions where p u l s e s ~ ore limited
Fig. 2. The pulses from fig, lb, shown after having been amplified and limited.
below 0.5 mA, at which time it switches rapidly back to zero. The presence of the tunnel diode eliminates the negative undershoot following the positive pulses and supplies a very stable base line for the pulses. NE-213 Scintillator RICI= I,O ~u.sec R2Cz= 0.20~ sec
o n ".
o~
Recoil Electrons
3. Apparatus and results The experimental arrangement is indicated in the block diagram of fig. 4 with the detailed circuit shown in fig. 5. Voltage pulses from the last dynode of a 56AVP photomultiplier are transmitted by 100 feet of R G l14/U cable to the central instrumentation area where connection is made to a linear amplifier system used for energy selection and slow coincidence gating. A parallel connection is made through C2 to a series of distributed line amplifiers. The 200-ohm input resistance combines with C2 to perform the desired R C differentiation. Output pulses from the series string of three amplifiers (two Hewlett-Packard 460 BR and one 460 AR) are applied to the tunnel diode pulse shaper.
PSS Output Pulse Height Fig. 3. Pulse-height spectrum present at the discriminator input in coincidence with dynode pulses of a fixed amplitude, corresponding to recoil protons of 0.8 MeV and recoil electrons of appropriately smaller energy to give the same pulse height.
These pulses are fed to the grid of a constant current tube which conducts during the time the signals are positive. By integrating this current, a signal is obtained which is proportional to the time during which the tube was conducting. These signals contain the
R'
fi r
~Constant
,
CurrentL_=.. Test I
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Gate Pulse Generator
~ Gate I Outputi =
.
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.
.
.
.
.
.
.
.
.
.
.
.
.
.
:
Fig. 4. Block diagram of the total pulse shape discrimination circuit. The portion outlined by the dashed line is shown in the detailed circuit diagram of fig. 5.
PULSE PULSE V2 6688
VI 6 A K 5
SHAPE
115
DISCRIMINATION
SHAPER V5 6AK5
TIME CONVERTER V4 6BN6
V5
6AK5
+;'00
o
o:,
01
+20
~ ~' +10
~QIt
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o~. ~ ~ o ~ :ib_
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+,~ ~v~
,~
+ ~ o ~ ~'~ +50 <;~?°'i~i ~
I 114,,,-c
Itl
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65/U
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18K (
[
Q01
1 1
[
f"
1
-__92 ~ GATING P U L S E GENERATOR
LEVEL ,2AU.
AO~OST
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2.2 K f
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001
~" _-
DISCRIMINATOR
v,
18K ~
d---
1°1--1
÷3oo
12AU7
,~
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j A-~o~,o K ~zw I ~ ~'~ ~ ~_,'r..~~ <+50 ~," :~,%~K 65/U
AMPLIFIER
rT]2r=
I
,~~_. - - - ~
--
> IOK
'1L
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! [
V~+;g;.~°°^~
150 rJ~j
>
,,e
IM ~K ~47F
,
O.i (
T
I
f
'q ~"
Fig. 5. Circuit diagram of pulse-shape discriminator.
3.1. GAMMA-ALPHA DISCRIMINATION
arrangement was obtained using alpha and gamma-ray sources with a stilbene scintillator. A two-parameter display of dynode pulse height vs pss (pulse shape sensor) test output pulse height is
The use of radioactive sources to initiate the events of interest is of particular convenience if any extended circuit testing is required. A rather convenient test
f PSS
output
(a) A l p h a - g a m m a discrimination
(b) N e u t r o n - g a m m a discrimination D y n o d e pulse heigt --->
Stilbene scintillator; A m ~41 + Co 6° RIC1 = 1.0 ~. sec;
NE-213 scintillator; Be 9 (~,n); E~ -- 2.0 MeV R,~C,~= 0.20 ~ sex:
Fig. 6. T h e photographs represent a two-parameter display of the output of the PSS circuit, indicating the separation of recoil electrons from alpha particles in ( a ) a n d from recoil protons in (b).
116
M.L. ROUSH et al.
shown in fig. 6. In fig. 6a one can see the continuous distribution of Compton electrons and the localized spot due to the relatively mona-energetic alpha-particles. Fig. 7 demonstrates the use of the pss signals to gate the dynode spectrum. In fig. 7b the spectrum in coincidence with the pss gate signals appears to be identical with the observed singles spectrum of alpha particles alone. The anti-coincidence spectrum of fig. 7c shows complete removal of the alpha peak indicating excellent separation of these two types of interactions.
7-~ Discriminator Setting (a) En=15MeV %_.
. , . ~t~_ ~I
i~. i
-
(b) En=6 MeV
;~
' ~
(c) En=0.60 MeV
n
PSS Output Pulse Height
i
i
-'.
6000
i
i
z41 80 Ungoted Spectrum of Am+Co Stilbene Scintillator
•
5000
!0.8! / i 0.4(
(d)
4000
o.o, 3000 ......
.'.........
2000
"'..........,.......,...'"',--...,.. --...
- I000 ¢ o ~
".,..., 0
~.4ooo
,b
E n (MeV}
~6
Fig. 8. Spectra at the input to the discriminator in coincidence with dynode pulses of selected pulse heights. The energies of proton recoils corresponding to the pulse amplitudes selected are indicated. The fraction of recoil-proton events of a given energy which will be accepted by the indicated discriminator setting is shown by curve (d).
PSS Gate Pulses In Coincidence
3000 2000
|000 .... ...-
5000
d,
....,
PSS Gate Pulses In Anticoincidence
"-. "%..
2000
/
""'"'"''"""'"'"''"v'""'""""'"'"""-.....
I000 L
"'........
2;
4'o
6'0
~o
,oo
Channel Number
Fig. 7. Spectrum observed with alpha particles from Am 241 and gamma rays from Co 60 incident upon a stilbene scintillator. The effect of requiring coincidence with the PSS gate pulses is shown in (b) and the effect of placing the gate pulses in anti-coincidence is shown in (c).
scintillator is not considered. Fig. 6b displays the output from the pss circuit using the reaction Beg(% n) as the source of neutrons in the presence of gamma rays. Quantitative vertical cross-sections of such a display are shown in fig. 8 a-c for three values of the dynode pulse amplitude. High energy neutrons and g a m m a rays were provided for the spectra using the reactions T(d, n) and Be9(p, 7). The vertical dotted line indicates a discriminator triggering level which can be seen to provide quite reasonable discrimination over a range of about 40 in pulse amplitude. Fig. 8d gives the acceptance efficiency of recoil proton events for the discriminator triggering level indicated. The results of fig. 8 were obtained using low photomultiplier gain to avoid photomultiplier saturation effects for the 1 5 - M e V recoil protons. Better performance for low-energy interactions is readily obtained by increasing the potential supplied to the phototube.
3.2. GAMMA-NEUTRON DISCRIMINATION
The separation of neutron produced events from a background of gamma-ray produced events was investigated for NE-213 liquid scintillator. For purposes of this investigation the possibility that energetic gamma rays or neutrons may produce specifically nuclear instead of atomic interactions within the
3.3. NOISE DISCRIMINATION Assuming that multiple photoelectron noise pulses result from the simultaneous release of electrons in the vicinity of the photocathodeT,S), the dynode current pulses produced will have a shape determined by the phototube transit time properties. According to
P U L S E SHAPE D I S C R I M I N A T I O N
117
PSS output
(a) Noise alone
(b) Noise + Fe 55 Dynode pulse height
Gamma-noise discrimination in Nal scintillator; E v = 5.8 keV; R 1 C 1 = 0.25 ~t sec, R z C ~ = 0.20 ~t sec Fig. 9. The photographs represent a two-parameter display of the output of the PSS circuit, indicating the separation of scintillation induced pulses from photomultiplier noise pulses of similar amplitude.
manufacturers specifications for the 56 AVP, a current pulse with a 2 nsec risetime and a 2 nsec pulse-width should result• This time distribution of anode current is sufficiently peaked to be readily distinguished from the current pulses associated with phosphor scintillations10). Since the noise pulses correspond principally to the emission of only a few photoelectrons, we should consider the discrete nature of this photocathode current• Let us examine the specific example of a noise pulse produced by two photoelectrons and a scintillation induced pulse resulting from the emission of two photo-electrons separated in time by 10 nsec. The resulting 5 nsec difference in zero-crossing times for the double R C differentiated signals will provide adequate separation of these two pulses• In practice, the time separation of photo-electrons will be influenced by statistical fluctuations in the emission of photons from the scintillator and fluctuations in the production of photo-electrons at the cathode. The resulting distribution of time separations precludes a clearcut distinction between noise pulses and scintillation produced pulses. The best separation will clearly result for pulses produced by at least several photoelectrons and for a scintillator with a long principal ceday constant. A thin-window NaI crystal was used to detect the 5.8-keV X-rays from an Fe ~5 source resulting in the two-parameter display of fig. 9. The use of the psd
I
I
I
I
[
I
I
I
t
NaT Scintillator
6000
F'e~6 Source
~ooo
E),. =5.8 keY
4000 3000
.,."-•... Ungated
• ' ...~
~. zooo = ~o "I 0 0 0 ~ g o o
"'...
o
•...,
...••..,.
3000
/ j Gated by PSD ./,J
aooo lOOO
o
"'......' I
20
40
i_
60
80
I
I00
•'i.......,.I
120
140
CHANNEL NUMBER
Fig. 10. Pulse-height spectrum of the 5.8-keV X-ray from an Fe 55 source detected in a NaI scintillator• The reduction in the number of noise pulses accomplished by pulse shape discrimination is demonstrated in spectrum (b).
118
M.L. ROUSH et al. 10 4
Tz, 500nsec
~
Tz. 2OOnsec
104El-
STILBENE
\
eutron
,o./
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(nsec)
/
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k E
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2~ .......
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'1 t STILBENE 1 Gamma- Neutr°n
I I
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~-----~
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RzCz(nsee)
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NE-213 oo=o-N.,o°
',
,
IO'i--
,,
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RiO, I~
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r~-~,:~'o .... 10 3
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v~/
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R2C2(nsec)
T2-T' =25 nsec~,~' I T2-T:20n sec- I ~ ~
(nse,)~_ /d
~
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/'X~x,(-~~X. ..
'
\\
r/ / /
' \h.[
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~""\',g "Tz=O'SFsec~'\ /Ta'LO,u'sec.
R,GI i"
I 104
Fig. 11. F i g u r e - o f - m e r i t values indicative of the ability of the circuit to separate g a m m a - r a y events from neutron-induced events in a stilbene scintillator. The dashed lines indicate the zero-crossing times for recoilp r o t o n events in (a) and the difference in crossing times in (b).
gating signals to reduce the noise background is demonstrated in fig. 10. 4. Circuit performance 4.1. CHOICEOF R C TIME CONSTANTS
Calculations were made to investigate the effect of the R C time constants. To obtain adequate performance of the pulse shape discrimination circuit over a wide range of pulse amplitudes requires careful
,T'-?=~:e1~..'
....
RzCz(n sec)
103
'
........ 104
Fig. 12. Figure-of-merit values indicative of the ability of the circuit to separate g a m m a - r a y events f r o m n e u t r o n - i n d u c e d events in an NE-213 scintillator. T h e dashed lines indicate the zero-crossing times for r e c o i l - p r o t o n events in (a) and the difference in crossing times in (b).
consideration for small pulses. The gradual slope of the small pulses as they cross zero makes it difficult to determine the crossing times accurately. The crossing times and slopes at crossing were calculated for various dynode time constants, R1C1, and various differentiator time constants, R2 C~, using equation (3). If Ta and 7"2 represent the zero-crossing times for the two types of events of interest, then the ability to separate the types of events depends upon the magnitude of (7"2 -- T1) as well as the accuracy of measure-
119
PULSE SHAPE D I S C R I M I N A T I O N 10 4
I0 4 \,
Tz : 500 nsec __..~
Tz • 1.0F sec ~ ) ~ , T2, O,5~ s e /
-//
i0 3 RICI
\~,-T z \ • t.5F sec
NoI
-
e
NE-150 Scintilla tor ~
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a
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(n sec)
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i NE-150 S c i n t i l l a t o r ~ Gamma-Neutron |
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IOs RzCz (n sec)
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t Tz~TQ,5 O n s _ e ~ I
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/ T . - T . - Z-5 0 n s e c . -
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|
i0 ¢
Fig. 13. Figure-of-merit values indicative of the ability of the circuit to separate gamma-ray events from neutron-induced events in an NE-150 scintillator. The dashed lines indicate the zero-crossing times for recoil-proton events in (a) and the difference in crossing times in (b).
I
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~ / ~ ' T ' 2 ; ; 0 0 0 n :22c ~
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_ I
I
.
IIIIIJl
IOZ RzCz(n sec)
' 10a
[
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I l l l J
104
Fig. 14. Figure-of-merit values indicative of the ability of the circuit to separate noise produced pulses from pulses resulting from scintillations within a NaI scintillator. The dashed lines indicate the zero-crossing times for scintillation events in (a) and the difference in crossing times in (b).
ment of crossing times. A figure-of-merit for a particular set of parameters may be defined as F.M. ~ (T2-- T]). (Slope)x[Tz
IO
]
(4)
where the slope at zero crossing is calculated for both pulses and the smaller of the two values is used for (Slope)xlmz. The details of the calculations are outlined in appendix A. The magnitude of the figures-ofmerit should be indicative of the actual experimental ability to separate the pulses of interest.
Fig. 11-13 indicate the results for g a m m a - n e u t r o n disrimination in the commonly used scintillators, stilbene, NE-213 and NE-150. The contours of constant value for the figure-of-merit indicate the region for optimum operation with each scintillator. There are, however, a number of auxiliary factors to be considered when selecting the time constants. The extent of the counting rate effects and the time delays necessary to allow gating will be determined by the time until zero crossing which is shown by the dashed
]20
M.L. ROUSH et al.
lines in part (a) of each figure. It is of course essential to verify that the difference in crossing times shown by the dashed lines in part (b) of each figure is sufficiently large to be compatible with the time resolution of the circuits used. When a single dynode output is used for both the linear analysis and the pulse shape discrimination, somewhat larger values of R C will be desirable. It should be recognized that selecting the maximum figure-of-merit is designed to optimize the ability of the circuit to separate events and not to optimize the time separation of the zero crossings. The degree to which the results depend upon the pulse shapes assumed, was studied by varying the decay constants and relative amplitudes. The results indicate that the magnitude of the figure-of-merit values is very sensitive to the shapes while the position of maximum varies only slightly. Even though the sensitive nature of the magnitude of the merit values makes the comparison between different scintillators particularly perilous, the large values obtained for NE-150 warrant special comment. The large values, when contrasted with the limited shape discrimination which has been obtained experimentally for this phosphor, must cast considerable doubt upon the reliability of the shape parameters availableg). If the shape parameters assumed are a reasonable representation of the actual case for a particular scintillator, then the comparison of the relative magnitude of merit values for different components in a single circuit or for different circuits should be quite reliable. Fig. 14 shows the results of calculations for the discrimination between noise pulses and scintillations within a NaI scintillator. The manufacturers specification of transit time properties for the 56 AVP photomultiplier indicate a width at half maximum for noise related anode current pulses of 2 nsec. To simplify the calculations, these current pulses were represented by a shape such as would be produced by scintillations with a decay constant of 2 nsec and negligible time dispersion within the phototube. The effect of this assumption was studied by making additional calculations for decay constants of 1 and 3 nsec. No significant differences were observed, indicating that the choice of this decay constant is not critical in predicting the region of optimum R C time constants. 4.2. OPTIMUM USE OF THE PULSE SHAPE DIFFERENCES Detailed statistical considerations have been made by Gatti and De Martini 11) to determine the optimum
treatment of pulses in order to extract the maximum possible information from the pulse shapes. The result of their calculation is that the optimum results will be obtained by weighting the input pulses by a time-dependent function, iz(t) -- ia(t) P(t) -- ie(t) + il (t)
(5)
where il(t) and iz(t) represent the average current pulse shapes resulting from the two types of exciting particles being considered. Thus, as might be expected, the maximum importance should be attached to those parts of the signal where the differences are relatively large. Consider the specific case of the discrimination between recoil protons and recoil electrons within a stilbene scintillator. The function P ( t ) which gives the optimum weighting of pulse amplitude is shown in fig. 15a.
P (t)
(a)
STILBENE SCINTILLATOR
f 40
8'0
~ (b) 40
120 ~ t (n sec)
160
RI CI,, R2C2 - 175 nsec STILBENE SCINTILLATOR
aOT~(nsac}~O
leo
Fig. 15. (a)Optimum weighting of input pulse amplitude. (b) Weighting of input pulse amplitude by measuring zerocrossing time.
It is of interest to estimate how efficiently the present technique uses the pulse shape information. The importance or weight assigned to the input signal at a given instant is determined by the sensitivity of the crossing time to the amplitude at that point. This sensitivity was determined by calculating the shift in crossing time produced by a small perturbation introduced at various positions along the signal. The shape of the doubly differentiated dynode pulse
PULSE SHAPE DISCRIMINATION for a recoil proton in stilbene, calculated from equation (3), was normalized to represent a total integrated charge produced by 200 photo-electrons. The signal produced by a single photo-electron released a time Te after the beginning of the basic pulse was also calculated. The signals were added linearly and the resulting shift in crossing time observed. The results obtained are displayed in fig. 15b. Thus the quantity to be measured (zero-crossing time) is determined by the integrated effect of the complete signal preceeding this time with the various portions of the signal weighted as shown. It is worthwhile to compare this fairly efficient use of the information with the more standard procedure of comparing the fast portion of a pulse with the total integrated pulse. The process of stretching a sharply differentiated anode pulse produces a signal rather closely proportional to the peak anode current. This peak current, determined by a small fraction of the total signal, will be subject to considerable statistical fluctuations. 4.3. COUNTING RATE EFFECTS Measurements described in section 3.2, made at low counting rates, indicate a rejection ratio of 500:1 for 0.3-10 MeV g a m m a - r a y events with negligible loss of neutron induced events within this range of pulse amplitude. At high counting rates a second g a m m a ray m a y be detected before the zero-crossing of the first, delaying the zero-crossing and hence causing the generation of an unwanted gating signal. The resolving time for overlap of such pulses is the zero-crossing time, which is roughly 1/zsec in length. The rejection ratio was observed to drop to 50:1 at a counting rate of l04 counts per second. This still provides a quite adequate reduction in the random background rate due to g a m m a rays without loss of the desired neutroninduced events.
121
5.1. R1C1-R2C2 CIRCUIT The removal of the cathode follower which has isolated the two RC sections, results in the circuit indicated in fig. 16. For a dynode current pulse represented by equation (I), the output signal will be,
V3(t) = K1 exp (--t/e) 6- K~. exp (--tiff) 66- Ks exp {--(L--M)t} 6- 1£4 exp {--(L6-m)t} (6) where rl = R1C1 and ~z = R2C2,
( RI -+-R21 1 M=½
(1 =
I_~16- (R16-R212
1
1 ½
A(1/7) CI(M--L6-1/e)(L6-M--1/~) B(]/~) CI(M--L6-1/fl)(L6-M--I/fl)
Kz = - -
1 [ A (M--L) B(M--L) ] 1(3 = ~ 2 M ( M - - L 6- l/e) ~- 2 M ( ~ - - L + 1/fl)J 1(4 =
l I-
:4(L+_M)
B(L+M)
M) +- 2 M(17 / Z-L-
1 M) "
It can be seen that the signal now depends upon the values of Ra and R2 as well as the two RC time constants. By assuming the distributed capacity of the dynode and related circuitry to be 30 pF, the value of R1 is automatically determined when ~1 is specified. Figure-of-merit calculations were made by specifying the value of R2 for each set. Fig. 16 displays the maximum figure-of-merit for a range of 25
c~
5. Alternate circuits
The double RC differentiation of dynode pulses which has been described in the previous sections was chosen for two principal reasons. The simplicity allows the functioning of the double RC circuit to be easily understood and the effects of varying the time constants to be readily predicted. Secondly, the use of a single dynode signal for both linear amplitude analysis and pulse shape discrimination provides convenience and simplicity. A number of modifications of this basic circuitry will now be considered. These circuits are still based upon the measurements of a crossingtime to indicate the character of an event.
ao
)_T__~ F_Eo v3(') i
| i 5 I0 z
I 103
II 04
I I0 ~
I IC
R2 { O h m s )
Fig. 16. Dependence of the maximum figure-of-merit upon the value of Rg. for the indicated circuitry.
122
M.L. ROUSH
et al.
values of R~. It is apparent that the low impedance presented by small values for R2 prevents the development of reasonable dynode pulses. An examination of the results reveals that for large Re the figure-ofmerit values approach those of fig. 11.
30
J
~J
5.2. RzCI-cF-R2LC2 CIRCUIT The difficulty of measuring the crossing times for small pulses can be reduced by adding an inductance in the RC circuit to produce a larger slope in the signal at the time of zero crossing. Care must be taken to insure that the circuit does not ring. Sets of figure-ofmerit values were calculated in which R1 C1 and R2 C2 were varied for fixed values of a parameter ~. The parameter ~ is defined as
C2
C
o u.
IC R2
IE
o
o'2
o14
o!~ "q
2
~1 : 4 L/R2 (22,
01B
~io
>
Fig. 17. Dependence of the maximum figure-of-merit upon the value of the inductance in the indicated circuit.
and thus relates to the degree of damping present for a particular inductance L compared to critical damping which would occur at an inductance value 2
Lc ~- ¼ R2 C2. For a dynode current pulse represented by equation (1), the output signal of the circuit shown in fig. 17 will be, V4(t) kl exp (--t/~) 4- kz exp (--t/fl) 44- ka exp (--t/RaG) 4- k5 exp {--(D 4- E)t} 4- k4 exp {--(D--E)t}
ka
~
m
:
where
D = ½ R2/L
E
4(7)
( _1 = 4 L 2 L C2]
k2 =
c1( R2
Ra Ca k3 = ~
1
+
Rz C1
1
D
Ra Ca
1 (0-
E-- ---R1 Ca )
E - -
-
B
--A
k4 =
1 R1 C1
-
r +
m
R1 Ca
4-
(E-- D) ( D - - E -R2)-~ k5 =
2 CIE
I/1
--A 1 (DR1 Ca )
B1
+ 1 RICI)
1 (D 4 - E - - R1C-----~) B
÷
R1 C1 + l
1 , i i ~zA >R:Cz 1
~D - E - - R--1C--~]
1
1
+1
1
fl
R1C,
>]
123
PULSE SHAPE D I S C R I M I N A T I O N
Fig. 17 shows the increase in figure-of-merit values resulting from the inclusion of the inductance. Figure-of-merit contours in fig. 18 indicate the region of optimum operation for the use of a critically damped R L C circuit for the "differentiation" process. 5.3. DELAY LINE PULSE SHAPING The use of a shorted delay line to produce the signal which crosses zero allows the use of relatively long R C time constants and an effective integration of the slow component of the pulse. The necessity of proper termination of the open end of the delay line produces a I0' Tz =400 nsee-
\ \ ~.~
$filbene
Gamma Neutron
R~C~
(nsec)
We would like to express our thanks to H.L. Fann for help in preparing the computer program used to make the figure-of-merit calculations and to J.E. Etter who constructed the electronics used to provide the two-parameter oscilloscope display. We also wish to express our appreciation to the University of Maryland Computer Science Center for providing computing time on the IBM 7090/1401 computer.
[0 ~'J
b,l_
/
/',
/
reduction by two of the pulse amplitude available. The figure-of-merit values including this reduction in amplitude are shown in fig. 19 and indicate that this circuit should operate well for pulses of small amplitude. Such circuitry has been tested and provided good discrimination over a limited range. Difficulty was experienced in attempting to obtain proper operation for a large range of pulse amplitude due to overload effects caused by the long large pulses. Alexander and Goulding 5) have followed a somewhat similar procedure, although their comparison of two zero-crossing times results in larger experimental uncertainties and requires more complex circuitry.
\'
',_
15
\
2o,,\ \
2T:DT
TZ=I00 nsec- "-"~'\ ,
,
,
,
.....
I0
104 .. ii
,
I0 z
i
RzCz(nsec)
Stilbene
I
Gamma -Neutron
L
/
/~
^_ r R~C2I
--ff~o.~ ~ LTJ
\\ . . . . . . .
/
\I i
,
,
,
,,,~j~
io ~
io 4
j ~T~_Tt= 15nsee I\\ I J
I
T,
--T2-TI=20 nsee [ i
,r
io'
l
T2-T,=5Onsec 1 1 r
/\/ //
/ \///,
(n sec) I0 ~
I
RjCl
~
i
l
I
Tz_Tl=25n___ /
RC
\
~ / //
(n sec iO2
STILBENE Gamma
-
Neutron
Delay - Line Clipped
I0:
: , IO
, , iiiiii
I i~1 ,tl/ll,lH 10 2 RzC2(n see) I0 ~
1n~/~
~
- - - / I
L_../.
i i l,,,,
I
I
i
i
I i
I i
!
J TZ: 200n sec--'1
I04
Fig. 18. Figure-of-merit values indicative of the ability of the circuit displayed in fig. 17 to separate gamma-ray events from neutron-induced events in a stilbene scintillator. The dashed lines indicate the zero-crossing times for recoil-proton events in (a) and the difference in crossing times in (b).
)
Jl
I IO
Tz=3OOnsec I i iiiiii lot
I 'J I'~l'~ =500nsec i I i i i iiii I j i iii DT(n sec)
103
104
Fig. 19. Figure-of-merit values indicative of the ability of the delay line shaping method to separate gamma-ray events from neutron-induced events in a stilbene scintillator.
M.L. ROUSH et al.
124
Appendix A FIGURE-OF MERIT CALCULATIONS The e x p e r i m e n t a l ability to separate small pulses is i n d i c a t e d b y the m a g n i t u d e of the f i g u r e - o f - m e r i t defined in section 4.1. T h e expression for V~(t) is n o t a n a l y t i c a l l y soluable for the z e r o - c r o s s i n g time. A c o m p u t e r code was d e v e l o p e d to locate the value of t for wich V~(t) changes sign, the size of the i n c r e m e n t s in TABLE 1
Scintillator and pulse characteristics.
B
Scintillator (nsec)
fl Refer(nsec) ence
t being decreased until the desired degree of a c c u r a c y in the crossing time was o b t a i n e d . T h e slope of the pulse as it crosses zero is then r e a d i l y o b t a i n e d by calculating the derivative of V2(t). These calculations were carried o u t for families of values for the R1 C1 a n d R2C2 time constants. The time c o n s t a n t s used to describe the shape of the current pulses are listed in table 1. T h e ratios of slow c o m p o n e n t to fast c o m p o n e n t were t a k e n f r o m the i n d i c a t e d references while the n o r m a l i z a t i o n is chosen so that each c u r r e n t pulse will p r o d u c e a d y n o d e voltage pulse of s t a n d a r d p e a k a m p l i t u d e using an R1 (71 of 7 #sec, such as is c o m m o n l y used in linear pulse analysis e q u i p m e n t .
References Stilbene
(e-)
(p+)
0.933 0.779
5.6 6.0
0.0616 0.0545
49 67
NE.-213
(e-) (p+)
1.05 0.824
5.2 5.6
0.0284 0.0288
107 138
NE-150
(e-) (p+)
1.004 0.794
3.4 4.0
0.0904 0.0532
54 100
NaI(TI) Noise
(e-)
0.0362 4.02
250. 2.0*
12
* Determined by photomultiplier characteristics (see section 4.1).
1) F.D. Brooks, Nucl. Instr. and Meth. 4 (1959) 151. z) R.B. Owen, I.R.E. Trans. Nucl. Sci. NS-8 (1961) 285. 8) M. Forte, K. Konski and C. Marazana, (UN Atomic Energy Agency Conf. Belgrade 1961 NE/59). 4) D.G. Foster, Hartford Report HW-74190. 5) K. Alexander and F.S. Goulding, Nucl. Instr. and Meth. 13 (1961) 244. 8) R.E. Green and R.B. Bell, Nucl. Instr. 3 (1958) 127. 7) J.A. Baicker, I.R.E. Trans. Nuclear Sci. NS-7 (1960) 74. 8) J.R. Prescott, Nucl. Instr. and Meth. 22 (1963) 256. 9) K. Peuckert, Nucl. Instr. and Meth. 17 (1962) 257. 10) Damerell, Nucl. Instr. and Meth. 15 (1962) 171. 11) E. Gatti and F. De Martini, Nucl. Electronics 2, p. 265; (UN Atomic Energy Conf., Belgrade 1961). 12) j. Sharpe, Nuclear radiation detectors (Wiley 1955).
© NORTH-HOLLAND PUBLISHING CO.
PULSE SHAPE DISCRIMINATION M.L. ROUSH, M.A. WILSON and W.F. HORNYAK Department of Physics and Astronomy, University of Maryland, College Park, Maryland
Received 5 June 1964
A method for separating the scintillation signals from particles with different specific ionization is described which is relatively insensitive to signal amplitude. It is based upon a measurement of the zero-crossing time for signals produced by RC differentiation of dynode voltage pulses. The circuit utilizes the signal from a single--photomultiplier dynode and operates satisfactorily at high counting rates. In the case of gamma-neutron discrimination in NE-213, excellent separation was observed over a range of amplitudes corresponding to recoil protons of 0.5 to 15 MeV energy. The acceptance efficiency for the recoil proton events drops to 5070 at 0.3 MeV. Good experimental results are also shown for gamma-alpha discrimination in a
1. Introduction Scintillation counters have been used for m a n y years for measuring the number, time of arrival and energy of nuclear radiations. Recently, information related to the specific ionization of the exciting particles has been obtained f r o m variations in signal pulse shape. Following the circuit first introduced by Brooks1), a considerable n u m b e r of other circuits have been developed for distinguishing particles of differing specific ionization2). These circuits have f o u n d a wide range of application to cases in which neutroninduced recoil protons in organic scintillators are to be detected in the presence of a g a m m a - r a y background. The present w o r k was undertaken to develop a circuit which would leave the photomultiplier anode signals undistorted for proper use with existing time-offlight equipment. Further, it was considered desirable to develop a system capable of operating at high count rates and functioning properly over a wide range of pulse amplitudes, with the ability to successfully accept small pulses, thus allowing high neutron acceptance efficiencies. The masking presence of noise pulses generated at the p h o t o c a t h o d e and early dynodes can present considerable difficulty in certain experimental situations. F o r example, when geometrical circumstances pievent proper optical coupling between scintillator and photomultiplier or when the n u m b e r of p h o t o n s generated in the scintillator per event is small, the desired signals m a y have an amplitude comparable to noise pulses. Consideration is given to discrimination against noise pulses, based u p o n their pulse shape 112
stilbene scintillator. In addition, the pulse-shape sensing circuitry was used to separate small dynode signals due to lowintensity scintillations in NaI from phototube noise pulses of comparable amplitude. Calculations were made to select those circuit parameters which provide the optimum experimental ability to separate events due to two kinds of particles. Graphical displays of this optimization are shown for gamma-neutron discrimination in stilbene, NE-213 and NE-150 scintillators. A detailed comparison is made between the present procedure and the theoretically optimum use of the shape differences. This method of shape discrimination appears to make very efficient use of the information available in the pulse shape. when the desired photo-electrons.
events
involve at least
several
2. Method for shape discrimination A l t h o u g h several circuits have been described in the literature 3-5) which use only dynode signals, thus allowing independent use of the anode signals for fast timing circuits, satisfactory results have been demonstrated for a rather limited range of pulse amplitudes. This limitation is caused by photomultiplier saturation for large pulses and failure of the diode pulse-stretcher for small pulses. The use of circuitry not requiring diodes allows the extension of operation to smaller pulse amplitudes. All the various methods of pulse-shape discrimination make use of information contained in the time dependence of the current pulse from a photomultiplier. In the present case this information is derived f r o m a double R C differentiated dynode current pulse by detecting the zero-crossing time of the resulting signal, a method similar in concept to the treatment by Forte 3) of anode current pulses. This zero-crossing time depends sensitively upon the decay time characteristics of light emission f r o m the scintillator and is independent of pulse amplitude. First, consider the dependence of d y n o d e pulse shape u p o n the decay time of the current pulse. It will be assumed that the n u m b e r of photo-electrons involved is sufficient to warrant treatment of a continuous current pulse following an average behavior. * Research supported in part by the U.S. Atomic Energy Commission.
113
PULSE SHAPE DISCRIMINATION
For pulses produced by only a few photo-electrons, large fluctuations from the average shape must be expected and will be considered later. Assuming the net current arriving at the dynode to be produced by scintillator light with two principal decay constants, we have
i(t) = A(E) exp (--t/=) + B(E) exp (--t/ff).
NE- 213 SCINTILLATOR RiCl= 1.0F"sec
I
Vl(t) recoil electrons ~ t._.~ il I )_.i___i_~Vi(tl
(1)
¢,~ ~
1~.,o
An expression is then obtained for the voltage pulse of the form A exp (--t/a) 1 1
Vl(t) --
O~
(a)
B exp (--tiff) --
R1 C1
1
1
ff
R(C1
o*.z
+
0'4 +(~.sec)~
0~.6
R2C2 = 0.01/~sec
T
(2)
v~t)
\\ The expected pulse shapes for an NE-213 scintillator have been calculated and are shown in fig. la for a dynode R C time constant of 1.0#sec.
~,.
--A exp (--t/a)
clll 1) R?c;
O~
+ ~
1
1
RIC1 ×
--A
1 -- R1C1)( 1
,e,,_ -v~(t) il~ \\/,,,-'protons TI~;IIC t roml
It is readily seen that the time at which the pulses reach their peak value depends upon the relative amount of slow component present. This time can be determined by differentiating the signal and measuring the time of zero crossing. To accomplish this the voltage pulse is applied to a simple R C differentiator, which yields,
v~(t) =
_~
(b)
o
oiz
o.,4
+(~,soc)--
Fig. 1. (a) The voltage pulse shapes developed at a dynode for recoil-electron and recoil-proton events within an NE-213 scintillator. (b) The voltage pulse shapes resulting from an RC differentiation of the dynode pulses. The insert displays the complete pulse for a recoil-proton event.
B exp (--tiff)
.,G) tl 1
1
+ ~--
ff
R(c~ - 1
-R1C1
"
R1 C1
@
-
C1
-
R2 C2
1 B/1 R2C2 -~x ) ( ~ - R I C ; ) ----
Figure lb shows the result of such a "differentiation" using R 1 C I = 1.0#sec and R2C2=0.01#sec. The differentiated pulses are amplified and limited with the result indicated in fig. 2. Signals resulting from very small dynode pulses will not be limited for their full length (if at all) and
!
A1
/7-1
R1-CI) @ (~
R1-CI
R2 C~
(1
R---~I) .(3,
the determination of zero-crossing time is hindered by the gradual slope. In the circuitry to be described, the signals are applied to a tunnel diode which is immediately driven into its high voltage state of approximately 1 V and remains in this state until its input current through a 60-ohm series resistor drops
114
M.L. ROUSH et al. NE-215 I.O/z sec
desired information and a discrimination level can be set to accept those events whose zero-crossing times are sufficiently long to produce signals above the trigger level. Subsequently, this discriminator output may be used to generate gating pulses. Figure 3 shows the distribution of pulse amplitudes at the discriminator input when an NE-213 scintillator is exposed to a combination of g a m m a rays and neutrons. A single-channel analyzer was used to select events producing dynode pulses of an amplitude corresponding to a recoil proton energy of 0.8 MeV. A great increase in sensitivity in the measurement of the zero-crossing times is accomplished by subtracting a constant time from all pulses. The pulses from the tunnel diode are applied to the first control grid of the constant current tube, (a 6BN6 operated in a manner similar to that of Green and Bell 6) and to a delay line of appropriate length. After a time delay A, the signals are applied to the second control grid and current will flow during the time both control grids are "pulsed
RlCl =
R~Cz= 0.0] p.sec
l __
~
Electrons
~-~
Protons
Regions where p u l s e s ~ ore limited
Fig. 2. The pulses from fig, lb, shown after having been amplified and limited.
below 0.5 mA, at which time it switches rapidly back to zero. The presence of the tunnel diode eliminates the negative undershoot following the positive pulses and supplies a very stable base line for the pulses. NE-213 Scintillator RICI= I,O ~u.sec R2Cz= 0.20~ sec
o n ".
o~
Recoil Electrons
3. Apparatus and results The experimental arrangement is indicated in the block diagram of fig. 4 with the detailed circuit shown in fig. 5. Voltage pulses from the last dynode of a 56AVP photomultiplier are transmitted by 100 feet of R G l14/U cable to the central instrumentation area where connection is made to a linear amplifier system used for energy selection and slow coincidence gating. A parallel connection is made through C2 to a series of distributed line amplifiers. The 200-ohm input resistance combines with C2 to perform the desired R C differentiation. Output pulses from the series string of three amplifiers (two Hewlett-Packard 460 BR and one 460 AR) are applied to the tunnel diode pulse shaper.
PSS Output Pulse Height Fig. 3. Pulse-height spectrum present at the discriminator input in coincidence with dynode pulses of a fixed amplitude, corresponding to recoil protons of 0.8 MeV and recoil electrons of appropriately smaller energy to give the same pulse height.
These pulses are fed to the grid of a constant current tube which conducts during the time the signals are positive. By integrating this current, a signal is obtained which is proportional to the time during which the tube was conducting. These signals contain the
R'
fi r
~Constant
,
CurrentL_=.. Test I
I
-rube i]out t
I
i I
I
[ I I [_
Gate Pulse Generator
~ Gate I Outputi =
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
:
Fig. 4. Block diagram of the total pulse shape discrimination circuit. The portion outlined by the dashed line is shown in the detailed circuit diagram of fig. 5.
PULSE PULSE V2 6688
VI 6 A K 5
SHAPE
115
DISCRIMINATION
SHAPER V5 6AK5
TIME CONVERTER V4 6BN6
V5
6AK5
+;'00
o
o:,
01
+20
~ ~' +10
~QIt
' , 5pf
o~. ~ ~ o ~ :ib_
'~o~
®-- -
~oK
@-
~..
+,~ ~v~
,~
+ ~ o ~ ~'~ +50 <;~?°'i~i ~
I 114,,,-c
Itl
V6
65/U
IlJ
18K (
[
Q01
1 1
[
f"
1
-__92 ~ GATING P U L S E GENERATOR
LEVEL ,2AU.
AO~OST
V8 8 . .
2.2 K f
QOI
[
Lil-°°'"°T
,2W
I8K
001
~" _-
DISCRIMINATOR
v,
18K ~
d---
1°1--1
÷3oo
12AU7
,~
[~,__o_.aTEST
j A-~o~,o K ~zw I ~ ~'~ ~ ~_,'r..~~ <+50 ~," :~,%~K 65/U
AMPLIFIER
rT]2r=
I
,~~_. - - - ~
--
> IOK
'1L
.I
! [
V~+;g;.~°°^~
150 rJ~j
>
,,e
IM ~K ~47F
,
O.i (
T
I
f
'q ~"
Fig. 5. Circuit diagram of pulse-shape discriminator.
3.1. GAMMA-ALPHA DISCRIMINATION
arrangement was obtained using alpha and gamma-ray sources with a stilbene scintillator. A two-parameter display of dynode pulse height vs pss (pulse shape sensor) test output pulse height is
The use of radioactive sources to initiate the events of interest is of particular convenience if any extended circuit testing is required. A rather convenient test
f PSS
output
(a) A l p h a - g a m m a discrimination
(b) N e u t r o n - g a m m a discrimination D y n o d e pulse heigt --->
Stilbene scintillator; A m ~41 + Co 6° RIC1 = 1.0 ~. sec;
NE-213 scintillator; Be 9 (~,n); E~ -- 2.0 MeV R,~C,~= 0.20 ~ sex:
Fig. 6. T h e photographs represent a two-parameter display of the output of the PSS circuit, indicating the separation of recoil electrons from alpha particles in ( a ) a n d from recoil protons in (b).
116
M.L. ROUSH et al.
shown in fig. 6. In fig. 6a one can see the continuous distribution of Compton electrons and the localized spot due to the relatively mona-energetic alpha-particles. Fig. 7 demonstrates the use of the pss signals to gate the dynode spectrum. In fig. 7b the spectrum in coincidence with the pss gate signals appears to be identical with the observed singles spectrum of alpha particles alone. The anti-coincidence spectrum of fig. 7c shows complete removal of the alpha peak indicating excellent separation of these two types of interactions.
7-~ Discriminator Setting (a) En=15MeV %_.
. , . ~t~_ ~I
i~. i
-
(b) En=6 MeV
;~
' ~
(c) En=0.60 MeV
n
PSS Output Pulse Height
i
i
-'.
6000
i
i
z41 80 Ungoted Spectrum of Am+Co Stilbene Scintillator
•
5000
!0.8! / i 0.4(
(d)
4000
o.o, 3000 ......
.'.........
2000
"'..........,.......,...'"',--...,.. --...
- I000 ¢ o ~
".,..., 0
~.4ooo
,b
E n (MeV}
~6
Fig. 8. Spectra at the input to the discriminator in coincidence with dynode pulses of selected pulse heights. The energies of proton recoils corresponding to the pulse amplitudes selected are indicated. The fraction of recoil-proton events of a given energy which will be accepted by the indicated discriminator setting is shown by curve (d).
PSS Gate Pulses In Coincidence
3000 2000
|000 .... ...-
5000
d,
....,
PSS Gate Pulses In Anticoincidence
"-. "%..
2000
/
""'"'"''"""'"'"''"v'""'""""'"'"""-.....
I000 L
"'........
2;
4'o
6'0
~o
,oo
Channel Number
Fig. 7. Spectrum observed with alpha particles from Am 241 and gamma rays from Co 60 incident upon a stilbene scintillator. The effect of requiring coincidence with the PSS gate pulses is shown in (b) and the effect of placing the gate pulses in anti-coincidence is shown in (c).
scintillator is not considered. Fig. 6b displays the output from the pss circuit using the reaction Beg(% n) as the source of neutrons in the presence of gamma rays. Quantitative vertical cross-sections of such a display are shown in fig. 8 a-c for three values of the dynode pulse amplitude. High energy neutrons and g a m m a rays were provided for the spectra using the reactions T(d, n) and Be9(p, 7). The vertical dotted line indicates a discriminator triggering level which can be seen to provide quite reasonable discrimination over a range of about 40 in pulse amplitude. Fig. 8d gives the acceptance efficiency of recoil proton events for the discriminator triggering level indicated. The results of fig. 8 were obtained using low photomultiplier gain to avoid photomultiplier saturation effects for the 1 5 - M e V recoil protons. Better performance for low-energy interactions is readily obtained by increasing the potential supplied to the phototube.
3.2. GAMMA-NEUTRON DISCRIMINATION
The separation of neutron produced events from a background of gamma-ray produced events was investigated for NE-213 liquid scintillator. For purposes of this investigation the possibility that energetic gamma rays or neutrons may produce specifically nuclear instead of atomic interactions within the
3.3. NOISE DISCRIMINATION Assuming that multiple photoelectron noise pulses result from the simultaneous release of electrons in the vicinity of the photocathodeT,S), the dynode current pulses produced will have a shape determined by the phototube transit time properties. According to
P U L S E SHAPE D I S C R I M I N A T I O N
117
PSS output
(a) Noise alone
(b) Noise + Fe 55 Dynode pulse height
Gamma-noise discrimination in Nal scintillator; E v = 5.8 keV; R 1 C 1 = 0.25 ~t sec, R z C ~ = 0.20 ~t sec Fig. 9. The photographs represent a two-parameter display of the output of the PSS circuit, indicating the separation of scintillation induced pulses from photomultiplier noise pulses of similar amplitude.
manufacturers specifications for the 56 AVP, a current pulse with a 2 nsec risetime and a 2 nsec pulse-width should result• This time distribution of anode current is sufficiently peaked to be readily distinguished from the current pulses associated with phosphor scintillations10). Since the noise pulses correspond principally to the emission of only a few photoelectrons, we should consider the discrete nature of this photocathode current• Let us examine the specific example of a noise pulse produced by two photoelectrons and a scintillation induced pulse resulting from the emission of two photo-electrons separated in time by 10 nsec. The resulting 5 nsec difference in zero-crossing times for the double R C differentiated signals will provide adequate separation of these two pulses• In practice, the time separation of photo-electrons will be influenced by statistical fluctuations in the emission of photons from the scintillator and fluctuations in the production of photo-electrons at the cathode. The resulting distribution of time separations precludes a clearcut distinction between noise pulses and scintillation produced pulses. The best separation will clearly result for pulses produced by at least several photoelectrons and for a scintillator with a long principal ceday constant. A thin-window NaI crystal was used to detect the 5.8-keV X-rays from an Fe ~5 source resulting in the two-parameter display of fig. 9. The use of the psd
I
I
I
I
[
I
I
I
t
NaT Scintillator
6000
F'e~6 Source
~ooo
E),. =5.8 keY
4000 3000
.,."-•... Ungated
• ' ...~
~. zooo = ~o "I 0 0 0 ~ g o o
"'...
o
•...,
...••..,.
3000
/ j Gated by PSD ./,J
aooo lOOO
o
"'......' I
20
40
i_
60
80
I
I00
•'i.......,.I
120
140
CHANNEL NUMBER
Fig. 10. Pulse-height spectrum of the 5.8-keV X-ray from an Fe 55 source detected in a NaI scintillator• The reduction in the number of noise pulses accomplished by pulse shape discrimination is demonstrated in spectrum (b).
118
M.L. ROUSH et al. 10 4
Tz, 500nsec
~
Tz. 2OOnsec
104El-
STILBENE
\
eutron
,o./
"~ /
(nsec)
/
~
1041-
k E
. . . . . .
~';I°°~':°
IOI
\
~
--
'~"' ~ 103
2~ .......
J
/ I i
I
~%
,0'~
I0 4
'1 t STILBENE 1 Gamma- Neutr°n
I I
-
'
~-----~
I01
RzCz(nsee)
10
NE-213 oo=o-N.,o°
',
,
IO'i--
,,
32
RiO, I~
"
-.
-
......
/ /
to-o-~
J
,2-,, :6un :ec.../
~ --:-
, ......... 10 3
, ...... RlCl(n see)
I01
<
I
I04
i
i
,,
---~
GNJEm~IS-Neuiron
'...\
----
i0 z
r~-~,:~'o .... 10 3
\?0
\ ~
t
i
~_T,:,5 nsec_1
lO 2
I0
v~/
\
R2C2(nsec)
T2-T' =25 nsec~,~' I T2-T:20n sec- I ~ ~
(nse,)~_ /d
~
"
"12\~
/'X~x,(-~~X. ..
'
\\
r/ / /
' \h.[
I0
~""\',g "Tz=O'SFsec~'\ /Ta'LO,u'sec.
R,GI i"
I 104
Fig. 11. F i g u r e - o f - m e r i t values indicative of the ability of the circuit to separate g a m m a - r a y events from neutron-induced events in a stilbene scintillator. The dashed lines indicate the zero-crossing times for recoilp r o t o n events in (a) and the difference in crossing times in (b).
gating signals to reduce the noise background is demonstrated in fig. 10. 4. Circuit performance 4.1. CHOICEOF R C TIME CONSTANTS
Calculations were made to investigate the effect of the R C time constants. To obtain adequate performance of the pulse shape discrimination circuit over a wide range of pulse amplitudes requires careful
,T'-?=~:e1~..'
....
RzCz(n sec)
103
'
........ 104
Fig. 12. Figure-of-merit values indicative of the ability of the circuit to separate g a m m a - r a y events f r o m n e u t r o n - i n d u c e d events in an NE-213 scintillator. T h e dashed lines indicate the zero-crossing times for r e c o i l - p r o t o n events in (a) and the difference in crossing times in (b).
consideration for small pulses. The gradual slope of the small pulses as they cross zero makes it difficult to determine the crossing times accurately. The crossing times and slopes at crossing were calculated for various dynode time constants, R1C1, and various differentiator time constants, R2 C~, using equation (3). If Ta and 7"2 represent the zero-crossing times for the two types of events of interest, then the ability to separate the types of events depends upon the magnitude of (7"2 -- T1) as well as the accuracy of measure-
119
PULSE SHAPE D I S C R I M I N A T I O N 10 4
I0 4 \,
Tz : 500 nsec __..~
Tz • 1.0F sec ~ ) ~ , T2, O,5~ s e /
-//
i0 3 RICI
\~,-T z \ • t.5F sec
NoI
-
e
NE-150 Scintilla tor ~
m
a
N
o
i
s
/ /
\
,'W'- !
(nsec)
(n sec)
".
35
"IO ~
I0 i
T2=200nsec !
t
i
i
i~laa
i
I0 =
I0
104
T2-T~=50. . . .
i
.~ i
i
i
RaCz(nsee) ~'~
I
I~al
e
i
I
i
'i
I
i
I
i ii1"1
i0 z
IO
104
i NE-150 S c i n t i l l a t o r ~ Gamma-Neutron |
i
I
lij
I0 3
:4Onset
I"
i"
/
r,, L , I "s 0
tO~ RICI
, J- ~
IIIIIT
I
I
I i I
I!
104
I0 ~
L
,
No Gamma-Noise
/ / ~
~ -
-- TUTI=300 n sec
-
BO I00 120
/,/
f.l"
R~CI
(n se c)
i
I
~ ~/"
/ i
i
R2Cz(nsec)
104 ~ /' F / ~ Tz-T~-400 n s e ~ . /
1
I
~
"
~
- -
(n see] -_
" 353
~
=i
J
T2%= 3 0 n s e ~ , ~.~_T2-Tj = 50 n sec Ta-TI= 40 n s e c - - ~ , I
I
I
I
I I I I I
Jl
I/
IOs RzCz (n sec)
I
~ I I I I
I
I03
I
I
t Tz~TQ,5 O n s _ e ~ I
I
/ T . - T . - Z-5 0 n s e c . -
I I1
|
i0 ¢
Fig. 13. Figure-of-merit values indicative of the ability of the circuit to separate gamma-ray events from neutron-induced events in an NE-150 scintillator. The dashed lines indicate the zero-crossing times for recoil-proton events in (a) and the difference in crossing times in (b).
I
I
I I I I I 1 ~
~ / ~ ' T ' 2 ; ; 0 0 0 n :22c ~
I
_ I
I
.
IIIIIJl
IOZ RzCz(n sec)
' 10a
[
I
I
I l l l J
104
Fig. 14. Figure-of-merit values indicative of the ability of the circuit to separate noise produced pulses from pulses resulting from scintillations within a NaI scintillator. The dashed lines indicate the zero-crossing times for scintillation events in (a) and the difference in crossing times in (b).
ment of crossing times. A figure-of-merit for a particular set of parameters may be defined as F.M. ~ (T2-- T]). (Slope)x[Tz
IO
]
(4)
where the slope at zero crossing is calculated for both pulses and the smaller of the two values is used for (Slope)xlmz. The details of the calculations are outlined in appendix A. The magnitude of the figures-ofmerit should be indicative of the actual experimental ability to separate the pulses of interest.
Fig. 11-13 indicate the results for g a m m a - n e u t r o n disrimination in the commonly used scintillators, stilbene, NE-213 and NE-150. The contours of constant value for the figure-of-merit indicate the region for optimum operation with each scintillator. There are, however, a number of auxiliary factors to be considered when selecting the time constants. The extent of the counting rate effects and the time delays necessary to allow gating will be determined by the time until zero crossing which is shown by the dashed
]20
M.L. ROUSH et al.
lines in part (a) of each figure. It is of course essential to verify that the difference in crossing times shown by the dashed lines in part (b) of each figure is sufficiently large to be compatible with the time resolution of the circuits used. When a single dynode output is used for both the linear analysis and the pulse shape discrimination, somewhat larger values of R C will be desirable. It should be recognized that selecting the maximum figure-of-merit is designed to optimize the ability of the circuit to separate events and not to optimize the time separation of the zero crossings. The degree to which the results depend upon the pulse shapes assumed, was studied by varying the decay constants and relative amplitudes. The results indicate that the magnitude of the figure-of-merit values is very sensitive to the shapes while the position of maximum varies only slightly. Even though the sensitive nature of the magnitude of the merit values makes the comparison between different scintillators particularly perilous, the large values obtained for NE-150 warrant special comment. The large values, when contrasted with the limited shape discrimination which has been obtained experimentally for this phosphor, must cast considerable doubt upon the reliability of the shape parameters availableg). If the shape parameters assumed are a reasonable representation of the actual case for a particular scintillator, then the comparison of the relative magnitude of merit values for different components in a single circuit or for different circuits should be quite reliable. Fig. 14 shows the results of calculations for the discrimination between noise pulses and scintillations within a NaI scintillator. The manufacturers specification of transit time properties for the 56 AVP photomultiplier indicate a width at half maximum for noise related anode current pulses of 2 nsec. To simplify the calculations, these current pulses were represented by a shape such as would be produced by scintillations with a decay constant of 2 nsec and negligible time dispersion within the phototube. The effect of this assumption was studied by making additional calculations for decay constants of 1 and 3 nsec. No significant differences were observed, indicating that the choice of this decay constant is not critical in predicting the region of optimum R C time constants. 4.2. OPTIMUM USE OF THE PULSE SHAPE DIFFERENCES Detailed statistical considerations have been made by Gatti and De Martini 11) to determine the optimum
treatment of pulses in order to extract the maximum possible information from the pulse shapes. The result of their calculation is that the optimum results will be obtained by weighting the input pulses by a time-dependent function, iz(t) -- ia(t) P(t) -- ie(t) + il (t)
(5)
where il(t) and iz(t) represent the average current pulse shapes resulting from the two types of exciting particles being considered. Thus, as might be expected, the maximum importance should be attached to those parts of the signal where the differences are relatively large. Consider the specific case of the discrimination between recoil protons and recoil electrons within a stilbene scintillator. The function P ( t ) which gives the optimum weighting of pulse amplitude is shown in fig. 15a.
P (t)
(a)
STILBENE SCINTILLATOR
f 40
8'0
~ (b) 40
120 ~ t (n sec)
160
RI CI,, R2C2 - 175 nsec STILBENE SCINTILLATOR
aOT~(nsac}~O
leo
Fig. 15. (a)Optimum weighting of input pulse amplitude. (b) Weighting of input pulse amplitude by measuring zerocrossing time.
It is of interest to estimate how efficiently the present technique uses the pulse shape information. The importance or weight assigned to the input signal at a given instant is determined by the sensitivity of the crossing time to the amplitude at that point. This sensitivity was determined by calculating the shift in crossing time produced by a small perturbation introduced at various positions along the signal. The shape of the doubly differentiated dynode pulse
PULSE SHAPE DISCRIMINATION for a recoil proton in stilbene, calculated from equation (3), was normalized to represent a total integrated charge produced by 200 photo-electrons. The signal produced by a single photo-electron released a time Te after the beginning of the basic pulse was also calculated. The signals were added linearly and the resulting shift in crossing time observed. The results obtained are displayed in fig. 15b. Thus the quantity to be measured (zero-crossing time) is determined by the integrated effect of the complete signal preceeding this time with the various portions of the signal weighted as shown. It is worthwhile to compare this fairly efficient use of the information with the more standard procedure of comparing the fast portion of a pulse with the total integrated pulse. The process of stretching a sharply differentiated anode pulse produces a signal rather closely proportional to the peak anode current. This peak current, determined by a small fraction of the total signal, will be subject to considerable statistical fluctuations. 4.3. COUNTING RATE EFFECTS Measurements described in section 3.2, made at low counting rates, indicate a rejection ratio of 500:1 for 0.3-10 MeV g a m m a - r a y events with negligible loss of neutron induced events within this range of pulse amplitude. At high counting rates a second g a m m a ray m a y be detected before the zero-crossing of the first, delaying the zero-crossing and hence causing the generation of an unwanted gating signal. The resolving time for overlap of such pulses is the zero-crossing time, which is roughly 1/zsec in length. The rejection ratio was observed to drop to 50:1 at a counting rate of l04 counts per second. This still provides a quite adequate reduction in the random background rate due to g a m m a rays without loss of the desired neutroninduced events.
121
5.1. R1C1-R2C2 CIRCUIT The removal of the cathode follower which has isolated the two RC sections, results in the circuit indicated in fig. 16. For a dynode current pulse represented by equation (I), the output signal will be,
V3(t) = K1 exp (--t/e) 6- K~. exp (--tiff) 66- Ks exp {--(L--M)t} 6- 1£4 exp {--(L6-m)t} (6) where rl = R1C1 and ~z = R2C2,
( RI -+-R21 1 M=½
(1 =
I_~16- (R16-R212
1
1 ½
A(1/7) CI(M--L6-1/e)(L6-M--1/~) B(]/~) CI(M--L6-1/fl)(L6-M--I/fl)
Kz = - -
1 [ A (M--L) B(M--L) ] 1(3 = ~ 2 M ( M - - L 6- l/e) ~- 2 M ( ~ - - L + 1/fl)J 1(4 =
l I-
:4(L+_M)
B(L+M)
M) +- 2 M(17 / Z-L-
1 M) "
It can be seen that the signal now depends upon the values of Ra and R2 as well as the two RC time constants. By assuming the distributed capacity of the dynode and related circuitry to be 30 pF, the value of R1 is automatically determined when ~1 is specified. Figure-of-merit calculations were made by specifying the value of R2 for each set. Fig. 16 displays the maximum figure-of-merit for a range of 25
c~
5. Alternate circuits
The double RC differentiation of dynode pulses which has been described in the previous sections was chosen for two principal reasons. The simplicity allows the functioning of the double RC circuit to be easily understood and the effects of varying the time constants to be readily predicted. Secondly, the use of a single dynode signal for both linear amplitude analysis and pulse shape discrimination provides convenience and simplicity. A number of modifications of this basic circuitry will now be considered. These circuits are still based upon the measurements of a crossingtime to indicate the character of an event.
ao
)_T__~ F_Eo v3(') i
| i 5 I0 z
I 103
II 04
I I0 ~
I IC
R2 { O h m s )
Fig. 16. Dependence of the maximum figure-of-merit upon the value of Rg. for the indicated circuitry.
122
M.L. ROUSH
et al.
values of R~. It is apparent that the low impedance presented by small values for R2 prevents the development of reasonable dynode pulses. An examination of the results reveals that for large Re the figure-ofmerit values approach those of fig. 11.
30
J
~J
5.2. RzCI-cF-R2LC2 CIRCUIT The difficulty of measuring the crossing times for small pulses can be reduced by adding an inductance in the RC circuit to produce a larger slope in the signal at the time of zero crossing. Care must be taken to insure that the circuit does not ring. Sets of figure-ofmerit values were calculated in which R1 C1 and R2 C2 were varied for fixed values of a parameter ~. The parameter ~ is defined as
C2
C
o u.
IC R2
IE
o
o'2
o14
o!~ "q
2
~1 : 4 L/R2 (22,
01B
~io
>
Fig. 17. Dependence of the maximum figure-of-merit upon the value of the inductance in the indicated circuit.
and thus relates to the degree of damping present for a particular inductance L compared to critical damping which would occur at an inductance value 2
Lc ~- ¼ R2 C2. For a dynode current pulse represented by equation (1), the output signal of the circuit shown in fig. 17 will be, V4(t) kl exp (--t/~) 4- kz exp (--t/fl) 44- ka exp (--t/RaG) 4- k5 exp {--(D 4- E)t} 4- k4 exp {--(D--E)t}
ka
~
m
:
where
D = ½ R2/L
E
4(7)
( _1 = 4 L 2 L C2]
k2 =
c1( R2
Ra Ca k3 = ~
1
+
Rz C1
1
D
Ra Ca
1 (0-
E-- ---R1 Ca )
E - -
-
B
--A
k4 =
1 R1 C1
-
r +
m
R1 Ca
4-
(E-- D) ( D - - E -R2)-~ k5 =
2 CIE
I/1
--A 1 (DR1 Ca )
B1
+ 1 RICI)
1 (D 4 - E - - R1C-----~) B
÷
R1 C1 + l
1 , i i ~zA >R:Cz 1
~D - E - - R--1C--~]
1
1
+1
1
fl
R1C,
>]
123
PULSE SHAPE D I S C R I M I N A T I O N
Fig. 17 shows the increase in figure-of-merit values resulting from the inclusion of the inductance. Figure-of-merit contours in fig. 18 indicate the region of optimum operation for the use of a critically damped R L C circuit for the "differentiation" process. 5.3. DELAY LINE PULSE SHAPING The use of a shorted delay line to produce the signal which crosses zero allows the use of relatively long R C time constants and an effective integration of the slow component of the pulse. The necessity of proper termination of the open end of the delay line produces a I0' Tz =400 nsee-
\ \ ~.~
$filbene
Gamma Neutron
R~C~
(nsec)
We would like to express our thanks to H.L. Fann for help in preparing the computer program used to make the figure-of-merit calculations and to J.E. Etter who constructed the electronics used to provide the two-parameter oscilloscope display. We also wish to express our appreciation to the University of Maryland Computer Science Center for providing computing time on the IBM 7090/1401 computer.
[0 ~'J
b,l_
/
/',
/
reduction by two of the pulse amplitude available. The figure-of-merit values including this reduction in amplitude are shown in fig. 19 and indicate that this circuit should operate well for pulses of small amplitude. Such circuitry has been tested and provided good discrimination over a limited range. Difficulty was experienced in attempting to obtain proper operation for a large range of pulse amplitude due to overload effects caused by the long large pulses. Alexander and Goulding 5) have followed a somewhat similar procedure, although their comparison of two zero-crossing times results in larger experimental uncertainties and requires more complex circuitry.
\'
',_
15
\
2o,,\ \
2T:DT
TZ=I00 nsec- "-"~'\ ,
,
,
,
.....
I0
104 .. ii
,
I0 z
i
RzCz(nsec)
Stilbene
I
Gamma -Neutron
L
/
/~
^_ r R~C2I
--ff~o.~ ~ LTJ
\\ . . . . . . .
/
\I i
,
,
,
,,,~j~
io ~
io 4
j ~T~_Tt= 15nsee I\\ I J
I
T,
--T2-TI=20 nsee [ i
,r
io'
l
T2-T,=5Onsec 1 1 r
/\/ //
/ \///,
(n sec) I0 ~
I
RjCl
~
i
l
I
Tz_Tl=25n___ /
RC
\
~ / //
(n sec iO2
STILBENE Gamma
-
Neutron
Delay - Line Clipped
I0:
: , IO
, , iiiiii
I i~1 ,tl/ll,lH 10 2 RzC2(n see) I0 ~
1n~/~
~
- - - / I
L_../.
i i l,,,,
I
I
i
i
I i
I i
!
J TZ: 200n sec--'1
I04
Fig. 18. Figure-of-merit values indicative of the ability of the circuit displayed in fig. 17 to separate gamma-ray events from neutron-induced events in a stilbene scintillator. The dashed lines indicate the zero-crossing times for recoil-proton events in (a) and the difference in crossing times in (b).
)
Jl
I IO
Tz=3OOnsec I i iiiiii lot
I 'J I'~l'~ =500nsec i I i i i iiii I j i iii DT(n sec)
103
104
Fig. 19. Figure-of-merit values indicative of the ability of the delay line shaping method to separate gamma-ray events from neutron-induced events in a stilbene scintillator.
M.L. ROUSH et al.
124
Appendix A FIGURE-OF MERIT CALCULATIONS The e x p e r i m e n t a l ability to separate small pulses is i n d i c a t e d b y the m a g n i t u d e of the f i g u r e - o f - m e r i t defined in section 4.1. T h e expression for V~(t) is n o t a n a l y t i c a l l y soluable for the z e r o - c r o s s i n g time. A c o m p u t e r code was d e v e l o p e d to locate the value of t for wich V~(t) changes sign, the size of the i n c r e m e n t s in TABLE 1
Scintillator and pulse characteristics.
B
Scintillator (nsec)
fl Refer(nsec) ence
t being decreased until the desired degree of a c c u r a c y in the crossing time was o b t a i n e d . T h e slope of the pulse as it crosses zero is then r e a d i l y o b t a i n e d by calculating the derivative of V2(t). These calculations were carried o u t for families of values for the R1 C1 a n d R2C2 time constants. The time c o n s t a n t s used to describe the shape of the current pulses are listed in table 1. T h e ratios of slow c o m p o n e n t to fast c o m p o n e n t were t a k e n f r o m the i n d i c a t e d references while the n o r m a l i z a t i o n is chosen so that each c u r r e n t pulse will p r o d u c e a d y n o d e voltage pulse of s t a n d a r d p e a k a m p l i t u d e using an R1 (71 of 7 #sec, such as is c o m m o n l y used in linear pulse analysis e q u i p m e n t .
References Stilbene
(e-)
(p+)
0.933 0.779
5.6 6.0
0.0616 0.0545
49 67
NE.-213
(e-) (p+)
1.05 0.824
5.2 5.6
0.0284 0.0288
107 138
NE-150
(e-) (p+)
1.004 0.794
3.4 4.0
0.0904 0.0532
54 100
NaI(TI) Noise
(e-)
0.0362 4.02
250. 2.0*
12
* Determined by photomultiplier characteristics (see section 4.1).
1) F.D. Brooks, Nucl. Instr. and Meth. 4 (1959) 151. z) R.B. Owen, I.R.E. Trans. Nucl. Sci. NS-8 (1961) 285. 8) M. Forte, K. Konski and C. Marazana, (UN Atomic Energy Agency Conf. Belgrade 1961 NE/59). 4) D.G. Foster, Hartford Report HW-74190. 5) K. Alexander and F.S. Goulding, Nucl. Instr. and Meth. 13 (1961) 244. 8) R.E. Green and R.B. Bell, Nucl. Instr. 3 (1958) 127. 7) J.A. Baicker, I.R.E. Trans. Nuclear Sci. NS-7 (1960) 74. 8) J.R. Prescott, Nucl. Instr. and Meth. 22 (1963) 256. 9) K. Peuckert, Nucl. Instr. and Meth. 17 (1962) 257. 10) Damerell, Nucl. Instr. and Meth. 15 (1962) 171. 11) E. Gatti and F. De Martini, Nucl. Electronics 2, p. 265; (UN Atomic Energy Conf., Belgrade 1961). 12) j. Sharpe, Nuclear radiation detectors (Wiley 1955).