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Measurement of the response of several organic scintillators to electrons, protons and deuterons
NUCLEAR
INSTRUMENTS
AND
MEASUREMENT
METHODS
64(1968)
OF THE RESPONSE TO ELECTRONS,
157-166;
0 NORTH-HOLLAND
OF SEVERAL
PROTONS
ORGANIC
PUBLISHING
CO.
SCINTILLATORS
AND DEUTERONS
D. L. SMITH, R. G. POLK and T. G. MILLER U.S.Army
Missile
Command,
Redstone
Arsenal,
Alabama
35809, U.S.A.
Received 16 May 1968 The response of several common organic scintillators to electrons, protons and deuterons has been measured using a photomultiplier tube with S-l 1 spectral sensitivity. The Compton scattering of monoenergetic beams of gamma rays and neutrons was used to induce the charged particles to recoil with specific energies in the interiors of the scintillators. Pulse-shape discrimination was employed to distinguish between neutron and gamma-ray events. The following scintillators were studied as indicated: anthracene (electrons: 0.234.99 MeV, protons: 0.24-15.0 MeV); stilbene (electrons: 0.31-0.99 MeV, protons: 0.35-15.0 MeV); NE-213 (electrons:
0.16-0.99 MeV, protons:
0.24-15.0 MeV); NE-102
1. Introduction Many organic crystals, liquids and plastics scintillate when bombarded with nuclear radiationsle3). Light is emitted in response to either primary or secondary ionization induced by the radiation. A number of these scintillators have found widespread use in nuclear spectroscopy because they possess desirable physical characteristics, exhibit good detection efficiency, and their response times allow relatively simple derivation of fast timing signals required for some experiments4). The response of these organic materials is usually critically dependent upon the specific ionization of the detected particles. Some of these scintillators respond to an ionizing particle by emitting a light pulse which is the sum of a short (a few nsec) and a long (tens to hundreds of nsec) decay-time component. The relative intensity of these components, and therefore the shape of the composite pulse, depends on the nature of the ionizing particle. Electronic circuits which distinguish between pulse shapes can be used in conjunction with these scintillators to detect a selected type of particle and discriminate against others5). The response of organic scintillators to electrons is quite linear with particle energy above approximately 100 keV. Furthermore, linear extrapolations of the electron response from higher energies generally pass within a few keV of the origin6). The response of these scintillators to heavier ionizing radiation is generally nonlinear with a light output which is substantially less than that for electrons of the same energy. Neutral gamma rays and neutrons can be detected by scintillators because they collide with particles in the
(electrons: 0.31-0.99 MeV, protons: 0.35-15.0 MeV); Pilot B (electrons: 0.31-0.99 MeV, protons: 0.35-15.0 MeV); and NE-230 (electrons: 0.16-0.99 MeV, deuterons: 0.25-11.0 MeV). Allmeasured responses for a given scintillator were normalized to its measured response for 0.525 MeV electrons (arbitrarily assigned the value 1000). These data were used to compute p/p and d/p ratios which are relatively insensitive to detector geometry. The scintillators studied exhibit ratios which are remarkably similar over a wide range of particle energies. They increase non-linearly from M 0.1 in the 0.3-1.0 MeV range to M 0.5 in the 12.0-15.0 MeV range.
scintillator material and cause them to recoil. In organic materials, gamma rays interact primarily with atomic electrons by the Compton effect whereas neutrons scatter elastically from the nuclei of the atoms in the scintillator. Organic scintillators are good neutron detectors since they contain light elements whose nuclear recoils can be detected easily. For several good scintillators, neutron events can be distinguished from gamma-ray events by the pulse-shape discrimination (PSD) techniques already mentioned. Information about the responses of organic scintillators to ionizing radiation is necessary for understanding the basic light producing mechanisms and is invaluable for laboratory applications3). Numerous studies of the response of these scintillators to various radiations have been reported. Birks has reviewed the literature rather thoroughly 1963 “). A partial list of references to more recent work is included in the present paper6-19). Most of the measurements reported have resulted from studies of only one or two scintillators at a time over narrow energy ranges. The present work is a systematic study of several commonly used organic scintillators over relatively wide ranges of particle energy. We have measured the response of anthracene, stilbene, NE-213, NE-102 and Pilot B scintillators to 0.3-15.0 MeV protons and of an NE-230 scintillator to 0.3-l 1.O MeV deuterons. The response of these scintillators to 0.3-1.0 MeV electrons was also investigated. 2. Experimental details The response of a scintillation detector (scintillator plus photomultiplier tube) to radiation depends on 157
158
D.
L. SMITH
several geometrical factors including the size and shape of the scintillator, the quality of the light reflector and location of the scintillation within the scintillator’7). For example, the size of the recorded pulses will be reduced when the light from the scintillations loses intensity due to reflections and transmission before reaching the photocathode. There are two methods for measuring the response of a scintillator to charged particles. The direct method is to bombard the scintillator with external beams of charged particles. The indirect method is to bombard the scintillator with monoenergetic beams of neutral particles such as gamma rays and neutrons of which a portion will elastically scatter from charged particles within the scintillator. Unique energy recoils will be recorded only if one detects the elastically scattered neutral radiation in a second detector, placed at a specific angle relative to the incident beam direction, in time coincidence with the recoil pulse. The indirect method was employed in the present work. This approach has the advantage of minimizing surface effects which can be important at low energies. Furthermore, the relative response of a scintillator to different charged particles can be measured with greater reliability since the penetrating primary radiations generally produce recoils rather uniformly throughout the bulk of the scintillator thereby minimizing the geometrical effects discussed previously. Measurements of relative response made by this method are reasonably insensitive to both geometrical and instrumental considerations. Fig. 1 is a schematic diagram of the apparatus used in the present experiment. A scintillator under study was attached to the center counter which scattered the primary radiation and detected the recoils. The side
et al.
detector was shielded from the primary radiations by lead and borated paraffin in order to reduce random coincidences. An approximately monoenergetic gammaray beam (gamma-ray energies of 1.17 MeV and 1.33 MeV) was produced by placing a sealed 1 mCi 6oCo source in a lead brick collimator which could be inserted in the shield opening. Monoenergetic neutron beams were produced by bombarding various targets with proton and deuteron beams from a 2 MV Van de Graaff accelerator. This scattering apparatus has been described in earlier communications*‘** ‘). The energies
_L__1
INCIDENT PARTICLES
YA
._ :.-. .,._
;
-
._ :
_ ._- :
/
I . r; . ; *.
SIDE
SHIELD
Fig. 1. Schematic diagram of the scattering apparatus used in the present study of scintillator response. A collimated beam of primary radiation is scattered by the center detector and detected in the side detector. The scatterers recoil and produce scintillations in the center detector.
TABLE
1
Sources of recoil particles. source of reaction
Accelerator Beam Energy (MeV)
DETECTOR
(MOVABLE)
Primary particles Energy Type (MeV)
Recoil particles Energy range Type (MeV) e-
0.16-0.99
1.77
1.17 and 1.33 0.97
1I
1.77
1.47
1.67
4.88
0.60
16.03
P d P d :
0.25-0.75 0.19-0.76 0.38-1.13 0.50-1.13 1.264.70 0.47-3.37 5X7-15.17 3.08-10.87
THE
RESPONSE
OF
ORGANIC
TABLE 2
Accelerator
targets.
Composition
Target
Preparation
Carbon
Natural carbon film (20 keV thick for 1.8 MeV protons) on a thin tantalum disc
Colloidal suspension of graphite in water evaporated on metal disc with a heat lamp
Deuterium
300 rug/cm2 deuterium absorbed into a 1 mg/cma layer of titanium on a copper disc
Commercial
Tritium
1 Ci/ina tritium absorbed into a 300 [&g/cm’ layer of titanium on a copper disc
159
SCINTILLATORS
of the recoil particles can be calculated from kinematics. Table 1 lists the ranges of recoil energies available for study in the present work. The accelerator beam energy was calibrated by detecting the 1880.7 f 0.4 keV threshold point for the 7Li(p,n)7Be reaction and the 1746.5 + 0.5 keV resonance of the 13C(p,y)14N reaction. The targets used in the present work are described in table 2. Corrections were made for energy losses in the targets, and these effects were considered in estimating uncertainties in the calculated recoil energies. Fig. 2 shows a breakdown of the various functions performed by the experimental apparatus. Pulse-height spectra from the center detector, representing the recoil particles, were recorded in a multichannel pulse-height analyzer, subject to fast coincidence and y-n discrimination condrtions in the center and/or side counters. These conditions could be switched in or out individually as required.
SCATTERED RECOIL DETECTOR (CENTER)
MOVEABLE SIDE DETECTOR
TIMING
PVLSE HEIGHT -
PULSE HEIGHT
1
. FAST
COINCIDENCE v
l
.
v
NEUTRON-
NEUTAON-
GAMMei RAY DlSCRlMlNeiTlON
GAMMA RAY DISCRIMINATION
-
SLOW
LOGIC AND
PULSE-HEIGHT
Fig. 2. Schematic
diagram showing the principal operations only neutron-induced events which
_
SIGNALS
performed by the electronics. The circuitry permits the option register fast coincidences between the two detectors.
of recording
160
L. SMITH
D.
et cd.
TABLE3 Scintillators. Scintillator
Anthracene (crystal) Stilbene (crystal) NE-213 (liquid) NE-230 (liquid) NE-102 (plastic) i Pilot B (plastic)
CENTER #_-______-__---
Source
Harshaw Chemical Co. Cleveland, Ohio, U.S.A. Nuclear Enterprises Ltd. Edinburgh, Scotland, U.K. Pilot Chemicals, Inc. U.S.A.
DETECTOR
_
_-
Detector geometry
Quality of y-neutron discrimination
l+” dia. x $” high cylinder 1” dia. x +” high cylinder 2” dia. x 2f” high cylinder 2” dia. x 24” high cylinder 2” dia x 2” high cylinder 1t” dia. x 2” high cylinder
good excellent excellent excellent none none
‘_______________~ SIDE
1
DETECTOR
, I I
DELAY I
I I
Re
7 J 1-
TO SELECTOR
TO SELEl
.----
LOGIC
SUBGROUP
BOUNDARIES
Fig. 3. Block diagram of the complete electronics circuitry. Dashed lines show logic subgroup boundaries for comparison
with fig. 2.
THE RESPONSE
OF ORGANIC
Table 3 gives a detailed description of the scintillators studied in the present work. The side detector utilized an NE-213 scintillator indentical to the one described in table 3 throughout the experiment. RCA 6810A photomultiplier tubes with Ashkin type voltage dividers were used in the scintillation probes22). Fig. 3 is a detailed block diagram of the electronics apparatus. Dashed lines divide the various electronics components into appropriate subgroupings to facilitate comparison with fig. 2. Fast timing signals were derived from the photomultiplier anode pulses by tunnel diode integral discriminators23). Pulses whose amplitudes were equivalent to the time integral of the scintillation light pulses were obtained by integrating dynode pulses with preamplifiers and shaping them with double-delay line (DDL) amplifiers. The fastcoincidence detector consisted of the more flexible TAC-SCA arrangement rather than the conventional pulse-overlap detector, with time resolutions of a few nsec achieved in practice using a good quality commercial time-to-amplitude converter and single-channel analyzer. These time resolutions were good enough to distinguish between low-energy (5 1.0 MeV) neutron events and gamma events on the basis of flight times between center and side counters separated by approximately 7 inch. Numerous schemes for achieving pulse-shape discrimination have been reported in the literature. References to a few of these methods are listed in the present paper 5Z24-35). Fig. 3 shows the electronics NEUTRON-GAMMA
9-
DISCRIMINATION
NE - 230
A
SCINTILLATOR
NEUTRON
RELATIVE
TIME
PEAK
( NANOSECONDSj
Fig. 4. Typical spectrum from the pulse-shape discrimination circuitry. The separation of electron and deuteron recoil events in an NE-230 scintillator is exhibited.
161
SCINTILLATORS
A RECOIL ELECTRON SPECTRUM I , I, I / I,, , r I,
1
/
7-
76?5
EOO-
700 -
600 ZI 2 500-• 3 5400 a
l
? 5 300 0 ”
~
. .. .
. . :.* . . .’ l
20
40
0
60
00
100
120
140
CHANNEL
Fig. 5. Typical recoil electron spectrum. Most of the counts in the low-pulse-height tail result from random coincidences. The procedure used in locating the recoil peak maximum is indicated and the estimated uncertainty is labelled.
apparatus we used to achieve y-n discrimination. Fast timing signals were produced by a single-channel analyzer which detected the zero crossovers in the pulses from the DDL amplifier. The time separations between the anode discriminator pulses and the zero crossover time pulses were measured by a TAC. Discrimination was possible because the time separation is different for neutron and gamma-ray events (fig. 4). The recoil pulse spectra were stored in a 400-channel pulse height analyzer. Pulses were recorded only when a linear gate, controlled by the appropriate fastcoincidence and/or y-n discrimination conditions, permitted them to pass through to the MCA. Several different signals could be sampled by means of a selector switch allowing fast setting up and checking of the various circuits. A pulser was used to simulate pulses from the center detector for the purpose of calibration. Accurate calibration was very important for this experiment because runs were made with a variety of operating conditions and the data had to be properly normalized. Most experimental runs were 30 min to 2 h long,
162
D.
A RECOIL
I IO
r
I
,
I
,
L. SMITH
depended critically on the choice of photomultiplier tube high voltage and DDL amplifier gain. The discrimination properties of NE-230 were reported recently3’). The present work supports the contention that good y-n discrimination can be obtained with NE-230 scintillators. Fig. 4 shows a time spectrum for NE-230 obtained with our circuitry. From fig. 4, the separation between the neutron and gamma-ray peaks is 8.9 nsec, and the full width at half maximum of the neutron peak is 4.2 nsec; hence the figure of merit for n-y separation, as defined in 35) is 2.1. For this curve, all pulses were rejected whose light outputs were less than the light output for a 0.25 MeV electron. The neutron source was the T(d,n)4He reaction. The recoil pulse height spectra recorded by the MCA were typed out and also punched onto paper tape. The data on paper tape were transferred to punched cards and were subsequently machine plotted.
PROTON SPECTRUM I’/
/
G
100
!
1
I
’
1 ’
152 212
.
80
d
5 a 5 a
f
70
6o
v) 50 t 5 8 40 30 20
.
. .
ANTHRACENE Ep =3.66 MeV
IO ,
l -
3. Data analysis /
.
0 40
80
120
160
200
240
et al.
280
$0
CHANNEL
Fig. 5 shows a typical electron recoil spectrum and fig. 6 shows a typical proton recoil spectrum. The electron recoil spectra were generally of good quality. Some of the proton and deuteron recoil spectra were difficult to analyze because the recoil peaks were broad
Fig. 6. Typical recoil proton spectrum. Most of the counts in the low-pulse-height tail result from random coincidences. The procedure used in locating the recoil peak maximum is indicated and the estimated uncertainty is labelled.
although some of the electron calibration points were obtained from overnight runs. All data were normalized to the response for 0.525 MeV electrons. Calibration runs were made whenever scintillators were removed from the photomultipliers or high voltages were changed. The buildup on the targets of carbon from the vacuum system and drive-in deuterons from the accelerator led to difficulties in identifying some of the recoil peaks since the neutron beams were not always purely monoenergetic. The best proton and deuteron recoil data were obtained when y-n discrimination was employed for both counters. This was not always possible because NE-102 and Pilot B do not exhibit PSD properties. Furthermore, our circuitry did not effectively discriminate proton or deuteron recoils from electron recoils for recoil energies below w 0.4 MeV. Time-of-flight discrimination using the fast-coincidence circuit proved to be useful for these low-energy recoil measurements. Stilbene, NE-213 and NE-230 exhibited excellent y-n discrimination, but good discrimination was also observed for anthracene. The quality of discrimination
ANTHRACENE
ELECTRO
.=i
E
8
_
co6(Jy
lYc?‘z i
102
d Tfp,n)He3 w Ci2(d,n)N I3 A D(d,n) He 3 q
‘I
1.0 PARTICLE
T(d,n)
I
He4
l>c”l
>
I
I,
IO.0
ENERGY
(MeV)
Fig. 7. Response of anthracene to electrons and protons.
THE RESPONSE
OF ORGANIC
163
SCINTILLATORS
STILBENE
NE-102 I
C”
“‘1”1
“I’
co60y
0
aT(p,n) n
He3
C’2(d,n)
I’
“““““”
A T(p,n)He3 = Cl2 (d,n) N13
N13
AD (d, n) He3
A D (d,n)
He3
m T (d,n)
0 T(d,n)
He4
He4
I PARTICLE
ENERGY
I.0 PARTICLE
(MeV)
Fig. 8. Response of stilbene to electrons and protons.
IO.0 ENERGY
(MeV)
Fig. 10. Response of NE-102 to electrons and protons.
PILOT
I-
B
, I’
A T (p,n) He3 n
q
I
0.1
I
!
1
I111111
I
T(d,n) I
I
He4
IllIll
1.0
PARTICLE
t 10.0
ENERGY
(MeV)
Fig. 9. Response of NE-213 to electrons and protons.
I
I
Cl2 (d,n)
N13
A D (d,n)
He3
a T (d,n)
He4
II111111
Il!lll 1.00 PARTICLE
ENERGY
I
10.0 (MeV)
Fig. 11. Response of Pilot B to electrons and protons.
164
D. L. SMITH
et al.
relative responses of the scintillators to 0.525 MeV electrons are given in table 4. These data are not as reliable as those presented in figs. 7-12 because of uncertainties associated with attaching the scintillators to the PM tube. Furthermore, it is difficult to compare the values in table 4 with others reported since absolute responses are a function of the sizes and shapes of the scintillators and other experimental factors17). Fig. 13 shows the p/p and d//I ratios which were computed from the response data in figs. 7-12. It is striking that these ratios are so similar over a wide range of energies even though the absolute responses which were measured differ by as much as a factor of 3 (table 4). The ratios are E 0.1 for particle energies in the range 0.20-1.0 MeV. They increase considerably between 1.0 MeV and 4.0 MeV and then level off to
ELECTRONS
TABLE 4
Measured
/
/‘ 4 C’2 (d,n) N I3 A D (d,n) He3 q T(d,n) He4
relative responses for the scintillators present work*.
used in the
n
/ 1.
I!(11
I .o PARTICLE ENERGY
I
!I,
Scintillator :’
Response to 0.525 MeV electrons relative to anthracene
10.0 (MeV)
Fig. 12. Response of NE-230 to electrons and deuterons.
and partially smeared by background. Several runs were required to obtain usable data for some of the lower energy recoils. The peak maxima were located and used in calculating the relative responses. Figs. 5 and 6 indicate a procedure used for locating the peak maxima and also show the corresponding estimated uncertainties. The uncertainties in locating the peak maxima were the major contributors to the overall uncertainties in the measured responses. An additional + 5% uncertainty was assigned to each data point to account for intangible instrumental effects such as possible amplifier gain drifts and high voltage instabilities. Uncertainties in the recoil energies of the electrons, protons and deuterons were also estimated. These were too small to be shown on plots of the final results.
anthracene stilbene NE-213 NE-230 NE-102 Pilot R
1 1.21 * 0.79+ 0.70 + 0.38 + 0.40 +
* The size and shapes of these scintillators is given in table 3. The measured response of a scintillator depends critically on its size and shape, quality of the reflector and other factors. This fact should be kept in mind when comparing the values in this table with other values reported in the literature17).
ANTHRACENE STILBENE PILOT B NE-213 NE-102
4. Results and discussion The results of the present series of measurements are shown in figs. 7-12. Although it is not entirely correct (section l), it has been assumed that the electron response is linear and passes through the origin. The relative response data are plotted on scales which arbitrarily assign the value 1000 for the response of each scintillator to 0.525 MeV electrons. The measured
0.15 0.11 0.09 0.05 0.05
D/B
ENERGY
-. -..-.. -...-.
RATIO
NE-230
PARTICLE
----
-
IMEVI
Fig. 13. Calculated relative responses. Ratios of p//3 and d/p are computed from the data presented in figs. 7-12.
THE
RESPONSE
OF ORGANIC
z 0.5 in the energy range 10.0-15.0 MeV. Anthracene and stiibene scintillators appear to have the largest p/P ratios at low energy. This fact, along with their good y-n discrimination properties, makes them desirable neutron detectors although large crystals are costly. Another interesting feature of the results presented in fig. 13 is that the p/p ratio for NE-213 is proportional to the d/b ratio for NE-230 over a large energy range. This may occur because these organic liquids are chemically similar. Many measurements of p//3 ratios have been reported in the literature. No attempt has been made by the authors to tabulate all of these; however, the results of
Comparison
Energy (MeV
.4nthracene
1 3 10 15 1.8 2.5 5.0 7.5 10.0 0.2 2 4 6 8 2.64
NE-102
Pilot B
* Estimated uncertainty
few
of
the
measurements’,6,11,12,14,~6,~9,36-4~)
are compared with the present work in table 5. Some of the values of p/p reported in a review article by Swank are much larger than those we have measured’). However, most of the measurements we have seen reported in the literature agree fairly well with our own to within our estimated 15-20% uncertainty. The authors would like to thank B. Sparling for considerable assistance during the experiment, R. Craun for help in preparing the figures, and Mrs. S. M. Ray for expertly typing the manuscript.
TABLE5 of some relative response measurements.
Scintillator
NE-213
a
165
SCINTILLATORS
Response ratio
Reference and reported result
Result from present work*
P/B
1) 0.54 0.64 0.73 0.75 3s) 0.26 0.27 0.36 0.45 0.48 33) 0.20 37) 0.23 0.33 0.42 0.47 16) 0.34
0.14 0.29 0.44 0.48 0.27 0.34 0.48 0.54 0.56 0.13 0.23 0.35 0.44 0.50 0.30
P/B
P/B P/B
PIP
15-20x.
References 1) R. K. Swank, Ann. Rev. Nucl. Sci. 4 (1954) 111. a) C. G. Bell, Jr. and F. N. Haynes, Liquidscintillation counting (Pergamon Press, Oxford, U.K., 1957). 3, J. B. Birks, The theory and practice of scintillation counting (Macmillan, New York, 1964). 4, J. B. Marion and J. L. Fowler, Fast neutron physics 1 (Interscience, New York, 1960). 5, F. D. Brooks, Nucl. Instr. and Meth. 4 (1959) 151. 6, F. T. Porter et al., Nucl. Instr. and Meth. 39 (1966) 35. 7) Scintillator Catalogue, Nuclear Enterprises, Ltd., Edinburgh Scotland. s) E. Brannen and G. L. Olde, Rad. Res. 16 (1962) 1. g, K. Peuckert, Nucl. Instr. and Meth. 17 (1962) 257. 10) R. Eisberg, S. Mayo and W. Schimmerling, Nucl. Instr. and Meth. 21 (1963) 232. ii) J. B. Czirr, Nucl. Instr. and Meth. 25 (1963) 106. r2) I. S. Sherman et al., IEEE Trans. Nucl. Sci. NS-11, no. 3 (1964) 20. i3) W. C. Kaiser et al., ibid., 29.
1”) M. Schumacher and A. Flammersfeld, Z. Physik 178 (1964) 11 r5) P. G. Sjolin, Nucl. Instr. and Meth. 37 (1965) 45. 16) T. G. Miller, Rev. Sci. Instr. 36 (1965) 847. r7) P. Kuijper et al., Nucl. Instr. and Meth. 42 (1966) 56. 1s) D. Aliaga-Kelly and D. R. Nicoll, Nucl. Instr. and Meth. 43 (1966) 110. is) L. P. Wishart et al., Nucl. Instr. and Meth. 57 (1967) 237. 20) T. G. Miller, Nucl. Instr. and Meth. 48 (1967) 154. 21) T. G. Miller, Nucl. Instr. and Meth. 32 (1965) 239. 22) I. Cernigoi et al., Nucl. Instr. and Meth. 9 (1960) 303. a3) R. Nutt, IEEE Trans. Nucl. Sci. NS-13, no. 1 (1966) 110. 24) H. 0. Funsten and G. C. Cobb, Rev. Sci. Instr. 31(1960) 571. 25) W. Daehnick and R. Sherr, Rev. Sci. Instr. 32 (1961) 666. 26) G. Walter et al., J. Phys. (Paris) 24 (1963) 1017. 27) M. L. Roush et al., Nucl. Instr. and Meth. 31 (1964) 112. as) E. Nadav and B. Kaufman, Nucl. Instr. and Meth. 33 (1965) 289. ag) R. Ftille et al., Nucl. Instr. and Meth. 35 (1965) 250. 30) W. Schweimer, Nucl. Instr. and Meth. 39 (1966) 343. 3l) B. SouEek andR. L. Chase, Nucl.Instr.and Meth.50(1967)71.
166
D.
L. SMITH
32) T. G. Miller, Response of deuterated benzene scintillators to neutrons and gamma rays, 34th Meeting Southeastern Section Am. Phys. Sot. (Clemson University, S.C., Nov., 1967). 33) F. A. Johnson, Nucl. Instr. and Meth. 58 (1968) 134. 34) C. E. Hollandsworth and W. P. Bucher, Rev. Sci. Instr. 39 (1968) 165. 35) T. G. Miller, Nucl. Instr. and Meth. 63 (1968) 121.
et al.
36) C. J. Taylor et al., Phys. Rev. 84 (1951) 1034. 37) H. C. Evans and E. H. Bellamy, Proc. Phys. Sot. (London) 74 (1959) 483. 38) M. Gettner and W. Selove, Rev. Sci. Instr. 31 (1960) 450. 3g) R. Batchelor et al., Nucl. Instr. and Meth. 13 (1961) 70. 40) J. R. Prescott and A. S. Rupaal, Can. J. Phys. 39 (1961) 221. 41) G. D. Badhwar et al., Nucl. Instr. and Meth. 57 (1967) 116.
INSTRUMENTS
AND
MEASUREMENT
METHODS
64(1968)
OF THE RESPONSE TO ELECTRONS,
157-166;
0 NORTH-HOLLAND
OF SEVERAL
PROTONS
ORGANIC
PUBLISHING
CO.
SCINTILLATORS
AND DEUTERONS
D. L. SMITH, R. G. POLK and T. G. MILLER U.S.Army
Missile
Command,
Redstone
Arsenal,
Alabama
35809, U.S.A.
Received 16 May 1968 The response of several common organic scintillators to electrons, protons and deuterons has been measured using a photomultiplier tube with S-l 1 spectral sensitivity. The Compton scattering of monoenergetic beams of gamma rays and neutrons was used to induce the charged particles to recoil with specific energies in the interiors of the scintillators. Pulse-shape discrimination was employed to distinguish between neutron and gamma-ray events. The following scintillators were studied as indicated: anthracene (electrons: 0.234.99 MeV, protons: 0.24-15.0 MeV); stilbene (electrons: 0.31-0.99 MeV, protons: 0.35-15.0 MeV); NE-213 (electrons:
0.16-0.99 MeV, protons:
0.24-15.0 MeV); NE-102
1. Introduction Many organic crystals, liquids and plastics scintillate when bombarded with nuclear radiationsle3). Light is emitted in response to either primary or secondary ionization induced by the radiation. A number of these scintillators have found widespread use in nuclear spectroscopy because they possess desirable physical characteristics, exhibit good detection efficiency, and their response times allow relatively simple derivation of fast timing signals required for some experiments4). The response of these organic materials is usually critically dependent upon the specific ionization of the detected particles. Some of these scintillators respond to an ionizing particle by emitting a light pulse which is the sum of a short (a few nsec) and a long (tens to hundreds of nsec) decay-time component. The relative intensity of these components, and therefore the shape of the composite pulse, depends on the nature of the ionizing particle. Electronic circuits which distinguish between pulse shapes can be used in conjunction with these scintillators to detect a selected type of particle and discriminate against others5). The response of organic scintillators to electrons is quite linear with particle energy above approximately 100 keV. Furthermore, linear extrapolations of the electron response from higher energies generally pass within a few keV of the origin6). The response of these scintillators to heavier ionizing radiation is generally nonlinear with a light output which is substantially less than that for electrons of the same energy. Neutral gamma rays and neutrons can be detected by scintillators because they collide with particles in the
(electrons: 0.31-0.99 MeV, protons: 0.35-15.0 MeV); Pilot B (electrons: 0.31-0.99 MeV, protons: 0.35-15.0 MeV); and NE-230 (electrons: 0.16-0.99 MeV, deuterons: 0.25-11.0 MeV). Allmeasured responses for a given scintillator were normalized to its measured response for 0.525 MeV electrons (arbitrarily assigned the value 1000). These data were used to compute p/p and d/p ratios which are relatively insensitive to detector geometry. The scintillators studied exhibit ratios which are remarkably similar over a wide range of particle energies. They increase non-linearly from M 0.1 in the 0.3-1.0 MeV range to M 0.5 in the 12.0-15.0 MeV range.
scintillator material and cause them to recoil. In organic materials, gamma rays interact primarily with atomic electrons by the Compton effect whereas neutrons scatter elastically from the nuclei of the atoms in the scintillator. Organic scintillators are good neutron detectors since they contain light elements whose nuclear recoils can be detected easily. For several good scintillators, neutron events can be distinguished from gamma-ray events by the pulse-shape discrimination (PSD) techniques already mentioned. Information about the responses of organic scintillators to ionizing radiation is necessary for understanding the basic light producing mechanisms and is invaluable for laboratory applications3). Numerous studies of the response of these scintillators to various radiations have been reported. Birks has reviewed the literature rather thoroughly 1963 “). A partial list of references to more recent work is included in the present paper6-19). Most of the measurements reported have resulted from studies of only one or two scintillators at a time over narrow energy ranges. The present work is a systematic study of several commonly used organic scintillators over relatively wide ranges of particle energy. We have measured the response of anthracene, stilbene, NE-213, NE-102 and Pilot B scintillators to 0.3-15.0 MeV protons and of an NE-230 scintillator to 0.3-l 1.O MeV deuterons. The response of these scintillators to 0.3-1.0 MeV electrons was also investigated. 2. Experimental details The response of a scintillation detector (scintillator plus photomultiplier tube) to radiation depends on 157
158
D.
L. SMITH
several geometrical factors including the size and shape of the scintillator, the quality of the light reflector and location of the scintillation within the scintillator’7). For example, the size of the recorded pulses will be reduced when the light from the scintillations loses intensity due to reflections and transmission before reaching the photocathode. There are two methods for measuring the response of a scintillator to charged particles. The direct method is to bombard the scintillator with external beams of charged particles. The indirect method is to bombard the scintillator with monoenergetic beams of neutral particles such as gamma rays and neutrons of which a portion will elastically scatter from charged particles within the scintillator. Unique energy recoils will be recorded only if one detects the elastically scattered neutral radiation in a second detector, placed at a specific angle relative to the incident beam direction, in time coincidence with the recoil pulse. The indirect method was employed in the present work. This approach has the advantage of minimizing surface effects which can be important at low energies. Furthermore, the relative response of a scintillator to different charged particles can be measured with greater reliability since the penetrating primary radiations generally produce recoils rather uniformly throughout the bulk of the scintillator thereby minimizing the geometrical effects discussed previously. Measurements of relative response made by this method are reasonably insensitive to both geometrical and instrumental considerations. Fig. 1 is a schematic diagram of the apparatus used in the present experiment. A scintillator under study was attached to the center counter which scattered the primary radiation and detected the recoils. The side
et al.
detector was shielded from the primary radiations by lead and borated paraffin in order to reduce random coincidences. An approximately monoenergetic gammaray beam (gamma-ray energies of 1.17 MeV and 1.33 MeV) was produced by placing a sealed 1 mCi 6oCo source in a lead brick collimator which could be inserted in the shield opening. Monoenergetic neutron beams were produced by bombarding various targets with proton and deuteron beams from a 2 MV Van de Graaff accelerator. This scattering apparatus has been described in earlier communications*‘** ‘). The energies
_L__1
INCIDENT PARTICLES
YA
._ :.-. .,._
;
-
._ :
_ ._- :
/
I . r; . ; *.
SIDE
SHIELD
Fig. 1. Schematic diagram of the scattering apparatus used in the present study of scintillator response. A collimated beam of primary radiation is scattered by the center detector and detected in the side detector. The scatterers recoil and produce scintillations in the center detector.
TABLE
1
Sources of recoil particles. source of reaction
Accelerator Beam Energy (MeV)
DETECTOR
(MOVABLE)
Primary particles Energy Type (MeV)
Recoil particles Energy range Type (MeV) e-
0.16-0.99
1.77
1.17 and 1.33 0.97
1I
1.77
1.47
1.67
4.88
0.60
16.03
P d P d :
0.25-0.75 0.19-0.76 0.38-1.13 0.50-1.13 1.264.70 0.47-3.37 5X7-15.17 3.08-10.87
THE
RESPONSE
OF
ORGANIC
TABLE 2
Accelerator
targets.
Composition
Target
Preparation
Carbon
Natural carbon film (20 keV thick for 1.8 MeV protons) on a thin tantalum disc
Colloidal suspension of graphite in water evaporated on metal disc with a heat lamp
Deuterium
300 rug/cm2 deuterium absorbed into a 1 mg/cma layer of titanium on a copper disc
Commercial
Tritium
1 Ci/ina tritium absorbed into a 300 [&g/cm’ layer of titanium on a copper disc
159
SCINTILLATORS
of the recoil particles can be calculated from kinematics. Table 1 lists the ranges of recoil energies available for study in the present work. The accelerator beam energy was calibrated by detecting the 1880.7 f 0.4 keV threshold point for the 7Li(p,n)7Be reaction and the 1746.5 + 0.5 keV resonance of the 13C(p,y)14N reaction. The targets used in the present work are described in table 2. Corrections were made for energy losses in the targets, and these effects were considered in estimating uncertainties in the calculated recoil energies. Fig. 2 shows a breakdown of the various functions performed by the experimental apparatus. Pulse-height spectra from the center detector, representing the recoil particles, were recorded in a multichannel pulse-height analyzer, subject to fast coincidence and y-n discrimination condrtions in the center and/or side counters. These conditions could be switched in or out individually as required.
SCATTERED RECOIL DETECTOR (CENTER)
MOVEABLE SIDE DETECTOR
TIMING
PVLSE HEIGHT -
PULSE HEIGHT
1
. FAST
COINCIDENCE v
l
.
v
NEUTRON-
NEUTAON-
GAMMei RAY DlSCRlMlNeiTlON
GAMMA RAY DISCRIMINATION
-
SLOW
LOGIC AND
PULSE-HEIGHT
Fig. 2. Schematic
diagram showing the principal operations only neutron-induced events which
_
SIGNALS
performed by the electronics. The circuitry permits the option register fast coincidences between the two detectors.
of recording
160
L. SMITH
D.
et cd.
TABLE3 Scintillators. Scintillator
Anthracene (crystal) Stilbene (crystal) NE-213 (liquid) NE-230 (liquid) NE-102 (plastic) i Pilot B (plastic)
CENTER #_-______-__---
Source
Harshaw Chemical Co. Cleveland, Ohio, U.S.A. Nuclear Enterprises Ltd. Edinburgh, Scotland, U.K. Pilot Chemicals, Inc. U.S.A.
DETECTOR
_
_-
Detector geometry
Quality of y-neutron discrimination
l+” dia. x $” high cylinder 1” dia. x +” high cylinder 2” dia. x 2f” high cylinder 2” dia. x 24” high cylinder 2” dia x 2” high cylinder 1t” dia. x 2” high cylinder
good excellent excellent excellent none none
‘_______________~ SIDE
1
DETECTOR
, I I
DELAY I
I I
Re
7 J 1-
TO SELECTOR
TO SELEl
.----
LOGIC
SUBGROUP
BOUNDARIES
Fig. 3. Block diagram of the complete electronics circuitry. Dashed lines show logic subgroup boundaries for comparison
with fig. 2.
THE RESPONSE
OF ORGANIC
Table 3 gives a detailed description of the scintillators studied in the present work. The side detector utilized an NE-213 scintillator indentical to the one described in table 3 throughout the experiment. RCA 6810A photomultiplier tubes with Ashkin type voltage dividers were used in the scintillation probes22). Fig. 3 is a detailed block diagram of the electronics apparatus. Dashed lines divide the various electronics components into appropriate subgroupings to facilitate comparison with fig. 2. Fast timing signals were derived from the photomultiplier anode pulses by tunnel diode integral discriminators23). Pulses whose amplitudes were equivalent to the time integral of the scintillation light pulses were obtained by integrating dynode pulses with preamplifiers and shaping them with double-delay line (DDL) amplifiers. The fastcoincidence detector consisted of the more flexible TAC-SCA arrangement rather than the conventional pulse-overlap detector, with time resolutions of a few nsec achieved in practice using a good quality commercial time-to-amplitude converter and single-channel analyzer. These time resolutions were good enough to distinguish between low-energy (5 1.0 MeV) neutron events and gamma events on the basis of flight times between center and side counters separated by approximately 7 inch. Numerous schemes for achieving pulse-shape discrimination have been reported in the literature. References to a few of these methods are listed in the present paper 5Z24-35). Fig. 3 shows the electronics NEUTRON-GAMMA
9-
DISCRIMINATION
NE - 230
A
SCINTILLATOR
NEUTRON
RELATIVE
TIME
PEAK
( NANOSECONDSj
Fig. 4. Typical spectrum from the pulse-shape discrimination circuitry. The separation of electron and deuteron recoil events in an NE-230 scintillator is exhibited.
161
SCINTILLATORS
A RECOIL ELECTRON SPECTRUM I , I, I / I,, , r I,
1
/
7-
76?5
EOO-
700 -
600 ZI 2 500-• 3 5400 a
l
? 5 300 0 ”
~
. .. .
. . :.* . . .’ l
20
40
0
60
00
100
120
140
CHANNEL
Fig. 5. Typical recoil electron spectrum. Most of the counts in the low-pulse-height tail result from random coincidences. The procedure used in locating the recoil peak maximum is indicated and the estimated uncertainty is labelled.
apparatus we used to achieve y-n discrimination. Fast timing signals were produced by a single-channel analyzer which detected the zero crossovers in the pulses from the DDL amplifier. The time separations between the anode discriminator pulses and the zero crossover time pulses were measured by a TAC. Discrimination was possible because the time separation is different for neutron and gamma-ray events (fig. 4). The recoil pulse spectra were stored in a 400-channel pulse height analyzer. Pulses were recorded only when a linear gate, controlled by the appropriate fastcoincidence and/or y-n discrimination conditions, permitted them to pass through to the MCA. Several different signals could be sampled by means of a selector switch allowing fast setting up and checking of the various circuits. A pulser was used to simulate pulses from the center detector for the purpose of calibration. Accurate calibration was very important for this experiment because runs were made with a variety of operating conditions and the data had to be properly normalized. Most experimental runs were 30 min to 2 h long,
162
D.
A RECOIL
I IO
r
I
,
I
,
L. SMITH
depended critically on the choice of photomultiplier tube high voltage and DDL amplifier gain. The discrimination properties of NE-230 were reported recently3’). The present work supports the contention that good y-n discrimination can be obtained with NE-230 scintillators. Fig. 4 shows a time spectrum for NE-230 obtained with our circuitry. From fig. 4, the separation between the neutron and gamma-ray peaks is 8.9 nsec, and the full width at half maximum of the neutron peak is 4.2 nsec; hence the figure of merit for n-y separation, as defined in 35) is 2.1. For this curve, all pulses were rejected whose light outputs were less than the light output for a 0.25 MeV electron. The neutron source was the T(d,n)4He reaction. The recoil pulse height spectra recorded by the MCA were typed out and also punched onto paper tape. The data on paper tape were transferred to punched cards and were subsequently machine plotted.
PROTON SPECTRUM I’/
/
G
100
!
1
I
’
1 ’
152 212
.
80
d
5 a 5 a
f
70
6o
v) 50 t 5 8 40 30 20
.
. .
ANTHRACENE Ep =3.66 MeV
IO ,
l -
3. Data analysis /
.
0 40
80
120
160
200
240
et al.
280
$0
CHANNEL
Fig. 5 shows a typical electron recoil spectrum and fig. 6 shows a typical proton recoil spectrum. The electron recoil spectra were generally of good quality. Some of the proton and deuteron recoil spectra were difficult to analyze because the recoil peaks were broad
Fig. 6. Typical recoil proton spectrum. Most of the counts in the low-pulse-height tail result from random coincidences. The procedure used in locating the recoil peak maximum is indicated and the estimated uncertainty is labelled.
although some of the electron calibration points were obtained from overnight runs. All data were normalized to the response for 0.525 MeV electrons. Calibration runs were made whenever scintillators were removed from the photomultipliers or high voltages were changed. The buildup on the targets of carbon from the vacuum system and drive-in deuterons from the accelerator led to difficulties in identifying some of the recoil peaks since the neutron beams were not always purely monoenergetic. The best proton and deuteron recoil data were obtained when y-n discrimination was employed for both counters. This was not always possible because NE-102 and Pilot B do not exhibit PSD properties. Furthermore, our circuitry did not effectively discriminate proton or deuteron recoils from electron recoils for recoil energies below w 0.4 MeV. Time-of-flight discrimination using the fast-coincidence circuit proved to be useful for these low-energy recoil measurements. Stilbene, NE-213 and NE-230 exhibited excellent y-n discrimination, but good discrimination was also observed for anthracene. The quality of discrimination
ANTHRACENE
ELECTRO
.=i
E
8
_
co6(Jy
lYc?‘z i
102
d Tfp,n)He3 w Ci2(d,n)N I3 A D(d,n) He 3 q
‘I
1.0 PARTICLE
T(d,n)
I
He4
l>c”l
>
I
I,
IO.0
ENERGY
(MeV)
Fig. 7. Response of anthracene to electrons and protons.
THE RESPONSE
OF ORGANIC
163
SCINTILLATORS
STILBENE
NE-102 I
C”
“‘1”1
“I’
co60y
0
aT(p,n) n
He3
C’2(d,n)
I’
“““““”
A T(p,n)He3 = Cl2 (d,n) N13
N13
AD (d, n) He3
A D (d,n)
He3
m T (d,n)
0 T(d,n)
He4
He4
I PARTICLE
ENERGY
I.0 PARTICLE
(MeV)
Fig. 8. Response of stilbene to electrons and protons.
IO.0 ENERGY
(MeV)
Fig. 10. Response of NE-102 to electrons and protons.
PILOT
I-
B
, I’
A T (p,n) He3 n
q
I
0.1
I
!
1
I111111
I
T(d,n) I
I
He4
IllIll
1.0
PARTICLE
t 10.0
ENERGY
(MeV)
Fig. 9. Response of NE-213 to electrons and protons.
I
I
Cl2 (d,n)
N13
A D (d,n)
He3
a T (d,n)
He4
II111111
Il!lll 1.00 PARTICLE
ENERGY
I
10.0 (MeV)
Fig. 11. Response of Pilot B to electrons and protons.
164
D. L. SMITH
et al.
relative responses of the scintillators to 0.525 MeV electrons are given in table 4. These data are not as reliable as those presented in figs. 7-12 because of uncertainties associated with attaching the scintillators to the PM tube. Furthermore, it is difficult to compare the values in table 4 with others reported since absolute responses are a function of the sizes and shapes of the scintillators and other experimental factors17). Fig. 13 shows the p/p and d//I ratios which were computed from the response data in figs. 7-12. It is striking that these ratios are so similar over a wide range of energies even though the absolute responses which were measured differ by as much as a factor of 3 (table 4). The ratios are E 0.1 for particle energies in the range 0.20-1.0 MeV. They increase considerably between 1.0 MeV and 4.0 MeV and then level off to
ELECTRONS
TABLE 4
Measured
/
/‘ 4 C’2 (d,n) N I3 A D (d,n) He3 q T(d,n) He4
relative responses for the scintillators present work*.
used in the
n
/ 1.
I!(11
I .o PARTICLE ENERGY
I
!I,
Scintillator :’
Response to 0.525 MeV electrons relative to anthracene
10.0 (MeV)
Fig. 12. Response of NE-230 to electrons and deuterons.
and partially smeared by background. Several runs were required to obtain usable data for some of the lower energy recoils. The peak maxima were located and used in calculating the relative responses. Figs. 5 and 6 indicate a procedure used for locating the peak maxima and also show the corresponding estimated uncertainties. The uncertainties in locating the peak maxima were the major contributors to the overall uncertainties in the measured responses. An additional + 5% uncertainty was assigned to each data point to account for intangible instrumental effects such as possible amplifier gain drifts and high voltage instabilities. Uncertainties in the recoil energies of the electrons, protons and deuterons were also estimated. These were too small to be shown on plots of the final results.
anthracene stilbene NE-213 NE-230 NE-102 Pilot R
1 1.21 * 0.79+ 0.70 + 0.38 + 0.40 +
* The size and shapes of these scintillators is given in table 3. The measured response of a scintillator depends critically on its size and shape, quality of the reflector and other factors. This fact should be kept in mind when comparing the values in this table with other values reported in the literature17).
ANTHRACENE STILBENE PILOT B NE-213 NE-102
4. Results and discussion The results of the present series of measurements are shown in figs. 7-12. Although it is not entirely correct (section l), it has been assumed that the electron response is linear and passes through the origin. The relative response data are plotted on scales which arbitrarily assign the value 1000 for the response of each scintillator to 0.525 MeV electrons. The measured
0.15 0.11 0.09 0.05 0.05
D/B
ENERGY
-. -..-.. -...-.
RATIO
NE-230
PARTICLE
----
-
IMEVI
Fig. 13. Calculated relative responses. Ratios of p//3 and d/p are computed from the data presented in figs. 7-12.
THE
RESPONSE
OF ORGANIC
z 0.5 in the energy range 10.0-15.0 MeV. Anthracene and stiibene scintillators appear to have the largest p/P ratios at low energy. This fact, along with their good y-n discrimination properties, makes them desirable neutron detectors although large crystals are costly. Another interesting feature of the results presented in fig. 13 is that the p/p ratio for NE-213 is proportional to the d/b ratio for NE-230 over a large energy range. This may occur because these organic liquids are chemically similar. Many measurements of p//3 ratios have been reported in the literature. No attempt has been made by the authors to tabulate all of these; however, the results of
Comparison
Energy (MeV
.4nthracene
1 3 10 15 1.8 2.5 5.0 7.5 10.0 0.2 2 4 6 8 2.64
NE-102
Pilot B
* Estimated uncertainty
few
of
the
measurements’,6,11,12,14,~6,~9,36-4~)
are compared with the present work in table 5. Some of the values of p/p reported in a review article by Swank are much larger than those we have measured’). However, most of the measurements we have seen reported in the literature agree fairly well with our own to within our estimated 15-20% uncertainty. The authors would like to thank B. Sparling for considerable assistance during the experiment, R. Craun for help in preparing the figures, and Mrs. S. M. Ray for expertly typing the manuscript.
TABLE5 of some relative response measurements.
Scintillator
NE-213
a
165
SCINTILLATORS
Response ratio
Reference and reported result
Result from present work*
P/B
1) 0.54 0.64 0.73 0.75 3s) 0.26 0.27 0.36 0.45 0.48 33) 0.20 37) 0.23 0.33 0.42 0.47 16) 0.34
0.14 0.29 0.44 0.48 0.27 0.34 0.48 0.54 0.56 0.13 0.23 0.35 0.44 0.50 0.30
P/B
P/B P/B
PIP
15-20x.
References 1) R. K. Swank, Ann. Rev. Nucl. Sci. 4 (1954) 111. a) C. G. Bell, Jr. and F. N. Haynes, Liquidscintillation counting (Pergamon Press, Oxford, U.K., 1957). 3, J. B. Birks, The theory and practice of scintillation counting (Macmillan, New York, 1964). 4, J. B. Marion and J. L. Fowler, Fast neutron physics 1 (Interscience, New York, 1960). 5, F. D. Brooks, Nucl. Instr. and Meth. 4 (1959) 151. 6, F. T. Porter et al., Nucl. Instr. and Meth. 39 (1966) 35. 7) Scintillator Catalogue, Nuclear Enterprises, Ltd., Edinburgh Scotland. s) E. Brannen and G. L. Olde, Rad. Res. 16 (1962) 1. g, K. Peuckert, Nucl. Instr. and Meth. 17 (1962) 257. 10) R. Eisberg, S. Mayo and W. Schimmerling, Nucl. Instr. and Meth. 21 (1963) 232. ii) J. B. Czirr, Nucl. Instr. and Meth. 25 (1963) 106. r2) I. S. Sherman et al., IEEE Trans. Nucl. Sci. NS-11, no. 3 (1964) 20. i3) W. C. Kaiser et al., ibid., 29.
1”) M. Schumacher and A. Flammersfeld, Z. Physik 178 (1964) 11 r5) P. G. Sjolin, Nucl. Instr. and Meth. 37 (1965) 45. 16) T. G. Miller, Rev. Sci. Instr. 36 (1965) 847. r7) P. Kuijper et al., Nucl. Instr. and Meth. 42 (1966) 56. 1s) D. Aliaga-Kelly and D. R. Nicoll, Nucl. Instr. and Meth. 43 (1966) 110. is) L. P. Wishart et al., Nucl. Instr. and Meth. 57 (1967) 237. 20) T. G. Miller, Nucl. Instr. and Meth. 48 (1967) 154. 21) T. G. Miller, Nucl. Instr. and Meth. 32 (1965) 239. 22) I. Cernigoi et al., Nucl. Instr. and Meth. 9 (1960) 303. a3) R. Nutt, IEEE Trans. Nucl. Sci. NS-13, no. 1 (1966) 110. 24) H. 0. Funsten and G. C. Cobb, Rev. Sci. Instr. 31(1960) 571. 25) W. Daehnick and R. Sherr, Rev. Sci. Instr. 32 (1961) 666. 26) G. Walter et al., J. Phys. (Paris) 24 (1963) 1017. 27) M. L. Roush et al., Nucl. Instr. and Meth. 31 (1964) 112. as) E. Nadav and B. Kaufman, Nucl. Instr. and Meth. 33 (1965) 289. ag) R. Ftille et al., Nucl. Instr. and Meth. 35 (1965) 250. 30) W. Schweimer, Nucl. Instr. and Meth. 39 (1966) 343. 3l) B. SouEek andR. L. Chase, Nucl.Instr.and Meth.50(1967)71.
166
D.
L. SMITH
32) T. G. Miller, Response of deuterated benzene scintillators to neutrons and gamma rays, 34th Meeting Southeastern Section Am. Phys. Sot. (Clemson University, S.C., Nov., 1967). 33) F. A. Johnson, Nucl. Instr. and Meth. 58 (1968) 134. 34) C. E. Hollandsworth and W. P. Bucher, Rev. Sci. Instr. 39 (1968) 165. 35) T. G. Miller, Nucl. Instr. and Meth. 63 (1968) 121.
et al.
36) C. J. Taylor et al., Phys. Rev. 84 (1951) 1034. 37) H. C. Evans and E. H. Bellamy, Proc. Phys. Sot. (London) 74 (1959) 483. 38) M. Gettner and W. Selove, Rev. Sci. Instr. 31 (1960) 450. 3g) R. Batchelor et al., Nucl. Instr. and Meth. 13 (1961) 70. 40) J. R. Prescott and A. S. Rupaal, Can. J. Phys. 39 (1961) 221. 41) G. D. Badhwar et al., Nucl. Instr. and Meth. 57 (1967) 116.