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Studies of a fast-timing, large-diameter photomultiplier as a fast neutron detector
NUCLEAR INSTRUMENTS AND METHODS 119 (1974)-364-572
© NORTH-HOLLAND PUßLtât:âN(à `'
STUDIES OF A FAST-TIMING, LARGE-DIAMETER PHOTOMULTIPLIER AS A FAST NEUTRON DETECTOR TOMONARI OTSUBO
Department of Physics, Tokyo Institute of Technology, Ookayama, Megro, Tokyo, Japan Received 4 April 1974 Timing andn-ypulse-shape discrimination of aneutron detector using a large-diameter photomultiplier, 60DVP, and NE213 liquid scintillator areinvestigated in orderto study the usefulness
of this system as an associated gamma-ray time-of-flight. s trometen
1. liatroduction
mentswith asingle large-diameter photomultiplier to 60DVP and NE213 liquid scintillator as a neutro detection assembly .
Theparticle-gamma-ray angular-correlation method hasserved as oneof the mostpowerful tools in nuclear spectroscopy . A largenumber of investigations hasbeen reported ofcharged-particle-gamma-ray angular correlations such as (d,py)`or (3He,py) reactions. On the
2. Apparatus A block diagram of the experimental arran for n-y two-dimens cnal measurements is shown in fig. 1 . A 7.5 cm diameter and 7.5 cm thickness Nal(!) scintillator was coupled to a Philips 58AVP photomultiplier as an AGCOF gamma-ray detector. 11ov-t'
othethand very little work has been done on neutrongamma-ray angular correlations. This fact is mainly due to the difficulty of obtaining adequate energy
resolution and high efficiency for neutrons . In some cases, however, themeasurements of (X,ny) reactions can provide fruitful information on the properties of excited states of nuclei which cannot be
plastic andGe(Li) detector were also used for measure-' ments of resolving times and gamma-ray energy. Linear and timing signals which correspond
easily reached by other reaction mechanisms [for a instance, °Ca(3He,ny)°2 Ti]. Recent developments of the fast-timing and pulse-shape processing techniques in the associated gamma-ray time-of-flight method (AGTOF)have givenresolving times of around 10-9 s.
gamma-ray energy and stop time of the time-to-puls ;-
Such resolving times could give adequate neutron energy resolution even under geometrically limited conditions') . The thickness of the detector along the neutron flight path must be thin enough to obtain the desired time resolution. However, the efficiency of detectors, especially that of the neutron detector, is another important factor that has to be considered whenever this AGTOF method is used. In order to increase the
neutron detectionefficiency useshould be made of such a scintillator-photomultiplier-tube combination of large sensitive area and fast timing characteristics . Some recent work of the AGTOF spectrometer has been reported by Joy) and Johnson et al .'). Johnson
et al ., employing a large annular-type scintillator and eight XPI040 photomultiplier tubes for neutron detection, obtained high efficiency andadequate energy resolution : In this report we will describe the results of measure-
Fig. 1 . A block diagram of the apparatus used in the a-y twodimensional measurements. LA - linear amplifier ; LG - linear gate ; TSCA-timing single-channel analyzer ; CFD-constantfraction discriminator ; INV-irvertsr ; THPC-time-to-pulse height converter; COIN -slow rnimidence; PSD-pulse-shape discriminator ; OSC-oscilloscope.
569
570
TOMONARIOTSUBO
height converter were extracted from the loth dynode and anode, respectively . To stabilize photomultiplier gain against widely fluctuating counting rate, a relatively low-resistance divider circuit was adopted with the last five dynodes directly bypassed to ground through capacitorO) . As for the neutron detector, an NE213 liquid scintillator contained in a cylindrical aluminium cell of 23.5 cm diameter and 3 cm thickness was used in conjunction with a 60DVP photomultiplier. The inside surfaceof thecell wascoated with NE561 reflector paint and dissolved oxygen in the. liquid scintillator' was removed by bubbling argon gas. The 60DVP photo, multiplier has characteristic features such as large useful diameter (20 cm) bi-alkali photocathode and fast rise time (2.5 ns at c. kV supply voltage). For an application of the photomultiplier in fast timing processes a negative high voltage is normally applied to the photocathode to achieve better transmission of the pulse shape and also to simplify the signal output circuits . In the present study we used this 60DVP x10° 10 W C
ryr ,
(a)
NE213(12.5cm)-5SW1P - . NE102(Scm)-56AVP PWB. source Flight Pa1h=50cm
C 0
»
â 45
N-20ns-1 En (MeV) " 6A3T1 OS ,~3-
V
c ô v
x101
w
.
0 =r
50
, "ß=2ons -.(
6543 2
50
with P50
100 (b)
"
without PSD. .
150
NE213(23 .5crn)-600VP NE102(5cm)-56AVP Pu-Be source Flight Path=50cm En (MeV) 0s 0.3-
100
150
channel number Fig. 2. Time-of-flight spectra obtained with and without pulseshape discrimination using the Pu-Be neutron source.
photomultiplier with a positive high voltage, since this created an appreciably lower noise level than -, negative high voltage. The last-dynode pulses from this tube were fed to the fast discriminator through a ferrite core inverter and a time-calibrated RG-8/U cable as in thecase for thegamma-ray detector, andthe anode pulses were fed to the neutron-gamma pulseshape discrimination (PSD) cin .uit . The fast timing discriminators used in this system were ORTEC Model 463 constant-fraction timing discriminators . The PSD circuit was constructed from the sample reported in refs 5and6.Some modifications were me.de to study the capability of the neutron-gamma-ray discrimination with this large-diameter scintillatorphotomultiplier combination. This PSD circuit provides two integrators in parallel with different time constants, a comparator, and several logic pulseprocessing sections. The optimum condition of this discriminator was searched with a Pu-Be neutron source and standard gamma-ray sources. For low energy neutrons the Tokyo Institute of Technology 4 MCV Van de Graaff accelerator was used to induce the 7Li(p,n)7 Be reaction. The neutron logic pulses (gate pulses) from this discriminator arefed to conventional slow timing circuits . 3. Experimental measumnents Characteristics of timing and pulse-shape discrimination of an NE213-58AVP assembly have been investigated by many authors'). They indicated that satisfactory performance could be obtained with this assembly undersome optimized conditions . Therefore, we compared the results obtained by our NE213-` 60DVP assembly with those obtained by the NE213-'' 58AVP assembly. The size of a liquid scintillator coupled to a 58AVP photomultiplier was 12.5 cm in diameter and 2.5 cm in thickness. This unit was prepared in thesame wayasin thecase ofthescintillator fora 60DVP photomultiplier. Figs 2a and 2b show the time-of-flight spectra obtained with and without pulse-shape discrimination using the Pu-Be neutron source. Prompt 7-7 coin-' cidence peaks on the left-hand side of the figures are due to gamma rays produced from the scattering or. absorption of neutrons by surroundingmatter includ ng scintillators . A gradual decrease of the neutron yield toward thelow-energy side represents theenergy spread of thealpharays emittedfrom Pu. The neutron energies. are calculated from neutron flight time. A maximum', neutron energy corresponds to neutrons going into the first excitedstate of theresidual t2 Cin the 9Be(a,n)'2C reaction.
PAST NEUTRON DETECTOR Gamma-ray rejection ratios for the NE213 (12.5 cm 0)-58AVP and NE213(23.5cmo}-60DVP assemblies were obtained from these figures and were determined to be larger than 99% and 94%, respectively .
The somewhat lower value for the NE213-( ,ODVP assembly seems to be resultingfrom poor discriminaii , for small signals. Counting losses of neutrons by the use of the pls >oshape discriminator depend mainly on the threshoA! energy of discrimination . This can be understood ¬roars the kinematic point of view of (n,p) scattering in tiao scintillator . A relationship between the counting lo s of neutronand neutron energy is shown in fig. 3.
s s
For thepurpose of measuring the resolving times of the detectors, the threshold level of the fast timing discriminator was set to 0.5 MeV for the gamma-ray detector and to the lowest possible level for the neutron detector . Fig. 4is theresult of measurements for
W
0
1
2
3 4 En (MeV)
5
several pairs of detectors using a 6° Co gamma-ray source. In the case of a 7.5 cm x 7.5 cm Nal(TI)-58AVP and 23 .5 cm x 3 cm NE213-60DVY assembly, it can be seen from fig. 4that theresolvingtime is 4.0 ns
6
Fig. 3. Thecounting loss of neutrons dueto theapplication of the pulse-shape discrimination technique:
and thecontributions to this value by each detector can be determined to be 2.0 and 3.3 ns, respectively, with the assumption of Gaussian line shapes for (b) , (c) and (d) of fig. 4.
x10' 4
(a) ..
0
NE213-564VP . NE102-56AVP
" . 9.6ns-~
416- l('
x103 E('He)=7.0MeV gamma ray single spectrum (90°)
3°P(1974-709 kev)
(b)
4
%00 -50AVP NE102-564VP
2
'.5
W c0
.P
(c)
û4 : -i. .3 .1
u, 2 E
..
"<-
û0 4
''(d)
_
0
(2937-67b k_ l
NE213-60DVP Nal(TD-58AVP at(NE213)-3 .3ns at(N.KTIP 2 .0-
4 .0
2
.
NE213-60DVP NE102-5fAVP
50
100 channel rxmber
150
Fig. 4. Resolvingtimesforseveral pairs of detectors using awCo gamma-ray source ; (a) 12.5 cmo x2.5cm NE213-58AVP and 3 cmo x 2.5 cm NE102-56AVP, (b) 7.5 cmox7.5 cm NaICf1)58AVP and 3 =0 x2.5 cm NE102-56AVP, (c) 23 .5 cmo x x 3 cm NEV3-6ODVP and3cmo x 2.5 cm NE102-56AVP, (d) 23.Scm0x3cm NE213-60DVP and 7.5 =0 x 7.5cm Nal(Tl)58AVP.
Fig. 5. Gamma-ray single and neutron-gated spectra of the 28Si(3He,ny)30 S reaction with a Gc(Li) detector.
572
TOMONARI OTSUBO
400
2 1
1300
n
200
c +itl11i1 ô " too fl,
4001
(c) 0" 51Mev
sponding neutron peak of the time spectrum. 2.20 and 1.19 MeV gamma-ray peaks are also seen in this spectrum as in the case of the Ge(Li) spectrum.
neutron spectrum with PSD
t+f,~t,,{t
IO 2o 30 channel number
é
40F(b) '21 1' i i
neutron spectrum without PSD
c
l .19M,V I
20 10 30 channel number "Si(°He, nr)°S E( He)=7.OMeV 7.5x7.scm :Nal(TO gamma ray spectrum (30r) 2.20Mev
50 channel numberloo
150
Fig. 6. Neutron and gamma-ray spectraof two-dimensional measurements from the ssSi(sHe,ny)s 0S reaction. 1 and 2 of figs. 6a and b correspond to first and second excited states of 30S. Cross marks in thegamma-rayspectra wereobtained on both sides of neutron groups. As an example for practical applications, the 28Si(3 He,ny)3aSreaction was performedwith a7.0MeV doubly ionized 3He-1-.eam delivered from the Van de Graafl accelerator. Figs 5a and 5b are part of the gamma-ray spectra obtained with a Ge(Li) detector. In the singlesspectrum (a), peaks which-can be seen are those of 30P through the 28Si(3He,py)3ap reaction, while in the n-ycoincidence spectrum (b), peaksdueto 30S can be clearly seen. The same reaction was also measured with the AGTOFmethod using theNaI(Tl)58AVP assembly as a gamma-ray detector and with a neutron flight path of 75 cm. Figs 6a and 6b show the neutron time-of-flight spectra in coincidence with the first excited gamma-ray group.Fig. ticgives thegammaray spectrum in coincidence with the sum of the first and second excited neutron groups shown in fig. 6a . The background gamma-ray spectrum was obtained from the lower and uppersides of thecorre-
4.C The measurement of fast neutrons often requires a detector system of alarge-diameter scintillator coupled with several photomultipliers. This complicates the preliminary adjustment of the electronic apparatus. In the present study properties of a large-diameter scintillator-photomultiplier NE213-60DVP assembly have been investigated and compared with thoseof the widespread photomultiplier 58AVP. The present study has shown the possibility of application of the NE213-WDVP unit for the fast neutron detector although its characteristic resolving time and n-y discrimination are somewhat inferior to those of the NE213-58AVPassembly. We adopted the` almost equal dynode voltage distribution which was' recommendedby themanufacturer, buta moreextensive study of the divider circuit may improve these properties. In measurements using a Pu-Be neutron source a simple method, whichis based on theAGTOFmethod, is employed to determine the energy dependence of the neutron detector with and without pulse-shape discrimination which affects seriously the detection of relatively low-energy neutrons. This is a convenient' method as atest of the AGTOF spectrometer. Theauthor would like to thank Prof. Y. Oda forhis advice and encouragement during the course of this work: Thanks arealso due to H. Abe, T. Kubo, S. Koh` and K. Kawasaki for their assistance with the electronics andoperation of theaccelerator. He wouldalso like to thank Prof. E. Arai for giving him the Pu-Bc' neutron source and Dr H. Kubo for hiscritical reading of the manuscript. References 1:) J. H. Neil r and W. M. Good, Fast matron physics (Inter. science, New York, 1960) Part 1, p. 509. T. Joy, Nuc1. Instr, and Meth 73'(1%9) 240. a) P. B. Johnson, J. M. G. Caraca, J.N. Pihl, It . A Gill and H. J. Rose, Nuci. Instr. andMeth.93 (1971) 417. 4) W.A. Gibson, Rev. Sci. Instr. 37 (1966) 631. s) D. W. Jones, IEEE Trans. Nucl . Sci. NS-15 (1968) 491. s) G. White, Nucl. Insts. and Meth . 4S (1966) 270; K. Kandiah andG. White, I.P.P.S.Conf. (Harwell, 1968). 7) J: Kalyna and I.J. Taylor, Nucl. Instr. and Meth. 88 (1970) 277.
© NORTH-HOLLAND PUßLtât:âN(à `'
STUDIES OF A FAST-TIMING, LARGE-DIAMETER PHOTOMULTIPLIER AS A FAST NEUTRON DETECTOR TOMONARI OTSUBO
Department of Physics, Tokyo Institute of Technology, Ookayama, Megro, Tokyo, Japan Received 4 April 1974 Timing andn-ypulse-shape discrimination of aneutron detector using a large-diameter photomultiplier, 60DVP, and NE213 liquid scintillator areinvestigated in orderto study the usefulness
of this system as an associated gamma-ray time-of-flight. s trometen
1. liatroduction
mentswith asingle large-diameter photomultiplier to 60DVP and NE213 liquid scintillator as a neutro detection assembly .
Theparticle-gamma-ray angular-correlation method hasserved as oneof the mostpowerful tools in nuclear spectroscopy . A largenumber of investigations hasbeen reported ofcharged-particle-gamma-ray angular correlations such as (d,py)`or (3He,py) reactions. On the
2. Apparatus A block diagram of the experimental arran for n-y two-dimens cnal measurements is shown in fig. 1 . A 7.5 cm diameter and 7.5 cm thickness Nal(!) scintillator was coupled to a Philips 58AVP photomultiplier as an AGCOF gamma-ray detector. 11ov-t'
othethand very little work has been done on neutrongamma-ray angular correlations. This fact is mainly due to the difficulty of obtaining adequate energy
resolution and high efficiency for neutrons . In some cases, however, themeasurements of (X,ny) reactions can provide fruitful information on the properties of excited states of nuclei which cannot be
plastic andGe(Li) detector were also used for measure-' ments of resolving times and gamma-ray energy. Linear and timing signals which correspond
easily reached by other reaction mechanisms [for a instance, °Ca(3He,ny)°2 Ti]. Recent developments of the fast-timing and pulse-shape processing techniques in the associated gamma-ray time-of-flight method (AGTOF)have givenresolving times of around 10-9 s.
gamma-ray energy and stop time of the time-to-puls ;-
Such resolving times could give adequate neutron energy resolution even under geometrically limited conditions') . The thickness of the detector along the neutron flight path must be thin enough to obtain the desired time resolution. However, the efficiency of detectors, especially that of the neutron detector, is another important factor that has to be considered whenever this AGTOF method is used. In order to increase the
neutron detectionefficiency useshould be made of such a scintillator-photomultiplier-tube combination of large sensitive area and fast timing characteristics . Some recent work of the AGTOF spectrometer has been reported by Joy) and Johnson et al .'). Johnson
et al ., employing a large annular-type scintillator and eight XPI040 photomultiplier tubes for neutron detection, obtained high efficiency andadequate energy resolution : In this report we will describe the results of measure-
Fig. 1 . A block diagram of the apparatus used in the a-y twodimensional measurements. LA - linear amplifier ; LG - linear gate ; TSCA-timing single-channel analyzer ; CFD-constantfraction discriminator ; INV-irvertsr ; THPC-time-to-pulse height converter; COIN -slow rnimidence; PSD-pulse-shape discriminator ; OSC-oscilloscope.
569
570
TOMONARIOTSUBO
height converter were extracted from the loth dynode and anode, respectively . To stabilize photomultiplier gain against widely fluctuating counting rate, a relatively low-resistance divider circuit was adopted with the last five dynodes directly bypassed to ground through capacitorO) . As for the neutron detector, an NE213 liquid scintillator contained in a cylindrical aluminium cell of 23.5 cm diameter and 3 cm thickness was used in conjunction with a 60DVP photomultiplier. The inside surfaceof thecell wascoated with NE561 reflector paint and dissolved oxygen in the. liquid scintillator' was removed by bubbling argon gas. The 60DVP photo, multiplier has characteristic features such as large useful diameter (20 cm) bi-alkali photocathode and fast rise time (2.5 ns at c. kV supply voltage). For an application of the photomultiplier in fast timing processes a negative high voltage is normally applied to the photocathode to achieve better transmission of the pulse shape and also to simplify the signal output circuits . In the present study we used this 60DVP x10° 10 W C
ryr ,
(a)
NE213(12.5cm)-5SW1P - . NE102(Scm)-56AVP PWB. source Flight Pa1h=50cm
C 0
»
â 45
N-20ns-1 En (MeV) " 6A3T1 OS ,~3-
V
c ô v
x101
w
.
0 =r
50
, "ß=2ons -.(
6543 2
50
with P50
100 (b)
"
without PSD. .
150
NE213(23 .5crn)-600VP NE102(5cm)-56AVP Pu-Be source Flight Path=50cm En (MeV) 0s 0.3-
100
150
channel number Fig. 2. Time-of-flight spectra obtained with and without pulseshape discrimination using the Pu-Be neutron source.
photomultiplier with a positive high voltage, since this created an appreciably lower noise level than -, negative high voltage. The last-dynode pulses from this tube were fed to the fast discriminator through a ferrite core inverter and a time-calibrated RG-8/U cable as in thecase for thegamma-ray detector, andthe anode pulses were fed to the neutron-gamma pulseshape discrimination (PSD) cin .uit . The fast timing discriminators used in this system were ORTEC Model 463 constant-fraction timing discriminators . The PSD circuit was constructed from the sample reported in refs 5and6.Some modifications were me.de to study the capability of the neutron-gamma-ray discrimination with this large-diameter scintillatorphotomultiplier combination. This PSD circuit provides two integrators in parallel with different time constants, a comparator, and several logic pulseprocessing sections. The optimum condition of this discriminator was searched with a Pu-Be neutron source and standard gamma-ray sources. For low energy neutrons the Tokyo Institute of Technology 4 MCV Van de Graaff accelerator was used to induce the 7Li(p,n)7 Be reaction. The neutron logic pulses (gate pulses) from this discriminator arefed to conventional slow timing circuits . 3. Experimental measumnents Characteristics of timing and pulse-shape discrimination of an NE213-58AVP assembly have been investigated by many authors'). They indicated that satisfactory performance could be obtained with this assembly undersome optimized conditions . Therefore, we compared the results obtained by our NE213-` 60DVP assembly with those obtained by the NE213-'' 58AVP assembly. The size of a liquid scintillator coupled to a 58AVP photomultiplier was 12.5 cm in diameter and 2.5 cm in thickness. This unit was prepared in thesame wayasin thecase ofthescintillator fora 60DVP photomultiplier. Figs 2a and 2b show the time-of-flight spectra obtained with and without pulse-shape discrimination using the Pu-Be neutron source. Prompt 7-7 coin-' cidence peaks on the left-hand side of the figures are due to gamma rays produced from the scattering or. absorption of neutrons by surroundingmatter includ ng scintillators . A gradual decrease of the neutron yield toward thelow-energy side represents theenergy spread of thealpharays emittedfrom Pu. The neutron energies. are calculated from neutron flight time. A maximum', neutron energy corresponds to neutrons going into the first excitedstate of theresidual t2 Cin the 9Be(a,n)'2C reaction.
PAST NEUTRON DETECTOR Gamma-ray rejection ratios for the NE213 (12.5 cm 0)-58AVP and NE213(23.5cmo}-60DVP assemblies were obtained from these figures and were determined to be larger than 99% and 94%, respectively .
The somewhat lower value for the NE213-( ,ODVP assembly seems to be resultingfrom poor discriminaii , for small signals. Counting losses of neutrons by the use of the pls >oshape discriminator depend mainly on the threshoA! energy of discrimination . This can be understood ¬roars the kinematic point of view of (n,p) scattering in tiao scintillator . A relationship between the counting lo s of neutronand neutron energy is shown in fig. 3.
s s
For thepurpose of measuring the resolving times of the detectors, the threshold level of the fast timing discriminator was set to 0.5 MeV for the gamma-ray detector and to the lowest possible level for the neutron detector . Fig. 4is theresult of measurements for
W
0
1
2
3 4 En (MeV)
5
several pairs of detectors using a 6° Co gamma-ray source. In the case of a 7.5 cm x 7.5 cm Nal(TI)-58AVP and 23 .5 cm x 3 cm NE213-60DVY assembly, it can be seen from fig. 4that theresolvingtime is 4.0 ns
6
Fig. 3. Thecounting loss of neutrons dueto theapplication of the pulse-shape discrimination technique:
and thecontributions to this value by each detector can be determined to be 2.0 and 3.3 ns, respectively, with the assumption of Gaussian line shapes for (b) , (c) and (d) of fig. 4.
x10' 4
(a) ..
0
NE213-564VP . NE102-56AVP
" . 9.6ns-~
416- l('
x103 E('He)=7.0MeV gamma ray single spectrum (90°)
3°P(1974-709 kev)
(b)
4
%00 -50AVP NE102-564VP
2
'.5
W c0
.P
(c)
û4 : -i. .3 .1
u, 2 E
..
"<-
û0 4
''(d)
_
0
(2937-67b k_ l
NE213-60DVP Nal(TD-58AVP at(NE213)-3 .3ns at(N.KTIP 2 .0-
4 .0
2
.
NE213-60DVP NE102-5fAVP
50
100 channel rxmber
150
Fig. 4. Resolvingtimesforseveral pairs of detectors using awCo gamma-ray source ; (a) 12.5 cmo x2.5cm NE213-58AVP and 3 cmo x 2.5 cm NE102-56AVP, (b) 7.5 cmox7.5 cm NaICf1)58AVP and 3 =0 x2.5 cm NE102-56AVP, (c) 23 .5 cmo x x 3 cm NEV3-6ODVP and3cmo x 2.5 cm NE102-56AVP, (d) 23.Scm0x3cm NE213-60DVP and 7.5 =0 x 7.5cm Nal(Tl)58AVP.
Fig. 5. Gamma-ray single and neutron-gated spectra of the 28Si(3He,ny)30 S reaction with a Gc(Li) detector.
572
TOMONARI OTSUBO
400
2 1
1300
n
200
c +itl11i1 ô " too fl,
4001
(c) 0" 51Mev
sponding neutron peak of the time spectrum. 2.20 and 1.19 MeV gamma-ray peaks are also seen in this spectrum as in the case of the Ge(Li) spectrum.
neutron spectrum with PSD
t+f,~t,,{t
IO 2o 30 channel number
é
40F(b) '21 1' i i
neutron spectrum without PSD
c
l .19M,V I
20 10 30 channel number "Si(°He, nr)°S E( He)=7.OMeV 7.5x7.scm :Nal(TO gamma ray spectrum (30r) 2.20Mev
50 channel numberloo
150
Fig. 6. Neutron and gamma-ray spectraof two-dimensional measurements from the ssSi(sHe,ny)s 0S reaction. 1 and 2 of figs. 6a and b correspond to first and second excited states of 30S. Cross marks in thegamma-rayspectra wereobtained on both sides of neutron groups. As an example for practical applications, the 28Si(3 He,ny)3aSreaction was performedwith a7.0MeV doubly ionized 3He-1-.eam delivered from the Van de Graafl accelerator. Figs 5a and 5b are part of the gamma-ray spectra obtained with a Ge(Li) detector. In the singlesspectrum (a), peaks which-can be seen are those of 30P through the 28Si(3He,py)3ap reaction, while in the n-ycoincidence spectrum (b), peaksdueto 30S can be clearly seen. The same reaction was also measured with the AGTOFmethod using theNaI(Tl)58AVP assembly as a gamma-ray detector and with a neutron flight path of 75 cm. Figs 6a and 6b show the neutron time-of-flight spectra in coincidence with the first excited gamma-ray group.Fig. ticgives thegammaray spectrum in coincidence with the sum of the first and second excited neutron groups shown in fig. 6a . The background gamma-ray spectrum was obtained from the lower and uppersides of thecorre-
4.C The measurement of fast neutrons often requires a detector system of alarge-diameter scintillator coupled with several photomultipliers. This complicates the preliminary adjustment of the electronic apparatus. In the present study properties of a large-diameter scintillator-photomultiplier NE213-60DVP assembly have been investigated and compared with thoseof the widespread photomultiplier 58AVP. The present study has shown the possibility of application of the NE213-WDVP unit for the fast neutron detector although its characteristic resolving time and n-y discrimination are somewhat inferior to those of the NE213-58AVPassembly. We adopted the` almost equal dynode voltage distribution which was' recommendedby themanufacturer, buta moreextensive study of the divider circuit may improve these properties. In measurements using a Pu-Be neutron source a simple method, whichis based on theAGTOFmethod, is employed to determine the energy dependence of the neutron detector with and without pulse-shape discrimination which affects seriously the detection of relatively low-energy neutrons. This is a convenient' method as atest of the AGTOF spectrometer. Theauthor would like to thank Prof. Y. Oda forhis advice and encouragement during the course of this work: Thanks arealso due to H. Abe, T. Kubo, S. Koh` and K. Kawasaki for their assistance with the electronics andoperation of theaccelerator. He wouldalso like to thank Prof. E. Arai for giving him the Pu-Bc' neutron source and Dr H. Kubo for hiscritical reading of the manuscript. References 1:) J. H. Neil r and W. M. Good, Fast matron physics (Inter. science, New York, 1960) Part 1, p. 509. T. Joy, Nuc1. Instr, and Meth 73'(1%9) 240. a) P. B. Johnson, J. M. G. Caraca, J.N. Pihl, It . A Gill and H. J. Rose, Nuci. Instr. andMeth.93 (1971) 417. 4) W.A. Gibson, Rev. Sci. Instr. 37 (1966) 631. s) D. W. Jones, IEEE Trans. Nucl . Sci. NS-15 (1968) 491. s) G. White, Nucl. Insts. and Meth . 4S (1966) 270; K. Kandiah andG. White, I.P.P.S.Conf. (Harwell, 1968). 7) J: Kalyna and I.J. Taylor, Nucl. Instr. and Meth. 88 (1970) 277.