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Energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523
Nuclear Instruments and Methods m Physics Research A 333 (1993) 492-501 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
Energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 Takahiko Aoyama, Kiyonari Honda and Chizuo Mori
Department of Nuclear Engineering, Nagoya University, Nagoya 464-01, Japan
Katsuhisa Kudo and Naoto Takeda
Electrotechnteal Laboratory, Tsukuba, Ibaraki 305, Japan
Received 9 October 1992 and in revised form 26 March 1993 The energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 was examined for the neutron energy range between 1.2 and 14 MeV. Pulse height spectra measured with monoenergetic neutrons evidenced that they were characterized by either a single peak with a shoulder or by double peaks . A Monte Carlo simulation of the neutron behavior in the detector reproduced the spectra by considering the nonlinear light yield against recoil proton energy Detection efficiencies ranging from 10% for 1.2 MeV to 0.6% for 14 MeV neutrons were obtained with a 12.7 cm diameter X 7.6 cm long scintillator. A high background rejection with a ratio of 2200 to 1 obtained without using any y-shielding makes the present neutron spectrometer attractive for the application to low-level environmental neutron measurements . 1. Introduction Energy measurement for extremely weak fluxes of neutrons requires the use of detectors which have a large discriminating power against -y-ray and charged particle backgrounds with a large detection efficiency for energetic neutrons . Full-energy-absorption scintillation spectrometers [1,2] have been devised for this purpose for a neutron energy range between about 0.5 and 20 MeV. These spectrometers were composed of a large volume of either boron-loaded plastic scintillator BC-454 or ordinary organic liquid scintillator BC-505 in which multtpieces of 6 Li glass scintillator plates were immersed . The former was used to measure the energy of space neutrons [3], and the latter was used to detect the emission of 2.45 MeV neutrons from "cold nuclear fusion" [4]. In these spectrometers the identification of neutron events was performed through a "double-pulse coincidence method", in which well defined neutron capture pulses were used to open the gate of an analog-to-digital converter (ADC) to analyze the height of preceding proton recoil pulses . A remarkable feature of these spectrometers is that the integrated light output by multiple proton recoils corresponds to the total energy loss of a neutron, i .e . the incident neutron energy . Hence the light outputs for monoenergetic neutrons have been believed to be constant, forming a single peak in the pulse height spec-
trum, even though the peak had a broad width. The spectrum, however, was proved in the present study to have essentially a complicated shape without having a single broad peak because of the nonlinearity of light yield against recoil-proton energy . The present paper describes the neutron energy response of an organic liquid scintillator BC-523 working as a full-energy-absorption neutron spectrometer . The scintillator, also boron-loaded as BC-454, has a higher light output and a larger hydrogen/carbon ratio than BC-454, and hence is expected to have a higher energy resolution and a larger detection efficiency than BC-454 . In this paper pulse height spectra and detection efficiencies measured with monoenergetic neutrons were compared with Monte Carlo calculations . 2. Construction and principle 2.1 . Detector
The detector used in the present neutron spectrometer is a 12 .7 cm diameter X 7.62 cm long liquid scintillator made of BC-523 (Bicron Corporation), which was coupled to a 12.7 cm diameter Hamamatsu R1512 photomultiplier tube (PMT). BC-523 is a 5% natural boron-loaded organic liquid with isotopic abundances
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved
T. Aoyama et al / Energy response of a full-energy-absorption neutron spectrometer
10'
lo,
10'
Neutron energy (eV)
1
10
io'
Fig. 1 . Mean neutron time-of-flight AH / u between elastic scatterings with hydrogen nuclei in the BC-523 scintillator as a function of neutron energy, where A1i is the mean free path and a is the velocity of the neutron. Time taken for a neutron to fly a 1 cm distance, 1 cm/ u, is also shown for reference. of 80 .2% 1 'B and 19 .8%' ° B, resulting in a 1° B content of about 1% by weight . The density and hydrogen/ carbon ratio of BC-523 quoted by Bicron are 0.916 g/cm; and 1 .738, respectively . Its light output is 65% of anthracene, as compared with 48% for BC-454 boron-loaded plastic scintillator [1]. 2.2. Operational principle
A fast neutron that enters the detector can be quickly slowed down in a few tens of nanoseconds, primarily by multiple scattering with hydrogen nuclei in the scintillator, and produces a prompt signal that provides a measure of its initial energy . Fig. 1 shows the mean neutron time-of-flight A H/u between elastic scatterings with hydrogen nuclei in the BC-523 scintillator as a function of neutron energy, where A H is the
493
mean free path and u is the velocity of the neutron . Since the mean neutron energy loss by elastic scattering with a hydrogen nucleus is half its initial energy, three to four scatterings are required on average for a fast neutron to be slowed down to less than 10% of its initial energy. It can be estimated from fig. 1 that it takes more than 10 ns for a 1 MeV neutron, as an example, to be slowed down to less than 100 keV. The slowed down neutron is finally captured by '() B in the scintillator if it is not scattered away from the scintillator, emitting a definite amount of light by the reaction 1°B(n, a) 7 Li . Detection and energy measurement of the reaction products provide a second pulse that signifies the capture of a neutron. The delayed capture occurs on a time scale of microseconds . Hence, if we perform a pulse-height analysis for the prompt recoil pulse only when the delayed capture pulse with a definite size appears, we can measure the total energy lost by a neutron through multiple elastic scattering, i.e . the energy of the incident neutron. 2.3. Electronics
Fig. 2 shows the block diagram of electronics for the present spectrometer . The energy signal was derived from the dynode of the PMT and the timing signal from the PMT anode through built-in emitter-follower preamplifiers with intrinsic rise time of less than 10 ns and decay time of 20 and 50 Ws for energy and timing signals, respectively . The energy signal was amplified with a linear amplifier with Gaussian pulse shaping. The shaping time constant of the amplifier was selected as short as possible to separate delayed capture pulses from prompt recoil pulses distributing in a short time differ-
Fig. 2. Block diagram of electronics for the BC-523 neutron spectrometer.
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer
494
ence, though it should have been kept long enough compared with the neutron slowing time. The latter was necessary to add all the light outputs emitted in the neutron slowing down process. Shaping time constants of 50 and 200 ns used for the present experiment, however, were not long enough, since the neutron slowing time was on the order of 10 ns as described above. This resulted in the attenuation of each recoil-proton signal produced in the neutron slowing down process when it was added to form a prompt recoil pulse. The amplified prompt recoil pulse was delayed with a linear gate stretcher (ORTEC 542) to be coincident with the arrival of the ADC gate signal generated by the delayed capture pulse . The timing electronics consist of a timing filter amplifier followed by a single-channel analyzer (SCA) with energy window set to respond to the delayed capture signal generated by the reaction t° B(n, a) 7Li . Fig. 3 shows the pulse height spectrum of neutron capture signals derived from the timing filter amplifier, which was obtained using a 252Cf neutron source with a 5 cm thick polyethylene moderator. The prominent peak corresponding to a-particles and 7Li recoils had an equivalent electron energy close to 60 keV, which was determined from the position of the full-energy peak by Z41Àm -y-rays . The right hand hump seen in fig. 3 is due to the Compton edge of 478 keV y-rays summed with the energy deposited by recoiling a.-particles and 7 Li nuclei, since the neutron capture reaction is accompanied by 478 keV -y-rays [11 with a probability of 94%, and the Compton scattering probability for the -y-rays was estimated to be 40% in the scintillator . The energy window of the SCA was selected to catch the peak alone, as shown in the figure,
to enhance the discriminating power against -y-background . The output of the SCA triggered a gate generator used to define the width of the coincidence gate pulse following the neutron capture pulse. The amplifier signal was also sent to a SCA operating with the maximum energy window and a low threshold level - corresponding to a proton energy of about 200 keV - that triggered a gate/delay generator used to define the width and the delay time of the coincidence gate pulse following the prompt recoil pulse. The delay was adjusted not to generate a coincidence signal for a single event which triggers both SCAB . The gate and gate/delay signals must be in coincidence to signify a valid neutron event in a given gate pulse width after the prompt signal . 2 .4 . Mean capture lifetime
The optimum width of the coincidence gate pulses was determined from the time distribution of the delayed neutron-capture pulse after the prompt recoil pulse. The distribution measured with a zs2Cf source is shown in fig. 4, where the time-independent component of accidental coincidences was subtracted . Fig. 4 reveals that the coincidence counting rate decreased exponentially with increasing time after the prompt pulse with a time constant of 2.20 ws . It has been shown that the time constant could be explained by the mean neutron capture lifetime independent of the neutron energy [1,5] though it slightly depended on the detector size [5] and on the scattering material around the detector [3]. The time constant for the present case was practically consistent with the mean capture lifetime of 2.24 ps calculated [1] using a
0 U
Fig. 3. Pulse height spectrum for neutron capture signals derived from the timing filter amplifier. The energy window of the SCA was selected to catch the peak alone generated by a-particles and 7Li recoils, the window of which is shown as the hatched area The upper level of the window should be broaden from ULl to UL2 to catch 100% of the neutron capture signal .
T. Aoyama et al / Energy response of a full-energy-absorption neutron spectrometer
495
small as possible to minimize accidental coincidence counts caused by background radiations . On the other
hand, the width should be large enough to obtain a large neutron detection efficiency . Since 90% of the
N
delayed capture pulses appear within 2.3 times the mean capture lifetime, it would be unnecessary to
ô
select a width larger than 2.3 ,r. Setting the total width of the coincidence gate pulses at 5.0 Vs as an optimum
v+
.C
we obtained an accidental coincidence rate of 0.13 cps for background -y-rays in our laboratory without any
ô
d
,y-shielding . The rate was 4.5 X 10 -4 times smaller than the background counting rate of 290 cps obtained with the same detector without coincidence operation.
73
's U
that the detector can handle was limited by the same gate-pulse width of 5.0 [Ls to be about 2000 cps to keep the accidental coinciThe maximum counting rate
dence rate within 1% of the true coincidence rate . The coincidence signals were used to open the
ADC gate to analyze the height of the prompt recoil pulses . The timing chart for a typical pulse is shown in
Fig. 4. Time distribution of delayed neutron capture pulse after the prompt recoil pulse.
fig . 5.
10 B content of 5 .29 X 10 20 at . /Cm; in the BC-523 scin-
tillator .
3. Energy response function
2.5. Background rejection
3.1 . Pulse height spectra for monoenergettc neutrons
The experimental mean capture lifetime
T
Monoenergetic neutrons were generated by using Cockcroft-Walton and Van de Graaff types of accelerators. Nuclear reactions used are shown in table 1 with
of 2.20
ws was used to determine the optimum width of the
coincidence gate pulses . The width should be kept as recoil TIMING FILTER AMPLIFIER COINCIDENCE Gate (recoil)
capture
I
Window
I
Disc .
i I I
I
I I
N
1 .4 us
i i
O .4 1 us ; ;
I I
I I
4 .7 us
i I i
I
COINCIDENCE Gate (capture)
I
I
1
0 .4 us .~i II
I I I i
r--0 .3 us
I I
ADC Gate
3 us
I I
I
ADC Input
v I I
i
fi
2 us Fig. 5. Timing chart for coincidence and ADC gate pulses and for ADC linear input pulses .
496
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer
Table 1 Nuclear reactions used to generate monoenergetic neutrons . Cockcroft-Walton and Van de Graaff types of accelerators were used. Nuclear reaction
Neutron energy [MeV1
a
T(p, n) 3 He
D(d, n) 3He
T(d, n) 4He
1 .18
2 .41 2 .60 2 .84 3 .0 5 .0 a
14 .6
a
cone, made of borated paraffin, 50 cm long, between the detector and the targets showed a large number of counts at lower pulse height and a monotonical count decrease with increasing pulse height for each neutron energy . Fig. 7 shows an example of a moderated neutron spectrum measured with the shadow cone (a) for a neutron energy of 2.4 MeV and of a direct neutron spectrum (b) for the same neutron energy, in which the latter was obtained by subtracting the spectrum of fig. 7a from the one shown in fig. 6b . Moderated neutron spectra observed at each neutron energy indicated that the upturn at a low pulse height observed at each spectrum of fig. 6 was due to the contribution of neutrons moderated in external materials. It is seen from figs. 6 and 7b that a pulse height spectrum for direct neutrons had a peak with a shoulder at the higher side of the peak for neutron energies below about 3 MeV and two peaks at higher neutron energies .
Van de Graaff.
the neutron energies obtained . The measurements of the energy response of the spectrometer were made in a standard neutron field established at the Electrotechnical Laboratory, consistent with those of other standardizing laboratories confirmed by a series of international intercomparisons held under the auspices of the Bureau International des Poids et Mesures, Sèvres [6]. The detector was supported with an aluminium stand about 1 m from the targets of the accelerators, and it was irradiated with neutrons on the flat front surface of the cylindrical detector. Fig. 6 shows pulse height spectra obtained with various energies of monoenergetic neutrons . Spectra obtained by inserting a shadow
3.2. Monte Carlo simulation
The origin of the complicated pulse height spectra observed for monoenergetic neutrons was examined using a Monte Carlo computer simulation for the behavior of fast neutrons entered into the detector. In 1500r-
2 .4 MeV
1000
N F2
O U
Û
500
v
t
ss
I 0
I
I
200
400
600
CHANNEL NUMBER
1000 5.0 MeV
~,
14 .6 MeV
800
4
600
3
z 400
200
0
0
I
I 200
I
, 400
CHANNEL NUMBER
1
iL..L._ .i
600
6
0
I
I 200
I
I 400
I
600
800
CHANNEL NUMBER
Fig. 6 . Pulse height spectra for monoenergetic neutrons with energies of (a) 1 .2 MeV, (b) 2 .4 MeV, (c) 5 .0 MeV and (d) 14 .6 MeV, respectively .
T Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer 500 F U0
0k 0
200
400
600
CHANNEL NUMBER
CHANNEL NUMBER
Fig. 7. Pulse height spectra for 2.4 MeV neutrons . (a) Externally moderated neutron spectra measured with the insertion of a shadow cone between the target and the scintillator . (b) Direct neutron spectrum obtained by subtracting (a) from the spectrum of fig. 6b . the calculation total light outputs by recoil protons were summed up in the neutron slowing down process. Neutron cross section data used were JENDLE-3 provided by the Japan Atomic Energy Research Institute (JAERI), which included the data of elastic scattering for I H, elastic and inelastic scatterings, and (n, a) reaction for I° B, elastic and inelastic scatterings for II B, eight different reactions for I2 C and three reactions for I6 0. The following simplification were adopted: - neutrons enter the detector uniformly and perpendicularly onto the flat front surface of the cylindrical detector ; - neutrons scattered away from the scintillator never come back again; - no neutrons enter the detector after scattering outside the scintillator ; - neutron scattering is isotropic in the center-ofmass-coordinate system ; - light emission is solely due to recoil protons. The light yield was assumed to be proportional to EP 5 [7], where EP is the recoil proton energy . This will be justified later. Fig. 8 shows a light output spectrum calculated for 2.4 MeV neutrons and the convolution of a Gaussian distribution function considering the energy resolution . A FWHM of 12% for the maximum light output was determined by the analysis of the Compton edge of ,y-rays [8] providing nearly the same light yield as that by 2.4 MeV protons. It is seen from fig. 8 that the light
49 7
output spectrum considering the energy resolution resembles the experimental pulse height spectrum of fig. 7b . In the calculation of the light output spectrum nonlinearity was assumed in the relation between proton energy and light yield, as described above. The nonlinearity is such that the sum of the light outputs for two protons having energies EI, E2 and giving a light output L, +2 = L(EI ) + L(E2 ) is less than that for a single proton having energy El +E 2 and giving L(EI + E2) > L I+2 [3]. This indicates that the maximum light output L P indicated in fig. 8 corresponds to the case where a 2.4 MeV neutron lost almost all its energy in a single head-on collision with a hydrogen nucleus, and therefore to the light yield of a 2.4 MeV proton . It is seen from fig. 8 that the light yield LP is consistent with the point providing approximately half of the counts at the shoulder for the light output spectra corrected for resolution . Since it was confirmed that the same relation for the light yield LP was realized in the light output spectra for the neutron energy range between 1 .2 and 14 MeV (see fig. 12), the values of the light yield LP were determined experimentally from pulse height spectra of fig. 6. Fig. 9 shows the relation between the light yield L P and proton energy, where the light yield was normalized at the electron light yield producing half the maximum counts of the Compton edge for 137CS y-rays [8]. It is seen from fig. 9 that the light yield is proportional to EP 5 for the range of proton energies EP from 1 .2 to 14 .6 MeV. In fig. 9 is also shown the energy dependence of the electron light yield, which was obtained by the analysis of the Compton edge [8] for various -y-ray energies . A linear relation between light yield and electron energy is seen in fig. 9 for the examined electron energy range from 0.3 to 2.4 MeV. Since the light yield for electrons was
MO X F
Uô
LIGHT OUTPUT
(ARB, UNITS)
Fig. 8. Light output spectrum obtained by a Monte Carlo calculation for 2.4 MeV neutrons and the convolution of Gaussian distribution function considering an energy resolution of 12% FWHM at the maximum light output Lp. The light output LP corresponds to the light yield of 2.4 MeV protons.
49 8
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer 1,5 r-
3 .0 MeV
M O .-1 X H Z
0,5
a Particle energy IMeV)
Fig. 9. Scintillation light yield of protons and electrons as a function of particle energy.
6E~xJ1,5 Me V
M O 0 .5 0 U
L-I- -_l.- ._ .lye 2 4
normalized in the same manner as for protons, the
I
LIGHT OUTPUT (ARB,
relation between proton and electron energies provid-
I
6I
I
8
10
UNITS)
ing the same light yield can be estimated from fig. 9. As described above,
since the light yield is not
proportional to the proton energy, the total light yield differs depending on the history of the neutron energy
loss in the neutron slowing down process even for a constant full energy absorption. Fig. l0a shows a calcu-
lated light output spectrum for 3.0 MeV neutrons, and fig. 10b shows that selected for the case alone in which a single energy loss of more than 1.5 MeV is included in the process of neutron slowing down . Comparing fig. l0a with fig. 10b we can conclude that the shoulder at
the maximum light output
seen
in fig. l0a corre-
sponded to the case of a large energy loss by a single scattering . This is consistent with our expectation . Fig. 11 shows a Monte Carlo calculation of captured-neutron energy spectrum for 3 MeV incident
Fig. 10 . Dependence of the light output spectra on the history of the neutron energy loss. (a) Light output spectrum calculated for 3.0 MeV neutrons . (b) The same spectrum but selected for the case alone in which a single energy loss of more than 1 .5 MeV is included m the neutron slowing down process.
convolution for each spectrum, since the dependence
of the FWHM on the proton energy was not known for the present scintillation detector . It is seen from fig. 12 2
En = 3 .0 MeV
neutron energy . It is seen from fig. 11 that captured
neutrons are not thermalized but have energies of the order of 10 eV . Fig. 11 also indicates that some of the neutrons are captured by ' ° B with large residual energies .
3.3. Calculated energy response function Light output spectra were calculated for nearly the same neutron energies as in the experiment . Results are shown in fig. 12 for neutron energies of 1.2, 3.0, 5.0
and 14 .0 MeV with the convolution of a Gaussian distribution function considering the energy resolution .
A FWHM of 12% observed with Compton electrons whose light output is equivalent to that by protons with energies of a few MeV was used
to calculate the
0 10-I
IO I Captured
103
105
neutron energy
IO7 (eV)
Fig. 11 . Captured neutron energy spectrum calculated for incident neutrons with an energy Ea of 3.0 MeV.
T Aoyama et al . / Energy response of a full-energy-absorption neutron spectrometer
49 9
linear amplifier used, where succeeding light outputs in the process of neutron slowing down were added after some attenuation . This would have effects to lower the peak position and broaden the peak width as seen in fig. 6, then producing the valley with increasing the height of the shoulders for large neutron energies .
that the ratio of counts at the shoulder to peak counts increased with neutron energy . This is because the mean free path of neutrons for elastic scatterings with hydrogen nuclei increases with neutron energy, which would result in selective capture of neutrons whose energy was lost in a large fraction at the first scattering. The ratio of counts at the shoulder to peak counts however is underestimated especially at the higher neutron energy, since the rate of neutron backscatterings by hydrogen nuclei, and therefore of the forward knock-on of protons, is slightly larger than that of isotropic scatterings of neutrons assumed in the calculation, and it increases with neutron energy from 0.4% at 1 MeV to 5.5% at 14 MeV [9]. Comparison of the calculated spectra of fig. 12 with experimental pulse height spectra of fig. 6 indicates that consistency was excellent for neutron energies of less than 3 MeV except for the upturn at a low pulse height side of experimental spectra due to externally moderated neutron components . For neutron energies of more than 5 MeV, however, there appear double peaks with a prominent valley for the experimental pulse height spectra, which have never been obtained by Monte Carlo calculations . The reason for this might be the relatively short shaping time constant of the
4. Detection efficiency The detection efficiency 71 for monoenergetic neutrons is defined as the ratio of the number of captured neutrons to that incident on the detector . The dependence of the efficiency rl n on the neutron energy was examined using Monte Carlo calculations, which were carried out for the neutron energy range between 1 and 20 MeV, setting the number of success events between 10 4 and 10 5 for each neutron energy . Results are shown in fig. 13 . The experimental detection efficiency 17ex was determined as follows. The counting rate n [s -1 ] for monoenergetic neutrons is given by the integral counting rate of the pulse height spectrum in fig. 6 subtracting the counting rate estimated from the spectrum measured with the shadow cone . Evaluating the neu-
MO H X
M O rl X
F Z O U
H Z O U
2
4
6
LIGHT OUTPUT (ARB,
8
10
12
UNITS)
14 .0 MeV
5 .0 MeV
r 0 U
2
4
6
8
LIGHT OUTPUT (ARB, UNITS)
10
12
0
2
4
6
8
LIGHT OUTPUT (ARB, UNITS)
10
12
Fig. 12. Light output spectra obtained by Monte Carlo calculations for neutron energies of (a) 1.2 MeV, (b) 3.0 MeV, (c) 5 .0 MeV and (d) 14 .0 MeV, and the convolution of Gaussian distribution function considering an energy resolution of 12% FWHM at the maximum light output for each spectrum .
500
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer
posited by recoiling a-particles and 7Li nuclei, rejecting the component of 478 keV -y-rays, the probability pg to generate capture-gate-signal was decreased to be 62% of the total neutron capture. In fig. 13 the values of 77,, divided by p e and pg are plotted as corrected experimental detection efficiencies . It is seen from fig. 13 that the corrected efficiencies agree well with the calculated curve, and that an efficiency of about 10% at 1.2 MeV neutrons decreases continuously to about 0.6% at 14 MeV.
s0
T v U
N _o U N N
5. Conclusions
Neutron energy (MeV) Fig. 13 . Detection efficiency as a function of neutron energy . Experimental detection efficiency was corrected for coincidence counting losses and the loss of coincidence-gate-pulse generation to compare with the efficiency obtained by Monte Carlo calculation .
tron flux density ¢ [cm-2 S- '] at the flat front surface of the detector with an area of A (= 127 cm 2) we can obtain the experimental efficiency ,Y)ex
= nIOA,
where the neutron flux density at 14 .6 MeV was determined by associated a-particle counting, while a proton-recoil telescope or a hydrogen gas proportional counter was used in the other energy region [6]. Comparison of the experimental efficiency qex with 77n requires corrections for ?7 ex to remove the difference in the conditions . Firstly, coincidence counting losses are included in il ex because of the finite pulse widths of the coincidence gate pulses and the delay between the pulses . The time distribution of the delayed capture pulse after the prompt recoil pulse was given by an exponentially decreasing function with a time constant ,r of 2.2 ~Ls as shown in fig. 4. Hence the coincidence probability pc is evaluated from a summed pulse width t W of 5.0 ws and a net delay time t d of 0.7 lts between coincidence-gate pulses to be pe = (
1-exp(-tW/-r)) exp(-td /T)=0 .65.
(2)
Secondarily, the loss of coincidence-gate-pulse generation is included in 71,x because of the narrow SCA window for neutron capture pulses . Since the window was adjusted to correspond solely to the energy de-
A full-energy-absorption neutron spectrometer was constructed using a boron-loaded liquid scintillator BC-523, and the energy response was examined experimentally and by Monte Carlo calculations for the neutron energy range between 1.2 and 14 MeV. The results are: - pulse height spectra measured with monoenergetic neutrons showed either a peak with a shoulder or double peaks instead of a single broad peak which had been expected ; - Monte Carlo calculations of the light emission in the course of the neutron slowing down in the scintillator indicated that the complicated pulse height spectra observed for monoenergetic neutrons were due to the nonlinearity of the light yield against recoil proton energy ; - the energy response functions calculated using the measured relation between light yield and recoilproton energy reproduced the characteristics of the experimental pulse height spectra; - the experimental neutron detection efficiency corrected for coincidence counting losses and the loss of coincidence-gate-pulse generation agreed well with the efficiency calculated by a Monte Carlo computer simulation. Although an accurate knowledge of the energy response functions is necessary for neutron spectrum unfolding, the difficulty of the precise determination of the response functions limits the application of the present spectrometer only to the unfolding of relatively simple neutron energy spectra with moderate energy resolution . Since the present scintillation spectrometer utilizes the double-pulse coincidence method for -y-ray rejection without using conventional pulse shape discrimination, the discriminating power would not degrade with increasing scintillator volume . Hence a spectrometer using a large volume of scintillator would be used effectively for the energy measurement of low neutron fluxes with intensities of 1 cm -2 s - I or less.
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer Acknowledgements We are grateful to Mr . S. Itoh of Nagoya University for his support in the development of the Monte Carlo computer program and to Mr . T. Narita of JAERI, who provided the neutron cross section data JENDLE3. Thanks are due to Nagoya University Computation Center for the approval to use the supercomputer and to Mr . Y. Takasuna for his experimental support. References [11 D.M . Drake, W.C . Feldman and C. Hurlbut, Nucl . Instr. and Meth . A247 (1986) 576.
501
[21 J.B . Czirr and G.L. Jensen, Nucl . Instr. and Meth . A284 (1989) 365. [3] W.C . Feldman, G.F . Auchampaugh and R.C . Byrd, Nucl . Instr. and Meth . A306 (1991) 350 . [4] S.E. Jones, E.P . Palmer, J.B . Czirr, D.L. Decker, G.L . Jensen, J.M. Thorne, S.F. Tayer and J. Rafelski, Nature 338 (1989) 737. [5] E.A . Kamykowski, Nucl . Instr. and Meth . A299 (1990) 105. [6] K. Kudo, T. Michikawa, T. Kinoshita, N. Kobayashi, A. Fukuda, Y. Hino and Y. Kawada, J. Nucl . Sci. Technol. 24 (1987) 684. [7] G.F . Knoll, Radiation Detection and Measurement, 2nd ed . (Wiley, New York, 1989) p. 224. [8] G. Dietze and H. Mein, Nucl . Instr. and Meth . 193 (1982) 549. [9] J.C. Hopkins and G. Breit, Nucl . Data Tables A9 (1971) 137.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
Energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 Takahiko Aoyama, Kiyonari Honda and Chizuo Mori
Department of Nuclear Engineering, Nagoya University, Nagoya 464-01, Japan
Katsuhisa Kudo and Naoto Takeda
Electrotechnteal Laboratory, Tsukuba, Ibaraki 305, Japan
Received 9 October 1992 and in revised form 26 March 1993 The energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 was examined for the neutron energy range between 1.2 and 14 MeV. Pulse height spectra measured with monoenergetic neutrons evidenced that they were characterized by either a single peak with a shoulder or by double peaks . A Monte Carlo simulation of the neutron behavior in the detector reproduced the spectra by considering the nonlinear light yield against recoil proton energy Detection efficiencies ranging from 10% for 1.2 MeV to 0.6% for 14 MeV neutrons were obtained with a 12.7 cm diameter X 7.6 cm long scintillator. A high background rejection with a ratio of 2200 to 1 obtained without using any y-shielding makes the present neutron spectrometer attractive for the application to low-level environmental neutron measurements . 1. Introduction Energy measurement for extremely weak fluxes of neutrons requires the use of detectors which have a large discriminating power against -y-ray and charged particle backgrounds with a large detection efficiency for energetic neutrons . Full-energy-absorption scintillation spectrometers [1,2] have been devised for this purpose for a neutron energy range between about 0.5 and 20 MeV. These spectrometers were composed of a large volume of either boron-loaded plastic scintillator BC-454 or ordinary organic liquid scintillator BC-505 in which multtpieces of 6 Li glass scintillator plates were immersed . The former was used to measure the energy of space neutrons [3], and the latter was used to detect the emission of 2.45 MeV neutrons from "cold nuclear fusion" [4]. In these spectrometers the identification of neutron events was performed through a "double-pulse coincidence method", in which well defined neutron capture pulses were used to open the gate of an analog-to-digital converter (ADC) to analyze the height of preceding proton recoil pulses . A remarkable feature of these spectrometers is that the integrated light output by multiple proton recoils corresponds to the total energy loss of a neutron, i .e . the incident neutron energy . Hence the light outputs for monoenergetic neutrons have been believed to be constant, forming a single peak in the pulse height spec-
trum, even though the peak had a broad width. The spectrum, however, was proved in the present study to have essentially a complicated shape without having a single broad peak because of the nonlinearity of light yield against recoil-proton energy . The present paper describes the neutron energy response of an organic liquid scintillator BC-523 working as a full-energy-absorption neutron spectrometer . The scintillator, also boron-loaded as BC-454, has a higher light output and a larger hydrogen/carbon ratio than BC-454, and hence is expected to have a higher energy resolution and a larger detection efficiency than BC-454 . In this paper pulse height spectra and detection efficiencies measured with monoenergetic neutrons were compared with Monte Carlo calculations . 2. Construction and principle 2.1 . Detector
The detector used in the present neutron spectrometer is a 12 .7 cm diameter X 7.62 cm long liquid scintillator made of BC-523 (Bicron Corporation), which was coupled to a 12.7 cm diameter Hamamatsu R1512 photomultiplier tube (PMT). BC-523 is a 5% natural boron-loaded organic liquid with isotopic abundances
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved
T. Aoyama et al / Energy response of a full-energy-absorption neutron spectrometer
10'
lo,
10'
Neutron energy (eV)
1
10
io'
Fig. 1 . Mean neutron time-of-flight AH / u between elastic scatterings with hydrogen nuclei in the BC-523 scintillator as a function of neutron energy, where A1i is the mean free path and a is the velocity of the neutron. Time taken for a neutron to fly a 1 cm distance, 1 cm/ u, is also shown for reference. of 80 .2% 1 'B and 19 .8%' ° B, resulting in a 1° B content of about 1% by weight . The density and hydrogen/ carbon ratio of BC-523 quoted by Bicron are 0.916 g/cm; and 1 .738, respectively . Its light output is 65% of anthracene, as compared with 48% for BC-454 boron-loaded plastic scintillator [1]. 2.2. Operational principle
A fast neutron that enters the detector can be quickly slowed down in a few tens of nanoseconds, primarily by multiple scattering with hydrogen nuclei in the scintillator, and produces a prompt signal that provides a measure of its initial energy . Fig. 1 shows the mean neutron time-of-flight A H/u between elastic scatterings with hydrogen nuclei in the BC-523 scintillator as a function of neutron energy, where A H is the
493
mean free path and u is the velocity of the neutron . Since the mean neutron energy loss by elastic scattering with a hydrogen nucleus is half its initial energy, three to four scatterings are required on average for a fast neutron to be slowed down to less than 10% of its initial energy. It can be estimated from fig. 1 that it takes more than 10 ns for a 1 MeV neutron, as an example, to be slowed down to less than 100 keV. The slowed down neutron is finally captured by '() B in the scintillator if it is not scattered away from the scintillator, emitting a definite amount of light by the reaction 1°B(n, a) 7 Li . Detection and energy measurement of the reaction products provide a second pulse that signifies the capture of a neutron. The delayed capture occurs on a time scale of microseconds . Hence, if we perform a pulse-height analysis for the prompt recoil pulse only when the delayed capture pulse with a definite size appears, we can measure the total energy lost by a neutron through multiple elastic scattering, i.e . the energy of the incident neutron. 2.3. Electronics
Fig. 2 shows the block diagram of electronics for the present spectrometer . The energy signal was derived from the dynode of the PMT and the timing signal from the PMT anode through built-in emitter-follower preamplifiers with intrinsic rise time of less than 10 ns and decay time of 20 and 50 Ws for energy and timing signals, respectively . The energy signal was amplified with a linear amplifier with Gaussian pulse shaping. The shaping time constant of the amplifier was selected as short as possible to separate delayed capture pulses from prompt recoil pulses distributing in a short time differ-
Fig. 2. Block diagram of electronics for the BC-523 neutron spectrometer.
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer
494
ence, though it should have been kept long enough compared with the neutron slowing time. The latter was necessary to add all the light outputs emitted in the neutron slowing down process. Shaping time constants of 50 and 200 ns used for the present experiment, however, were not long enough, since the neutron slowing time was on the order of 10 ns as described above. This resulted in the attenuation of each recoil-proton signal produced in the neutron slowing down process when it was added to form a prompt recoil pulse. The amplified prompt recoil pulse was delayed with a linear gate stretcher (ORTEC 542) to be coincident with the arrival of the ADC gate signal generated by the delayed capture pulse . The timing electronics consist of a timing filter amplifier followed by a single-channel analyzer (SCA) with energy window set to respond to the delayed capture signal generated by the reaction t° B(n, a) 7Li . Fig. 3 shows the pulse height spectrum of neutron capture signals derived from the timing filter amplifier, which was obtained using a 252Cf neutron source with a 5 cm thick polyethylene moderator. The prominent peak corresponding to a-particles and 7Li recoils had an equivalent electron energy close to 60 keV, which was determined from the position of the full-energy peak by Z41Àm -y-rays . The right hand hump seen in fig. 3 is due to the Compton edge of 478 keV y-rays summed with the energy deposited by recoiling a.-particles and 7 Li nuclei, since the neutron capture reaction is accompanied by 478 keV -y-rays [11 with a probability of 94%, and the Compton scattering probability for the -y-rays was estimated to be 40% in the scintillator . The energy window of the SCA was selected to catch the peak alone, as shown in the figure,
to enhance the discriminating power against -y-background . The output of the SCA triggered a gate generator used to define the width of the coincidence gate pulse following the neutron capture pulse. The amplifier signal was also sent to a SCA operating with the maximum energy window and a low threshold level - corresponding to a proton energy of about 200 keV - that triggered a gate/delay generator used to define the width and the delay time of the coincidence gate pulse following the prompt recoil pulse. The delay was adjusted not to generate a coincidence signal for a single event which triggers both SCAB . The gate and gate/delay signals must be in coincidence to signify a valid neutron event in a given gate pulse width after the prompt signal . 2 .4 . Mean capture lifetime
The optimum width of the coincidence gate pulses was determined from the time distribution of the delayed neutron-capture pulse after the prompt recoil pulse. The distribution measured with a zs2Cf source is shown in fig. 4, where the time-independent component of accidental coincidences was subtracted . Fig. 4 reveals that the coincidence counting rate decreased exponentially with increasing time after the prompt pulse with a time constant of 2.20 ws . It has been shown that the time constant could be explained by the mean neutron capture lifetime independent of the neutron energy [1,5] though it slightly depended on the detector size [5] and on the scattering material around the detector [3]. The time constant for the present case was practically consistent with the mean capture lifetime of 2.24 ps calculated [1] using a
0 U
Fig. 3. Pulse height spectrum for neutron capture signals derived from the timing filter amplifier. The energy window of the SCA was selected to catch the peak alone generated by a-particles and 7Li recoils, the window of which is shown as the hatched area The upper level of the window should be broaden from ULl to UL2 to catch 100% of the neutron capture signal .
T. Aoyama et al / Energy response of a full-energy-absorption neutron spectrometer
495
small as possible to minimize accidental coincidence counts caused by background radiations . On the other
hand, the width should be large enough to obtain a large neutron detection efficiency . Since 90% of the
N
delayed capture pulses appear within 2.3 times the mean capture lifetime, it would be unnecessary to
ô
select a width larger than 2.3 ,r. Setting the total width of the coincidence gate pulses at 5.0 Vs as an optimum
v+
.C
we obtained an accidental coincidence rate of 0.13 cps for background -y-rays in our laboratory without any
ô
d
,y-shielding . The rate was 4.5 X 10 -4 times smaller than the background counting rate of 290 cps obtained with the same detector without coincidence operation.
73
's U
that the detector can handle was limited by the same gate-pulse width of 5.0 [Ls to be about 2000 cps to keep the accidental coinciThe maximum counting rate
dence rate within 1% of the true coincidence rate . The coincidence signals were used to open the
ADC gate to analyze the height of the prompt recoil pulses . The timing chart for a typical pulse is shown in
Fig. 4. Time distribution of delayed neutron capture pulse after the prompt recoil pulse.
fig . 5.
10 B content of 5 .29 X 10 20 at . /Cm; in the BC-523 scin-
tillator .
3. Energy response function
2.5. Background rejection
3.1 . Pulse height spectra for monoenergettc neutrons
The experimental mean capture lifetime
T
Monoenergetic neutrons were generated by using Cockcroft-Walton and Van de Graaff types of accelerators. Nuclear reactions used are shown in table 1 with
of 2.20
ws was used to determine the optimum width of the
coincidence gate pulses . The width should be kept as recoil TIMING FILTER AMPLIFIER COINCIDENCE Gate (recoil)
capture
I
Window
I
Disc .
i I I
I
I I
N
1 .4 us
i i
O .4 1 us ; ;
I I
I I
4 .7 us
i I i
I
COINCIDENCE Gate (capture)
I
I
1
0 .4 us .~i II
I I I i
r--0 .3 us
I I
ADC Gate
3 us
I I
I
ADC Input
v I I
i
fi
2 us Fig. 5. Timing chart for coincidence and ADC gate pulses and for ADC linear input pulses .
496
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer
Table 1 Nuclear reactions used to generate monoenergetic neutrons . Cockcroft-Walton and Van de Graaff types of accelerators were used. Nuclear reaction
Neutron energy [MeV1
a
T(p, n) 3 He
D(d, n) 3He
T(d, n) 4He
1 .18
2 .41 2 .60 2 .84 3 .0 5 .0 a
14 .6
a
cone, made of borated paraffin, 50 cm long, between the detector and the targets showed a large number of counts at lower pulse height and a monotonical count decrease with increasing pulse height for each neutron energy . Fig. 7 shows an example of a moderated neutron spectrum measured with the shadow cone (a) for a neutron energy of 2.4 MeV and of a direct neutron spectrum (b) for the same neutron energy, in which the latter was obtained by subtracting the spectrum of fig. 7a from the one shown in fig. 6b . Moderated neutron spectra observed at each neutron energy indicated that the upturn at a low pulse height observed at each spectrum of fig. 6 was due to the contribution of neutrons moderated in external materials. It is seen from figs. 6 and 7b that a pulse height spectrum for direct neutrons had a peak with a shoulder at the higher side of the peak for neutron energies below about 3 MeV and two peaks at higher neutron energies .
Van de Graaff.
the neutron energies obtained . The measurements of the energy response of the spectrometer were made in a standard neutron field established at the Electrotechnical Laboratory, consistent with those of other standardizing laboratories confirmed by a series of international intercomparisons held under the auspices of the Bureau International des Poids et Mesures, Sèvres [6]. The detector was supported with an aluminium stand about 1 m from the targets of the accelerators, and it was irradiated with neutrons on the flat front surface of the cylindrical detector. Fig. 6 shows pulse height spectra obtained with various energies of monoenergetic neutrons . Spectra obtained by inserting a shadow
3.2. Monte Carlo simulation
The origin of the complicated pulse height spectra observed for monoenergetic neutrons was examined using a Monte Carlo computer simulation for the behavior of fast neutrons entered into the detector. In 1500r-
2 .4 MeV
1000
N F2
O U
Û
500
v
t
ss
I 0
I
I
200
400
600
CHANNEL NUMBER
1000 5.0 MeV
~,
14 .6 MeV
800
4
600
3
z 400
200
0
0
I
I 200
I
, 400
CHANNEL NUMBER
1
iL..L._ .i
600
6
0
I
I 200
I
I 400
I
600
800
CHANNEL NUMBER
Fig. 6 . Pulse height spectra for monoenergetic neutrons with energies of (a) 1 .2 MeV, (b) 2 .4 MeV, (c) 5 .0 MeV and (d) 14 .6 MeV, respectively .
T Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer 500 F U0
0k 0
200
400
600
CHANNEL NUMBER
CHANNEL NUMBER
Fig. 7. Pulse height spectra for 2.4 MeV neutrons . (a) Externally moderated neutron spectra measured with the insertion of a shadow cone between the target and the scintillator . (b) Direct neutron spectrum obtained by subtracting (a) from the spectrum of fig. 6b . the calculation total light outputs by recoil protons were summed up in the neutron slowing down process. Neutron cross section data used were JENDLE-3 provided by the Japan Atomic Energy Research Institute (JAERI), which included the data of elastic scattering for I H, elastic and inelastic scatterings, and (n, a) reaction for I° B, elastic and inelastic scatterings for II B, eight different reactions for I2 C and three reactions for I6 0. The following simplification were adopted: - neutrons enter the detector uniformly and perpendicularly onto the flat front surface of the cylindrical detector ; - neutrons scattered away from the scintillator never come back again; - no neutrons enter the detector after scattering outside the scintillator ; - neutron scattering is isotropic in the center-ofmass-coordinate system ; - light emission is solely due to recoil protons. The light yield was assumed to be proportional to EP 5 [7], where EP is the recoil proton energy . This will be justified later. Fig. 8 shows a light output spectrum calculated for 2.4 MeV neutrons and the convolution of a Gaussian distribution function considering the energy resolution . A FWHM of 12% for the maximum light output was determined by the analysis of the Compton edge of ,y-rays [8] providing nearly the same light yield as that by 2.4 MeV protons. It is seen from fig. 8 that the light
49 7
output spectrum considering the energy resolution resembles the experimental pulse height spectrum of fig. 7b . In the calculation of the light output spectrum nonlinearity was assumed in the relation between proton energy and light yield, as described above. The nonlinearity is such that the sum of the light outputs for two protons having energies EI, E2 and giving a light output L, +2 = L(EI ) + L(E2 ) is less than that for a single proton having energy El +E 2 and giving L(EI + E2) > L I+2 [3]. This indicates that the maximum light output L P indicated in fig. 8 corresponds to the case where a 2.4 MeV neutron lost almost all its energy in a single head-on collision with a hydrogen nucleus, and therefore to the light yield of a 2.4 MeV proton . It is seen from fig. 8 that the light yield LP is consistent with the point providing approximately half of the counts at the shoulder for the light output spectra corrected for resolution . Since it was confirmed that the same relation for the light yield LP was realized in the light output spectra for the neutron energy range between 1 .2 and 14 MeV (see fig. 12), the values of the light yield LP were determined experimentally from pulse height spectra of fig. 6. Fig. 9 shows the relation between the light yield L P and proton energy, where the light yield was normalized at the electron light yield producing half the maximum counts of the Compton edge for 137CS y-rays [8]. It is seen from fig. 9 that the light yield is proportional to EP 5 for the range of proton energies EP from 1 .2 to 14 .6 MeV. In fig. 9 is also shown the energy dependence of the electron light yield, which was obtained by the analysis of the Compton edge [8] for various -y-ray energies . A linear relation between light yield and electron energy is seen in fig. 9 for the examined electron energy range from 0.3 to 2.4 MeV. Since the light yield for electrons was
MO X F
Uô
LIGHT OUTPUT
(ARB, UNITS)
Fig. 8. Light output spectrum obtained by a Monte Carlo calculation for 2.4 MeV neutrons and the convolution of Gaussian distribution function considering an energy resolution of 12% FWHM at the maximum light output Lp. The light output LP corresponds to the light yield of 2.4 MeV protons.
49 8
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer 1,5 r-
3 .0 MeV
M O .-1 X H Z
0,5
a Particle energy IMeV)
Fig. 9. Scintillation light yield of protons and electrons as a function of particle energy.
6E~xJ1,5 Me V
M O 0 .5 0 U
L-I- -_l.- ._ .lye 2 4
normalized in the same manner as for protons, the
I
LIGHT OUTPUT (ARB,
relation between proton and electron energies provid-
I
6I
I
8
10
UNITS)
ing the same light yield can be estimated from fig. 9. As described above,
since the light yield is not
proportional to the proton energy, the total light yield differs depending on the history of the neutron energy
loss in the neutron slowing down process even for a constant full energy absorption. Fig. l0a shows a calcu-
lated light output spectrum for 3.0 MeV neutrons, and fig. 10b shows that selected for the case alone in which a single energy loss of more than 1.5 MeV is included in the process of neutron slowing down . Comparing fig. l0a with fig. 10b we can conclude that the shoulder at
the maximum light output
seen
in fig. l0a corre-
sponded to the case of a large energy loss by a single scattering . This is consistent with our expectation . Fig. 11 shows a Monte Carlo calculation of captured-neutron energy spectrum for 3 MeV incident
Fig. 10 . Dependence of the light output spectra on the history of the neutron energy loss. (a) Light output spectrum calculated for 3.0 MeV neutrons . (b) The same spectrum but selected for the case alone in which a single energy loss of more than 1 .5 MeV is included m the neutron slowing down process.
convolution for each spectrum, since the dependence
of the FWHM on the proton energy was not known for the present scintillation detector . It is seen from fig. 12 2
En = 3 .0 MeV
neutron energy . It is seen from fig. 11 that captured
neutrons are not thermalized but have energies of the order of 10 eV . Fig. 11 also indicates that some of the neutrons are captured by ' ° B with large residual energies .
3.3. Calculated energy response function Light output spectra were calculated for nearly the same neutron energies as in the experiment . Results are shown in fig. 12 for neutron energies of 1.2, 3.0, 5.0
and 14 .0 MeV with the convolution of a Gaussian distribution function considering the energy resolution .
A FWHM of 12% observed with Compton electrons whose light output is equivalent to that by protons with energies of a few MeV was used
to calculate the
0 10-I
IO I Captured
103
105
neutron energy
IO7 (eV)
Fig. 11 . Captured neutron energy spectrum calculated for incident neutrons with an energy Ea of 3.0 MeV.
T Aoyama et al . / Energy response of a full-energy-absorption neutron spectrometer
49 9
linear amplifier used, where succeeding light outputs in the process of neutron slowing down were added after some attenuation . This would have effects to lower the peak position and broaden the peak width as seen in fig. 6, then producing the valley with increasing the height of the shoulders for large neutron energies .
that the ratio of counts at the shoulder to peak counts increased with neutron energy . This is because the mean free path of neutrons for elastic scatterings with hydrogen nuclei increases with neutron energy, which would result in selective capture of neutrons whose energy was lost in a large fraction at the first scattering. The ratio of counts at the shoulder to peak counts however is underestimated especially at the higher neutron energy, since the rate of neutron backscatterings by hydrogen nuclei, and therefore of the forward knock-on of protons, is slightly larger than that of isotropic scatterings of neutrons assumed in the calculation, and it increases with neutron energy from 0.4% at 1 MeV to 5.5% at 14 MeV [9]. Comparison of the calculated spectra of fig. 12 with experimental pulse height spectra of fig. 6 indicates that consistency was excellent for neutron energies of less than 3 MeV except for the upturn at a low pulse height side of experimental spectra due to externally moderated neutron components . For neutron energies of more than 5 MeV, however, there appear double peaks with a prominent valley for the experimental pulse height spectra, which have never been obtained by Monte Carlo calculations . The reason for this might be the relatively short shaping time constant of the
4. Detection efficiency The detection efficiency 71 for monoenergetic neutrons is defined as the ratio of the number of captured neutrons to that incident on the detector . The dependence of the efficiency rl n on the neutron energy was examined using Monte Carlo calculations, which were carried out for the neutron energy range between 1 and 20 MeV, setting the number of success events between 10 4 and 10 5 for each neutron energy . Results are shown in fig. 13 . The experimental detection efficiency 17ex was determined as follows. The counting rate n [s -1 ] for monoenergetic neutrons is given by the integral counting rate of the pulse height spectrum in fig. 6 subtracting the counting rate estimated from the spectrum measured with the shadow cone . Evaluating the neu-
MO H X
M O rl X
F Z O U
H Z O U
2
4
6
LIGHT OUTPUT (ARB,
8
10
12
UNITS)
14 .0 MeV
5 .0 MeV
r 0 U
2
4
6
8
LIGHT OUTPUT (ARB, UNITS)
10
12
0
2
4
6
8
LIGHT OUTPUT (ARB, UNITS)
10
12
Fig. 12. Light output spectra obtained by Monte Carlo calculations for neutron energies of (a) 1.2 MeV, (b) 3.0 MeV, (c) 5 .0 MeV and (d) 14 .0 MeV, and the convolution of Gaussian distribution function considering an energy resolution of 12% FWHM at the maximum light output for each spectrum .
500
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer
posited by recoiling a-particles and 7Li nuclei, rejecting the component of 478 keV -y-rays, the probability pg to generate capture-gate-signal was decreased to be 62% of the total neutron capture. In fig. 13 the values of 77,, divided by p e and pg are plotted as corrected experimental detection efficiencies . It is seen from fig. 13 that the corrected efficiencies agree well with the calculated curve, and that an efficiency of about 10% at 1.2 MeV neutrons decreases continuously to about 0.6% at 14 MeV.
s0
T v U
N _o U N N
5. Conclusions
Neutron energy (MeV) Fig. 13 . Detection efficiency as a function of neutron energy . Experimental detection efficiency was corrected for coincidence counting losses and the loss of coincidence-gate-pulse generation to compare with the efficiency obtained by Monte Carlo calculation .
tron flux density ¢ [cm-2 S- '] at the flat front surface of the detector with an area of A (= 127 cm 2) we can obtain the experimental efficiency ,Y)ex
= nIOA,
where the neutron flux density at 14 .6 MeV was determined by associated a-particle counting, while a proton-recoil telescope or a hydrogen gas proportional counter was used in the other energy region [6]. Comparison of the experimental efficiency qex with 77n requires corrections for ?7 ex to remove the difference in the conditions . Firstly, coincidence counting losses are included in il ex because of the finite pulse widths of the coincidence gate pulses and the delay between the pulses . The time distribution of the delayed capture pulse after the prompt recoil pulse was given by an exponentially decreasing function with a time constant ,r of 2.2 ~Ls as shown in fig. 4. Hence the coincidence probability pc is evaluated from a summed pulse width t W of 5.0 ws and a net delay time t d of 0.7 lts between coincidence-gate pulses to be pe = (
1-exp(-tW/-r)) exp(-td /T)=0 .65.
(2)
Secondarily, the loss of coincidence-gate-pulse generation is included in 71,x because of the narrow SCA window for neutron capture pulses . Since the window was adjusted to correspond solely to the energy de-
A full-energy-absorption neutron spectrometer was constructed using a boron-loaded liquid scintillator BC-523, and the energy response was examined experimentally and by Monte Carlo calculations for the neutron energy range between 1.2 and 14 MeV. The results are: - pulse height spectra measured with monoenergetic neutrons showed either a peak with a shoulder or double peaks instead of a single broad peak which had been expected ; - Monte Carlo calculations of the light emission in the course of the neutron slowing down in the scintillator indicated that the complicated pulse height spectra observed for monoenergetic neutrons were due to the nonlinearity of the light yield against recoil proton energy ; - the energy response functions calculated using the measured relation between light yield and recoilproton energy reproduced the characteristics of the experimental pulse height spectra; - the experimental neutron detection efficiency corrected for coincidence counting losses and the loss of coincidence-gate-pulse generation agreed well with the efficiency calculated by a Monte Carlo computer simulation. Although an accurate knowledge of the energy response functions is necessary for neutron spectrum unfolding, the difficulty of the precise determination of the response functions limits the application of the present spectrometer only to the unfolding of relatively simple neutron energy spectra with moderate energy resolution . Since the present scintillation spectrometer utilizes the double-pulse coincidence method for -y-ray rejection without using conventional pulse shape discrimination, the discriminating power would not degrade with increasing scintillator volume . Hence a spectrometer using a large volume of scintillator would be used effectively for the energy measurement of low neutron fluxes with intensities of 1 cm -2 s - I or less.
T. Aoyama et al. / Energy response of a full-energy-absorption neutron spectrometer Acknowledgements We are grateful to Mr . S. Itoh of Nagoya University for his support in the development of the Monte Carlo computer program and to Mr . T. Narita of JAERI, who provided the neutron cross section data JENDLE3. Thanks are due to Nagoya University Computation Center for the approval to use the supercomputer and to Mr . Y. Takasuna for his experimental support. References [11 D.M . Drake, W.C . Feldman and C. Hurlbut, Nucl . Instr. and Meth . A247 (1986) 576.
501
[21 J.B . Czirr and G.L. Jensen, Nucl . Instr. and Meth . A284 (1989) 365. [3] W.C . Feldman, G.F . Auchampaugh and R.C . Byrd, Nucl . Instr. and Meth . A306 (1991) 350 . [4] S.E. Jones, E.P . Palmer, J.B . Czirr, D.L. Decker, G.L . Jensen, J.M. Thorne, S.F. Tayer and J. Rafelski, Nature 338 (1989) 737. [5] E.A . Kamykowski, Nucl . Instr. and Meth . A299 (1990) 105. [6] K. Kudo, T. Michikawa, T. Kinoshita, N. Kobayashi, A. Fukuda, Y. Hino and Y. Kawada, J. Nucl . Sci. Technol. 24 (1987) 684. [7] G.F . Knoll, Radiation Detection and Measurement, 2nd ed . (Wiley, New York, 1989) p. 224. [8] G. Dietze and H. Mein, Nucl . Instr. and Meth . 193 (1982) 549. [9] J.C. Hopkins and G. Breit, Nucl . Data Tables A9 (1971) 137.