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Performance of a phoswich detector composed of an inner NaI(Tl) crystal and surrounding NE102A plastic scintillator for neutron spectrometry
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Nuclear Instruments and Methods in Physics Research A 587 (2008) 20–28 www.elsevier.com/locate/nima
Performance of a phoswich detector composed of an inner NaI(Tl) crystal and surrounding NE102A plastic scintillator for neutron spectrometry T. Watanabea,, H. Arakawaa, T. Kajimotoa, Y. Iwamotoa,1, D. Satoha,1, S. Kuniedaa,1, S. Nodaa, N. Shigyoa, K. Ishibashia, T. Nakamurab, R.C. Haightc a
Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Cyclotron and Radioisotope Center, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan c LANSCE-NS, Los Alamos National Laboratory, MS H855, Los Alamos, NM 87545, USA Received 24 January 2007; received in revised form 1 January 2008; accepted 2 January 2008 Available online 6 January 2008
Abstract We have developed a phoswich detector for neutron spectrometry, which adopts a reversed configuration of slow- and fast-decay-time scintillators in its inner and surrounding outer regions, respectively, in the detection of recoil protons from a hydrogenous radiator. The phoswich detector consists of an inner slow, NaI(Tl) scintillator, and an outer fast, plastic scintillator. This configuration allows us to discriminate protons of full kinetic-energy deposition only in the NaI(Tl) scintillator and those not stopping in the inner scintillator. The response functions of the phoswich detector were measured for neutron energies ranging from 100 to 350 MeV. The experiment used the recoil-proton method and pulse-shape discrimination with the two-gate integration technique using a spallation neutron source at the WNR facility of the Los Alamos Neutron Science Center (LANSCE). The experimental results were consistent with calculations by the Particle and Heavy Ion Transport code System (PHITS). To evaluate the effectiveness of the phoswich configuration, full energydeposition fraction was calculated. This fraction is defined as the ratio of the integration around the full-energy peak to that of the entire energy region. The calculation confirmed that the phoswich detector with a reversed configuration is useful for neutron measurements. r 2008 Elsevier B.V. All rights reserved. PACS: 29.30.Hs; 29.40.Mc Keywords: Reversed-type phoswich detector; Full energy-deposition fraction; Pulse-shape discrimination; Recoil-proton method; Neutron spectrometry; PHITS
1. Introduction Unfolding methods, with liquid organic scintillators such as NE213 and BC501A, are useful for neutron spectrometry when the time-of-flight method is inapplicable. However, it is difficult for the unfolding procedure to reproduce the neutron energy spectrum that has sharpCorresponding author. Tel./fax: +81 92 802 3484.
E-mail address: [email protected] (T. Watanabe). 1 Present address: Japan Atomic Energy Agency, 2–4 Shirakata, Tokaimura, Naka-gun, Ibaraki 319-1195, Japan. 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.01.001
peak structures, because the response function of NE213 (conventional size of 12.7 cm in diameter by 12.7 cm in length) changes slowly with neutron energy, for neutrons above 100 MeV. The recoil-proton method, employing a neutron-toproton converter and an inorganic scintillator, is frequently used in neutron measurements; however, its neutrondetection efficiency is very low, typically less than 0.01%. To obtain reasonable detection efficiency, it is necessary to detect both protons knocked out by head-on collision and those recoiled forward in a certain range of solid angle. This degrades the neutron energy resolution to some extent. When the forward solid angle is taken wide to make
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2. Phoswich detector 2.1. Configuration Top and cross-sectional views of the phoswich detector are shown in Fig. 1. The phoswich detector is composed of a regular trapezoid body of NaI(Tl) crystal-inorganic scintillator and an NE102A plastic scintillator, whose thickness is 10 mm. The plastic scintillator surrounds the NaI(Tl) crystal, except for the side on which particles are incident. The NaI(Tl) crystal is 362.5-mm long, which approximately corresponds to the full-stop range of 400-MeV protons. Light from the optically coupled scintillators is transmitted to a photomultiplier tube
10
102.5
39
Plastic scintillator, NE102A
NaI(Tl)
Aluminium housing
Photomultiplier tube
Aluminium housing
89
a high detection efficiency, some recoiled protons may get out sideward from the inorganic scintillator and lead to undesired events producing the partial energy deposition. To eliminate the protons that deposit part of their energies in the scintillator, a phoswich-type configuration with a fast decay-constant plastic scintillator surrounding the inorganic one can be used as a recoil-proton detector. This phoswich configuration is expected to reduce background events ascribed to charged particles entering from the outer plastic scintillator. Use of the phoswich detector consisting of two different decay-time scintillators, allows us by pulse-shape discrimination (PSD) [1,2] to distinguish the events that the scintillation light has occurred in only one scintillator from those that the scintillation light has occurred in both. Some types of phoswich detectors have been previously developed. A detector was fabricated with an inner NE213 liquid scintillator surrounded by a thin CaF2 ðEuÞ crystal wall, except for the front surface [3]. Takada et al. [4] fabricated a phoswich detector consisting of an inner NE213 liquid scintillator enclosed by an NE115 plastic scintillator that distinguishes photon and neutron events in a charged-particle mixed field. The phoswich detectors mentioned above are configured with a slow decay-constant scintillator surrounding a fast one. The phoswich detector in the present, however, has a reversed-type configuration, in which a slow NaI(Tl) scintillator is surrounded by a fast plastic scintillator. In this reversed-type configuration, the outer, fast scintillator creates an inhibit signal, but may be influenced and smeared by the dominant inner scintillator light. A phoswich detector with reversed configuration has not been reported so far. Therefore, in this paper, we present the new, reversedtype phoswich detector, and its response-function measurements. The measurements were taken with the spallation neutron source at the Los Alamos Neutron Science Center (LANSCE). We compared experimental response functions with the calculations by the Particle and Heavy-Ion Transport code System (PHITS) [5]. The full energydeposition fraction was calculated to clarify the effectiveness of the phoswich configuration.
21
10
Photomultiplier tube
NaI(Tl)
362.5
Light guide
Fig. 1. Cross-sectional structures of the phoswich detector (unit:mm): (a) top view and (b) side view.
Table 1 Properties of the scintillators used in the phoswich detector [14] Scintillator
Decay constant (ns)
Density ðg=cm3 Þ
NaI(Tl) NE102A
230 24
3.67 1.032
through a thin light guide connected to the back side of the plastic scintillator using BC600 optical cement. 2.2. Principle of operation General properties of both NaI(Tl) inorganic and NE102A plastic scintillators are given in Table 1. The plastic scintillator has a fast decay time (a few ns), while the NaI(Tl) scintillator has a slow decay time of 230 ns. From the difference between the decay-time constants, we can determine whether the output scintillation comes from the NaI(Tl) or the plastic scintillator or both. This is based on the two different-gate charge integrations [1], which is schematically shown in Fig. 2. Fig. 2 shows a schematic description of the three types of events and their signals from the phoswich proton detector. Fig. 2(a) indicates the case where a proton deposits all of its energy only in the NaI(Tl) scintillator— scintillation is produced only in NaI(Tl). In this case, the output signal has a relatively small fraction of the fast component, compared to the total scintillation light in the right side of Fig. 2(a). In Fig. 2(b), a proton passes through both the NaI(Tl) and the plastic scintillators. The resultant output signal has a comparatively larger fast component than in Fig. 2(a). Fig. 2(c) presents the situation where the recoil proton loses its energy only in the outer plastic scintillator and thereby produces only a fast signal.
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NE102A NaI(Tl)
Signal from the phoswich detector
Time
Proton Fast gate Total gate
100 ns 300 ns
NE102A NaI(Tl)
Signal from the phoswich detector
Time
Proton Fast gate Total gate
100 ns 300 ns
NE102A NaI(Tl)
Proton
Signal from the phoswich detector
Time
Fast gate Total gate
100 ns 300 ns
Fig. 2. Schematic description of the principle of discriminating recoil protons with the phoswich detector: (a) recoil-proton deposits its energy in the NaI(Tl) crystal; (b) recoil-proton deposits its energy in both scintillators; and (c) recoil-proton deposits its energy in the plastic scintillator.
The charge integrations of the signal are made during time durations specified by the fast gate and the total gate, respectively. The fast gate (typically 100 ns) is adjusted at the plastic scintillator’s scintillation peak. The total gate (typically 300 ns) is wider to cover the long tail of the NaI(Tl) scintillator signal. 3. Experiment 3.1. Setup The experimental arrangement for the phoswich detector response-function measurements is illustrated in Fig. 3. The measurement was carried out with the spallation neutron source on the 4FP15L beam line at the Weapons Neutron Research (WNR) facility of the LANSCE [6]. The neutron beams were generated by the bombardment of a tungsten target (Target-4) with an 800-MeV pulsed proton beam from the LANSCE linear accelerator. The proton
beam has a typical time structure of a 650-ms-wide macropulse every 8.3 ms. Each macropulse consists of 360-ps-wide micropulses separated by 1:8 ms. The flight path of 4FP15L at 15 left to the proton beam is approximately 90 m. The neutron beam was collimated to 7 mm in diameter in the experimental area, and the incident neutrons were monitored by a 238U fission ionization chamber [7] that was placed at about 2-m upstream from the phoswich detector. Kinetic energies of neutrons incident on the detector were calculated by measuring the time of flight (TOF) between the spallation target and a DE detector. The TOF measurement was also performed with the fission ionization chamber to obtain the incident neutron flux. Either a 20-mm thick polyethylene or a 7.5-mm thick graphite disk, each 5 cm in diameter, served as a radiator, which converts the incident neutrons into charged particles for energy measurement by the phoswich detector. The graphite disk was designed to include the same amount of
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Phoswich detector Fission ionization chamber
Radiator CH2 (φ 5 x 2 cm) or C (φ 5 x 0.75 cm)
Radiator
am
e
nb
ΔE detector
o utr
Ne
Phoswich detector
90 m ΔE detector 800 MeV Proton beam
15 deg. Target-4
Collimator (φ 7 mm) Neutron beam
Shield (Iron and lead)
Fig. 3. Arrangement for the phoswich detector response-function measurement.
Fig. 4. Block diagram of the circuit for response-function measurement.
carbon atoms as the polyethylene. The radiator was located 10-cm upstream from the front surface of the phoswich detector. The DE detector (2-mm thick NE102 plastic scintillator) was placed in front of the phoswich detector to identify protons that recoiled from the radiators. Neutron shields consisting of iron and lead blocks, placed between the fission ionization chamber and the phoswich detector reduced the level of background neutrons from the spallation target. 3.2. Electronics and data acquisition system Fig. 4 presents a simplified block diagram of the electronics used to process the signals from the phoswich detector and DE detector. The DE detector provided the start signal for the TOF measurement on the condition that the recoil proton was detected. Accelerator pulses for timing were delayed to a considerable extent to serve as the TOF stop signal. Both signals reached a time-to-digital converter (TDC). When the proton beam bombarded Target-4, it generated some flash g-rays in addition to a number of neutrons. The flash g-rays provided a useful
time reference for converting TOF into incident neutron energy. Output signals from the phoswich detector were divided into three signals after a timing filter amplifier (ORTEC 474), where a time constant of 50 ns was chosen. Use of the timing filter amplifier enhanced the signal from the outer, fast scintillator. One of the divided signals was fed into a constant-fraction discriminator (CFD), and the other two were used for fast- and total-charge integrations, whose time intervals were 100 and 300 ns, respectively. The gate signal for the charge integration was generated in a gate generator (shown as G.G. in Fig. 4). The three signals were sent to CAMAC analog-to-digital converters (ADCs). If the signals from the phoswich detector and the DE counter simultaneously reached a coincidence module (shown as COIN in Fig. 4) within a specified time duration (1600 ns) after the signals from the proton beams entered the COIN, the generated trigger signals were sent to CAMAC modules such as the ADCs and the TDC. A network crate controller (TOYO Corp. Model CC/NET) was used as a CAMAC controller. It takes about 300 ms for the ADC to convert the signals and
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transfer them to a hard disk drive on a personal computer connected to the crate controller via Ethernet. 4. Analysis 4.1. Incident neutron beam 4.1.1. Time-of-flight The kinetic energy of the incident neutron projectile was measured by the TOF method. Fig. 5 shows an example of the measured TDC spectrum. The time difference between the flash g-ray peak and the individual events was measured. This was converted to neutron energy with the flight path length from the spallation target to the DE detector. Fig. 5 shows the derived incident neutron energy in MeV. 4.1.2. Neutron-induced 238U fission cross-section Neutron-induced 238U fission cross-sections up to several hundred MeV were required to obtain the number of incident neutrons. Although 238U fission cross-sections below 20 MeV are well known, few measurements [8–12] above 20 MeV have been reported, as mentioned in Ref. [10]. Lisowski et al. [10] and Shcherbakov et al. [12] measured the fission cross-section data from a spallation source in a wide energy range up to about 200 MeV. Also, there are the results of Goldanskiy et al. [8] at 120 and 380 MeV, Eismont et al. [11] at 135 and 162 MeV, and those of Pankratov et al. [9] covering the range from 5 to 37 MeV, with 5 points above 30 MeV. The experimental results of Lisowski et al. were for the incident neutron energies below 200 MeV. Fission crosssections above 200 MeV were calculated by Fiscal [13], which adopts the systematics to reproduce experimental data of proton-, neutron- and photon-induced fission cross-sections for Ag–243Am (29 isotopes). The calculation
results by the Fiscal are shown in Fig. 6, together with the experimental data mentioned above. The Fiscal calculations and the Lisowski et al. experimental data, measured at the same facility, agreed well above 100 MeV. There is a discrepancy between the calculation and the experimental value of Goldanskiy et al. [8] at 120 and 380 MeV; the experimental data above 200 MeV is not sufficient enough to make a meaningful discussion of this discrepancy. 4.1.3. Flux The number of incident neutrons was obtained by the use of fission chamber. The TOF measurement and subsequent fission-event analysis have been described in Ref. [7]. Fission chamber data in each data set gave events of insufficient statistics due to the use of thin 238 U-deposition layer. The fission data in all data sets were summed to calculate the averaged flux of acceptable statistics. Fig. 7 shows the measured flux of the incident neutron beam. The measured flux was expressed by a relative-flux form in units of neutrons/MeV/sr/count, where sr and count stand for the solid angle subtended by the collimated beam relative to the neutron-production target and the number of fission events, respectively. 4.2. Selection of full energy-deposition events Fig. 8 shows a correlation between the total gate ADC signal for the phoswich detector and that for the DE detector, using which particles produced from the radiator can be identified. This figure shows three regions: (A), (B), and (C). In region (A), the scintillations from DE and the phoswich detectors are relatively small. The events in region (A) indicate events associated with generation of g-rays and neutrons penetrating the radiator. In region (B),
106
105 30
100
Goldanskiy et al. (1955) Pankratov et al. (1963) Lisowski et al. (1991) Eismont et al. (1996) Shcherbakov et al. (2001) Fiscal
2.5
300
Incident neutron energy (MeV)
Cross section (b)
104 Counts
3.0
Flash γ-rays 200
103
102
2.0
1.5
1.0
101
0.5 100
0
500
1000
1500
2000 2500 TDC (ch)
3000
3500
4000
Fig. 5. TDC spectrum by the DE detector. Time conversion is about 0.5 ns/ch.
0.0 50
100
150 200 250 300 Neutron energy (MeV)
Fig. 6. Calculated and experimental
238
350
Uðn; fÞ cross-sections.
400
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800
1010
600 Fast gate ADC (ch)
Neutron relative-flux (n/MeV/sr/count)
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108
25
(C)
400
200 (B)
107
(A)
0
0
100
200
300 400 500 600 Neutron energy (MeV)
700
0
800
200
400 Total gate ADC (ch)
600
800
Fig. 9. PSD by the two-gate integration method for the phoswich detector.
Fig. 7. Measured neutron relative flux.
1000
600
500
600
Total gate ADC (ch)
ADC for ΔE (ch)
800
(C)
400 (B) 200
400
300
200
(A)
100 0 0
200 400 Total gate ADC for phoswich (ch)
600
Fig. 8. Typical two-dimensional spectrum of the total gate signal for the phoswich detector versus the DE detector.
the ADC for DE decreases as the total gate ADC for the phoswich detector increases. Region (B) represents the detection of protons produced mainly by elastic collisions with hydrogen and the 12 Cðn; pÞ reaction. Compared to region (B), the scintillation of DE is larger in region (C). Region (C) indicates the detection of the deuterons produced in the radiator. The result of the PSD for the phoswich detector is shown in Fig. 9, where events in all regions in Fig. 8 are plotted. One can see three types of regions. In region (A), the scintillation light was emitted by the NaI(Tl) scintillator. In region (B), the scintillation occurred in both the NaI(Tl) and the plastic scintillators. The events in region (C) were detected when only the plastic scintillator emitted scintillation light. The events in region (A) were selected as the full-stopping proton events.
0
0
100
200 300 Deposited energy (MeV)
400
Fig. 10. Calibration of phoswich detector.
4.3. Calibration For energy calibration, the relationship between the deposition energy and the ADC channel for the NaI(Tl) scintillator is required. However, it is difficult to determine the value of the full energy deposition in the experiments. The calibration procedure was as follows. First, the deposition energy was calculated by PHITS, at the peak position generated from the H(n,p) reaction for an incident neutron energy. Then, the calculated deposition energy was related to the ADC channel of the full-energy peak in the response function that was measured for the same incident neutron energy. Fig. 10 shows the calibration result. A linear dependence of the ADC channel on the deposition energy was observed, while slight saturation effect was observed at deposition energies above 350 MeV.
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In PHITS calculation, the incident energies were set to mean values of the individual energy bins, and the neutron source had a 3.5-mm radius. The deposited energies in the DE detector, the NaI(Tl), and the plastic scintillators of the phoswich detector were calculated. Using these deposited energies, the response functions were calculated with a procedure consistent with the data analysis in Section 4.2. The calculations were performed for both the polyethylene and graphite converters. After subtracting the carbon contributions from the polyethylene calculation, the calculation result was broadened by a Gaussian function to consider the energy resolution of the detector. The energy resolutions were from 9 to 25%, and are listed in Table 2.
5. Results 5.1. Response functions of the phoswich detector Fig. 11 shows the phoswich detector response functions for incident neutron energies of 95–105, 145–155, 195–205, 245–255, 290–310, and 340–360 MeV, with the calculation results by PHITS. The error bars for the present data include systematic and statistical errors. The systematic errors derived from the uncertainty of 238U fission crosssections were set at 3 and 10%, below and above 200 MeV, respectively. The experimental response functions show good agreement with the calculations in the peak region, with the suitable energy resolutions.
(× 10-5)
PHITS (As calculated) PHITS (Broadened) Present
6.0
Neutron energy: 95−105 MeV
4.0
2.0
Response (1/source/MeV)
Response (1/source/MeV)
(× 10-5)
0.0
PHITS (As calculated) PHITS (Broadened) Present
3.0
Neutron energy: 145−155 MeV
2.0
1.0
0.0 0
30 60 90 Proton deposited energy (MeV)
0
(× 10-5)
(× 10-5)
PHITS (As calculated) PHITS (Broadened) Present Neutron energy: 195−205 MeV
1.0
0.5
Response (1/source/MeV)
Response (1/source/MeV)
2.0
1.5
PHITS (As calculated) PHITS (Broadened) Present
1.0
Neutron energy: 245−255 MeV
0.5
0.0
0.0 0
50 100 150 200 Proton deposited energy (MeV)
0
PHITS (As calculated) PHITS (Broadened) Present
6.0
Neutron energy: 290−310 MeV
4.0
2.0
0.0 0
50 100 150 200 250 Proton deposited energy (MeV)
300
(× 10-5) 5.0
100 200 300 Proton deposited energy (MeV)
Response (1/source/MeV)
(× 10-5)
Response (1/source/MeV)
50 100 150 Proton deposited energy (MeV)
PHITS (As calculated) PHITS (Broadened) Present
4.0
Neutron energy: 340−360 MeV
3.0 2.0 1.0 0.0 0
100 200 300 Proton deposited energy (MeV)
400
Fig. 11. Experimental response functions of the phoswich detector with the calculations by the PHITS.
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Table 2 Improvement of the full energy-deposition fraction and the energy resolution
95–105 145–155 195–205 245–255 290–310 340–360
Mean energy (MeV)
100 150 200 250 300 350
Full energy-deposition fraction (%)
Energy resolution (%)
E2DE
E2DE and Phoswich PSD
E2DE
E2DE and Phoswich PSD
95.1 81.8 80.7 62.8 55.1 37.6
95.8 85.5 87.7 69.2 63.5 52.2
25.4 17.9 13.3 11.9 11.6 9.7
22.5 15.6 12.8 10.6 10.1 9.1
To examine the effectiveness of the phoswich configuration, the full energy-loss fraction and the energy resolution for the response function were calculated. The response function was obtained after choosing the recoil-proton events labeled (B) (recoil proton) in Fig. 8, and the full energy-deposition events labeled (A) in Fig. 9. For comparison, the same examination was performed with the response function after selection of region (B) in Fig. 8. Table 2 shows these quantities with and without the phoswich PSD. It was found that the adoption of the phoswich configuration increased the full energy-loss fraction of the incident neutron energy. At maximum, an improvement of about 140% was obtained in the 340–360 incident neutron energy bin. In addition, the energy resolution with the phoswich configuration was slightly better. The improvement of the full energy-loss fraction was relatively low in lower energy bins. This is because neutrons of 7-mm diameter beam were used, and the fullstopping range of the proton was short at lower energies. The effectiveness of the phoswich configuration was estimated to be greater for a point-like neutron source than for the narrow neutron beam used in the present experiment. The response for a point neutron source was calculated at the neutron energy of 300 MeV, using PHITS. The arrangement of the source and detectors for the calculation is shown in Fig. 12(a). The point source was located 33.7 cm upstream from the front surface of the phoswich detector, where the extended side surfaces of the phoswich detector crossed. Fig. 12(b) shows the calculation results for the response function of the phoswich detector to a point neutron source, with and without the phoswich PSD. The response functions without the phoswich PSD have been estimated to have a large fraction in the tail region. The same comparison, concerning the full energy-loss fraction, was performed. The result showed that the fraction increased from 26 to 72%. When the TOF technique is inapplicable for measuring secondary neutrons, the phoswich detector developed is expected to be effective in experiments such as the measurement of ðn; xnÞ double-differential cross-sections, where neutrons like the point source are emitted from a sample target.
Phoswich detector
ΔE detector
Radiator
2.5
mm
36
Point neutron source 6.8
mm
33
(× 10-6) Response (1/source/MeV)
Energy bin (MeV)
E−ΔE& Phoswich PSD (Broadened) E−ΔE (Broadened)
6.0
Neutron energy: 300 MeV
4.0
2.0
0.0 0
100 200 300 Proton deposited energy (MeV)
Fig. 12. (a) Illustration of the arrangement of the point source and the phoswich detector and (b) calculated response functions of the reversed phoswich detector to the point source by PHITS.
6. Conclusions The reversed-type phoswich detector configuration was developed and tested. It consisted of an inner NaI(Tl) inorganic scintillator and a surrounding outer plastic scintillator (NE102A). The response functions of the phoswich detector to 7-mm diameter beam of neutrons were measured in the mean energy range of 100–350 MeV at LANSCE. Comparison of the response with the PHITS prediction gave a good agreement within 10% in the peak region at incident energies below 300 MeV, and a certain discrepancy up to 30% at 350 MeV.
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The use of PSD for the reversed-type phoswich configuration was verified to successfully work and be useful in achieving improvements in the full energy-loss fraction. The improvement was a few percent below the neutron energy of 150 MeV and increased with the neutron energy, reaching 140% at 350 MeV. An estimate, obtained with PHITS, showed that the full energy-loss fraction for a point neutron source is 2.8 times larger than that for a source with a 7-mm diameter collimated neutron beam. This configuration is considered to be useful in measuring secondary neutrons from a sample target, for discriminating protons of full kinetic-energy deposition in the energy range 150–350 MeV in the NaI(Tl) scintillator and those penetrating the outer plastic scintillator. The response function with the phoswich PSD has a small fraction in the tail and is suitable for the unfolding method in neutron spectrometry.
the use of the Los Alamos Neutron Science Center at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy. This work was also supported by the Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 12480141 and No. 15206110).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgments We are grateful to A. Bridge, G. Chaparro, and J.M. O’Donnell for their generous assistance in the experiments, and the LANSCE operations staff for providing a highquality pulsed proton beam. This work has benefited from
[11] [12] [13] [14]
F.D. Brooks, Nucl. Instr. and Meth. A 4 (1959) 151. M.L. Roush, et al., Nucl. Instr. and Meth. A 31 (1964) 112. M. Takada, et al., Nucl. Instr. and Meth. A 379 (1996) 293. M. Takada, et al., Nucl. Instr. and Meth. A 465 (2001) 498. H. Iwase, et al., J. Nucl. Sci. Technol. 39 (2002) 1142. P.W. Lisowski, et al., Nucl. Sci. Eng. 106 (1990) 208. S.A. Wender, et al., Nucl. Instr. and Meth. A 336 (1993) 226. V.I. Goldanskiy, et al., Dokl. Akad. Nauk 101 (1955) 1027. V.M. Pankratov, Atomnaja Energ. 14 (1963) 177. P.W. Lisowski, et al., Fission cross sections in the intermediate energy region, Proceedings of the Specialists’ Meeting on Neutron Cross Section Standards for the Energy Region above 20 MeV, Uppsala, Sweden, 1991, p. 177. V.P. Eismont, et al., Phys. Rev. C 53 (1996) 2911. O. Shcherbakov, et al., J. Nucl. Sci. Technol. 2 (Suppl.) (2002) 230. T. Fukahori, et al., J. Nucl. Sci. Technol. 2 (Suppl.) (2002) 25. G.F. Knoll, Radiation Detection and Measurement, Wiley, New York, 1989.
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Performance of a phoswich detector composed of an inner NaI(Tl) crystal and surrounding NE102A plastic scintillator for neutron spectrometry T. Watanabea,, H. Arakawaa, T. Kajimotoa, Y. Iwamotoa,1, D. Satoha,1, S. Kuniedaa,1, S. Nodaa, N. Shigyoa, K. Ishibashia, T. Nakamurab, R.C. Haightc a
Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Cyclotron and Radioisotope Center, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan c LANSCE-NS, Los Alamos National Laboratory, MS H855, Los Alamos, NM 87545, USA Received 24 January 2007; received in revised form 1 January 2008; accepted 2 January 2008 Available online 6 January 2008
Abstract We have developed a phoswich detector for neutron spectrometry, which adopts a reversed configuration of slow- and fast-decay-time scintillators in its inner and surrounding outer regions, respectively, in the detection of recoil protons from a hydrogenous radiator. The phoswich detector consists of an inner slow, NaI(Tl) scintillator, and an outer fast, plastic scintillator. This configuration allows us to discriminate protons of full kinetic-energy deposition only in the NaI(Tl) scintillator and those not stopping in the inner scintillator. The response functions of the phoswich detector were measured for neutron energies ranging from 100 to 350 MeV. The experiment used the recoil-proton method and pulse-shape discrimination with the two-gate integration technique using a spallation neutron source at the WNR facility of the Los Alamos Neutron Science Center (LANSCE). The experimental results were consistent with calculations by the Particle and Heavy Ion Transport code System (PHITS). To evaluate the effectiveness of the phoswich configuration, full energydeposition fraction was calculated. This fraction is defined as the ratio of the integration around the full-energy peak to that of the entire energy region. The calculation confirmed that the phoswich detector with a reversed configuration is useful for neutron measurements. r 2008 Elsevier B.V. All rights reserved. PACS: 29.30.Hs; 29.40.Mc Keywords: Reversed-type phoswich detector; Full energy-deposition fraction; Pulse-shape discrimination; Recoil-proton method; Neutron spectrometry; PHITS
1. Introduction Unfolding methods, with liquid organic scintillators such as NE213 and BC501A, are useful for neutron spectrometry when the time-of-flight method is inapplicable. However, it is difficult for the unfolding procedure to reproduce the neutron energy spectrum that has sharpCorresponding author. Tel./fax: +81 92 802 3484.
E-mail address: [email protected] (T. Watanabe). 1 Present address: Japan Atomic Energy Agency, 2–4 Shirakata, Tokaimura, Naka-gun, Ibaraki 319-1195, Japan. 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.01.001
peak structures, because the response function of NE213 (conventional size of 12.7 cm in diameter by 12.7 cm in length) changes slowly with neutron energy, for neutrons above 100 MeV. The recoil-proton method, employing a neutron-toproton converter and an inorganic scintillator, is frequently used in neutron measurements; however, its neutrondetection efficiency is very low, typically less than 0.01%. To obtain reasonable detection efficiency, it is necessary to detect both protons knocked out by head-on collision and those recoiled forward in a certain range of solid angle. This degrades the neutron energy resolution to some extent. When the forward solid angle is taken wide to make
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2. Phoswich detector 2.1. Configuration Top and cross-sectional views of the phoswich detector are shown in Fig. 1. The phoswich detector is composed of a regular trapezoid body of NaI(Tl) crystal-inorganic scintillator and an NE102A plastic scintillator, whose thickness is 10 mm. The plastic scintillator surrounds the NaI(Tl) crystal, except for the side on which particles are incident. The NaI(Tl) crystal is 362.5-mm long, which approximately corresponds to the full-stop range of 400-MeV protons. Light from the optically coupled scintillators is transmitted to a photomultiplier tube
10
102.5
39
Plastic scintillator, NE102A
NaI(Tl)
Aluminium housing
Photomultiplier tube
Aluminium housing
89
a high detection efficiency, some recoiled protons may get out sideward from the inorganic scintillator and lead to undesired events producing the partial energy deposition. To eliminate the protons that deposit part of their energies in the scintillator, a phoswich-type configuration with a fast decay-constant plastic scintillator surrounding the inorganic one can be used as a recoil-proton detector. This phoswich configuration is expected to reduce background events ascribed to charged particles entering from the outer plastic scintillator. Use of the phoswich detector consisting of two different decay-time scintillators, allows us by pulse-shape discrimination (PSD) [1,2] to distinguish the events that the scintillation light has occurred in only one scintillator from those that the scintillation light has occurred in both. Some types of phoswich detectors have been previously developed. A detector was fabricated with an inner NE213 liquid scintillator surrounded by a thin CaF2 ðEuÞ crystal wall, except for the front surface [3]. Takada et al. [4] fabricated a phoswich detector consisting of an inner NE213 liquid scintillator enclosed by an NE115 plastic scintillator that distinguishes photon and neutron events in a charged-particle mixed field. The phoswich detectors mentioned above are configured with a slow decay-constant scintillator surrounding a fast one. The phoswich detector in the present, however, has a reversed-type configuration, in which a slow NaI(Tl) scintillator is surrounded by a fast plastic scintillator. In this reversed-type configuration, the outer, fast scintillator creates an inhibit signal, but may be influenced and smeared by the dominant inner scintillator light. A phoswich detector with reversed configuration has not been reported so far. Therefore, in this paper, we present the new, reversedtype phoswich detector, and its response-function measurements. The measurements were taken with the spallation neutron source at the Los Alamos Neutron Science Center (LANSCE). We compared experimental response functions with the calculations by the Particle and Heavy-Ion Transport code System (PHITS) [5]. The full energydeposition fraction was calculated to clarify the effectiveness of the phoswich configuration.
21
10
Photomultiplier tube
NaI(Tl)
362.5
Light guide
Fig. 1. Cross-sectional structures of the phoswich detector (unit:mm): (a) top view and (b) side view.
Table 1 Properties of the scintillators used in the phoswich detector [14] Scintillator
Decay constant (ns)
Density ðg=cm3 Þ
NaI(Tl) NE102A
230 24
3.67 1.032
through a thin light guide connected to the back side of the plastic scintillator using BC600 optical cement. 2.2. Principle of operation General properties of both NaI(Tl) inorganic and NE102A plastic scintillators are given in Table 1. The plastic scintillator has a fast decay time (a few ns), while the NaI(Tl) scintillator has a slow decay time of 230 ns. From the difference between the decay-time constants, we can determine whether the output scintillation comes from the NaI(Tl) or the plastic scintillator or both. This is based on the two different-gate charge integrations [1], which is schematically shown in Fig. 2. Fig. 2 shows a schematic description of the three types of events and their signals from the phoswich proton detector. Fig. 2(a) indicates the case where a proton deposits all of its energy only in the NaI(Tl) scintillator— scintillation is produced only in NaI(Tl). In this case, the output signal has a relatively small fraction of the fast component, compared to the total scintillation light in the right side of Fig. 2(a). In Fig. 2(b), a proton passes through both the NaI(Tl) and the plastic scintillators. The resultant output signal has a comparatively larger fast component than in Fig. 2(a). Fig. 2(c) presents the situation where the recoil proton loses its energy only in the outer plastic scintillator and thereby produces only a fast signal.
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NE102A NaI(Tl)
Signal from the phoswich detector
Time
Proton Fast gate Total gate
100 ns 300 ns
NE102A NaI(Tl)
Signal from the phoswich detector
Time
Proton Fast gate Total gate
100 ns 300 ns
NE102A NaI(Tl)
Proton
Signal from the phoswich detector
Time
Fast gate Total gate
100 ns 300 ns
Fig. 2. Schematic description of the principle of discriminating recoil protons with the phoswich detector: (a) recoil-proton deposits its energy in the NaI(Tl) crystal; (b) recoil-proton deposits its energy in both scintillators; and (c) recoil-proton deposits its energy in the plastic scintillator.
The charge integrations of the signal are made during time durations specified by the fast gate and the total gate, respectively. The fast gate (typically 100 ns) is adjusted at the plastic scintillator’s scintillation peak. The total gate (typically 300 ns) is wider to cover the long tail of the NaI(Tl) scintillator signal. 3. Experiment 3.1. Setup The experimental arrangement for the phoswich detector response-function measurements is illustrated in Fig. 3. The measurement was carried out with the spallation neutron source on the 4FP15L beam line at the Weapons Neutron Research (WNR) facility of the LANSCE [6]. The neutron beams were generated by the bombardment of a tungsten target (Target-4) with an 800-MeV pulsed proton beam from the LANSCE linear accelerator. The proton
beam has a typical time structure of a 650-ms-wide macropulse every 8.3 ms. Each macropulse consists of 360-ps-wide micropulses separated by 1:8 ms. The flight path of 4FP15L at 15 left to the proton beam is approximately 90 m. The neutron beam was collimated to 7 mm in diameter in the experimental area, and the incident neutrons were monitored by a 238U fission ionization chamber [7] that was placed at about 2-m upstream from the phoswich detector. Kinetic energies of neutrons incident on the detector were calculated by measuring the time of flight (TOF) between the spallation target and a DE detector. The TOF measurement was also performed with the fission ionization chamber to obtain the incident neutron flux. Either a 20-mm thick polyethylene or a 7.5-mm thick graphite disk, each 5 cm in diameter, served as a radiator, which converts the incident neutrons into charged particles for energy measurement by the phoswich detector. The graphite disk was designed to include the same amount of
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Phoswich detector Fission ionization chamber
Radiator CH2 (φ 5 x 2 cm) or C (φ 5 x 0.75 cm)
Radiator
am
e
nb
ΔE detector
o utr
Ne
Phoswich detector
90 m ΔE detector 800 MeV Proton beam
15 deg. Target-4
Collimator (φ 7 mm) Neutron beam
Shield (Iron and lead)
Fig. 3. Arrangement for the phoswich detector response-function measurement.
Fig. 4. Block diagram of the circuit for response-function measurement.
carbon atoms as the polyethylene. The radiator was located 10-cm upstream from the front surface of the phoswich detector. The DE detector (2-mm thick NE102 plastic scintillator) was placed in front of the phoswich detector to identify protons that recoiled from the radiators. Neutron shields consisting of iron and lead blocks, placed between the fission ionization chamber and the phoswich detector reduced the level of background neutrons from the spallation target. 3.2. Electronics and data acquisition system Fig. 4 presents a simplified block diagram of the electronics used to process the signals from the phoswich detector and DE detector. The DE detector provided the start signal for the TOF measurement on the condition that the recoil proton was detected. Accelerator pulses for timing were delayed to a considerable extent to serve as the TOF stop signal. Both signals reached a time-to-digital converter (TDC). When the proton beam bombarded Target-4, it generated some flash g-rays in addition to a number of neutrons. The flash g-rays provided a useful
time reference for converting TOF into incident neutron energy. Output signals from the phoswich detector were divided into three signals after a timing filter amplifier (ORTEC 474), where a time constant of 50 ns was chosen. Use of the timing filter amplifier enhanced the signal from the outer, fast scintillator. One of the divided signals was fed into a constant-fraction discriminator (CFD), and the other two were used for fast- and total-charge integrations, whose time intervals were 100 and 300 ns, respectively. The gate signal for the charge integration was generated in a gate generator (shown as G.G. in Fig. 4). The three signals were sent to CAMAC analog-to-digital converters (ADCs). If the signals from the phoswich detector and the DE counter simultaneously reached a coincidence module (shown as COIN in Fig. 4) within a specified time duration (1600 ns) after the signals from the proton beams entered the COIN, the generated trigger signals were sent to CAMAC modules such as the ADCs and the TDC. A network crate controller (TOYO Corp. Model CC/NET) was used as a CAMAC controller. It takes about 300 ms for the ADC to convert the signals and
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transfer them to a hard disk drive on a personal computer connected to the crate controller via Ethernet. 4. Analysis 4.1. Incident neutron beam 4.1.1. Time-of-flight The kinetic energy of the incident neutron projectile was measured by the TOF method. Fig. 5 shows an example of the measured TDC spectrum. The time difference between the flash g-ray peak and the individual events was measured. This was converted to neutron energy with the flight path length from the spallation target to the DE detector. Fig. 5 shows the derived incident neutron energy in MeV. 4.1.2. Neutron-induced 238U fission cross-section Neutron-induced 238U fission cross-sections up to several hundred MeV were required to obtain the number of incident neutrons. Although 238U fission cross-sections below 20 MeV are well known, few measurements [8–12] above 20 MeV have been reported, as mentioned in Ref. [10]. Lisowski et al. [10] and Shcherbakov et al. [12] measured the fission cross-section data from a spallation source in a wide energy range up to about 200 MeV. Also, there are the results of Goldanskiy et al. [8] at 120 and 380 MeV, Eismont et al. [11] at 135 and 162 MeV, and those of Pankratov et al. [9] covering the range from 5 to 37 MeV, with 5 points above 30 MeV. The experimental results of Lisowski et al. were for the incident neutron energies below 200 MeV. Fission crosssections above 200 MeV were calculated by Fiscal [13], which adopts the systematics to reproduce experimental data of proton-, neutron- and photon-induced fission cross-sections for Ag–243Am (29 isotopes). The calculation
results by the Fiscal are shown in Fig. 6, together with the experimental data mentioned above. The Fiscal calculations and the Lisowski et al. experimental data, measured at the same facility, agreed well above 100 MeV. There is a discrepancy between the calculation and the experimental value of Goldanskiy et al. [8] at 120 and 380 MeV; the experimental data above 200 MeV is not sufficient enough to make a meaningful discussion of this discrepancy. 4.1.3. Flux The number of incident neutrons was obtained by the use of fission chamber. The TOF measurement and subsequent fission-event analysis have been described in Ref. [7]. Fission chamber data in each data set gave events of insufficient statistics due to the use of thin 238 U-deposition layer. The fission data in all data sets were summed to calculate the averaged flux of acceptable statistics. Fig. 7 shows the measured flux of the incident neutron beam. The measured flux was expressed by a relative-flux form in units of neutrons/MeV/sr/count, where sr and count stand for the solid angle subtended by the collimated beam relative to the neutron-production target and the number of fission events, respectively. 4.2. Selection of full energy-deposition events Fig. 8 shows a correlation between the total gate ADC signal for the phoswich detector and that for the DE detector, using which particles produced from the radiator can be identified. This figure shows three regions: (A), (B), and (C). In region (A), the scintillations from DE and the phoswich detectors are relatively small. The events in region (A) indicate events associated with generation of g-rays and neutrons penetrating the radiator. In region (B),
106
105 30
100
Goldanskiy et al. (1955) Pankratov et al. (1963) Lisowski et al. (1991) Eismont et al. (1996) Shcherbakov et al. (2001) Fiscal
2.5
300
Incident neutron energy (MeV)
Cross section (b)
104 Counts
3.0
Flash γ-rays 200
103
102
2.0
1.5
1.0
101
0.5 100
0
500
1000
1500
2000 2500 TDC (ch)
3000
3500
4000
Fig. 5. TDC spectrum by the DE detector. Time conversion is about 0.5 ns/ch.
0.0 50
100
150 200 250 300 Neutron energy (MeV)
Fig. 6. Calculated and experimental
238
350
Uðn; fÞ cross-sections.
400
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800
1010
600 Fast gate ADC (ch)
Neutron relative-flux (n/MeV/sr/count)
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108
25
(C)
400
200 (B)
107
(A)
0
0
100
200
300 400 500 600 Neutron energy (MeV)
700
0
800
200
400 Total gate ADC (ch)
600
800
Fig. 9. PSD by the two-gate integration method for the phoswich detector.
Fig. 7. Measured neutron relative flux.
1000
600
500
600
Total gate ADC (ch)
ADC for ΔE (ch)
800
(C)
400 (B) 200
400
300
200
(A)
100 0 0
200 400 Total gate ADC for phoswich (ch)
600
Fig. 8. Typical two-dimensional spectrum of the total gate signal for the phoswich detector versus the DE detector.
the ADC for DE decreases as the total gate ADC for the phoswich detector increases. Region (B) represents the detection of protons produced mainly by elastic collisions with hydrogen and the 12 Cðn; pÞ reaction. Compared to region (B), the scintillation of DE is larger in region (C). Region (C) indicates the detection of the deuterons produced in the radiator. The result of the PSD for the phoswich detector is shown in Fig. 9, where events in all regions in Fig. 8 are plotted. One can see three types of regions. In region (A), the scintillation light was emitted by the NaI(Tl) scintillator. In region (B), the scintillation occurred in both the NaI(Tl) and the plastic scintillators. The events in region (C) were detected when only the plastic scintillator emitted scintillation light. The events in region (A) were selected as the full-stopping proton events.
0
0
100
200 300 Deposited energy (MeV)
400
Fig. 10. Calibration of phoswich detector.
4.3. Calibration For energy calibration, the relationship between the deposition energy and the ADC channel for the NaI(Tl) scintillator is required. However, it is difficult to determine the value of the full energy deposition in the experiments. The calibration procedure was as follows. First, the deposition energy was calculated by PHITS, at the peak position generated from the H(n,p) reaction for an incident neutron energy. Then, the calculated deposition energy was related to the ADC channel of the full-energy peak in the response function that was measured for the same incident neutron energy. Fig. 10 shows the calibration result. A linear dependence of the ADC channel on the deposition energy was observed, while slight saturation effect was observed at deposition energies above 350 MeV.
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In PHITS calculation, the incident energies were set to mean values of the individual energy bins, and the neutron source had a 3.5-mm radius. The deposited energies in the DE detector, the NaI(Tl), and the plastic scintillators of the phoswich detector were calculated. Using these deposited energies, the response functions were calculated with a procedure consistent with the data analysis in Section 4.2. The calculations were performed for both the polyethylene and graphite converters. After subtracting the carbon contributions from the polyethylene calculation, the calculation result was broadened by a Gaussian function to consider the energy resolution of the detector. The energy resolutions were from 9 to 25%, and are listed in Table 2.
5. Results 5.1. Response functions of the phoswich detector Fig. 11 shows the phoswich detector response functions for incident neutron energies of 95–105, 145–155, 195–205, 245–255, 290–310, and 340–360 MeV, with the calculation results by PHITS. The error bars for the present data include systematic and statistical errors. The systematic errors derived from the uncertainty of 238U fission crosssections were set at 3 and 10%, below and above 200 MeV, respectively. The experimental response functions show good agreement with the calculations in the peak region, with the suitable energy resolutions.
(× 10-5)
PHITS (As calculated) PHITS (Broadened) Present
6.0
Neutron energy: 95−105 MeV
4.0
2.0
Response (1/source/MeV)
Response (1/source/MeV)
(× 10-5)
0.0
PHITS (As calculated) PHITS (Broadened) Present
3.0
Neutron energy: 145−155 MeV
2.0
1.0
0.0 0
30 60 90 Proton deposited energy (MeV)
0
(× 10-5)
(× 10-5)
PHITS (As calculated) PHITS (Broadened) Present Neutron energy: 195−205 MeV
1.0
0.5
Response (1/source/MeV)
Response (1/source/MeV)
2.0
1.5
PHITS (As calculated) PHITS (Broadened) Present
1.0
Neutron energy: 245−255 MeV
0.5
0.0
0.0 0
50 100 150 200 Proton deposited energy (MeV)
0
PHITS (As calculated) PHITS (Broadened) Present
6.0
Neutron energy: 290−310 MeV
4.0
2.0
0.0 0
50 100 150 200 250 Proton deposited energy (MeV)
300
(× 10-5) 5.0
100 200 300 Proton deposited energy (MeV)
Response (1/source/MeV)
(× 10-5)
Response (1/source/MeV)
50 100 150 Proton deposited energy (MeV)
PHITS (As calculated) PHITS (Broadened) Present
4.0
Neutron energy: 340−360 MeV
3.0 2.0 1.0 0.0 0
100 200 300 Proton deposited energy (MeV)
400
Fig. 11. Experimental response functions of the phoswich detector with the calculations by the PHITS.
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Table 2 Improvement of the full energy-deposition fraction and the energy resolution
95–105 145–155 195–205 245–255 290–310 340–360
Mean energy (MeV)
100 150 200 250 300 350
Full energy-deposition fraction (%)
Energy resolution (%)
E2DE
E2DE and Phoswich PSD
E2DE
E2DE and Phoswich PSD
95.1 81.8 80.7 62.8 55.1 37.6
95.8 85.5 87.7 69.2 63.5 52.2
25.4 17.9 13.3 11.9 11.6 9.7
22.5 15.6 12.8 10.6 10.1 9.1
To examine the effectiveness of the phoswich configuration, the full energy-loss fraction and the energy resolution for the response function were calculated. The response function was obtained after choosing the recoil-proton events labeled (B) (recoil proton) in Fig. 8, and the full energy-deposition events labeled (A) in Fig. 9. For comparison, the same examination was performed with the response function after selection of region (B) in Fig. 8. Table 2 shows these quantities with and without the phoswich PSD. It was found that the adoption of the phoswich configuration increased the full energy-loss fraction of the incident neutron energy. At maximum, an improvement of about 140% was obtained in the 340–360 incident neutron energy bin. In addition, the energy resolution with the phoswich configuration was slightly better. The improvement of the full energy-loss fraction was relatively low in lower energy bins. This is because neutrons of 7-mm diameter beam were used, and the fullstopping range of the proton was short at lower energies. The effectiveness of the phoswich configuration was estimated to be greater for a point-like neutron source than for the narrow neutron beam used in the present experiment. The response for a point neutron source was calculated at the neutron energy of 300 MeV, using PHITS. The arrangement of the source and detectors for the calculation is shown in Fig. 12(a). The point source was located 33.7 cm upstream from the front surface of the phoswich detector, where the extended side surfaces of the phoswich detector crossed. Fig. 12(b) shows the calculation results for the response function of the phoswich detector to a point neutron source, with and without the phoswich PSD. The response functions without the phoswich PSD have been estimated to have a large fraction in the tail region. The same comparison, concerning the full energy-loss fraction, was performed. The result showed that the fraction increased from 26 to 72%. When the TOF technique is inapplicable for measuring secondary neutrons, the phoswich detector developed is expected to be effective in experiments such as the measurement of ðn; xnÞ double-differential cross-sections, where neutrons like the point source are emitted from a sample target.
Phoswich detector
ΔE detector
Radiator
2.5
mm
36
Point neutron source 6.8
mm
33
(× 10-6) Response (1/source/MeV)
Energy bin (MeV)
E−ΔE& Phoswich PSD (Broadened) E−ΔE (Broadened)
6.0
Neutron energy: 300 MeV
4.0
2.0
0.0 0
100 200 300 Proton deposited energy (MeV)
Fig. 12. (a) Illustration of the arrangement of the point source and the phoswich detector and (b) calculated response functions of the reversed phoswich detector to the point source by PHITS.
6. Conclusions The reversed-type phoswich detector configuration was developed and tested. It consisted of an inner NaI(Tl) inorganic scintillator and a surrounding outer plastic scintillator (NE102A). The response functions of the phoswich detector to 7-mm diameter beam of neutrons were measured in the mean energy range of 100–350 MeV at LANSCE. Comparison of the response with the PHITS prediction gave a good agreement within 10% in the peak region at incident energies below 300 MeV, and a certain discrepancy up to 30% at 350 MeV.
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The use of PSD for the reversed-type phoswich configuration was verified to successfully work and be useful in achieving improvements in the full energy-loss fraction. The improvement was a few percent below the neutron energy of 150 MeV and increased with the neutron energy, reaching 140% at 350 MeV. An estimate, obtained with PHITS, showed that the full energy-loss fraction for a point neutron source is 2.8 times larger than that for a source with a 7-mm diameter collimated neutron beam. This configuration is considered to be useful in measuring secondary neutrons from a sample target, for discriminating protons of full kinetic-energy deposition in the energy range 150–350 MeV in the NaI(Tl) scintillator and those penetrating the outer plastic scintillator. The response function with the phoswich PSD has a small fraction in the tail and is suitable for the unfolding method in neutron spectrometry.
the use of the Los Alamos Neutron Science Center at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy. This work was also supported by the Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 12480141 and No. 15206110).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgments We are grateful to A. Bridge, G. Chaparro, and J.M. O’Donnell for their generous assistance in the experiments, and the LANSCE operations staff for providing a highquality pulsed proton beam. This work has benefited from
[11] [12] [13] [14]
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