Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 60 (2014) 349 – 355
Union of Compact Accelerator-Driven Neutron Sources (UCANS) III & IV
Neutron TOF experiments using transparent rubber sheet type neutron detector with dispersed small pieces of LiCaAlF6 scintillator Dai Sugimotoa*, Kenichi Watanabea, Katsuya Hirotaa, Atushi Yamazakia, Akira Uritania, Tetsuo Iguchia, Kentaro Fukudab, Sumito Ishidub, Noriaki Kawaguchib, Takayuki Yanagidac, Yutaka Fujimotoa, Akira Yoshikawad, Hiroyuki Hasemie, Kouichi Kinoe, Yoshiaki Kiyanagie a
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan b Tokuyama Corp., 3 Shibuya Shibuya-ku, Tokyo 150–8383, Japan c Kyushu Institute of Technology, 2-4, Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan d Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan e Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
Abstract We developed a new type of neutron detector using a transparent rubber sheet in which small pieces of Eu doped LiCaAlF6 scintillator are dispersed. We demonstrate that the proposed rubber sheet type neutron detector can show a clear neutron absorption peak in the pulse height spectrum and can easily eliminate gamma-ray events with pulse height discrimination in neutron TOF experiments at the 45 MeV electron Linac facility of Hokkaido University. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of UCANS (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of UCANS III and UCANS IV Keywords: neutron detector; TOF experiment; Eu:LiCaAlF6 scintillator.
1. Introduction Recently, neutron science plays an important role in various fields, such as not only nuclear engineering but also material science and bio-science. In neutron science, nuclear reactors and
* Corresponding author. Tel.: +81-52-789-3846; fax: +81-52-789-3843. E-mail address:
[email protected]
1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of UCANS III and UCANS IV doi:10.1016/j.phpro.2014.11.047
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accelerator-based neutron generators are used as neutron sources. Nuclear reactors generally have quite high neutron yield but requires a lot of complicate works for facility management, such as managements of nuclear materials. On the other hand, accelerator-based neutron generators require only easier managements than reactors because neutrons are emitted only under operation and accelerators require no nuclear material management. In addition, accelerator-based neutron generator is relatively easy to be operated in the pulse mode. In the pulse mode operation, neutron time-of-flight (TOF) measurements can be performed. Since the TOF corresponds to the neutron energy or wavelength, the TOF measurements provide energy or wavelength resolved information in analyses. The accelerator-based neutron generators are divided into two types, which are charged particle accelerators and electron accelerators. Most of charged particle accelerators are proton accelerators. The proton accelerator-based neutron generators are based on high-energy proton induced nuclear reactions, such as 7Li(p,n), 9Be(p,n) and spallation reactions. On the other hand, the electron accelerator-based neutron generators are based on (J,n) reactions induced by bremsstrahlung radiations generated when high-energy electrons collide into a heavy target, such as a lead target. This type of accelerator emits not only neutrons but also a large number of bremsstrahlung radiations in addition to neutron induced gamma rays. Intense neutron sources, such as nuclear reactors and accelerator-based neutron generators, can perform various applications, such as neutron diffraction analysis and neutron radiography. These applications require neutron sensitive detectors with no gamma-ray sensitivity because gamma rays cause undesirable signal events. Especially, the electron accelerator-based neutron generators require detectors with extremely high n/J sensitivity ratio because of its high gamma-ray yield. So far, standard neutron detectors have used 3He gas due to its high neutron sensitivity and gamma-ray insensitivity. In recent years, because the 3He shortage has been facing severe problem, alternatives to 3 He neutron detectors have been required to be developed. One of the promising candidates is a scintillator-based neutron detector. We have developed a Li-loaded scintillator Eu:LiCaAlF6 (Eu:LiCAF) as an alternative neutron detector. In this paper, we propose a new type of neutron detector, that is a transparent rubber sheet with dispersed small pieces of Eu:LiCAF scintillators. We demonstrate that the proposed rubber sheet type neutron detector can eliminate gamma-ray events with pulse height discrimination in neutron TOF experiments at the 45 MeV electron Linac facility of Hokkaido University, which is a reference neutron facility with relatively high gamma-ray backgrounds.
2. New transparent rubber sheet type neutron scintillator 2.1. Neutron scintillators The properties of typical neutron scintillators are shown in Table 1. A Li glass scintillator has been one of the conventional standard neutron scintillators [1-2]. Since the Li glass is transparent, it shows a clear neutron absorption peak in the pulse height spectrum. However, D/E ratio, which is defined as the ratio of scintillation efficiency for alpha particles to that for electrons, is relatively low and the light yield is not so high. These properties make it difficult in discrimination between neutron and gamma-ray events with the pulse height. LiF-ZnS composite scintillator is also conventional type of neutron detectors [4]. This scintillator shows extremely high n/J sensitivity ratio but shows no neutron absorption peak because of nonuniformity in scintillation light collection efficiency due to its opacity. This property means that neutron sensitivity, which is determined with counts above a discrimination level, can easily change when the system gain changes by ambient conditions, such as temperature. LiI and Cs2LiYCl6 scintillators have quite high light yield and high α/β ratio [3,9]. It is, however, difficult to make a large size crystal and to
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fabricate into various configurations because of its strong hygroscopicity, so their applications are restricted into limited fields. Table 1. Properties of typical neutron scintillators [1-9] Light Density Li content Scintillator yield (g/cm3) (Li atoms /cm3) (phts/n)
Emission peak (nm)
Decay time (ns)
D/E ratio
Transparency
Hygroscopicity
Pulse shape Discri.
Ce:Li glass
6,000
2.5
~1.6×1022
395
75
0.23
Yes
No
-
Eu:LiI
49,500
4.06
1.8×1022
470
1,400
0.9
Yes
Yes
-
LiF-ZnS
-
2.6/4.0
~2.0×1022*
450
200
-
Opaque
No
-
Ce:LiCaAlF6
5,000
2.99
9.6×1021
285
40
0.4
Yes
No
Yes
Eu:LiCaAlF6
40,000
2.99
9.6×1021
375
1,600
0.2
Yes
No
No
3.31
21
370
1/50 /1,000
0.67
Yes
Yes
Yes
Ce:Cs2LiYCl6
64,000
3.5×10
* LiF:Zns = 2:1 (weight ratio), Grain packing ratio: 75%
2.2. Eu:LiCAF scintillator We have developed LiCAF scintillators as the new neutron scintillators [6-8]. This scintillator has some excellent properties. This scintillator can easily be fabricated into various configurations because of no hygroscopicity and also can show a clear neutron absorption peak due to its transparency. Especially, Eu:LiCAF scintillators also have relatively high light yield but have difficulty in gamma-ray rejection because of its low D/E ratio. If Eu:LiCAF can reduce gamma-ray sensitivity, it can be an ideal neutron scintillator. As mentioned above, bulk Eu:LiCAF scintillators with larger size more than mm order suffer from interference of gamma rays with high energy of MeV order in pulse height spectra because of its low D/E ratio. In order to reduce interference of gamma rays, controlling the size of scintillators is useful. When scintillator size is controlled smaller than the range of fast electrons induced by gamma rays but larger than the range of reaction products in 6Li(n,t)D reactions, gamma-ray events can be easily distinguished in the pulse height spectra. Figs. 1 and 2 show the pulse height spectra obtained from a bulk and a small piece of Eu:LiCAF scintillator with the size less than 1 mm, respectively. In a small piece scintillator, fast
3
252 Cf 60
Co
2 1 0 0
100 200 300 400 500 Pulse height [channel number]
Fig. 1. Pulse height spectra obtained from a bulk Eu:LiCAF scintillator with size of 10×10×2 mm when irradiated by a moderated Cf-252 neutron source and Co-60 gamma ray source.
0.0006 Count rate [counts/sec]
Count rate [counts/sec]
4
0.0005
252 Cf 60
Co
0.0004 0.0003 0.0002 0.0001 0 0
100 200 300 400 500 Pulse height [channel number]
Fig. 2. Pulse height spectra obtained from a small piece of Eu:LiCAF scintillator with less than 1 mm size when irradiated by a moderated Cf-252 neutron source and Co-60 gamma ray source.
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electrons induced by gamma rays easily escape from the scintillator after depositing only a small fraction of its initial energy. Consequently, a small piece scintillator can make the signal pulse height for gammaray induced events lower compared with that for neutron events but has quite low detection efficiency for neutrons. 2.3. Transparent rubber sheet with dispersed small pieces of Eu:LiCAF scintillator In order to increase the detection efficiency with keeping the gamma-ray rejection ability, a large number of small pieces of scintillators can be arranged over a sensitive area of a photo detector. This configuration has difficulty in uniformly collecting scintillation photons because small pieces of scintillators are difficult to be fabricated with uniform size and shape. In order to suppress variation of the scintillation photon collection efficiency caused by the refractions and reflections on interfaces of the scintillators, we propose to disperse small pieces of scintillators into transparent rubber with a similar refractive index, which is approximately 1.4, and to form the rubber into a sheet shape. In this configuration, a large number of small scintillator pieces can be arranged with uniform light collection efficiency. This type of neutron scintillator sheet has both relatively high neutron sensitivity and high gamma-ray suppression ability. In addition, the rubber sheet is flexible and allows easy handling in contrast to thin crystal or glass scintillators, which are fragile and require delicate handling. In this configuration, we should consider the density of dispersed small scintillators. In the rubber sheet with densely dispersed scintillators, fast electrons induced by gamma rays can re-enter into neighbor scintillators and the gamma-ray event suppression ability can be deteriorated.
3. Experimental setup We experimentally demonstrated that the proposed rubber sheet type neutron detector had discrimination ability between neutron and gamma-ray events with pulse height discrimination in neutron TOF experiments at the 45 MeV electron Linac facility of Hokkaido University. Fig. 3 shows the experimental setup. In these experiments, we used the cold neutron source using a lead conversion target surrounded by a solid methane moderator. The pulse repetition rate was 25 Hz. The neutron fight path length was 6 m. As a photo detector, a position sensitive photomultiplier tube using resistive charge division method (RPMT, Hamamatsu R3292) was used. We prepared two type of 50×50×5 mm3 rubber sheets with dispersed small pieces of Eu:LiCAF scintillators with low density and high density. The scintillator size
Scintillator Moderator RPMT
HV Signal processing circuits
B-loaded PE
Pb target
Electron beam
Fig. 3. Setup of neutron TOF experiments at the 45 MeV electron Linac facility of Hokkaido University.
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was controlled less than 1 mm. The low and high density rubber sheet has the 6Li content of 3×1020 and 3×1021 atoms/cm3, respectively. The rubber sheet type scintillator was mounted on the RPMT surface. The RPMT was shielded from ambient light. The lateral and rear side of the scintillation detector was surrounded with B-loaded polyethylene in order to shield scattered neutrons from surrounding materials. The signals from the RPMT were fed into the signal processing circuits and the total pulse height, centerof-gravity position of scintillations and elapsed time from the neutron pulse were recorded into a control PC. In the experiments of the gamma-ray response evaluation, the front surface of the detector was also shielded with a Cd plate to prevent incoming thermal neutrons.
4. Results and discussion We evaluated the response of the transparent rubber scintillator with dispersed small Eu:LiCAF pieces to neutrons and gamma rays occurring at the detector position. In order to except the influence of fast and epithermal neutrons, which cannot be shielded with the Cd plate, only events with the flight time from 1 to 30 msec, corresponding neutron energy from 0.2 to 190 meV, were recorded. In this range of the flight time or neutron energy, neutrons are perfectly shielded with the Cd plate and the pure gamma ray response can be evaluated. Figs. 4 and 5 show the pulse height spectra obtained from the transparent rubber sheet scintillators with dispersed small Eu:LiCAF pieces with the high and low density, a)
20000 10000 0 0
b)
101 100
0 0
200 400 600 800 Pulse height [channel number]
102
0
200 400 600 800 Pulse height [channel number]
Fig. 4. Pulse height spectra obtained from the transparent rubber sheet scintillator with densely dispersed small Eu:LiCAF pieces. a) and b) show the neutron and gamma-ray responses, respectively. They were obtained without and with a Cd thermal neutron shielding, respectively.
a)
5000
200 400 600 800 Pulse height [channnel number]
b) Counts [600 sec]
Counts [300 sec]
103
10000 Counts [300 sec]
Counts [300 sec]
30000
102
101
100
0
200 400 600 800 Pulse height [channel number]
Fig. 5. Pulse height spectra obtained from a transparent rubber sheet scintillator with sparsely dispersed small Eu:LiCAF pieces. a) and b) show the neutron and gamma-ray responses, respectively.
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respectively. From the neutron response obtained from the rubber with densely dispersed small scintillators shown in Fig. 4 a), we can see a clear neutron absorption peak. The peak width for the transparent rubber sheet scintillator is comparable to that for a bulk Eu:LiCAF scintillator shown in Fig. 1. The uniform scintillation photon collection can be achieved even in the transparent rubber with dispersed small scintillator pieces. The lower side tail component of the neutron peak is more significant than that for a bulk Eu:LiCAF. This tail component is considered to be the wall effect, in which alphas and/or tritons produced in 6Li(n,t)α reactions escape to outside the scintillator before they deposit the full energy. Since small scintillator pieces have high ratio of surface area to volume, the tail component is more significant than that of a bulk scintillator. For the rubber with densely dispersed small scintillators, signals induced by gamma rays shown in Fig.4 b) have relatively large pulse height. This is because fast electrons induced by gamma rays, which can have path length more than 1 mm, can re-enter into neighbor scintillators and relatively large energy can be deposited into a few pieces of the small scintillators. Consequently, the gamma-ray event suppression ability is deteriorated compared with a single small piece of Eu:LiCAF scintillator. From the neutron response of the rubber with sparsely dispersed small scintillators shown in Fig. 5 a), a clear neutron peak can also be confirmed. However, the signals induced by gamma rays are lower than that by neutrons. This means that the gamma-ray events can be distinguished only with the signal pulse height. We conclude that the transparent rubber sheet scintillator with dispersed small Eu:LiCAF pieces with the adequate density can measure only neutrons with suppressing the gamma-ray response even at the electron accelerator-based neutron source facility. Of course, since the neutron sensitivity depends on the Eu:LiCAF density in the rubber, the Eu:LiCAF density should be optimized according to applications. 5. Conclusion We developed a new type of neutron detector using a transparent rubber sheet with dispersed small pieces of Eu:LiCAF scintillator. We successfully demonstrated that the proposed rubber sheet type neutron detector can show a clear neutron absorption peak in the pulse height spectrum and can easily eliminate gamma-ray events with pulse height discrimination in neutron TOF experiments at the electron Linac facility of Hokkaido University. The density of the dispersed small Eu:LiCAF scintillators in the rubber is the important parameter, that determines the neutron sensitivity and gamma-ray suppression ability. This parameter, therefore, should be optimized according to applications. Acknowledgements This study is partially supported by the Adaptable and Seamless Technology transfer Program through targetdriven R&D of Japan Science and Technology Agency. References [1] A. R. Spowart, Measurement of the absolute scintillation efficiency of granular and glass neutron scintillators, Nucl. Instrum. Methods 1969; 75: 35-42. [2] Lithium Glass Scintillators Product Data Sheet, Saint-Gobain Ceramics & Plastics, Inc., 2007. www.detectors.saintgobain.com/uploadedFiles/SGdetectors/Documents/Product_Data_Sheets/Glass-Scintillator-Data-Sheet.pdf (accessed October 11th) [3] Brochure on 6LiI(Eu), Detec, 2011. www.detec-rad.com/dl/Scint_Brochure.pdf (accessed October 11th)
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[4] G, F, Knoll: Radiation detection and measurement, John Wiley & Sons, Inc., 2010. [5] M. Nikl, et al., Scintillation Decay of LiCaAlF6:Ce3+ Single Crystals, Phys. Stat. Sol. (a) 2001; 187: R1-R3. [6] A. Gektin et al., LiCaAlF6:Ce crystal: a new scintillator, Nucl. Instrum. Methods A 2002; 486: 274-277. [7] A. Yoshikawa et al., Single crystal growth, optical properties and neutron responses of Ce3+ doped LiCaAlF6, IEEE Trans. Nucl. Sci. 2009; 56: 3796-3799. [8] A. Yamazaki et al., Neutron–gamma discrimination based on pulse shape discrimination in a Ce:LiCaAlF6 scintillator, Nucl. Instrum. Methods A 2011; 652: 435-438. [9] Gamma-Neutron Scintillation Detector CLYC, RMD, 2013, rmdinc.com/wp-content/uploads/2013/03/CLYC_Properties.pdf (accessed October 11th)
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