Development of a small scintillation detector with an optical fiber for fast neutrons

Applied Radiation and Isotopes 69 (2011) 539–544

Contents lists available at ScienceDirect

Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

Development of a small scintillation detector with an optical fiber for fast neutrons T. Yagi a,, H. Unesaki b, T. Misawa b, C.H. Pyeon b, S. Shiroya b,1, T. Matsumoto c, H. Harano c a

Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan Research Reactor Institute, Kyoto University, Asashiro-nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan c National Institute of Advanced Industrial Science and Technology, Umezomo, Tsukuca, Ibaraki 305-8668, Japan b

a r t i c l e in f o

abstract

Article history: Received 27 July 2010 Received in revised form 4 November 2010 Accepted 19 November 2010

To investigate the characteristics of a reactor and a neutron generator, a small scintillation detector with an optical fiber with ThO2 has been developed to measure fast neutrons. However, experimental facilities where 232Th can be used are limited by regulations, and S/N ratio is low because the background counts of this detector are increase by alpha decay of 232Th. The purpose of this study is to develop a new optical fiber detector for measuring fast neutrons that does not use nuclear material such as 232Th. From the measured and calculated results, the new optical fiber detector which uses ZnS(Ag) as a converter material together with a scintillator have the highest detection efficiency among several developed detectors. It is applied for the measurement of reaction rates generated from fast neutrons; furthermore, the absolute detection efficiency of this detector was obtained experimentally. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Fast neutron Optical fiber ZnS(Ag) Scintillation detector ThO2 D-T neutron

1. Introduction Measurement of fast neutron flux is important for investigations of the neutronic characteristics of reactors and neutron generators. Neutron detectors such as fission chambers and activation foils have been used for the measurement of neutrons. However, fission chambers cannot be inserted into narrow spaces such as the gap between fuel assemblies, although they can measure neutrons in real time. In the activation method using thin activation foils, neutron flux cannot be measured in real time, although the foils can be inserted into narrow spaces. In recent years, a scintillation detector with an optical fiber (hereafter referred to as ‘‘the optical fiber detector’’) has been developed (Mori et al., 1994). This detector consists of a plastic or quartz optical fiber whose tip is covered with a mixture of scintillators such as ZnS(Ag) and neutron converter materials such as 6LiF for the detection of the thermal neutrons. The common converter materials for fast neutrons are 232Th or 238U because these materials only undergoes fission reaction with fast neutrons and fission products produce scintillation by collision with ZnS(Ag). The detector with 232Th is used to measure fast neutron flux across

 Corresponding author. Tel.: +81 72 451 2359; fax: + 81 72 451 2603.

E-mail address: [email protected] (T. Yagi). Present address: Nuclear Safety Commission of Japan, 3-1-1 Kasumigaseki, Chiyoda-ku, Tokyo 100-8970, Japan. 1

0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.11.021

the tritium target of a D-T neutron generator and in a research reactor (Yamane et al., 1998, 1999). However, experimental facilities where 232Th and 238U can be used are limited by regulations because those are nuclear materials. In addition, the signal-tonoise ratio of those detectors is low because the background counts of this detector are increased by the alpha decay of 232Th. The purpose of this study was to develop a new optical fiber detector without using 232Th for measuring fast neutrons and to investigate the characteristics of this detector including its detection efficiency through experiments conducted in D-T neutron fields.

2. Optical fiber detector for measuring fast neutrons 2.1. Principle The optical fiber detector was fabricated in the following way: (1) the neutron converter material and scintillator powder were mixed, (2) the mixed material was attached to the tip of an optical fiber with adhesive, (3) black paint was coated onto the surface of the mixed material. A sketch of the structure of the optical fiber detector and measurement system is shown in Fig. 1. The neutron converter material emits charged particles when neutrons collide with it, and then the charged particles emit light via the scintillator. Signals from a photomultiplier tube (Hamamatsu R1635) are then amplified by a pre-amplifier and a linear amplifier, before being analyzed by an MCA (multi-channel analyzer) or counted by a scaler through a SCA

540

T. Yagi et al. / Applied Radiation and Isotopes 69 (2011) 539–544

Fig. 1. Structure of the optical fiber detector and measurement system.

Table 1 Neutron converter materials for the measurement of fast neutrons. Material

Nuclide

Reaction

Cross-section at 14 MeV (barns)a

Threshold (MeV)

ThO2

232

n,f n,na n,p n,d n,a n,na n,np n,p n, a n,p n, a n,na n,np n,p n,d n,t n, a n,p n,na n,np n,p n, a n,na n,np n,p n, a n,na n,np n,p n, a n,na n,p n,a

0.350 0.037 0.044 0.015 0.109 0.002 0.280 0.078 0.122 0.007 0.028 0.359 0.043 0.015 0.034 0.015 0.021 0.149c, 0.193d 0.081 0.076 0.254 0.160 0.114 0.043 0.214 0.137 0.132 0.005 0.104 0.060 0.008 0.001 0.009

0.4b 7.613 10.24 10.53 2.355 10.47 8.581 1.896 3.249 3.185 – 4.226 8.417 4.251 6.074 7.959 1.603 0.1b 7.167 9.144 0.957 – 7.335 9.864 – – 8.158 11.2 4.445 1.374 9.252 9.243 4.065

Th O

16

Al

27

LiF

6

Al

Li

19

F

ZnS

64

Zn S

32

33

S

34

S

36

S

Table 2 Properties of the optical fiber detectors. Optical fiber

Diameter (mm) Component

1.2 Poly(methyl methacrylate)– (CH2C(CH3)COOCH3)n–

Converter and scintillator

Weight ratio

1:1

Mean thickness (mm)

0.3

Density (g/cm3)

ZnS(Ag) Al 6 LiF ThO2

4.10 2.70 2.64 9.86

to produce charged particles or to allow fission reactions with fast neutrons, (2) it should have threshold reactions for fast neutrons when it is used at a mixed neutron field including low energy neutrons or non-threshold reaction when it is used at a fast neutron field, and (3) the handling of materials should be simple and safe. According to these characteristics, ThO2, Al, 6LiF, and ZnS are selected as candidates for neutron converter materials for the fast neutrons. These converter materials have several charged particle reactions in D-T neutron fields, as listed in Table 1. The response of these detectors is the summation of those several reactions and depends on the neutron energy spectrum. Among these materials, 6 LiF has both threshold and non-threshold reactions, it can be used only in neutron field without thermal or epithermal neutrons such as a D-T neutron field. ZnS also has non-threshold reactions; however, cross-sections in lower energy region are much smaller than those in fast energy region and it can be classified as a threshold reaction material.

a

JENDL-3.3. JENDL Dosimetry File 91. JENDL Dosimetry File 99. d Joint European Fusion File (JEF-2.2). b c

(single channel pulse height analyzer). In addition, the optical fiber detector can be withdrawn at a constant speed by a specially designed drive unit which can be controlled remotely, meaning that the reaction rate distribution can be measured remotely. The desirable characteristics of a converter material used to measure fast neutrons are as follows: (1) it has large cross-sections

2.2. Experiment in KUCA In the present experiments, four types of optical fiber detectors were used; namely, ThO2 +ZnS(Ag), Al+ZnS(Ag), 6LiF+ZnS(Ag), and ZnS(Ag) alone. Several properties of the optical fiber detector are listed in Table 2. These experiments were conducted in the D-T neutron field using a neutron generator of a Cockcroft–Walton type accelerator in the Kyoto University Critical Assembly (KUCA) (Ichihara et al., 1983; Pyeon et al., 2007), as shown in Fig. 2. The target was consisted of a copper plate coated with titanium that contained tritium, and the beam

T. Yagi et al. / Applied Radiation and Isotopes 69 (2011) 539–544

spot size was approximately 2.5 cm in diameter. These detectors were withdrawn in an upward direction at specific intervals by the drive unit. The fibers were inserted into a Teflon tube which was attached to the Al sheath in front of the target to guide the optical fiber detector.

Fig. 2. Measurement position of the four types of optical fiber detectors across the tritium target of the accelerator in KUCA. The vertical axis shows the distance from the target center.

541

2.3. Results and analysis 2.3.1. Pulse height spectra To compare characteristics of the optical fiber detectors with ThO2 + ZnS(Ag) and the other detectors, pulse height spectra were measured in front of the target with and without operation of the accelerator. The pulse height spectra measured by the optical fiber detector with ZnS(Ag), Al+ZnS(Ag) and 6LiF + ZnS(Ag) are shown in Fig. 3(a)–(c). Those results show that the background pulse height spectra without neutrons appeared at a region of low pulse height, which indicates that the background counts were caused by g-rays and electrical noise signals. The discrimination level of the pulse height spectra was set as shown in Fig. 3 to discriminate neutron signals from g-rays and electrical noise signals because these pulse heights were lower than the pulse height in operation. The discrimination level can be calibrated using the pulse height of the background measurement and a standard signal such as from a light pulser. On the other hand, the pulse height spectra of the optical fiber detector with ThO2 + ZnS(Ag) are shown in Fig. 3(d). This figure shows that the background pulse height spectrum appears up to the region of high pulse height. The result shows that the pulse height spectrum was caused by g-rays, electrical noise, and alpha-particles generated by the alpha decay of 232Th and its daughter nuclides. The signals from g-rays and electrical noise were eliminated by setting the discrimination level appropriately, and the alpha-particle signal was subtracted from the count rate

Fig. 3. Pulse height spectra and discrimination level of the optical fiber detector in front of the tritium target. (a) The detector with ZnS, (b) The detector with Al+ZnS, (c) The detector with 6LiF+ZnS and (d) The detector with ThO2+ZnS.

542

T. Yagi et al. / Applied Radiation and Isotopes 69 (2011) 539–544

obtained with background measurement. According to these results, the signal-to noise ratio of the three detectors that did not contain ThO2 were higher than that of the detector with ThO2. To investigate the scintillation light emission mechanism of these detectors and to calculate interaction and transport of charged particles created in nuclear reactions, energy deposition spectra in the detectors were calculated with Monte Carlo particle transport calculation code, PHITS (Particle and Heavy Ion Transport Code System) (Iwase et al., 2002). JENDL-3.3 (Shibata et al., 2002) and ENDF/B-VII.0 (Chadwick et al., 2006) were employed as the evaluated nuclear data library. The energy deposition spectra of proton, deuteron, triton, alpha, and all particles in the volume (1-mm-diam.0:3mm  height) in front of 14 MeV neutron source were calculated with four detectors, as shown in Fig. 4. It is found that the amount of deposition is large at low energy region. In the detectors with ZnS and Al+ZnS, alpha-particles deposited high energy compared with proton and deuteron. Moreover, triton also deposited high energy to the detector with 6LiF+ ZnS. However, in the low energy deposit region, particles other than proton, deuteron, alpha, and triton have much contribution, and it was found that electron has very important role in this low energy deposition. On the other hand, the results for the detector with ThO2 + ZnS in Fig. 4(d) show that although fission fragments deposited much higher energy compared with other particles,

the particles appeared in lower energy regions such as alpha or protons also deposited large amount of energy to the detector. 2.3.2. New detector for measuring fast neutrons To investigate the relative detection efficiencies of four detectors at D-T neutron fields, reaction rates in front of the target were compared. Table 3 shows the measured reaction rate and the macroscopic cross-sections taken into consideration of the weight ratio of the converter material and scintillator. Macroscopic crosssections show that the order of magnitude is (1) 6LiF+ ZnS, (2) ZnS, (3) Al+ ZnS, and (4) ThO2 +ZnS. However, the measured reaction rate of ZnS(Ag) was the highest among these detectors. This result shows that using ZnS(Ag) alone as a converter material together with a scintillator is effective from the view point of the detection efficiency. The measured result of the detector with ThO2 + ZnS was too low, which means the epoxy adhesive of the detector was considered to be discolored because this detector was made about 10 years ago. To examine the ability of the fast neutron measurement with the new detectors, reaction rate distributions were measured across the tritium target. The measured and calculated results are shown in Fig. 5, the vertical values of which have been normalized to the position of the target center. Sum of reaction rates of charged particles listed in Table 1 were calculated using

Fig. 4. Calculated energy deposition spectra in the detectors. (a) ZnS, (b) Al+ZnS, (c) 6LiF+ZnS and (d) ThO2+ZnS.

T. Yagi et al. / Applied Radiation and Isotopes 69 (2011) 539–544

Table 3 Comparison between measured reaction rate and normalized macroscopic crosssections at 14 MeV neutrons. Detector

6

LiF+ ZnS Al +ZnS ThO2 + ZnS ZnS

Measured reaction rate

0:68 7 0:01 0:48 7 0:01 0:17 7 0:01 1:00 7 0:02

Table 4 Estimation of background signals caused by an optical fiber and Teflon tube. Calculation conditions

Distance from the target center (cm)

Normalized energy deposition

Without fiber and Teflon tube

4 4

1:00 7 0:01 0:99 7 0:01

With fiber

4 4

1:00 7 0:01 1:11 7 0:01

With Teflon tube

4 4

0:98 7 0:01 0:98 7 0:01

Normalized macroscopic cross-section (1)

(2)

1.08 0.99 0.88 1.00

1.04 0.95 0.87 1.00

543

(1) JENDL-3.3 and JENDL/D91. (2) JENDL-3.3 and JEF-2.2.

Fig. 5. Reaction rate distribution of the optical fiber detectors along the axial direction across the tritium target. The vertical values have been normalized to the position of the target center.

MCNP5 code (X-5 Monte Carlo Team, 2003) and ENDF/B-VII.0, where isotropic source definition was employed. Fig. 5 shows that the shapes of the distributions measured by these three detectors were similar in the range of  4 to 4 cm region at full width at half maximum. In the region away from the target, the measured and the calculated reaction rates of 6LiF + ZnS(Ag) were higher than those of the other detectors as shown in Fig. 5. The reason is that the optical fiber detector with 6LiF + ZnS(Ag) which had non-threshold energy, and it reacted with epithermal neutrons generated by collisions with surrounding structures. From these results, these detectors can measure fast neutrons around the target. However, discrepancies of measured and calculated results were appeared at lower position from the target center. This reason is isotropic source definition employed with calculation and background signals from the detector itself. To examine these discrepancies of measured and calculated values, background signals in the scintillation spectra were estimated with PHITS code as the same procedures mentioned above. Energy depositions in the detector with and without an optical fiber and a Teflon tube whose major component was -(CF2-CF2)n- were calculated at full width at half maximum and symmetrical position. Table 4 shows energy depositions normalized at 4 cm without the fiber and the Teflon tube. The result with an optical fiber at  4 cm from the target was only higher than the other results. The reason of increasing the sensitivity at the lower position is considered to be the influence of the recoil protons from the plastic optical fiber. However, these results show that this problem can be solved by withdrawing in an upward and downward from the target center separately.

Fig. 6. g-rays pulse height spectrum of the Zn foil placed in front of the target.

From these results, all types of optical fiber detectors were able to measure fast neutrons; however, measured results depended on position of detectors. The optical fiber detector using only ZnS(Ag) was the most effective for measuring fast neutrons because it had the highest detection efficiency and threshold reaction.

3. Detection efficiency of the new optical fiber detector 3.1. Principle To calibrate the new optical fiber detector with ZnS(Ag), the detection efficiency was evaluated with combination of the foil activation method in D-T neutron fields. The absolute value of the fast neutron flux was obtained using a Zn foil because this threshold energy is almost same as that of the charged particle reactions in ZnS. In this case, the saturation activity of Zn foil was approximately by the following equation since the D-T neutron field was a mono-energetic field. D1 ¼ V  SZn ðE0 ÞfZn ðE0 Þ,

ð1Þ

where V is the volume of Zn foil, SZn is the macroscopic crosssection of the 64Zn(n, p)64Cu reaction, fZn is the neutron flux, and E0 is 14 MeV. After the irradiation, the saturation activity was measured using the detection efficiency of germanium detector, and the absolute value of fast neutron flux was obtained. The detection efficiency of the optical fiber detector was defined by the following equation:

e  n=f,

ð2Þ

544

T. Yagi et al. / Applied Radiation and Isotopes 69 (2011) 539–544

where n (s  1) is the counting rate and f (s  1 cm  2) is the neutron flux. 3.2. Experiment in AIST Experiments were conducted using D-T reactions generated by a Cockcroft–Walton accelerator installed in National Institute of Advanced Industrial Science and Technology (AIST) (Kudo et al., 2002). The target consisted of evaporated Ti–T on a 0.5 mm Cu backing. To compare its reaction rate with that of the optical fiber detector with ZnS(Ag), a foil activation method involving Zn foil (2  2  0.2 cm3) was used. The irradiation time of the foils was 20 min. The fast neutron flux distribution obtained by the Zn foils was compared with the distribution of the optical fiber detector, and the detection efficiency of the optical fiber detector was obtained from Eq. (2). 3.3. Detection efficiency The g-rays energy spectrum of the Zn foil placed in front of the target is shown in Fig. 6. The peak at 511 keV generated by the 64 Zn(n, p)64Cu reaction was observed. Zn foil at 5 cm upper position from the target was used as calibration because background signals from the optical fiber are low as mentioned in Section 2.3.2. In the present experiment, the detection efficiency e of the optical fiber detector with ZnS(Ag) was 1:2  106 [cps/f14 MeV ] in the D-T neutron field.

4. Conclusion In order to detect fast neutrons, several new optical fiber detectors without 232Th have been developed, and the characteristics of each detector were examined in D-T neutron fields. From the measured and calculated results, the optical fiber detectors with Al+ ZnS(Ag), 6LiF+ ZnS(Ag) and ZnS(Ag) can measure fast neutrons effectively. Among these detectors, a new optical fiber

detector in which only ZnS(Ag) was used as a converter material together with a scintillator had the highest detection efficiency. Furthermore, the absolute detection efficiency of this detector was obtained experimentally.

Acknowledgements The authors would like to thank all KUCA staff members for carrying out these experiments. References Chadwick, M.B., Oblozˇinsky´, P., Herman, M., et al., 2006. ENDF/B-VII.0: next generation evaluated nuclear data library for nuclear science and technology. Nucl. Data Sheets 107, 2931–3060. Ichihara, C., Nakamura, H., Kobayashi, K., Ikegawa, R., Tsuruta, T., Kanda, K., Shibata, T., 1983. Characteristics of KUCA pulsed neutron generator, KURRI-TR-240, Kyoto University Research Reactor Institute (in Japanese). Iwase, H., Niita, K., Nakamura, T., 2002. Development of general-purpose particle and heavy ion transport Monte carlo code. J. Nucl. Sci. Technol. 39 (11), 1142–1151. Kudo, K., Uritani, A., Takeda, N., 2002. NMIJ report for CCRI key comparison of neutron fluence measurements at energies of 144 keV, 5.0 MeV and 14.8 MeV. AIST Bull. Metrol. 2 (1), 61–71. Mori, C., Osada, T., Yanagida, K., Aoyama, T., Uritani, A., Miyahara, H., Yamane, Y., Kobayashi, K., Ichihara, C., Shiroya, S., 1994. Simple and quick measurement of neutron flux distribution by using an optical fiber with scintillator. J. Nucl. Sci. Technol. 31 (3), 248–249. Pyeon, C.H., Hirano, Y., Misawa, T., Unesaki, H., Ichihara, C., Iwasaki, T., Shiroya, S., 2007. Preliminary experiments on accelerator-driven subcritical reactor with pulsed neutron generator in Kyoto University Critical Assembly. J. Nucl. Sci. Technol. 44 (11), 1368–1378. Shibata, K., Kawano, T., Nakagawa, T., et al., 2002. Japanese evaluated nuclear data library version 3 revision-3: JENDL-3.3. J. Nucl. Sci. Technol. 39 (11), 1125–1136. X-5 Monte Carlo Team, 2003. MCNP—a general Monte Carlo N-particle transport code, version 5, LA-UR-03-1987, Los Alamos National Laboratory. Yamane, Y., Linde´n, P., Karlsson, J.K-H., Pa´zsit, I., 1998. Measurement of 14.1 MeV neutrons with a Th-scintillator optical fibre detector. Nucl. Instrum. Methods A 416, 371–380. Yamane, Y., Uritani, A., Misawa, T., Karlsson, J.K.-H., Pa´zsit, I., 1999. Measurement of the thermal and fast neutron flux in a research reactor with a Li and Th loaded optical fibre detector. Nucl. Instrum. Methods A 432, 403–409.