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First performance test of newly developed plastic scintillator for radiation detection
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First Performance Test of Newly Developed Plastic Scintillator for Radiation Detection M. Watanabe, M. Katsumata, H. Ono, T. Suzuki, H. Miyata, Y. Itoh, K. Ishida, M. Tamura, Y. Yamaguchi
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Nuclear Instruments and Methods in Physics Research A
Received date: 16 January 2014 Revised date: 16 September 2014 Accepted date: 3 October 2014 Cite this article as: M. Watanabe, M. Katsumata, H. Ono, T. Suzuki, H. Miyata, Y. Itoh, K. Ishida, M. Tamura, Y. Yamaguchi, First Performance Test of Newly Developed Plastic Scintillator for Radiation Detection, Nuclear Instruments and Methods in Physics Research A, http://dx.doi.org/10.1016/j.nima.2014.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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First Performance Test of Newly Developed Plastic Scintillator for Radiation Detection✩ M. Watanabea,∗∗, M. Katsumatab, H. Onoc , T. Suzukia , H. Miyatab , Y. Itoha , K. Ishidaa , M. Tamurad, Y. Yamaguchid
5
a Graduate
6
b Faculty
7 8
9
School of Science and Technology, Niigata University, Niigata 950-2181, Japan of Science, Physics Department, Niigata University, Niigata 950-2181, Japan c Nippon Dental University, School of Life Dentistry at Niigata, Niigata 951-8580, Japan d Carlit Holdings Co., Ltd., Chiyoda, Tokyo 101-0024, Japan
Abstract We present a plastic scintillator, developed in collaboration with Carlit Holdings Co., Ltd., that is fabricated using a liquid parent material cured at room temperature by adding a hardener. The new scintillator can incorporate heatlabile functional materials such as gadolinium to enhance neutron sensitivity. The characteristics of the new scintillator, in particular the light yield and attenuation length, were evaluated using a
90
Sr β-ray source. The light yield was
measured 7% Anthracene on the basis of a comparison with commerically available scintillator (BC-408) at a distance of 18 cm from the photodetector surface. This light yield is dependent on the distance between the luminous point and the photodetector because of light attenuation. The attenuation length of the Gd-doped scintillator was about 50 cm. 10
Keywords: plastic scintillator, neutron detectors, gadolinium, reactor,
11
safeguards, neutrino
12
1. Introduction
13
Scintillation counters are widely used as radiation detectors in nuclear and
14
high-energy physics and have recently found new applications especially in med-
15
ical fields [1-3].
16
Plastic scintillators are known to be easy to handle and chemically stable
17
owing to the solid state of the material even though relatively expensive than ✩
Preprint submitted to Elsevier ∗ ∗∗ Tel.:
+81252626138; Fax.: +81252626138 Email address: [email protected] (M. Watanabe) URL: http://www.hep.sc.niigata-u.ac.jp (M. Watanabe)
October 8, 2014
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liquid scintillator (LS), and plastic scintillator technology is well developed.
19
Several studies have been conducted to include functionalities such as radiation
20
hardness [4] and neutron sensitivity [5-7] as well as reducing manufacturing
21
costs [8, 9]. Nuclear and high-energy physics researchers require reasonably
22
priced large plastic scintillator detectors for calorimeters [10-13] and reactor
23
neutrino detectors [14].
24
In response to this, we have been working for the reactor neutrino detection
25
application toward the development of the lower cost plastic scintillator 1 . The
26
anti-neutrinos are detected via the inverse beta decay process, ν¯e + p → e+ +
27
n. Here the hydrogen in scintillator works as target proton. Some projects
28
employ Gd-doped liquid scintillator to achieve a large detection volume with
29
low cost and to enhance neutron detection [15-17] 2 . The flammability of the
30
LS, however, is problematic when the measurement were done close to reactor.
31
To rectify the problem, we developed a Gd-doped plastic scintillator, that is
32
relatively flame resistance than LS. The reactor monitor made of Gd doped
33
plastic scintillator is supposed to be about one cubic meter (about one ton) in
34
size [18].
35
In this study, we developed two types of plastic scintillators, undoped and Gd
36
doped, based on new cost-effective materials. The light yield and attenuation
37
length of these scintillators were measured and compared with the commercial
38
standard plastic scintillator (BC-408) made by Saint-Gobain Co., and the results
39
are presented here. 1 For
example, the International Atomic Energy Agency (IAEA) suggests a safeguard nu-
clear regime to prevent fissile material from civil nuclear fuel being converted into nuclear weapons. Anomalous reactor activity is monitored through the large number of reactor antineutrinos (ν¯e ) originating from a series of fission processes in the nuclear reactor. 2 Gd has large cross section for thermal neutron and generates higher energy gammas, whose total energy is 8 MeV.
2
40
2. New manufacturing method
41
Commercially available plastic scintillators are composed of a parent mate-
42
rial such as polystyrene or polyvinyl toluene and wavelength shifters. Primary
43
and secondary wavelength shifters are typically 2,5-diphenyloxazole (PPO) and
44
1,4-bis(5-phenyl-2-oxazolyl)benzene (POPOP), respectively. To incorporate the
45
wavelength shifters, the parent material is heated to a temperature above its
46
melting point (∼100 ◦ C for polystyrene or polyvinyl toluene [19]), and the wave-
47
length shifters are then dispersed in the parent material. To prevent cracking
48
upon cooling, a typical polystyrene plastic scintillator is cooled slowly after this
49
process.
50
The new plastic scintillator (Fig. 1) has been developed in collaboration
51
with Carlit Holdings Co., Ltd., with a focus on reducing production costs. It is
52
based on a parent material (bisphenol A type epoxy resin) that can be cured at
53
room temperature by the adding of a hardener (amine-based curing agent). In
54
order to obtain better mixing of the wavelength shifters (PPO: 1 wt%, POPOP:
55
0.03 wt%), the parent material and hardener, we heat the parent material to 60
56 57
◦
C. The temperature is lower than flash points of the materials. The density of
the material is 1.1 g/cm3 .
58
The flexibility of the mold selection extends the range of shapes and sizes
59
that can be fabricated, and the scintillator manufacturing apparatus can be
60
simplified. Furthermore, the low-temperature process reduces the cooling time
61
and the amount of electricity needed for heating. Since we do not need to
62
control the temperature precisely, staff cost in production is reduced. Roughly
63
estimating the price of new material scintillator is dominated by material cost
64
and is reduced to one tenth of the conventional one. In addition, the lower
65
temperature process allows additional functional materials that are heat labile or
66
chemically unstable to be incorporated into the scintillator. We have developed a
67
Gd-doped plastic scintillator that is expected to enhance the neutron sensitivity
68
[5, 6] and was not commercially available.
3
Fig. 1. Newly developed plastic scintillators, 20 × 200 × 3 mm3 in size, illuminated by ultraviolet light (254 nm) from above: undoped (a) JCT05-1 and (b) JCT05-2 and Gd-dopedtype (c) JCT05-G1 and (d) JCT05-G2.
4
69
3. Experimental setup
70
The experimental setup for the scintillation light measurements is shown in
71
Fig. 2. We prepared six 20 × 200 × 3 mm3 scintillator samples: two undoped
72
samples (JCT05-1/2), two 0.1 wt%-Gd-doped samples (JCT05-G1/2), and two
73
reference standard scintillators (BC-408-1/2: produced by Saint-Gobain Co.).
74
The samples with new material were cut from the same lot. The samples of
75
the reference scintillator (BC-408) are cut of the large size one purchased from
76
manufacturer (Saint-Gobain Co.). The thickness of the samples as measured
77
by a micrometer to within an accuracy of 2 µm differ by a maximum of 1.0
78
mm (see Table 1). Each scintillator was polished with buffing machine and
79
optically connected to the photomultiplier tube (PMT) with optical grease (BC-
80
630: Saint-Gobain Co.) and wrapped with an aluminized polyester film and
81
black tape before being covered with a black sheet [20]. We know the size of the
82
sample is not enough to detect gammas from neutron captured by Gd. Use of Gd
83
itself is well established technique to enhance neutron detection. Deterioration
84
of the scintillation performance due to Gd dope, however, is crucial issue. The
85
main task of the study is to confirm the basic performance, light yield and
86
attenuation length, of new material scintillator.
87
Collimated β-rays from a
90
Sr β-ray source (Emax =2.28 MeV 1.95 MBq)
88
were used to assess the light yield and transmission. The collimator was a
89
10 mm-thick aluminum plate with a 2 mm-diameter hole in the center. To
90
select only penetrating β-rays, we placed a 40 × 40 × 2 mm3 trigger scintillator
91
underneath the sample scintillator.
92
The scintillation light is emitted from the energy deposited in the plastic
93
scintillator by the β-rays, and this light is detected by a H7195 Hamamatsu
94
PMT that has a gain of 3 × 106 and is supplied with −2100 V.
95
The PMT output is converted into a digital signal by a Hoshin 12 bits C009
96
charge-integrated analog-to-digital converter (ADC) that has a sensitivity of
97
0.24 pC/count with a 200 ns gate provided by a trigger counter.
98
To measure the position (x) dependence of the light yield in a range of
5
99
6−18 cm from the PMT surface, the β-ray source and trigger counter were
100
moved in intervals of 1 cm along the longitudinal direction of the scintillator.
101
The measurement position was determined by the supporter that was affixed
102
with both the sample scintillator and the β-ray collimator. This supporter has
103
grooves at 1 cm intervals (accurate to 50 µm) along the longitudinal direction.
104
4. Results
105
The pulse shapes of the new plastic scintillators were similar to those of the
106
reference scintillators BC-408-1/2 (Fig. 3), and the pulse widths (10−20 ns)
107
were also the same as those of the reference scintillators.
108
4.1. Light yield
109 110
The light yield is evaluated as the signal pulse height (PH), which is extracted from the mean of the ADC distribution by subtracting the pedestal mean.
111
Figure 4 shows typical PH distributions from each scintillator sample irra-
112
diated by β-rays from a source 6 cm from the PMT surface. We see that all the
113
scintillators have a similar PH distribution, but those of the JCT05 and JCT05-
114
G are more Gaussian-like, while those of the BC-408-1/2 are more Landau-like.
115
The Gaussian-like distribution of the new scintillators is caused by random pro-
116
cesses such as PMT noise due to the low light yield.
117
The mean, root mean square (RMS), and resolution of each PH distribution
118
are summarized in Table 2. The resolution is defined as the histogram RMS
119
divided by the mean. The maximum PHs of the JCT05 scintillators are lower
120
than those of the reference BC-408 scintillators, but to compare the light yields,
121
we normalized the PHs by the thicknesses in Table 1. We found that the PH of
122
the JCT05 scintillator was 11% of that of the BC-408 scintillator at a distance of
123
18 cm from the PMT surface. It corresponds to the light yield of 7% Anthracene.
124
However, this value depends on the distance between the luminous point and
125
the PMT surface because of light attenuation.
126
The systematic error in the PH mean that is taken into account is dominated
127
by the instability of the coupling between the sample scintillator and PMT. We 6
Black box
Collimator (2 mm diameter) Sample scintillator 90 Sr Source PMT
Supporter Supporter PMT
Trigger scintillator
Supporter
Fig. 2. Schematic view and photograph of the scintillator measurement setup. The trigger scintillator is placed under the sample scintillator, and the 90 Sr β-ray source and its collimator (2 mm diameter) are placed on the sample scintillator.
7
Fig. 3. Raw-pulse distributions of the (a) JCT05-1, (b) JCT05-2, (c) JCT05-G1, (d) JCT05G2, (e) BC-408-1, and (f) BC-408-2 scintillators for the 90 Sr β-ray source located at a distance of 12 cm from the PMT surface. Pulse shapes were measured with a digital oscilloscope. Ten pulse shapes are overlaid in each figure.
8
Table 1. Thicknesses of the scintillator samples. The average thickness Av and difference between the maximum and minimum thickness ∆T are also listed for each sample.
Position x
JCT05-1
JCT05-2
JCT05-G1
JCT05-G2
BC-408-1
BC-408-2
(cm)
(mm)
(mm)
(mm)
(mm)
(mm)
(mm)
0
3.938
3.856
3.958
3.887
3.253
3.242
1
3.911
3.811
4.022
3.921
3.244
3.236
2
3.933
3.819
4.142
4.016
3.243
3.266
3
3.947
3.840
4.229
4.105
3.245
3.283
4
3.964
3.857
4.321
4.199
3.246
3.295
5
3.970
3.856
4.366
4.288
3.268
3.309
6
3.957
3.827
4.397
4.332
3.275
3.310
7
3.933
3.797
4.413
4.397
3.279
3.314
8
3.901
3.755
4.401
4.449
3.281
3.316
9
3.841
3.707
4.370
4.500
3.283
3.320
10
3.782
3.652
4.329
4.542
3.278
3.318
11
3.723
3.583
4.272
4.572
3.274
3.314
12
3.644
3.519
4.211
4.592
3.267
3.310
13
3.556
3.443
4.141
4.595
3.264
3.301
14
3.458
3.361
4.062
4.580
3.253
3.292
15
3.364
3.276
3.969
4.553
3.246
3.280
16
3.257
3.179
3.872
4.514
3.238
3.274
17
3.153
3.086
3.761
4.472
3.232
3.273
18
3.067
3.025
3.664
4.380
3.232
3.274
19
2.976
2.905
3.580
4.313
3.222
3.281
20
2.958
2.885
3.595
4.270
3.217
3.283
Av
3.630
3.526
4.099
4.356
3.254
3.200
∆T
1.012
0.972
0.833
0.708
0.066
0.083
9
128
evaluated the instability of the coupling by performing five PH measurements
129
after reconnection. The systematic error is estimated as the standard deviation
130
of the results of the five measurements. 600
600
(b)
400
Entries
Entries
(a)
200 0 200
400 200 0 200
400 600 Pulse height (ch)
600
600
(d)
400
Entries
Entries
(c)
200 0
400 200 0
200 400 Pulse height (ch)
2000
(f) Entries
Entries
200 400 Pulse height (ch)
2000
(e) 1000
0
400 600 Pulse height (ch)
2000
1000
0
3000 Pulse height (ch)
2000
3000 Pulse height (ch)
Fig. 4. Typical PH distributions of (a) JCT05-1, (b) JCT05-2, (c) JCT05-G1, (d) JCT05G2, (e) BC-408-1, and (f) BC-408-2 irradiated by β-rays from a source 6 cm from the PMT surface.
131
4.2. Light attenuation length
132
The attenuation length of the light propagating through the plastic scin-
133
tillator represents the transparency of the material and is evaluated from the
134
position dependence of the light output along the longitudinal direction of the
135
scintillator [21, 22]. To take into account the variation in the thickness, the
136
mean value of the light output is divided by the thickness of the measured posi-
137
tion to obtain normalized values per millimeter. Figure 5 shows the normalized
138
mean PHs as a function of the 90 Sr β-ray source position. The light attenuation
10
Table 2. The mean, root mean square (RMS), and resolution of the PH distribution of each scintillator.
JCT05-1
JCT05-2
JCT05-G1
JCT05-G2
BC-408-1
BC-408-2
Position x
Mean
RMS
Resolution
(cm)
(ch)
(ch)
(%)
6
435 ± 11
70
14
12
283 ± 11
55
19
18
220 ± 11
47
21
6
470 ± 10
76
14
12
296 ± 10
56
19
18
222 ± 10
48
21
6
234 ± 9
45
19
12
154 ± 9
35
23
18
116 ± 9
30
26
6
278 ± 12
50
18
12
173 ± 12
36
21
18
134 ± 12
32
24
6
2441 ± 78
285
12
12
2225 ± 78
265
12
18
2154 ± 78
259
12
6
2345 ± 35
279
12
12
2209 ± 35
265
12
18
2164 ± 35
263
12
11
139
length is obtained by fitting the data with B f = A + 2 · e−x/L , x
(1)
140
where A and B are amplitudes, x is the horizontal distance between the source
141
center and the PMT surface, and L is the light attenuation length[23]. The first
142
term denotes light reflected in the scintillator, while the second term denotes
143
light directly transmitted from the luminous point taking into account the solid-
144
angle effect.
145
The attenuation lengths are summarized in Table 3. Because of the limited
146
length (200 mm) of the scintillators, the errors tend to be relatively larger for
147
longer attenuation lengths. The new scintillator can be used for a relatively
148
small detector of 1 m or less, even though the attenuation length is shorter than
149
that of the BC-408. The shorter attenuation length of the JCT05-G scintilla-
150
tor is assumed to be caused by impurity contamination introduced during Gd
151
doping. The attenuation length measured with BC-408 scintillator is several
152
meters and is near to the catalog value (210 cm) [24]. The difference is due to
153
the different size of the sample measured 3 .
154
We note that the attenuation length of the JCT05 scintillator is much longer
155
than its length. Less light is emitted by the JCT05 scintillator in comparison
156
with the BC-408 scintillator because of the lower luminescence of the parent ma-
157
terial. Thus, to improve the light yield, the parent material should be optimized,
158
and this is a subject for further study.
159
A typical neutrino target of a nuclear reactor monitor placed close (20−30m)
160
to the reactor core is about 1 m3 in size with a weight of 1 ton [14,18]. The
161
length of the scintillator bar used for the target will be about 1 m, and the
162
attenuation length of the new JCT05-G scintillator is approximately 50 cm.
163
Therefore, this new Gd-doped scintillator can be used if the scintillation light
164
is read by the PMT at both ends of the scintillator bar. Further development
165
is planned to improve the attenuation length of the JCT05-G scintillator. 3 Larger
sample (1 × 2 × 200 cm3 ) is used at measurement by Saint-Gobain Co.
12
Normalized mean (ch)
Normalized mean (ch)
150 140 130 120 110 100 90 80 70 60 50 4
(a)
6
8
10 12 14 16 Sr source position (cm)
18
20
150 140 130 120 110 100 90 80 70 60 50 4
(b)
6
70 65 60 55 50 45 40 35 30 25 20 4
(c)
6
8
10 12 14 16 Sr source position (cm)
18
20
70 65 60 55 50 45 40 35 30 25 20 4
8
Normalized mean (ch)
Normalized mean (ch)
10 12 14 16 Sr source position (cm)
18
20
6
8
10 12 14 16 Sr source position (cm)
18
20
18
20
90
(e)
6
10 12 14 16 Sr source position (cm)
(d)
90
800 780 760 740 720 700 680 660 640 620 600 4
8
90
Normalized mean (ch)
Normalized mean (ch)
90
18
20
800 780 760 740 720 700 680 660 640 620 600 4
(f)
6
90
8
10 12 14 16 Sr source position (cm)
90
Fig. 5. Normalized mean PH of (a) JCT05-1, (b) JCT05-2, (c) JCT05G-1, (d) JCT05G-2, (e) BC-408-1, and (f) BC-408-2 as a function of the source position (x). The mean values are normalized by the thickness at the respective position. The errors are smaller than the size of the marker in all figures.
Table 3. Attenuation length of each scintillator.
Attenuation length (cm) JCT05-1
144 ± 17
JCT05-2
78 ± 7
JCT05-G1
51 ± 4
JCT05-G2
49 ± 5
BC-408-1
615 ± 405
BC-408-2
856 ± 410
13
166
5. Summary
167
We have presented a new plastic scintillator produced with simple low-
168
temperature process using a liquid-based parent material with a hardener. The
169
scintillator is cost effective and can be doped with Gd.
170
We confirmed that the new plastic scintillators JCT05 and JCT05-G emitted 90
Sr β-rays and that they function well as plastic scin-
171
light when irradiated by
172
tillators. The light attenuation lengths of the JCT05-G and JCT05 scintillators
173
were ∼50 cm and 1 m, respectively. We assume the shorter attenuation length
174
of the JCT05-G scintillator is caused by impurity contamination introduced
175
during Gd doping.
176
Further improvement is expected to result in improved light yields, and the
177
attenuation length can be increased by optimizing the pure parent material,
178
wavelength shifters, and dopant, which will allow the development of larger
179
radiation detectors. Even at the current performance level, the new plastic
180
scintillator has capability to be molded into various shapes, doped with a range
181
of functional materials, and formed into a relatively small (1 m) detector.
182
6. Acknowledgment
183
We are grateful for the assistance provided by Mr. Hiroki Morii, a technical
184
staff member in the Faculty of Science, Niigata University. This work was
185
supported by JSPS KAKENHI Grant Nos. 19340057 and 23340063. This study
186
was also supported in 2007 by the Sasaki Environment Technology Foundation.
14
187
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[16] C. Aberle, C. Buck, F.X. Hartmann, S. Sch¨onert, Chem. Phys. Lett. 516 (2011) 257. [17] M. Katsumata, H. Miyata, N. Tamura, T. Kawasaki, Nucl. Instr. Meth. A 629 (2011) 50.
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[18] N.S. Bowden, A. Bernstein, M. Allen, J.S. Brennan, M. Cunningham, J.K.
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Estrada, C.M.R. Greaves, C. Hagmann, J. Lund, W. Mengesha, T.D. Wein-
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beck, C.D. Winant, Nucl. Instr. and Meth. A. 572 (2007) 985.
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[19] M. Bowen, S. Majewski, D. Pettey, J. Walker, R. Wojcik, C. Zorn, Nucl. Instr. Meth. A 276 (1989) 391.
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[20] E.P. Jacosalem, S. Iba, N. Nakajima, H. Ono, A.L.C. Sanchez, A.M. Bacala,
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H. Miyata, GLD Calorimeter Group, Pramana 69, No. 6 (2007) 1051.
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Deppman, E. De Sanctis, V. Giourdjian, P. Levi Sandri, S. Mansanta, M.
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Mirazita, V. Muccifora, A.R. Reolon, M. Taiuti, A. Rottura, M. Anghinolfi, 16
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M. Battaglieri, P. Corvisiero, E. Golovach, A. Longhi, V.I. Mokeev, M.
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[22] M. Gierlik, T. Batsch, R. Marcinkowski, M. Moszynski, T. Sworobowicz, Nucl. Instr. Meth. A 593 (2008) 426. [23] W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer-Verlag. [24] Saint-Gobain Co.,
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(Last accessed: December 5, 2013)
17
First Performance Test of Newly Developed Plastic Scintillator for Radiation Detection M. Watanabe, M. Katsumata, H. Ono, T. Suzuki, H. Miyata, Y. Itoh, K. Ishida, M. Tamura, Y. Yamaguchi
www.elsevier.com/locate/nima
PII: DOI: Reference:
S0168-9002(14)01122-X http://dx.doi.org/10.1016/j.nima.2014.10.005 NIMA57083
To appear in:
Nuclear Instruments and Methods in Physics Research A
Received date: 16 January 2014 Revised date: 16 September 2014 Accepted date: 3 October 2014 Cite this article as: M. Watanabe, M. Katsumata, H. Ono, T. Suzuki, H. Miyata, Y. Itoh, K. Ishida, M. Tamura, Y. Yamaguchi, First Performance Test of Newly Developed Plastic Scintillator for Radiation Detection, Nuclear Instruments and Methods in Physics Research A, http://dx.doi.org/10.1016/j.nima.2014.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2
3 4
First Performance Test of Newly Developed Plastic Scintillator for Radiation Detection✩ M. Watanabea,∗∗, M. Katsumatab, H. Onoc , T. Suzukia , H. Miyatab , Y. Itoha , K. Ishidaa , M. Tamurad, Y. Yamaguchid
5
a Graduate
6
b Faculty
7 8
9
School of Science and Technology, Niigata University, Niigata 950-2181, Japan of Science, Physics Department, Niigata University, Niigata 950-2181, Japan c Nippon Dental University, School of Life Dentistry at Niigata, Niigata 951-8580, Japan d Carlit Holdings Co., Ltd., Chiyoda, Tokyo 101-0024, Japan
Abstract We present a plastic scintillator, developed in collaboration with Carlit Holdings Co., Ltd., that is fabricated using a liquid parent material cured at room temperature by adding a hardener. The new scintillator can incorporate heatlabile functional materials such as gadolinium to enhance neutron sensitivity. The characteristics of the new scintillator, in particular the light yield and attenuation length, were evaluated using a
90
Sr β-ray source. The light yield was
measured 7% Anthracene on the basis of a comparison with commerically available scintillator (BC-408) at a distance of 18 cm from the photodetector surface. This light yield is dependent on the distance between the luminous point and the photodetector because of light attenuation. The attenuation length of the Gd-doped scintillator was about 50 cm. 10
Keywords: plastic scintillator, neutron detectors, gadolinium, reactor,
11
safeguards, neutrino
12
1. Introduction
13
Scintillation counters are widely used as radiation detectors in nuclear and
14
high-energy physics and have recently found new applications especially in med-
15
ical fields [1-3].
16
Plastic scintillators are known to be easy to handle and chemically stable
17
owing to the solid state of the material even though relatively expensive than ✩
Preprint submitted to Elsevier ∗ ∗∗ Tel.:
+81252626138; Fax.: +81252626138 Email address: [email protected] (M. Watanabe) URL: http://www.hep.sc.niigata-u.ac.jp (M. Watanabe)
October 8, 2014
18
liquid scintillator (LS), and plastic scintillator technology is well developed.
19
Several studies have been conducted to include functionalities such as radiation
20
hardness [4] and neutron sensitivity [5-7] as well as reducing manufacturing
21
costs [8, 9]. Nuclear and high-energy physics researchers require reasonably
22
priced large plastic scintillator detectors for calorimeters [10-13] and reactor
23
neutrino detectors [14].
24
In response to this, we have been working for the reactor neutrino detection
25
application toward the development of the lower cost plastic scintillator 1 . The
26
anti-neutrinos are detected via the inverse beta decay process, ν¯e + p → e+ +
27
n. Here the hydrogen in scintillator works as target proton. Some projects
28
employ Gd-doped liquid scintillator to achieve a large detection volume with
29
low cost and to enhance neutron detection [15-17] 2 . The flammability of the
30
LS, however, is problematic when the measurement were done close to reactor.
31
To rectify the problem, we developed a Gd-doped plastic scintillator, that is
32
relatively flame resistance than LS. The reactor monitor made of Gd doped
33
plastic scintillator is supposed to be about one cubic meter (about one ton) in
34
size [18].
35
In this study, we developed two types of plastic scintillators, undoped and Gd
36
doped, based on new cost-effective materials. The light yield and attenuation
37
length of these scintillators were measured and compared with the commercial
38
standard plastic scintillator (BC-408) made by Saint-Gobain Co., and the results
39
are presented here. 1 For
example, the International Atomic Energy Agency (IAEA) suggests a safeguard nu-
clear regime to prevent fissile material from civil nuclear fuel being converted into nuclear weapons. Anomalous reactor activity is monitored through the large number of reactor antineutrinos (ν¯e ) originating from a series of fission processes in the nuclear reactor. 2 Gd has large cross section for thermal neutron and generates higher energy gammas, whose total energy is 8 MeV.
2
40
2. New manufacturing method
41
Commercially available plastic scintillators are composed of a parent mate-
42
rial such as polystyrene or polyvinyl toluene and wavelength shifters. Primary
43
and secondary wavelength shifters are typically 2,5-diphenyloxazole (PPO) and
44
1,4-bis(5-phenyl-2-oxazolyl)benzene (POPOP), respectively. To incorporate the
45
wavelength shifters, the parent material is heated to a temperature above its
46
melting point (∼100 ◦ C for polystyrene or polyvinyl toluene [19]), and the wave-
47
length shifters are then dispersed in the parent material. To prevent cracking
48
upon cooling, a typical polystyrene plastic scintillator is cooled slowly after this
49
process.
50
The new plastic scintillator (Fig. 1) has been developed in collaboration
51
with Carlit Holdings Co., Ltd., with a focus on reducing production costs. It is
52
based on a parent material (bisphenol A type epoxy resin) that can be cured at
53
room temperature by the adding of a hardener (amine-based curing agent). In
54
order to obtain better mixing of the wavelength shifters (PPO: 1 wt%, POPOP:
55
0.03 wt%), the parent material and hardener, we heat the parent material to 60
56 57
◦
C. The temperature is lower than flash points of the materials. The density of
the material is 1.1 g/cm3 .
58
The flexibility of the mold selection extends the range of shapes and sizes
59
that can be fabricated, and the scintillator manufacturing apparatus can be
60
simplified. Furthermore, the low-temperature process reduces the cooling time
61
and the amount of electricity needed for heating. Since we do not need to
62
control the temperature precisely, staff cost in production is reduced. Roughly
63
estimating the price of new material scintillator is dominated by material cost
64
and is reduced to one tenth of the conventional one. In addition, the lower
65
temperature process allows additional functional materials that are heat labile or
66
chemically unstable to be incorporated into the scintillator. We have developed a
67
Gd-doped plastic scintillator that is expected to enhance the neutron sensitivity
68
[5, 6] and was not commercially available.
3
Fig. 1. Newly developed plastic scintillators, 20 × 200 × 3 mm3 in size, illuminated by ultraviolet light (254 nm) from above: undoped (a) JCT05-1 and (b) JCT05-2 and Gd-dopedtype (c) JCT05-G1 and (d) JCT05-G2.
4
69
3. Experimental setup
70
The experimental setup for the scintillation light measurements is shown in
71
Fig. 2. We prepared six 20 × 200 × 3 mm3 scintillator samples: two undoped
72
samples (JCT05-1/2), two 0.1 wt%-Gd-doped samples (JCT05-G1/2), and two
73
reference standard scintillators (BC-408-1/2: produced by Saint-Gobain Co.).
74
The samples with new material were cut from the same lot. The samples of
75
the reference scintillator (BC-408) are cut of the large size one purchased from
76
manufacturer (Saint-Gobain Co.). The thickness of the samples as measured
77
by a micrometer to within an accuracy of 2 µm differ by a maximum of 1.0
78
mm (see Table 1). Each scintillator was polished with buffing machine and
79
optically connected to the photomultiplier tube (PMT) with optical grease (BC-
80
630: Saint-Gobain Co.) and wrapped with an aluminized polyester film and
81
black tape before being covered with a black sheet [20]. We know the size of the
82
sample is not enough to detect gammas from neutron captured by Gd. Use of Gd
83
itself is well established technique to enhance neutron detection. Deterioration
84
of the scintillation performance due to Gd dope, however, is crucial issue. The
85
main task of the study is to confirm the basic performance, light yield and
86
attenuation length, of new material scintillator.
87
Collimated β-rays from a
90
Sr β-ray source (Emax =2.28 MeV 1.95 MBq)
88
were used to assess the light yield and transmission. The collimator was a
89
10 mm-thick aluminum plate with a 2 mm-diameter hole in the center. To
90
select only penetrating β-rays, we placed a 40 × 40 × 2 mm3 trigger scintillator
91
underneath the sample scintillator.
92
The scintillation light is emitted from the energy deposited in the plastic
93
scintillator by the β-rays, and this light is detected by a H7195 Hamamatsu
94
PMT that has a gain of 3 × 106 and is supplied with −2100 V.
95
The PMT output is converted into a digital signal by a Hoshin 12 bits C009
96
charge-integrated analog-to-digital converter (ADC) that has a sensitivity of
97
0.24 pC/count with a 200 ns gate provided by a trigger counter.
98
To measure the position (x) dependence of the light yield in a range of
5
99
6−18 cm from the PMT surface, the β-ray source and trigger counter were
100
moved in intervals of 1 cm along the longitudinal direction of the scintillator.
101
The measurement position was determined by the supporter that was affixed
102
with both the sample scintillator and the β-ray collimator. This supporter has
103
grooves at 1 cm intervals (accurate to 50 µm) along the longitudinal direction.
104
4. Results
105
The pulse shapes of the new plastic scintillators were similar to those of the
106
reference scintillators BC-408-1/2 (Fig. 3), and the pulse widths (10−20 ns)
107
were also the same as those of the reference scintillators.
108
4.1. Light yield
109 110
The light yield is evaluated as the signal pulse height (PH), which is extracted from the mean of the ADC distribution by subtracting the pedestal mean.
111
Figure 4 shows typical PH distributions from each scintillator sample irra-
112
diated by β-rays from a source 6 cm from the PMT surface. We see that all the
113
scintillators have a similar PH distribution, but those of the JCT05 and JCT05-
114
G are more Gaussian-like, while those of the BC-408-1/2 are more Landau-like.
115
The Gaussian-like distribution of the new scintillators is caused by random pro-
116
cesses such as PMT noise due to the low light yield.
117
The mean, root mean square (RMS), and resolution of each PH distribution
118
are summarized in Table 2. The resolution is defined as the histogram RMS
119
divided by the mean. The maximum PHs of the JCT05 scintillators are lower
120
than those of the reference BC-408 scintillators, but to compare the light yields,
121
we normalized the PHs by the thicknesses in Table 1. We found that the PH of
122
the JCT05 scintillator was 11% of that of the BC-408 scintillator at a distance of
123
18 cm from the PMT surface. It corresponds to the light yield of 7% Anthracene.
124
However, this value depends on the distance between the luminous point and
125
the PMT surface because of light attenuation.
126
The systematic error in the PH mean that is taken into account is dominated
127
by the instability of the coupling between the sample scintillator and PMT. We 6
Black box
Collimator (2 mm diameter) Sample scintillator 90 Sr Source PMT
Supporter Supporter PMT
Trigger scintillator
Supporter
Fig. 2. Schematic view and photograph of the scintillator measurement setup. The trigger scintillator is placed under the sample scintillator, and the 90 Sr β-ray source and its collimator (2 mm diameter) are placed on the sample scintillator.
7
Fig. 3. Raw-pulse distributions of the (a) JCT05-1, (b) JCT05-2, (c) JCT05-G1, (d) JCT05G2, (e) BC-408-1, and (f) BC-408-2 scintillators for the 90 Sr β-ray source located at a distance of 12 cm from the PMT surface. Pulse shapes were measured with a digital oscilloscope. Ten pulse shapes are overlaid in each figure.
8
Table 1. Thicknesses of the scintillator samples. The average thickness Av and difference between the maximum and minimum thickness ∆T are also listed for each sample.
Position x
JCT05-1
JCT05-2
JCT05-G1
JCT05-G2
BC-408-1
BC-408-2
(cm)
(mm)
(mm)
(mm)
(mm)
(mm)
(mm)
0
3.938
3.856
3.958
3.887
3.253
3.242
1
3.911
3.811
4.022
3.921
3.244
3.236
2
3.933
3.819
4.142
4.016
3.243
3.266
3
3.947
3.840
4.229
4.105
3.245
3.283
4
3.964
3.857
4.321
4.199
3.246
3.295
5
3.970
3.856
4.366
4.288
3.268
3.309
6
3.957
3.827
4.397
4.332
3.275
3.310
7
3.933
3.797
4.413
4.397
3.279
3.314
8
3.901
3.755
4.401
4.449
3.281
3.316
9
3.841
3.707
4.370
4.500
3.283
3.320
10
3.782
3.652
4.329
4.542
3.278
3.318
11
3.723
3.583
4.272
4.572
3.274
3.314
12
3.644
3.519
4.211
4.592
3.267
3.310
13
3.556
3.443
4.141
4.595
3.264
3.301
14
3.458
3.361
4.062
4.580
3.253
3.292
15
3.364
3.276
3.969
4.553
3.246
3.280
16
3.257
3.179
3.872
4.514
3.238
3.274
17
3.153
3.086
3.761
4.472
3.232
3.273
18
3.067
3.025
3.664
4.380
3.232
3.274
19
2.976
2.905
3.580
4.313
3.222
3.281
20
2.958
2.885
3.595
4.270
3.217
3.283
Av
3.630
3.526
4.099
4.356
3.254
3.200
∆T
1.012
0.972
0.833
0.708
0.066
0.083
9
128
evaluated the instability of the coupling by performing five PH measurements
129
after reconnection. The systematic error is estimated as the standard deviation
130
of the results of the five measurements. 600
600
(b)
400
Entries
Entries
(a)
200 0 200
400 200 0 200
400 600 Pulse height (ch)
600
600
(d)
400
Entries
Entries
(c)
200 0
400 200 0
200 400 Pulse height (ch)
2000
(f) Entries
Entries
200 400 Pulse height (ch)
2000
(e) 1000
0
400 600 Pulse height (ch)
2000
1000
0
3000 Pulse height (ch)
2000
3000 Pulse height (ch)
Fig. 4. Typical PH distributions of (a) JCT05-1, (b) JCT05-2, (c) JCT05-G1, (d) JCT05G2, (e) BC-408-1, and (f) BC-408-2 irradiated by β-rays from a source 6 cm from the PMT surface.
131
4.2. Light attenuation length
132
The attenuation length of the light propagating through the plastic scin-
133
tillator represents the transparency of the material and is evaluated from the
134
position dependence of the light output along the longitudinal direction of the
135
scintillator [21, 22]. To take into account the variation in the thickness, the
136
mean value of the light output is divided by the thickness of the measured posi-
137
tion to obtain normalized values per millimeter. Figure 5 shows the normalized
138
mean PHs as a function of the 90 Sr β-ray source position. The light attenuation
10
Table 2. The mean, root mean square (RMS), and resolution of the PH distribution of each scintillator.
JCT05-1
JCT05-2
JCT05-G1
JCT05-G2
BC-408-1
BC-408-2
Position x
Mean
RMS
Resolution
(cm)
(ch)
(ch)
(%)
6
435 ± 11
70
14
12
283 ± 11
55
19
18
220 ± 11
47
21
6
470 ± 10
76
14
12
296 ± 10
56
19
18
222 ± 10
48
21
6
234 ± 9
45
19
12
154 ± 9
35
23
18
116 ± 9
30
26
6
278 ± 12
50
18
12
173 ± 12
36
21
18
134 ± 12
32
24
6
2441 ± 78
285
12
12
2225 ± 78
265
12
18
2154 ± 78
259
12
6
2345 ± 35
279
12
12
2209 ± 35
265
12
18
2164 ± 35
263
12
11
139
length is obtained by fitting the data with B f = A + 2 · e−x/L , x
(1)
140
where A and B are amplitudes, x is the horizontal distance between the source
141
center and the PMT surface, and L is the light attenuation length[23]. The first
142
term denotes light reflected in the scintillator, while the second term denotes
143
light directly transmitted from the luminous point taking into account the solid-
144
angle effect.
145
The attenuation lengths are summarized in Table 3. Because of the limited
146
length (200 mm) of the scintillators, the errors tend to be relatively larger for
147
longer attenuation lengths. The new scintillator can be used for a relatively
148
small detector of 1 m or less, even though the attenuation length is shorter than
149
that of the BC-408. The shorter attenuation length of the JCT05-G scintilla-
150
tor is assumed to be caused by impurity contamination introduced during Gd
151
doping. The attenuation length measured with BC-408 scintillator is several
152
meters and is near to the catalog value (210 cm) [24]. The difference is due to
153
the different size of the sample measured 3 .
154
We note that the attenuation length of the JCT05 scintillator is much longer
155
than its length. Less light is emitted by the JCT05 scintillator in comparison
156
with the BC-408 scintillator because of the lower luminescence of the parent ma-
157
terial. Thus, to improve the light yield, the parent material should be optimized,
158
and this is a subject for further study.
159
A typical neutrino target of a nuclear reactor monitor placed close (20−30m)
160
to the reactor core is about 1 m3 in size with a weight of 1 ton [14,18]. The
161
length of the scintillator bar used for the target will be about 1 m, and the
162
attenuation length of the new JCT05-G scintillator is approximately 50 cm.
163
Therefore, this new Gd-doped scintillator can be used if the scintillation light
164
is read by the PMT at both ends of the scintillator bar. Further development
165
is planned to improve the attenuation length of the JCT05-G scintillator. 3 Larger
sample (1 × 2 × 200 cm3 ) is used at measurement by Saint-Gobain Co.
12
Normalized mean (ch)
Normalized mean (ch)
150 140 130 120 110 100 90 80 70 60 50 4
(a)
6
8
10 12 14 16 Sr source position (cm)
18
20
150 140 130 120 110 100 90 80 70 60 50 4
(b)
6
70 65 60 55 50 45 40 35 30 25 20 4
(c)
6
8
10 12 14 16 Sr source position (cm)
18
20
70 65 60 55 50 45 40 35 30 25 20 4
8
Normalized mean (ch)
Normalized mean (ch)
10 12 14 16 Sr source position (cm)
18
20
6
8
10 12 14 16 Sr source position (cm)
18
20
18
20
90
(e)
6
10 12 14 16 Sr source position (cm)
(d)
90
800 780 760 740 720 700 680 660 640 620 600 4
8
90
Normalized mean (ch)
Normalized mean (ch)
90
18
20
800 780 760 740 720 700 680 660 640 620 600 4
(f)
6
90
8
10 12 14 16 Sr source position (cm)
90
Fig. 5. Normalized mean PH of (a) JCT05-1, (b) JCT05-2, (c) JCT05G-1, (d) JCT05G-2, (e) BC-408-1, and (f) BC-408-2 as a function of the source position (x). The mean values are normalized by the thickness at the respective position. The errors are smaller than the size of the marker in all figures.
Table 3. Attenuation length of each scintillator.
Attenuation length (cm) JCT05-1
144 ± 17
JCT05-2
78 ± 7
JCT05-G1
51 ± 4
JCT05-G2
49 ± 5
BC-408-1
615 ± 405
BC-408-2
856 ± 410
13
166
5. Summary
167
We have presented a new plastic scintillator produced with simple low-
168
temperature process using a liquid-based parent material with a hardener. The
169
scintillator is cost effective and can be doped with Gd.
170
We confirmed that the new plastic scintillators JCT05 and JCT05-G emitted 90
Sr β-rays and that they function well as plastic scin-
171
light when irradiated by
172
tillators. The light attenuation lengths of the JCT05-G and JCT05 scintillators
173
were ∼50 cm and 1 m, respectively. We assume the shorter attenuation length
174
of the JCT05-G scintillator is caused by impurity contamination introduced
175
during Gd doping.
176
Further improvement is expected to result in improved light yields, and the
177
attenuation length can be increased by optimizing the pure parent material,
178
wavelength shifters, and dopant, which will allow the development of larger
179
radiation detectors. Even at the current performance level, the new plastic
180
scintillator has capability to be molded into various shapes, doped with a range
181
of functional materials, and formed into a relatively small (1 m) detector.
182
6. Acknowledgment
183
We are grateful for the assistance provided by Mr. Hiroki Morii, a technical
184
staff member in the Faculty of Science, Niigata University. This work was
185
supported by JSPS KAKENHI Grant Nos. 19340057 and 23340063. This study
186
was also supported in 2007 by the Sasaki Environment Technology Foundation.
14
187
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