Measurement of spatial distribution of neutrons and gamma rays for BNCT using multi-imaging plate system

Measurement of spatial distribution of neutrons and gamma rays for BNCT using multi-imaging plate system

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Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Contents lists available at ScienceDirect

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

Measurement of spatial distribution of neutrons and gamma rays for BNCT using multi-imaging plate system Kenichi Tanaka a,n, Yoshinori Sakurai b, Hiroki Tanaka b, Tsuyoshi Kajimoto a, Takushi Takata b, Jun Takada c, Satoru Endo a a

Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, Japan Research Reactor Institute, Kyoto University, Kumatori, Japan c Center of Medical Education, Sapporo Medical University, Sapporo, Japan b

ar t ic l e i nf o

a b s t r a c t

Article history: Received 28 January 2015 Received in revised form 3 June 2015 Accepted 26 July 2015

Quality assurance of the spatial distributions of neutrons and gamma rays was tried using imaging plates (IPs) and converters to enhance the beam components in the epithermal neutron mode of the Kyoto University Reactor. The converters used were 4 mm thick epoxy resin with B4C at 6.85 weight-percent (wt%) 10B for epithermal neutrons, and 3 mm thick carbon for gamma rays. Results suggested that the IP signal does not need a sensitivity correction regardless of the incident radiation that produces it. & 2015 Elsevier Ltd. All rights reserved.

Keywords: BNCT Dosimetry Imaging plate Beam component

1. Introduction

2. Materials and methods

Measurement of the spatial distributions of neutrons and gamma rays is important for the quality assurance of boron neutron capture therapy (BNCT). It is desirable to measure the beam components such as the thermal, epithermal, fast neutrons and gamma rays, separately. This study designed and tested the multiimaging plate (IP) system for this purpose. The multi-IP system consists of converters which are intended to enhance the beam components, i.e., to increase the contribution of the components to the IP signal, and IPs (Amemiya and Miyahara, 1988). Previous works have demonstrated that IP doped with Gd enabled the measurement of the thermal neutron distribution via the 157 Gd(n,γ)158Gd reaction (Takahashi et al., 1996; Tanaka et al., 2010). The feasibility of enhancing thermal neutrons by the 10 B(n,α)7Li reaction was also suggested (Thoms, 1999; Tanaka et al., 2011). Furthermore, we have investigated the configuration of converters which can enhance and separate the neutron and gamma ray components using Monte Carlo simulation (Tanaka et al., 2014). This paper describes the experimental demonstration of the measurement using the multi-IP system. Also, the characteristics of the analysis for the multi-IP system was be investigated.

The principle of the multi-IP system is as follows: (1) thermal and epithermal neutrons will be enhanced by the secondary particles from the 10B(n,α)7Li reaction in the epoxy resin doped with boron, (2) fast neutrons enhanced by the recoiled protons from the epoxy resin, and (3) gamma rays enhanced by the carbon. Carbon is not expected to enhance neutrons due to its low neutron interaction cross section, but will consequently enhance the gamma rays via the secondary electrons produced. Then, enhanced components will be detected with the IPs. We previously proposed (Tanaka et al., 2014) potential combinations of the converters for 7Li(p,n) neutrons with the proton incident energy at 2.5 MeV from a simulation study using the PHITS code (Iwase et al., 2002). The proposed converters were about 1 mm and 5 mm thick epoxy resin doped with 10B at about 10 wt% for thermal and epithermal neutrons, and about 1–20 mm thick carbon for gamma rays. They were applicable to a setup where each IP was irradiated separately. Based on further calculations for conditions where all the IPs are irradiated simultaneously, the converter configuration shown in Fig. 1 was chosen for the standard epithermal neutron irradiation mode at the Kyoto University Reactor Heavy Water Neutron Irradiation Facility (KURHWNIF) which is currently used for BNCT treatments (Sakurai and Kobayashi, 2000). The converter configuration chosen for this study is shown in

n

Corresponding author. Fax: þ 81 82 424 7619. E-mail address: [email protected] (K. Tanaka).

http://dx.doi.org/10.1016/j.apradiso.2015.07.048 0969-8043/& 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: Tanaka, K., et al., Measurement of spatial distribution of neutrons and gamma rays for BNCT using multiimaging plate system. Appl. Radiat. Isotopes (2015), http://dx.doi.org/10.1016/j.apradiso.2015.07.048i

K. Tanaka et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎

2

(mm)

3

5

3

1

3

2

5

Carbon

IP

100

90

89

Incident Beam

Epoxy with 6.85wt% 10B

Fig. 1. Cross sectional view of the IP and converter. The figure is not in scale in order to show the geometry clearly.

Fig. 1 with an epoxy resin was that was infused with B4C, where 10B was at natural abundance. The IP at 1 mm into the epoxy was for thermal neutrons, and the one at 4 mm into the epoxy was for epithermal neutrons. The converters and IPs were covered with 5 mm carbon to shield them from visible light. Enhancement of the fast neutrons using the recoiled protons in epoxy was not investigated since our previous study suggested that IP was very sensitive to the gamma rays (Tanaka et al., 2014). The IP used was “BAS-TR” from Fuji Film Corporation, Japan. It had a dimension of 89  89 mm2. The sensitive region of the BASTR is not covered with the packing material, therefore, the secondary charged particles from the converter such as electron, proton, alpha particle, 7Li ion reach the sensitive region of the IP. The experiment was performed with the standard epithermal neutron irradiation mode of KUR-HWNIF at 1 MW. The beam size was set to about 100  100 mm2 using the collimator. The irradiation was delivered for 20 min. The nominal value of the flux at the center of the collimator aperture was 7.07  106 cm  2 s  1, 1.33  108 cm  2 s  1 and 1.38  107 cm  2 s  1 for thermal, epithermal and fast neutrons, respectively. The gamma ray flux was 1.25  107 cm  2 s  1. After irradiation, the IPs were kept in a lightshielding cassette. About 41 h after the irradiation, the IP signal (PSLi) was measured. Here, i identifies the IP wherein; 1 – denotes the IP in carbon, 2 – the IP at 1 mm into the epoxy, 3 – the IP at 4 mm into the epoxy. The fluence ϕj of each component was determined using the following model;

Table 1 Calculated sensitivity, aij (MeV/incident particle).

⎛ ϕ1⎞ ⎛ PSL1 ⎞ ⎛ a a 11 12 a13 ⎞ ⎜ ⎟ ⎜ ⎟ PSL = ⎜ PSL2 ⎟ = ⎜ a21 a22 a22 ⎟ ⎜ ϕ2 ⎟ = A⋅ϕ ⎜ ⎟ ⎜⎝ a a a ⎟⎠ ⎜ ⎟ 31 32 33 ⎝ ϕ ⎠ ⎝ PSL3⎠ 3

(1)

The values with the under-bar show the contribution of the component aimed to detect. The values are for the nominal beam component intensity at the center of the collimator aperture at 7.07  106 cm  2 s  1, 1.33  108 cm  2 s  1 and 1.38  107 cm  2 s  1 for thermal, epithermal and fast neutrons, and 1.25  107 cm  2 s  1 for gamma rays.

ϕ = A−1⋅PSL

(2)

where aij denotes the sensitivity of the ith IP for the component j. For the values of j, 1 – denotes the gamma-ray component, 2 and 3 – denote the thermal and epithermal neutron components, respectively. The sensitivities were determined from the calculations using the PHITS code. The energy deposition (MeV/incident particle) at the IP sensitive region was computed as a representative of the sensitivity. Since the purpose of this study was to estimate the relative distribution of the component fluences, the result of Eq. (2) were expressed in arbitrary units.

3. Results and discussion The sensitivities of the IPs in Fig. 1 for the beam components were calculated with the PHITS code and are listed in Table 1. The contributions of the beam components to the energy depositions in the IPs for the converter configuration used are shown in

Converter configuration

Carbon (i¼ 1) 1 mm into epoxy with 6.85 wt% 10 B (i¼ 2) 4 mm into epoxy with 6.85 wt% 10 B (i¼ 3)

Beam component Gamma-ray (j¼ 1)

Thermal neutron (j¼2)

Epithermal neutron (j¼3)

Fast neutron

2.97  10  4 2.32  10  4

4.23  10  5 1.32  10  4

1.70  10  5 5.83  10  5

7.20  10  6 1.36  10  5

1.85  10  4

4.56  10  5

3.78  10  5

1.31  10  5

Gamma-ray Thermal neutron

Epithermal neutron

Fast neutron

54 21

5 8

39 69

2 2

26

4

68

2

Table 2 Calculated energy deposition contribution (%) in IP. Converter configuration

Carbon 1 mm into epoxy with 6.85 wt% 10B 4 mm into epoxy with 6.85 wt% 10B

Component

Table 2. Since the epithermal neutron irradiation mode was assumed, the calculated contribution of thermal neutrons to the IP at 1 mm into the epoxy was only about 8%. The fluence estimation method was tested with the Eq. (2), using the sensitivities in Table 1. The fluence estimated using the results for the three IPs are shown in Fig. 2. Since there was a trouble in the IP reader, the data was available only at the lower half for the collimator aperture with an area of about 45  89 mm2. The thermal neutron fluence computed from Eq. (2) yielded negative values because, as stated above, the thermal neutron component in the beam used is low. Hence, only gamma rays and epithermal neutron fluences were computed without using the IP signals for thermal neutron converter. In this case, the contribution of the thermal neutron component is not included or corrected. However, it is only several percents as shown in Table 2, for the beam component flux at nominal values. The results for two component estimation are shown in Fig. 3. In this case, both the gamma rays and epithermal neutrons are

Please cite this article as: Tanaka, K., et al., Measurement of spatial distribution of neutrons and gamma rays for BNCT using multiimaging plate system. Appl. Radiat. Isotopes (2015), http://dx.doi.org/10.1016/j.apradiso.2015.07.048i

K. Tanaka et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎

0

2

Vertical Horizontal

1 0 -5 -4 -3 -2 -1

0

1

2

3

4

5

Distance from collimator center (cm)

Epithermal neutron fluence (Arb.Unit)

Vertical Horizontal

3 Thermal neutron fluence (Arb.Unit)

Gamma-ray fluence (Arb.Unit)

4

3

-0.5

-1

-5 -4 -3 -2 -1 0

1

2

3

Distance from collimator center (cm)

4

5

10

5

0

Vertical Horizontal -5 -4 -3 -2 -1

0

1

2

3

4

5

Distance from collimator center (cm)

Fig. 2. Fluence distribution estimated with three IPs.(a) Gamma rays, (b) thermal neutrons, (c) epithermal neutrons The positive value of the distance specifies the upper side for the vertical direction, and right hand side viewed from the downstream of the beam for the horizontal direction.

positive. Each distribution has its maximum close to the center of the collimator aperture, and it decreased smoothly as the distance from the center increased. The estimated components were judged to have plausible distributions. Using the obtained data, the characteristics of the analysis in this study was investigated in the following subsections. 3.1. Dependence on radiation type of enhancement of beam components The IP signal intensity is expected to depend on the incident particle to the IP (Thoms, 1996), and consequently on the enhanced component. For example, in relation to the attenuation of the readout light emitted from the IP in its sensitive region (Bonnet et al., 2013), the efficiency of converting the energy deposition into the IP signal will be higher for the alpha particles than that for the gamma rays by possibly a few tens of percents (Tanaka et al., 2014). The time dependence of the fading of the IP signal has been reported to be independent of the radiation type (Thoms, 1996). However, the results of investigation by Nakamura et al. (1996) showed that the IP signal produced by alpha particles reduces more rapidly than those by X-rays. At 30 min. or more after the irradiation, the difference was reported to be about 30–40%. These factors, i.e., the attenuation of the readout light and the time dependence of the fading of the IP signal, have opposite influences, i.e., the former increases the sensitivity to the alpha particles, and the latter decreases. In total, the type of radiation could double the IP signal. The impact of these factors is discussed here, from the viewpoint of confirming the temporal change of the beam component distribution. In including the variation of the IP sensitivity for the beam component (j), the coefficient aij should be adjusted. For example,

if the sensitivity for the secondary particles of 10B(n,α)7Li reaction that enhances the epithermal neutron component is β times, ai3 for all three IPs should be replaced with βai3. This is because the multi-IP system in this study uses the same IP for all the components evaluated. Then, the solution of the Eq. (2) will be ϕepi′¼ ϕepi /β. This means that including the dependence of the efficiency in converting the energy deposition to the IP signal on the type of the radiation does not change the relative distribution of the resultant fluences, which is what this study aimed to confirm. The analysis without considering the dependence of IP signal on radiation type will be enough to assure the temporal change in fluence distribution in the irradiation field. However, it is important that the beam component of interest has a significant contribution to the IP signal. Otherwise, the spatial fluctuation in other components will distort the estimated distribution of the said component. 3.2. Sensitivity of the IPs to three beam components The present study utilizes the same IP model for all three beam components. In this analysis, the variation in the type of the radiation which deposits the energy to the IP is assumed to have the same influence on three IPs as mentioned earlier. However, the difference in the experimental procedure applied to each IP will affect the individual IPs differently. This will decrease the accuracy in the estimation of the component distribution. As a test, factors were used to scale the sensitivity for each IP. In Fig. 4, the sensitivity for the IP at 1 mm into the epoxy, a2j was multiplied by a constant regardless of the component j, while the sensitivities of the other IPs, a1j and a3j were not changed. This corresponds to the situation in the experiment where light shielding, temperature, waiting time from the irradiation to the IP readout, is different for the IP used for the thermal neutron component from those for the other IPs. This resulted in a drastic change in the estimated fluence

3 2 1

Epithermal neutron fluence (Arb. Unit)

Gamma-ray fluence (Arb. Unit)

4

Vertical Horizontal

0 -5 -4 -3 -2 -1 0 1 2 3 4 Distance from collimator center (cm)

5

10

5

Vertical Horizontal

0 -5 -4 -3 -2 -1 0 1 2 3 4 Distance from collimator center (cm)

5

Fig. 3. Fluence distribution estimated with two IPs. (a) Gamma rays, (b) epithermal neutrons.

Please cite this article as: Tanaka, K., et al., Measurement of spatial distribution of neutrons and gamma rays for BNCT using multiimaging plate system. Appl. Radiat. Isotopes (2015), http://dx.doi.org/10.1016/j.apradiso.2015.07.048i

K. Tanaka et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎

4

Fluence (Arb. Unit)

10 5

gamma thermal epithermal

the multi-IP system in this study uses the same IP for all the components evaluated. On the other hand, whether positive values can be obtained for the evaluated component fluences depends on experimental procedure, especially its difference between the IPs, and the correctness of the sensitivities of the IPs utilized.

0 0.1

1

10

-5

-10

Scaling factor for sensitivity of IP at 1 mm into epoxy

Fig. 4. Estimated fluence at the center of the collimator aperture dependent on sensitivity of IP at 1 mm into epoxy. The sensitivity for the IP at 1 mm into the epoxy resin was multiplied by a scaling factor, while those for the other IPs were kept at initial values as shown in Table 1. Lines are visual guide.

as shown in Fig. 4. Thermal neutrons and the other components resulted in positive values for scaling factors of about 0.5–0.9. These scaling factors reduce the signal for only the IP at 1 mm into the epoxy, i.e. IP for estimating the thermal neutron component. A possible cause of this is the exposure of the IP to visible light in the experiment. The factors that influences whether a positive solution can be obtained from Eq. (2) is experimental procedure, especially the difference in methods used between the IPs, and the correctness of the sensitivities aij. From this viewpoint again, using the same IPs for all the converters is advantageous because it will be easy to set the conditions of the IPs to be the same.

4. Conclusion A trial to measure the spatial distribution of the neutrons dependent on their energy and gamma rays separately was demonstrated by using a commercially available IP combined with converters to enhance specific beam components. Additionally, potential usability of multi IPs combined with the converters made of epoxy resin infused with 10B and carbon was shown for two components, i.e., epithermal neutrons and gamma rays. It was shown that no sensitivity correction is needed for the IP signal, regardless of the type of radiation used to enhance specific beam components, in order to confirm the temporal change in the spatial fluence distribution in the irradiation field. This is because

Acknowledgment Part of the present study was supported by Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science under Grants #24659568 and 26293281. The authors express their sincere appreciation to Mr. Yoshiyuki Kanazawa in Sapporo Medical University, Japan for their support in the investigations, and Dr. Gerard Bengua in Auckland City Hospital, New Zealand for his support in editing the manuscript.

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Please cite this article as: Tanaka, K., et al., Measurement of spatial distribution of neutrons and gamma rays for BNCT using multiimaging plate system. Appl. Radiat. Isotopes (2015), http://dx.doi.org/10.1016/j.apradiso.2015.07.048i