Evaluation of the energy resolution of a prompt gamma-ray imaging detector using LaBr3(Ce) scintillator and 8 × 8 array MPPC for an animal study of BNCT

doi:10.1016/j.apradiso.2020.109214

Applied Radiation and Isotopes

Volume 163, September 2020, 109214

https://doi.org/10.1016/j.apradiso.2020.109214 Get rights and content

Highlights

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A prompt gamma-ray imaging detector was developed with a LaBr₃(Ce) scintillator and a multi-pixel photon counter (MPPC).
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This system was able to obtain energy resolution as full width at half maximum (FWHM) at 511 keV of 5.0 ± 0.2% .
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It was confirmed that this system could discriminate 511 keV gamma rays as background and 478 keV prompt gamma rays.
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The two-dimensional counts distribution of 478 keV gamma rays from 50 ppm sample was measured.

Abstract

To develop boron neutron capture therapy (BNCT), it is desired to measure ¹⁰B concentration and obtain a two-dimensional ¹⁰B distribution in animal studies. In this research, we develop a prompt gamma-ray imaging detector to measure ¹⁰B distribution using a 50 mm × 50 mm x 10 mm LaBr₃(Ce) scintillator and a multi-pixel photon counter (MPPC). To measure a two-dimensional ¹⁰B distribution, the 478 keV gamma-ray emitted from ¹⁰B(n,α)⁷Li reaction should be measured with the discrimination from 511 keV background gamma rays in each MPPC. Furthermore, as a characteristic of the detector, it is necessary to investigate whether the 478 keV events individually incident on the MPPC can be measured in two dimensions. In this study, we evaluated the energy resolution and performed a two-dimensional distribution measurement using a thermal neutron beam and prompt gamma rays from a boron sample.

This system was able to obtain energy resolution as full width at half maximum at 511 keV of 5.0 ± 0.2% in all MPPC pixels, better than the 6.5% energy resolution required to discriminate between 478 and 511 keV gamma rays.

When the region of interest (ROI) was set up from -3σ to – σ (first ROI) and -3σ to the median (second ROI) for the Gaussian distribution of a 478 keV gamma-ray peak using a 6.25 ppm sample, the detector count rate of the 478 keV gamma rays was 0.03 and 0.11 cps, respectively, without a collimator. Moreover, the effect due to the overlapping 511 keV gamma ray peak was approximately 2.0% in the first ROI and approximately 3.2% in the second ROI. In addition, the counts of 478 keV gamma rays were visualized in two-dimensional.

Introduction

Boron neutron capture therapy (BNCT) is generally employed to treat cancer cells using a¹⁰B compound and a neutron beam. BNCT can destroy cancer cells using alpha particles and Li atomic nuclei produced by the ¹⁰B(n, α)⁷Li reaction between ¹⁰B and thermal neutrons. In addition, the range of these particles is shorter than the diameter of a cell; therefore, this treatment modality can treat cancer cells selectively, provided that ¹⁰B is accumulated in cancer cells, not normal cells.

In BNCT basic research field, it is desired to develop the new boron compound that is well accumulating in tumor cells. To confirm the ¹⁰B distribution in each organ of mice administrated boron compound as basic research, prompt gamma ray analysis have been applied at the Kyoto University Research Reactor (KUR) (Ono et al., 1996, 1999). The information of ¹⁰B concentration in each organs of mice is obtained using a high-purity germanium detector that analyzes 478 keV prompt gamma rays emitted from the nuclear reaction between ¹⁰B and thermal neutrons with a probability of approximately 94% (Kobayashi and Kanda, 1983; Laakso et al., 2001). However, this method is not able to obtain the information in two dimensions and in real-time. From the biological point of view, it is desired to evaluate where and how much ¹⁰B is accumulated in the alive mice administrated new boron compound.

For positron emission tomography, detectors for gamma-ray imaging in two dimensions with some scintillators and a multi-pixel photon counter have been developed. However, the detection area is small and the energy resolution for 511 keV peak is not sufficient for the discrimination between 478 and 511 keV peak (Kataoka et al., 2015; Shimizu et al., 2013). Minsky et al. measured the tomographic image using four LaBr₃(Ce) scintillators with a lead collimator and rotating phantom (Minsky et al., 2011). To obtain information on a¹⁰B distribution in a mouse simultaneously in two dimensions, a detection area has to be 50 mm × 50 mm that is larger than a mouse size. Also, there are annihilation gamma rays on the BNCT field that originate from electron-pair annihilation caused by the reaction between a heavy material and 2.22 MeV gamma rays produced by the ¹H(n, γ)²H reaction; as a result, a detector system has to have better energy resolution for 511 keV peak than 6.5% to discriminate between 478 and 511 keV peak because numerically the peak at 511 keV should appear where it is away from 1σ of 478 keV peak.

Thus, we are developing a prompt gamma-ray imaging detector system for an animal study of BNCT. It comprises a 50 mm × 50 mm x 10 mm LaBr₃(Ce) scintillator and an 8 × 8 channel multi-pixel photon counter (MPPC), a 64-channel amplifier, a shaper, and analog-to-digital converters (ADCs). This system can measure gamma-ray spectra for 64 channels; as a result, two-dimensional distribution can be obtained. This paper reports the concept underlying this two-dimensional measure system and the results of energy resolution of this system using different ¹⁰B concentration samples.

Section snippets

Materials and methods

The dimensions of the LaBr₃(Ce) scintillator (Saint-Gobain Co., Ltd.) was 50 mm × 50 mm × 10 mm (Van Loef et al., 2001). The scintillator was placed in front of an 8 × 8 array MPPC. An MPPC is a type of silicon photomultiplier (SiPM) manufactured by Hamamatsu Photonics K. K. (type S13360-6050VS). The 8 × 8 array MPPC was assembled by CLEAR-PULSE CO., LTD for the study design because of the necessity of the sensitivity for the scintillation wavelength of LaBr₃(Ce) of 380 nm. The effective active

Results and discussion

The energy resolution as full width at half maximum (FWHM) for all channels for the 511 keV gamma rays is shown in Fig. 3, and the average energy resolution was 5.0 ± 0.2%. Since 64 channels can be read out individually, it has been confirmed that uniform energy resolution can be obtained.

Gamma-ray spectra were obtained when thermal neutrons irradiated the boron sample with 6.25, 25, 50 and 5000 ppm. In addition, Gaussian distributions for 478 and 511 keV gamma rays were defined to discriminate

Conclusion

The energy resolution of an imaging system that comprised a LaBr₃(Ce) scintillator and an MPPC was evaluated. The average energy resolution at 511 keV using an ²²Na source was approximately 5.0%, which is better than the 6.5% required to discriminate between 478 and 511 keV gamma rays. The good linearity between the count rate at 478 keV gamma rays in the ROIs that was defined from -3σ to –σ and from -3σ to the median, and ¹⁰B concentration below 50 ppm was confirmed. Finally, we were able to

CRediT authorship contribution statement

Keita Okazaki: Validation, Investigation, Methodology. Hiroki Tanaka: Project administration, Supervision, Conceptualization, Funding acquisition. Takushi Takata: Resources. Shinji Kawabata: Resources. Kiyotaka Akabori: Resources. Yoshinori Sakurai: Resources.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: This study was partially funded by Sumitomo Heavy Industries, Ltd.

Acknowledgement

This study was partially supported by Japan Society for the Promotion of Science(JSPS) KAKENHI Grant Number JP25282155 and 16H03193.

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Radiobiological evidence suggesting heterogeneous microdistribution of boron compounds in tumors: its relation to quiescent cell population and tumor cure in neutron capture therapy
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