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

To develop boron neutron capture therapy (BNCT), it is desired to measure B concentration and obtain a two-dimensional B distribution in animal studies. In this research, we develop a prompt gamma-ray imaging detector to measure B distribution using a 50 mm × 50 mm x 10 mm LaBr(Ce) scintillator and a multi-pixel photon counter (MPPC). To measure a two-dimensional B distribution, the 478 keV gamma-ray emitted from B(n,α)Li 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.