Development of a 2- to 10-MeV Gamma-Ray Field Using an Fe-PE-Cd Moderator Assembly with a p-Li Neutron Source

In the irradiation field of boron neutron capture therapy (BNCT), contaminant gamma rays with maximum energy above 10 MeV are produced by the neutron interactions with the surrounding materials. These contaminant gamma rays result in a nonselective dose to both tumor tissue and a large volume of normal tissue. In this study, a 2- to 10-MeV gamma-ray field, with characteristic gamma rays at 2.22, 3.68, 4.95, 5.92, 6.02, 6.38, 6.51, 7.28, 7.64, 8.89, 9.30, and 10.04 MeV, was developed for validation of the gamma-ray dosimeters and detectors used in BNCT. This gamma-ray field was generated using a moderator assembly (Fe-polyethylene-Cd) with an ~2.95-MeV proton-Li (thick target) neutron source at Fast Neutron Laboratory, Tohoku University, Japan. The gamma-ray dose rate D˙γ was measured as 3.51 × 10−2 μSv·μC−1 using a radio-photoluminescence glass dosimeter (RPLGD), consistent with the simulation results of 3.57 × 10−2 μSv·μC−1 and 3.56 × 10−2 μSv·μC−1 from MCNP6 and the Particle and Heavy Ion Transport code System (version 3.320), respectively. Additionally, it was discovered that a small number of secondary electrons were emitted from the moderator assembly. Since the electron absorbed dose rate was comparable to the gamma-ray absorbed dose rate, D˙eglass:D˙γglass ≈ 1:1, it is essential to correct the electron dose contribution to a RPLGD. This study provides a standard gamma-ray field for dosimetry studies in BNCT.