Monte Carlo Simulation Using Japanese Voxel Phantoms to Analyze the Contribution of Particle Types and Their Energy Distributions to Organ Doses upon External Neutron Exposure

A simulation method to analyze all particle behavior inside a human body upon neutron exposure has been developed by incorporating the Japanese voxel phantoms into the Monte Carlo particle transport code PHITS. The absorbed doses in organs were calculated for idealized irradiation geometries in the energy range from 10−9 to 150 MeV, and compared with the data of ICRP Publication 74 and the Visible Chinese Human phantom. While they agreed within 30% in most organs, some deviations due to variations in the internal structure of anthropomorphic phantoms were observed. The contributions of particle types and their energy distributions to absorbed doses were quantitatively clarified by utilizing the event generator mode of PHITS. It was found that the ratio of energies imparted to organs by charged particles significantly varies with the depth of target organs from the surface of the body. Upon 10−8 MeV irradiation, the proton contribution was about 10 times higher in the skin than in the stomach.