Calculations of energy deposition distributions in a plastic phantom irradiated by 20-500 MeV neutrons

Full text: From the viewpoint of radiation safety at high-energy radiation fields such as accelerator facilities, estimations of dose rate caused by high-energy neutrons above 20 MeV are important. In order to prove the validity of the particle and heavy ion transport code system, PHITS, as a dose estimation method for neutrons above 20 MeV, a measurement of absorbed dose distributions in a plastic phantom with a tissue equivalent proportional counter (TEPC) is planned at 140-350 MeV p-7Li neutron field of the RCNP (Research Center of Nuclear Physics) ring cyclotron, Osaka University. Before the measurements, distributions of energy deposition in the plastic phantom were calculated for incident neutrons from 20 to 500 MeV with the PHITS code. In the calculations, the 30 x 30 x 30 cm3 slab phantom made of polymethyl methacrylate (PMMA) was placed at the position of 1140 cm from the neutron source. The detector was modelled as a sphere of about 2.0 cm in diameter, and a sphere shell of TE plastic wall with 1.27-mm internal diameter was placed as an estimator in the detector. Deposition energy in the estimator placed in front of the phantom, and 2-, 5-, 10-, 15- and 20-cm depths in the phantom, was calculated with the PHITS code. The JENDL-3.2 cross section library was used in the calculations for neutrons below 20 MeV. It was found that deposited energies on the surface of the phantom was smaller than those inside the phantom. It is caused that charged particles from neutron-induced reactions at thin spherical wall in the detector can only contribute to the deposited energies on the surface of the phantom, while many charged particles from the reactions in the phantom can contribute to the energies inside the phantom. The deposited energy were maximum at the position of 15- and 20-cm depths for 150- and 350-MeV sources, respectively, while the values were maximum at the position of 2- and 5-cm depths for 20- and 70-MeV sources, respectively. Calculated results show that the contribution of protons caused by the neutron-induced reaction is dominant to total deposited energy. The contributions of each particle except proton to total energy depositions are about 1∼5 % for all the source energies, and that of pion is about 3 % for 500-MeV neutron sources. (author)