Comparative Study of Neutron Flux Produced by Proton Beam Irradiation on Mo-100 and Y-89 Targets at Energies of 10-30 MeV Using PHITS Simulation

Abstract

Radioisotopes such as Tc-99m and Zr-89 play an important role in nuclear medicine and can be produced through proton irradiation on Mo-100 and Y-89 targets, respectively. In addition to producing the desired radioisotopes, this irradiation process also generates secondary neutrons that need to be characterized for radiation safety purposes. This study aims to compare the neutron flux produced from proton beam irradiation on Mo-100 and Y-89 targets at energies ranging from 10-30 MeV using PHITS Monte Carlo simulation version 3.36. Simulations were performed on cylindrical target geometry (radius 0.5 cm, height 1.0 cm) with five proton energy variations for each target, using the [T-Track] tally with region mesh. The simulation results show that the total neutron flux increases significantly with increasing proton energy for both targets, and the Mo-100 target consistently produces higher neutron flux compared to Y-89 across all energy variations, with relative differences ranging from 36.7% to 63.8%. The most significant flux increase occurs in the lower energy range (10-15 MeV), then slows at higher energies. The neutron energy spectra for both targets show a dominance of low-energy neutrons (0-2 MeV), consistent with the characteristics of evaporation neutrons in proton-induced nuclear reactions. The difference in neutron flux between the two targets is attributed to differences in mass number and nuclear reaction cross sections. These findings can serve as a reference for radiation safety considerations in cyclotron-based proton irradiation facilities for medical radioisotope production.