Analysis of the Effect of Proton Energy Variation on Radioactivity Yield and Radioactive Impurities in a Natural Zinc Oxide Target (ZnO) Using PHITS Simulation

Abstract

Radionuclide production for nuclear medicine purposes is generally carried out through proton irradiation of a specific target; however, this process may also produce impurity radionuclides that affect the purity of the final product. This study aims to analyze the effect of proton energy variation on neutron flux, radioactivity yield, and the formation of radioactive impurities in a natural zinc oxide (ZnO) target. The research method used was Monte Carlo simulation using PHITS software, varying the proton energy at 5, 10, 15, 20, 25, and 30 MeV on ZnO target with a density of 5.61 g/cm3, using T-Track, T-Yield, and T-Dchain tallies to obtain the neutron flux, reaction yield, and radionuclide inventory resulting from activation. The results show that the neutron flux increased consistently from undetected at 5 MeV to 6.5338 × 109 neutron/cm2/lethargy/source at 30 MeV, consistent with the threshold energy concept based on Coulomb's law. The formation of new radionuclides was first observed at 20 MeV in the form of Zn-67 and Zn-68 depletion, and increased significantly at 30 MeV along with the appearance of new radionuclides such as C-12, O-16, Ca-40, Ni-60, and Cu-63, indicating the activation of spallation reactions. These findings indicate a trade-off between the desired radionuclide yield and the purity level of the final product, so that the selection of an optimal proton energy must consider the balance between the two in a ZnO-based radioisotope production scheme.