Investigations on ion irradiation induced modifications in Th 5f occupancy in thoria

Abstract

The predominant type of defect that are created by ion/neutron irradiation, in oxide nuclear fuel materials like thoria, is oxygen vacancy. In the present study, the effect of O-vacancies on the f-occupancy of thorium is studied using experiments and density functional theory (DFT) calculations. Thoria samples are irradiated with low energy (100 keV He+) and high energy (100 MeV Ag+) ions and the effects are characterized with electron energy loss spectroscopy (EELS). Ion irradiation induced crater formation and annealing of craters are observed under scanning electron microscopy, and the crater formation is due to the deposition of large electronic energy density (∼20 keV/nm3). The branching ratio of the N45 edges and the expectation values of the 5f spin-orbit interaction per core hole (〈w100〉/nh) clearly reveal that there is a transition in the f electron occupancy upon ion irradiation, towards that of atomic thorium. The electron charge density difference, calculated from DFT calculations, also support the experimental observations.