Positron annihilation characteristics in UO₂: For lattice and vacancy defects induced by electron irradiation

In this work, both 22Na based positron lifetime spectroscopy (PALS) and slow positron beam based Doppler annihilation-ray broadening spectrometry (SPBDB) have been used to characterize respectively the bulk and the first micron under the surface of sintered UO₂ disks that have been polished and annealed at high temperature (1700 °C/24 h/ArH₂). Results show the presence of negative ions that are tentatively identified to negatively charged oxygen atoms located in interstitial sites. The positron annihilation characteristics in the UO₂ lattice have been determined and are equal to S_L(UO₂) = 0.371(5), W_L(UO ₂) = 0.078(7), τ_L(UO₂) = 169 ± 1 ps. Such disks have been irradiated at room temperature with electrons and a particles at different fluences. After irradiation, SPBDB and PALS measurements show the formation of U-related vacancy defects after a 2.5 MeV electrons irradiation whereas no defects are detected for an irradiation at 1 MeV.