Thermal evolution of the vacancy defects distribution in 1 MeV helium implanted sintered UO2

Abstract Evolution of vacancy defects as a function of temperature was studied by positron annihilation spectroscopy in 1 MeV helium implanted sintered UO 2 disks. A slow positron beam coupled with a Doppler broadening spectrometer was used to measure the low and high momentum annihilation fractions, S and W respectively, as a function of positron energy E in UO 2 disks implanted at the fluence of 1 × 10 16 3 He cm −2 before and after annealing. The S(E) and W(E) behaviours change with the annealing temperature in the range from 160 to 1300 °C indicating that the vacancy defects distribution evolves. The positron trapping rate at vacancy defects decreases with the increasing annealing temperature with two stages around 250 °C and 800 °C. We will discuss the phenomena that could explain these annealing stages in comparison with other positron annihilation spectroscopy results and with the 3 He depth profile evolution as the function of temperature measured by Nuclear Reaction Analysis.