Thermal evolution of vacancy defects induced in sintered UO ₂ disks by helium implantation

A slow positron beam coupled with Doppler broadening (DB) spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO ₂ disks implanted with different 1 MeV ³ He fluences and annealed in ArH ₂ or in vacuum. The S(E) and W(E) behaviors indicate that for fluences in the range from 2 × 10 ¹⁴ to 2 × 10 ^{16 3} He cm ^-2 , the vacancy defects distribution evolves with the annealing temperature in the range from 264 to 700 °C under ArH ₂ . This evolution is found to be dependent on the ³ He fluence implanted in the sintered UO ₂ disks. For the lowest fluence of 2 × 10 ^{14 3} He cm ^-2 , the S(W) plot with positron energy as the running parameter suggests that only the concentration of vacancy defects decreases when annealing temperature increases. For the highest implantation fluences (from 5 × 10 ¹⁵ to 2 × 10 ^{16 3} He cm ^-2 ) the S(W) plot suggests that the nature of the vacancy defects changes in the annealing temperature range from 260 to 400 °C. Measurements performed in implanted UO ₂ disks annealed in vacuum have revealed a partial recovery of the vacancy defects possibly due to their recombination with mobile oxygen interstitials. The role of the hydrogen infusion into the disk is also discussed.