The effect of matrix oxidation upon the diffusion and trapping of gas atoms was studied by implanting Kr, and by monitoring its release in air. It was recalled that the trapping of gas atoms during migration had previously been observed in pure ThO2 at the same implantation dose. Before implantation, the specimens were annealed at 1400C in air or H so as to remove polishing damage. The Kr release was measured in air at temperatures which were increased from 700 to 1500C. The H2-annealed material, which was stoichiometric at the time of implantation, soon became oxidized during heat treatment. The Kr release from the oxidized specimens revealed only mild or weak trapping. That is, the fractional release was much higher than that from ThO2. The latter had exhibited much stronger trapping at similar doses. The enhanced release was in accord with theoretical predictions of a lowered solution energy for noble gases in the anion-excess oxide. Specimens which were oxidized before implantation suffered U depletion at the surface, due to preferential vaporization. Consequently, release from pre-oxidized specimens exhibited a strong trapping behavior.

T.Ogawa, R.A.Verrall, H.Matzke, P.G.Lucata: Solid State Ionics, 1991, 49, 211-6