Amorphization of zirconolite: alpha-decay event damage versus krypton ion irradiation

Abstract

Natural zirconolite (CaZrTi2O7) can receive α-decay event doses of > 1026 alpha-decay events/m3 (~2 dpa), and can thus become amorphous (metamict) due to the radioactive decay of 238U, 235U and 232Th and their daughter products which occur as substitutional impurities (the zirconolite contains approximately 20 wt% ThO2 and is 550 million years old). In this study, the zirconolite sample was recrystallized by thermal annealing at 1130°C for 8 hours in air. Amorphization was induced with 1.5 MeV Kr+ ions using the HVEM-Tandem Facility at Argonne National Laboratory. The Kr dose rate used during the irradiation was 3.4 × 1011 ions/cm2s. which is a damage rate 2 × 1012 times higher than that which has occurred in the natural sample due to decay of the actinides. In-situ transmission electron microscopy was completed during the ion irradiation. The recrystallized zirconolite also formed isolated thorianite (ThO2) crystals, 25–100 nm in size, in the matrix of the polycrystalline zirconolite. After a Kr+ dose of 4 × 1014 ions/cm2 (= 0.3 dpa). the zirconolite grains were reamorphized: however, the thorianite grains remained crystalline even after a Kr+ dose of 6 × 1014 ions/cm2.