AMORPHIZATION OF ZIRCONOLITE - ALPHA-DECAY EVENT DAMAGE VERSUS KRYPTON ION IRRADIATION

Natural zirconolite (CaZrTi2O7) can receive alpha-decay event doses of > 10(26) alpha-decay events/m3 (approximately 2 dpa), and can thus become amorphous (metamict) due to the radioactive decay of U-238, U-235 and Th-232 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-degrees-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 x 10(11) ions/cm2s, which is a damage rate 2 x 10(12) 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 x 10(14) ions/cm2 (= 0.3 dpa), the zirconolite grains were reamorphized; however, the thorianite grains remained crystalline even after a Kr+ dose of 6 x 10(14) ions/cm2.