Alpha-decay event damage in zircon

Based on density measurements, X-ray diffraction analysis, and high-resolution transmission electron microscopy of a suite of natural zircon samples from Sri Lanka (0.06 × 10¹⁵ to 6.8 × 10 ¹⁵ α-decay events/mg), three stages of damage accumulation may be delineated. Stage I (<3 × 10¹⁵ α-decay events/mg) is characterized by sharp Bragg diffraction maxima with a minor contribution from the diffuse-scattering component Electron diffraction patterns were sharp. Damage is dominated by the accumulation of isolated point defects, which cause unit-cell expansion and distortion that account for most of the decrease in density. These defects may partially anneal over geologic periods of time. Stage II (3 × 10¹⁵ to 8 × 10¹⁵ α-decay events/mg) is characterized by significant decreases in the intensity of the Bragg diffraction maxima, which become asymmetric from increased contributions of the diffuse-scattering component High-resolution transmission electron microscopy indicated that the microstructure consists of distorted crystalline regions and amorphous "tracks" caused by α-recoil nuclei. With increasing a-decay dose, damaged crystalline regions are converted into aperiodic regions but with no further significant expansion of the unit cell in the remaining crystalline regions. Stage III (>8 × 10¹⁵ α-decay events/mg) consists of material that is entirely aperiodic as far as can be determined by X-ray or electron diffraction. There was no evidence for the formation of ZrO₂ or SiO₂ as final products during the last stage of metamictization. Based on modeled density changes, aperiodic regions continue to experience a change In structure as they are redamaged. During stage II of the process, the modeled density of aperiodic regions changes from 4.5 g/cm³ to 4.1 g/cm³. Fission fragment damage does not contribute to the process of metamictization. The amorphization process is consistent with a model for the multiple overlap of displacement cascades, suggesting amorphization occurs as a result of defect accumulation rather than directly within a single displacement cascade. Comparison of results for natural zircon with those for Pu-doped zircon showed that dose-rate variations (even as great as a factor of 10⁸) had no substantial effect on the damage accumulation process. Unit-cell parameters increased and density decreased more for the Pu-doped zircon than for natural zircon in the early stages of damage accumulation (<3 × 10¹⁵ α-decay events/mg), suggesting that annealing of point defects In the early stages of the damage accumulation process occurs in natural zircon under ambient conditions. This accounts for the distinct sigmoidal shape of the damage curves for natural zircon and the apparent Incubation period before the onset of amorphization.