Defect accumulation in ThO2 irradiated with swift heavy ions

Polycrystalline ThO2 was irradiated with 2.2 GeV Au ions and characterized by synchrotron X-ray diffraction, X-ray absorption spectroscopy, and Raman spectroscopy. The diffraction measurements indicated an increase in the unit cell parameter and the accumulation of heterogeneous microstrain with increasing ion fluence, both of which are consistent with a single-impact model of damage accumulation. An analytical fit of the data to a single-impact model yielded a saturation unit cell expansion of 0.049 ± 0.002% and a saturation strain of 10.4 ± 0.2%. Cross-section data determined from the model values yielded effective ion track diameters of 1.9 ± 0.2 nm and 3.2 ± 0.3 nm for the two modifications, respectively, indicating that the tracks consist of a core region in which swelling and strain have occurred and a defect-rich halo in which microstrain is present but the initial unit cell parameter has not changed significantly. The spectroscopic analysis revealed the presence of significant local structural distortion in the irradiated material, but no evidence of systematic modification to the electronic state or chemical environment of the cations. This indicates that swift heavy ion irradiation of ThO2 primarily produces simple point defects or defect agglomerates.