Single crystals with various stoichiometries were irradiated with 50keV Kr+ ions so as to create defect cascades that were similar to those which resulted from fast neutron irradiation. Two-beam dark-field transmission electron microscopy under weak dynamic conditions was used to measure the defect cascade size distribution along principal axes for 4 different O stoichiometries. High-resolution electron microscopy was used to verify the mean size of the cascades. It was found that the mean size of the defect cascades which were generated was independent of O stoichiometries of between 6.35 and 6.9. This indicated that magnetic flux pinning did not vary because of any association between the O concentration and the cascade size. When the Kr+ ions were incident near to the c-axis, the defect cascades tended to have a circular cross-section (3.5nm diameter) that was perpendicular to the ion beam. An elliptical cross-section was found for ions that were incident near to the x-axis. The ellipse minor axis (3.5nm) was in the [001] direction and the major axis (5.5nm) was in the [010] direction. This aspect ratio (1.6) matched the one which was found after high-energy heavy-ion track damage. This implied that the shape was determined not at the recoil damage stage, but during cooling or relaxation stages; where an anisotropy of the thermal conductivity dictated the defect shape.

B.G.Storey, M.A.Kirk, J.A.Osborne, L.D.Marks, P.Kostic, B.W.Veal: Philosophical Magazine A, 1996, 74[3], 617-28