Overlap of swift heavy ion tracks in Al2O3

A structure in overlapping swift heavy ion track regions in Al₂O₃ is studied using numerical and experimental techniques. A swift heavy ion impact is modeled with Monte-Carlo code TREKIS, describing excitation of the electronic and atomic systems, and with classical molecular dynamics tracing subsequent lattice relaxation. This combined approach is applied to simulate (predict) structure and phase transformations in tracks of heavy ions. The results of simulations are validated by high resolution transmission electron microscopy of irradiated crystals. The data obtained for 167 MeV Xe and 700 MeV Bi ion impacts in Al₂O₃ demonstrate that relaxation of the excess lattice energy results in formation of a cylinder-like discontinuous disordered region of about 1.8 nm and 3.4 nm in diameter, respectively. The simulation of the overlap of Xe ion tracks revealed the recovery of the pre-existing tracks, whereas higher electronic energy losses around the Bi ion trajectory lead to both damage recovery and formation of a damaged region depending on the distance between tracks, which are in a good agreement with experimental observations.