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Response of dielectric crystals: MgO, Al 2 O 3 and Y 3 Al 5 O 12 (YAG) to irradiation with 167 MeV Xe ions decelerating in the electronic stopping regime is studied. Comprehensive simulations demonstrated that despite similar ion energy losses and the initial excitation kinetics of the electronic systems and lattices, significant differences occur among final structures of ion tracks in these materials, supported by experiments. No ion tracks appeared in MgO, whereas discontinuous distorted crystalline tracks of ~2 nm in diameter were observed in Al 2 O 3 and continuous amorphous tracks were detected in YAG. These track structures in Al 2 O 3 and YAG were confirmed by high resolution TEM data. The simulations enabled us to identify recrystallization as the dominant mechanism governing formation of detected tracks in these oxides. We analyzed effects of the viscosity in molten state, lattice structure and difference in the kinetics of metallic and oxygen sublattices at the crystallization surface on damage recovery in tracks.

Structure changes and their formation threshold in swift heavy ion (SHI) tracks in Al2O3 are studied using a combined start-to-end numerical model. The hybrid approach consists of the Monte-Carlo code TREKIS, describing kinetics of the electronic subsystem, and classical Molecular Dynamics for lattice atoms. The developed approach is free from a posteriori fitting parameters. Simulations of Xe 167 MeV ion impacts show that relaxation of an excess lattice energy results in formation of a cylindrical discontinuous disordered region of about 2 nm in diameter. Recent transmission electron microscopy observations agree with these results. The threshold of an SHI track formation is estimated to be ~6.1 keV nm−1. Calculated x-ray diffraction patterns of irradiated material demonstrate more pronounced damage of the Al atoms sublattice near SHI trajectories. Modeling of Xe ion tracks overlapping demonstrates that the damaged area can be restored to a near virgin state. Estimations give 6.5 nm as the minimal distance between the Xe ion trajectories resulting in recovery of the transformed structure produced by the previous ion.

The event-by-event Monte Carlo code, TREKIS, was recently developed to describe excitation of the electron subsystems of solids in the nanometric vicinity of a trajectory of a nonrelativistic swift heavy ion (SHI) decelerated in the electronic stopping regime. The complex dielectric function (CDF) formalism was applied in the used cross sections to account for collective response of a matter to excitation. Using this model we investigate effects of the basic assumptions on the modeled kinetics of the electronic subsystem which ultimately determine parameters of an excited material in an SHI track.In particular, (a) effects of different momentum dependencies of the CDF on scattering of projectiles on the electron subsystem are investigated. The 'effective one-band' approximation for target electrons produces good coincidence of the calculated electron mean free paths with those obtained in experiments in metals. (b) Effects of collective response of a lattice appeared to dominate in randomization of electron motion. We study how sensitive these effects are to the target temperature. We also compare results of applications of different model forms of (quasi-) elastic cross sections in simulations of the ion track kinetics, e.g. those calculated taking into account optical phonons in the CDF form vs. Mott's atomic cross sections. (c) It is demonstrated that the kinetics of valence holes significantly affects redistribution of the excess electronic energy in the vicinity of an SHI trajectory as well as its conversion into lattice excitation in dielectrics and semiconductors. (d) It is also shown that induced transport of photons originated from radiative decay of core holes brings the excess energy faster and farther away from the track core, however, the amount of this energy is relatively small.Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (№ K3-2016-035). The results were obtained using computational resources of MCC NRC "Kurchatov Institute"

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