Structural changes induced by fast heavy ion irradiation of pure metals

Abstract

Irradiation of pure metal targets with swift heavy ions (GeV energy) results in electronic excitation and latent track formation, beyond a threshold value of stopping power. Experiments show that the cores of the cylindrical damage tracks are homogeneous; thus it is supposed that every ion homogeneously deposits its energy over all atoms of the crystal involved in the interaction process. Energy deposition results in the ionization of the target, over a cylindrical region (ionization cylinder) coaxial with the damage cylinder, with the conditions that each atom within the ionization cylinder is considered as isolated and that it undergoes n multiple ionization events. Following a criterion of minimum energy expense, the single target atom Z A is progressively stripped of its electrons, beginning with the outer shell and it changes its atomic configuration from Z A to (Z A − n) ∗ . The ionized Z B ∗ = (Z A − n) ∗ atoms are ejected out of the ionization cylinder, being spread in the damage cylinder where they form a ( Z A Z B ∗ ) starting compound. In the frame of the Segregation-Charge Transfer (SCT) model, at the interface between the crystalline Z A matrix and a damage cylinder, containing the starting compound, enrichment either of Z A, or of Z B ∗ compound component occurs, giving origin to non-equilibrium profiles, both compositional, and of electronic density. The local trend towards restoration of the bulk, equilibrium charge density profile is simulated by charge transfer reactions. Each reaction product is a dimer, considered as a cluster of an effective compound. The energy cost to introduce in the matrix an effective compound dimer is calculated and qualitative differences are found between metals undergoing amorphization or, respectively crystal structure formation under fast heavy ion bombardment.