STRUCTURAL-CHANGES INDUCED BY FAST HEAVY-ION IRRADIATION OF PURE METALS

Irradiation of pure metal targets with swift heavy ions (GeV energy) r esults in electronic excitation and latent track formation, beyond a t hreshold value of stopping power. Experiments show that the cores of t he cylindrical damage tracks are homogeneous; thus it is supposed that every ion homogeneously deposits its energy over all atoms of the cry stal involved in the interaction process. Energy deposition results in the ionization of the target, over a cylindrical region (ionization c ylinder) coaxial with the damage cylinder, with the conditions that ea ch atom within the ionization cylinder is considered as isolated and t hat it undergoes n multiple ionization events. Following a criterion o f minimum energy expense, the single target atom Z(A) is progressively stripped of its electrons, beginning with the outer shell and it chan ges 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, bei ng spread in the damage cylinder where they form a (Z(A)Z(B)) Startin g compound. In the frame of the Segregation-Charge Transfer (SCT) mode l, at the interface between the crystalline Z(A) matrix and a damage c ylinder, containing the starting compound, enrichment either of Z(A), or of Z(B) compound component occurs, giving origin to non-equilibriu m profiles, both compositional, and of electronic density. The local t rend towards restoration of the bulk, equilibrium charge density profi le 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 calcul ated and qualitative differences are found between metals undergoing a morphization or, respectively crystal structure formation under fast h eavy ion bombardment.