Experimental study of point-defect creation in high-energy heavy-ion tracks.

Abstract

Thin platelets of LiF crystals have been bombarded on the side with Ne (40 MeV/amu), Ar (60 MeV/amu), Kr (42 MeV/amu), and Xe (27 MeV/amu) ions at room temperature in the dose range from ${10}^{8}$ to ${10}^{13}$ ions ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Taking into account the large penetration depths of these high-energy ions (\ensuremath{\simeq}1.4, 1.8, 0.6, and 0.2 mm for Ne, Ar, Kr, and Xe, respectively), it was possible to measure the depth distribution profiles of primary point defects (F centers) and aggregated defects (${\mathit{F}}_{2}$ centers) using a microspectrophotometric technique. These defects are localized in tracks surrounding the ion trajectories in which the energy is deposited by the \ensuremath{\delta} rays emitted. Concerning the creation of primary defects, it has been shown that each individual track is saturated with F centers (\ensuremath{\simeq}4\ifmmode\times\else\texttimes\fi{}${10}^{18}$ F centers/${\mathrm{cm}}^{3}$). From the evolution of the F center depth profiles as a function of the ion doses, using a model of saturated tracks, it has been possible to determine the radii of the tracks all along the ion trajectories.These radii, which are of the order of 7.5, 8, 14, and 32 nm at the entrance in the crystals for Ne, Ar, Kr, and Xe, respectively, increase continuously up to the values of 12, 16, 20, and 44 nm during the slowing down of the ions up to the end of the trajectories. In the wide range of energy deposition into electronic processes studied (from 0.2 to 20 MeV \ensuremath{\mu}${\mathrm{m}}^{\mathrm{\ensuremath{-}}1}$), a continuous behavior of the primary-defect creation is observed. This seems to indicate that the same excitonic mechanism is responsible of the primary-Frenkel-pair creation in the volume of the track irradiated by the secondary electrons and other mechanisms such as Coulomb explosion or melting, which could take place in the tracks above a certain dissipated-energy threshold, must be ruled out. Finally, the specificity of damaging with ions compared with other irradiation modes (electrons or electromagnetic radiation) is mainly observed with aggregated defects. Due to the high energy density dissipated in the tracks and saturation with isolated primary defects, a great number of other primary defects are stabilized in the form of various aggregate centers in the anionic and cationic sublattices. With such a complex damage microstructure, the recombination, aggregation, and annealing mechanisms observed are nonconventional. For example, it is shown that the aggregation of F centers into ${\mathit{F}}_{2}$ centers is markedly influenced by the nature and energy of the projectile.