Ion tracks in amorphous silica
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Investigations of the structural modifications induced in amorphous silica by ion irradiations in a wide energy range from ;1 MeV to ;1 GeV are reviewed. Several characterization methods such as infrared spectroscopy, chemical etching, dimensional measurements, and small-angle x-ray scattering have been used to measure the damage induced by individual ions and to analyze its evolution as a function of the energy released by the irradiating species. The comparison of the obtained results shows that high-energy ions lead to the formation along the ion trajectories of damaged zones (called ion tracks) above an electronic energy loss threshold depending on the ion specific energy. This threshold can be as low as ;1.4 keV/nm for ion beams of 0.2 MeV/u and increases to ;2.4 keV/nm at ;5 MeV/u, in agreement with the velocity effect which predicts a narrower radial distribution of the deposited electronic energy with low-velocity ions than with high-velocity ions. Above these threshold values, track radii increase approximately with the square root of the electronic energy loss. In addition, for Au beams between 0.3 and 27 MeV, the generated damage exhibits a U-shaped dependence on the incident ion energy, suggesting a combined effect of the nuclear and electronic energy loss in this energy range. A unified thermal spike model taking into account the contributions of both energy losses allows to reproduce the whole experimental data.
I. INTRODUCTION
When an energetic ion penetrates into a solid, it loses its energy mainly via two nearly independent processes, the relative magnitude of each is related to the ion velocity: (i) nuclear energy loss dominant at low energy (i.e., ;keV/u) and due to a direct transfer of kinetic energy to the target nuclei (elastic collisions); (ii) electronic energy loss dominant at high energy (i.e., ;MeV/u) and due to electronic excitation and/or ionization of the target atoms (inelastic collisions) which may lead to the formation of an electron cascade around the ion trajectory. The first process is known to directly generate atomic displacements leading ultimately to defect formation and structural transformations within the atomic lattice. The second process may also indirectly induce significant atomic rearrangements in the wake of swift heavy ions through energy transfer from the electron cascade to the lattice atoms. Depending on the nature of the irradiated material and the amount of electronic energy loss deposited in the wake of the swift heavy ions, this process can give rise to different
Contributing Editor: William J. Weber a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.75 J. Mater. Res., 2015
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types of atomic movements and structural modifications. The latter are illustrated, for instance, by: (i) the giant anisotropic deformation in amorphous solids (the so-called ion hammering effect) 1–4; (ii) the recovery of the defects concomitantly created by nuclear collisions in metals5,6 ; (i
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