Ion tracks in amorphous silica

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 (calledion 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.