Latent tracks in sapphire induced by 20-MeV fullerene beams

Single crystals of α-Al₂O₃ were irradiated with 20-MeV fullerene beams in a fluence range from 1.0×10¹⁰ to 2.2×10¹¹ C₆₀⁺cm^-2. The cluster electronic stopping power (dE/dx)ₑ was approximately 62keVnm^-1. Two complementary techniques were employed to assess the modifications induced by these irradiations: Rutherford-backscattering spectrometry in channeling geometry (RBS-C) and transmission electron microscopy (TEM). The disorder induced by electronic processes is clearly determined by the RBS-C analysis. A damage cross section Aₑ of about 2.2×10^-12cm² has been extracted from the disorder kinetics, which corresponds to a track radius of ≈8.5nm. From lattice-disorder profiling, a maximal decorrelation length of the C₆₀ clusters in the crystal was estimated to be ≈150nm. TEM micrographs exhibit cylindrical latent tracks formed around the projectile trajectory, while the high-resolution observations evidence the amorphization of sapphire in the core of these tracks. The present results have been interpretated within a model of high locally deposited energy densities in the cluster irradiation regime.