Track formation in SiO2 quartz and the thermal-spike mechanism

α-quartz has been irradiated with heavy ions: ¹⁹F, ³²S, and ⁶³Cu at an energy of about 1 MeV/amu in order to cover a range of electronic stopping powers dE/dx between 2.4 and 9 keV/nm and ⁵⁸Ni, ⁸⁶Kr, ¹²⁸Te, ¹²⁹Xe, ¹⁸¹Ta, and ²⁰⁸Pb between 1 and 5.8 MeV/amu for dE/dx>7 keV/nm. The extent of the induced damage is determined using Rutherford backscattering ion channeling with a 2-MeV ⁴He beam. The damage cross section A is obtained using a Poisson law F_d=1-exp(-Aφt), where φ is the flux and t the irradiation time. This damage cross section is linked to the effective radius R_e through the relation A=πR²_e, where R_e is the radius of an equivalent cylinder of damage. Using high-resolution electron microscopy, cylinders of amorphous matter have been observed, whose radius corresponds to R_e when the track is continuous (i.e., for A>=1.3×10^-13 cm²; R_e>=2 nm). A thermal-spike model is applied to calculate the radii of the observed tracks assuming that the observed amorphous cylinders correspond to a rapid quench of a molten liquid phase along the ion path. The model is applied only when the latent track is continuous and cylindrical. A good agreement is obtained taking into account that the initial spatial energy deposition on the electrons depends on the ion velocity.