TRACK FORMATION IN SIO2 QUARTZ AND THE THERMAL-SPIKE MECHANISM

Alpha-quartz has been irradiated with heavy ions: F-19, S-32, and Cu-6 3 at an energy of about 1 MeV/amu in order to cover a range of electro nic stopping powers dE/dx between 2.4 and 9 keV/nm and Ni-58, Kr-86, T e-128, Xe-129, Ta-181, and Pb-208 between 1 and 5.8 MeV/amu for dE/dx > 7 keV/nm. The extent of the induced damage is determined using Ruthe rford backscattering ion channeling with a 2-MeV 4He beam. The damage cross section A is obtained using a Poisson law F(d) = 1-exp(-Aphit), where phi is the flux and t the irradiation time. This damage cross se ction is linked to the effective radius R(e) through the relation A = piR(e)2, where R(e) is the radius of an equivalent cylinder of damage. Using high-resolution electron microscopy, cylinders of amorphous mat ter have been observed, whose radius corresponds to R(e) when the trac k is continuous (i.e., for A greater-than-or-equal-to 1.3 X 10(-13) cm 2 ; R(e) greater-than-or-equal-to 2 nm). A thermal-spike model is appl ied to calculate the radii of the observed tracks assuming that the ob served amorphous cylinders correspond to a rapid quench of a molten li quid phase along the ion path. The model is applied only when the late nt track is continuous and cylindrical. A good agreement is obtained t aking into account that the initial spatial energy deposition on the e lectrons depends on the ion velocity.