Does the Latent Track Occurrence in Amorphous Materials Result from a Transient Thermal Process?
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STRACT Heavy ion irradiations in the electronic stopping power (S.) regime have been performed in amorphous materials. Latent tracks have been observed in amorphous semiconductors (a-Ge, a-Si) and their radii have been deduced from a phenomenological analysis in an amorphous metallic alloy, in vitreous silica and "polymer" like amorphous carbon. A transient thermal model is developed describing the energy diffusion by the electron gas, by the atomic lattice and the energy exchange between the two subsystems. According to Fick's law, the classical equations of heat flow in the two subsystems (electrons and atoms) are numerically solved in a cylindrical geometry taking into account the temperature dependence of all the parameters. A simulation of annealing of nuclear collisions induced defects in crystalline iron allows to determine a local temperature. Electronic defect creation occurs when S. increases and becomes larger than a threshold which is correlated with the appearance of a molten phase. Using such a criterion, the radii of latent tracks are reproduced in both a - Ge and a - Si with the same value of the electron-phonon coupling despite large differences in their lattice thermodynamic parameters. Such a model is applied to amorphous metallic alloy FessB s5, vitreous silica and amorphous carbon.
INTRODUCTION Two models have been proposed in order to explain the appearance of latent tracks induced in matter by the slowing down process of incident ions in the electronic stopping power regime. The first one was the thermal spike proposed by Desauer [1] and reconsidered for metals by Seitz and Koehler [2]. The second one was the ionic spike, proposed by Fleischer et al. [3] that explains that metals are insensitive to the electronic excitation produced by fission fragments irradiation. In both models the key is the high mobility of the electrons in metals. In the ionic spike model for metals the Coulomb repulsion between lattice ions was considered as too quickly screened by the return electrons which inhibits the ionic impulse. In the thermal spike model for metals the electronic energy was considered as spread out in a too large volume to induce a significant increase of the lattice temperature. Now a systematic use of heavy ion accelerators has enlarged the number of materials (metals, semiconductors and insulators) [4-12] which present defect creation induced by heavy ions in electronic stopping power (S,) regime. Especially all amorphous materials for which the electron mobility is greatly reduced are sensitive [10]. Moreover due to their weak electron mobility they are more sensitive [4-14] than the same materials in their crystalline phase. Hence both models must be 99 Mat. Res. Soc. Symp. Proc. Vol. 504 ©1998 Materials Research Society
reconsidered. In the course of time the ionic spike arises in 10-14 - 1013 s while the increase of lattice temperature occurs in 10-13 - 10-12 s. As the thermal spike appears after the ionic spike, it can anneal all the previous atomic displacements. Then it is necessary
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