Evolution of cascade region during the thermal spike

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Abstract
Evolution taking place under irradiation of cascade region (CR) in copper during the thermal spike has been simulated by molecular dynamics technique. It is shown that over time intervals of ∼10−12 s in CR with sufficiently big sizes there is a possibility of quasimelting, during which the majority of atoms change their initial positions. The defect structure of the cascade region does not qualitatively change immediately after crystallization. However, at T≃0.7–0.8 Tm vacancies, because of the temperature gradients, move preferentially towards the centre of CR. It is shown that at “fast” cooling (∼6 1014 K/s) they form finally a depleted zone with higher (∼2 times) concentration of vacancies as compared to the initial one; and at “slow” cooling (∼2 1014 K/s) they form a void of irregular form, which at lower temperatures (∼0.7-0.4 Tm can transform, because of the thermal pressure in CR, into the Frank dislocation loop over the period of ∼10−10 s. An analysis has been made of the experimental data indicating the possibility of “slow” cooling in metals located in the middle of the periodic system and of “fast” cooling-in metals located at the end of it.