The evolution of irradiation damage cascades was simulated by using molecular dynamics methods and a many-body potential. Some 200 cascades were produced, with random knock-on directions and with primary knock-on atom energies ranging from 60eV to 10keV. The starting temperature for these simulations was 100K. Cascade evolution was usually monitored for times of up to about 10ps, and sometimes for up to 30ps. The cascades were characterized by the sudden emission of replacement collision sequences and by shape variations which were due to local channelling events. At higher energies, the core was shown to have a liquid-like structure with cavitation. The annealing phase left loosely clustered vacancies at the cascade center, but collapse to a vacancy loop was not generally observed. A feature of the more energetic cascades was the production, via a ballistic mechanism, of interstitial atom clusters at the periphery of the cascades. The large number of simulations permitted the efficiency of point defect production to be analyzed as a function of primary knock-on atom energy. The value which was obtained fell sharply, from the classical value of 0.8, to 0.37 at 250eV; followed by a steady decline to 0.28 at 2keV and 0.15 at 10keV. It was concluded that the creation of point defects in metals during irradiation was likely to be markedly lower than the usual value of 0.8 which was used in irradiation dose assessments. The effect of pre-existing features upon cascade evolution was also investigated by adding features such as vacancy and interstitial loops and small voids. These structures were assumed to be in equilibrium at 100K, before the initiation of a 1keV cascade. In all of these simulations, an appreciable reduction in final defect production was observed.

A.J.E.Foreman, W.J.Phythian, C.A.English: Radiation Effects and Defects in Solids, 1994, 129[1-2], 25-30