Molecular Dynamics Simulations of Surface Collision Cascades in Nickel

Collisions cascades initiated by Ni atoms with energies E = 5, 10, 15, and 20 keV on the surface of nickel at temperatures T = 100, 300, 600, 900, and 1200 K were simulated using the molecular dynamics method. To obtain statistically significant values of the number N-FP of Frenkel pairs and fractions of vacancies sigma(vac) and interstitial atoms sigma(SIA) in clusters of point defects as functions of (E, T), representative sampling of 24 collision cascades was generated for all energies of incident particles and irradiation temperatures. It was found that the values of < N-FP >, <sigma(vac)>, and <sigma(SIA)> averaged over all surface collision cascades with equal (E, T) parameters exceed the corresponding parameters of collision cascades in bulk nickel modeled under identical conditions. Point defects produced in collision cascades on the surface of nickel tend to form clusters. The primary physical mechanism increasing < N-FP >, <sigma(vac)>, and <sigma(SIA)> in surface cascades is the spatial separation of vacancies and interstitial atoms induced by the elastic interaction of point defects with the free surface.