The effect of free surfaces on cascade damage production in iron

The first results have been obtained from a study intended to provide a quantitative measure of the effect of free surfaces on cascade damage evolution. A sufficient number of 10 keV iron cascades were completed to statistically evaluate variations between surface-influenced and bulk cascades. Two sets of near-surface cascades were completed; surface atoms served as the primary knockon atom (PKA) in the first set, while the PKA were located 10 lattice parameters below the surface in the second. Relative to bulk cascades, stable point defect production increased for cascades 10ao below the surface. Stable vacancy production increased further for cascades initiated at the surface, while the number of surviving interstitials decreased. The difference between the surviving vacancies and interstitials arises from sputtering and surface absorption of mobile interstitial defects. The fraction of vacancies contained in clusters increased as the cascade initiation site approached the surface, and larger vacancy clusters were formed. Conversely, no significant change in the in-cascade interstitial clustering fraction or the interstitial cluster size distribution was observed.