Nucleation and growth process of defect clusters in copper during helium ion irradiation

The formation process of interstitial dislocation loops in copper under He ¹ ion irradiation has been studied by using transmission electron microscopy. Wedge shaped specimens of pure copper are subjected to irradiation with 5 to 100 keV He⁺ ions at irradiation temperatures between 300 to 570 K. The minimum thickness where interstitial loops are formed increases with increasing He⁺ ion energy and irradiation temperature, although this thickness scarcely depends on He⁺ ion dose rate. The density of loops shows a linear increase against dose for irradiation with 20 and 30 keV He⁺ ions but has a higher order dependence with respect to doses greater than 2 under irradiation with 5 and 10 keV He⁺ ions. On the basis of the results induced by He⁺ ion irradiation, we have concluded that a low fraction of high energy PKAs play an important role for the nucleation of interstitial loops for energies higher than about 20 keV.