Defect-cluster formation in high-energy-density cascades in gold

Defect-cluster formation in Au following room-temperature atomic (Bi + ) and diatomic (Bi 2 + ) heavy-ion irradiation at low energies (10 and 20 keV atom −1 , respectively) has been investigated by transmission electron microscopy. Both the effect of energy density in individual cascades ('spike effects') at low doses (≲10 11 cm −2 ) and the effect of spatial overlap of cascades at high doses (≲10 14 cm −2 ) on defect-cluster formation were investigated by determining cluster parameters such as interstitial or vacancy type, yield Q, size distribution, cascade efficiency and cluster depth distributions. The microscopic analysis showed mainly clustering of vacancies into dislocation loops under all conditions investigated. At low doses the fraction of visible cascade events (Q < 1) increased with increasing energy and energy density. Evaluation of the size distributions indicated a more efficient separation of self-interstitial atoms and vacancies in denser cascades. Cluster depth distributions were compared with calculated damage distributions and with vacancy distributions obtained from MARLOWE computer simulations. Experimental and calculated distributions are in reasonable agreement for monatomic projectiles; diatomic projectiles led to much greater average cluster depths. This new effect, first observed in Au, has also been found in Si and Ge, which indicates that it is a rather general phenomenon under such conditions. At higher doses the interaction of cascades with preformed defect structures led to an increase of the average size of vacancy loops as saturation was approached at a cluster density of about 10 14 cm −2 .