On the experimental determination of the migrating defect fraction under cascade damage conditions

Information on the fraction of defects surviving intracascade recombination, escaping the cascade volume and migrating until their annihilation ("migrating defect fraction", MDF) can be obtained from the analysis of radiation enhanced diffusion (RED) or radiation induced segregation (RIS) and maximum swelling rates. RED and RIS yield the ratio of the MDF over the effective sink strength whereas maximum swelling rates give lower bound estimates of the MDF. The basic assumptions made in the previous analysis of RED (RIS) and swelling are critically examined in the light of the present understanding of defect production in displacement cascades. MDF values deduced previously from RED are found to be clearly below the lower bound estimates obtained from maximum swelling rates. The discrepancy becomes even larger if the conventional monodefect dislocation bias is used in the analysis of swelling. Possible reasons for this discrepancy are discussed: (1) differences in the contribution of mobile defect clusters produced in cascades to RED and swelling, and (2) an underestimation of both the sink strength evolving during cascade damage conditions and the driving force for the swelling. We argue that the conventional method to deduce the sink strength from sink densities observed in TEM and the application of the conventional monodefect dislocation bias indeed yield only lower bound estimates for the sink strength and for the swelling rate, respectively. If the MDF were established by some other method RED (or RIS) could be used to measure the sink strength.