Materials with low atomic weight, such as aluminum, exhibit more spatially diffuse displacement cascades than high atomic weight materials due to the increase in nuclear and electronic stopping power with in­creasing atomic weight. For example, the calculated average vacancy concentration in Au displacement cascades is about two to three times higher than in Al cascades for a wide range of PKA energies.57 This increased energy density and compactness in the spa­tial extent of displacement cascades can produce enhanced clustering of point defects within the ener­getic displacement cascades of high atomic weight materials. Electrical resistivity isochronal annealing studies of fission neutron-irradiated metals have con­firmed that the amount of defect recovery during Stage I annealing decreases with increasing atomic weight,79 which is an indication of enhanced SIA clustering within the displacement cascades. The importance of atomic weight on defect clustering depends on the material-specific critical energy for subcascade formation compared to the average PKA energy. For example, in the fcc noble metal series Cu, Ag, Au, the subcascade formation energy increases slightly with mass (10, 13, and 14 keV, respectively), and very little qualitative difference exists in the defect cluster accumulation behavior of these three materi — als.13,56 In general, there is not a universal relation between atomic weight and microstructural para­meters such as overall defect production,1 1 defect cluster yield,122,123 or visible defect cluster size.56