Among the features visible in the two cascades shown in Figure 6 are a number of small interstitial clusters. For example, the cascade debris from the 1 keV cascade in Figure 6(c) contains only seven stable interstitials, but five of them (71%) are in clusters: one di-interstitial and one tri-interstitial. This tendency for point defects to cluster is charac­teristic of energetic displacement cascades, and it differentiates neutron and ion irradiation from typi­cal 0.5 to 1 MeV electron irradiation, which primarily produces only isolated Frenkel pair defects. The differences between in-cascade vacancy and intersti­tial clustering discussed below, and the fact that their migration behavior is also quite different, have a profound influence on radiation-induced micro­structural evolution at longer times. This impact of point defect clusters on microstructural evolution is discussed in detail in Chapter 1.13, Radiation Damage Theory.