Dual-beam irradiation and simultaneous observation with 30 keV Xe+ ions and 250 or 1000 keV electrons have been performed for understanding the structure and the accumulation process of cascades and for attaining insight into the synergistic effect of electron and/or free point defects on the properties of cascades. Covalent crystals such as Si, Ge and Ge-20 at% Si show contrast in transmission electron microscopy through the overlap of cascades, which stabilizes high concentration of point defects and induces an amorphous-like phase in covalent crystals. In ionic crystals such as MgO, Al2O3 and MgAl2O4, recombination of Frenkel pairs is of predominance and a small number of point defects are rather homogeneously distributed even when heavy ion impacts are introduced. The effects of electrons are to suppress the formation of vacancy clusters and to assist the nucleation of interstitial loops under the dual-beam irradiation.
