Interstitial and vacancy loops in graphite irradiated at high temperatures

Abstract

An electron microscope examination of neutron damage in graphite single crystals irradiated at 900°C and 1200°C reveals two types of defect, which have been identified by detailed studies of their diffraction contrast. Large interstitial loops of radius about 350 Å are surrounded by small vacancy loops which appear to form in their stress fields. After annealing at 1820°C some of the interstitial loops have sheared to a configuration of lower stacking fault energy. This behaviour is unexpected and casts doubt on previous identifications of defect loops in graphite. A possible mechanism for this phenomenon is discussed and the `A over A' stacking fault energy estimated to be about 70 erg cm-2.