An electron microscopic examination of neutron damage in graphite single crystals irradiated at 900 or 1200C revealed two types of defect; identified by detailed study of their diffraction contrast. Large interstitial loops with a radius of about 350Å were surrounded by small vacancy loops which appeared to form in their stress fields. After annealing at 1820C, some of the interstitial loops had sheared into a configuration of lower stacking-fault energy. This behaviour was unexpected and cast doubt on previous identifications of defect loops in graphite. A possible mechanism for this phenomenon was considered, and the so-called A-over-A stacking-fault energy was estimated to be about 70mJ/m2.

Interstitial and Vacancy Loops in Graphite Irradiated at High Temperatures. P.A.Thrower: British Journal of Applied Physics, 1964, 15[10], 1153-9