By combining positron annihilation data, and ab initio calculations, abundant planar V6 rings were identified in heavily neutron-irradiated samples. The calculations showed that the V6 ring was particularly stable, and annealing experiments demonstrated that the V6 ring could survive at up to 1500C. The results suggested that vacancies aggregated in so-called magic numbers in graphite.

Stable Vacancy Clusters in Neutron-Irradiated Graphite: Evidence for Aggregations with a Magic Number. Z.Tang, M.Hasegawa, T.Shimamura, Y.Nagai, T.Chiba, Y.Kawazoe, M.Takenaka, E.Kuramoto, T.Iwata: Physical Review Letters, 1999, 82[12], 2532-5