The influence of boron on the clustering of radiation damage in graphite

Abstract

Graphite single crystals have been doped with either ¹⁰B or ¹¹B in the range 30 p.p.m. to 10⁴ p.p.m. After irradiation to a dose of 7·6 × 10¹⁹ n/cm² at 650°C or 900°C, the number and size of the interstitial loops were determined using thin foil electron microscopy. It was found that the loop density increases as the boron content to the one-half power and that the loop radius decreases as the boron content to the minus one-sixth power. The ¹⁰B transmutation to lithium and helium has no influence on the nucleation of the interstitial loops. Different effects are observed upon annealing the irradiated crystals, depending upon whether boron is introduced before or after irradiation. If the boron is introduced before irradiation, large vacancy loops are formed on annealing; if after irradiation, large interstitial loops are seen. The annealing results are interpreted on the basis of mutual annihilation between vacancies and interstitials and the loss of defects to the surface.