The nucleation of radiation damage in graphite

Abstract

A quantitative survey has been carried out of the sizes and densities of the defects found by transmission electron microscopy of irradiated graphite crystals. Three regimes are considered, viz. isothermal irradiations between 150° and 1200°c, seeding irradiations in which the crystals have been given a short pre-irradiation at a lower temperature before the isothermal irradiation, and the behaviour during thermal annealing. In each regime a comparison has been made with the corresponding macroscopic property changes observed on polycrystalline material. This behaves like crystals with added traps about 0·37 ev in depth, causing heterogeneous nucleation of interstitial clusters, at a density of about one per crystallite plane. The vacant lattice sites have an energy of 5·5 ± 1 ev, only about half that predicted by recent theoretical calculations. Interstitial loops migrate by a conservative climb process with an activation energy of 2·6 ev, which is reduced to 1·8 ev in the presence of continued irradiation. The difference is attributed to the thermal activation energy of formation of jogs on the loops. At higher temperatures, vacancy-controlled annihilation is associated with an activation energy of 5·4 ev which refers to interplanar vacancy motion. The behaviour of some macroscopic property changes on irradiation and annealing implies the presence of a saturating population of sub-microscopic defects which are susceptible to radiation annealing. The chief limitation to the model at present is the comparative ignorance of the properties of vacancies and their interactions with interstitials.