The growth of interstitial clusters in graphite under irradiation

Abstract

After high doses of fast neutron irradiation graphite is observed to contain circular clusters of interstitial atoms, each one atomic layer thick inserted between the existing hexagonal layers of the lattice. In the electron microscope these are visible as unsheared dislocation loops having Burgers vector c/2 in the c-axis direction. They grow by the diffusion to them of the mobile interstitial atoms produced by the irradiation. This diffusion-controlled growth is discussed in the present paper with particular reference to the macroscopic growth of graphite crystals in the c-direction. The fractional macroscopic growth ΔX_c/X_c has been observed to exceed the fractional growth of the c-spacing, Δc/c, by an amount which increases rapidly with dose. The present paper shows that this behaviour is understandable for a model which contains, in addition to mobile interstitials, also numerous small immobile interstitial clusters. It is argued that there is strong evidence for such defects, of high stability, from changes under irradiation in various other physical properties.