The annealing of graphite irradiated with electrons at 80°K

Abstract

Abstract After fast electron irradiation at 80°K graphite exhibits, in its electrical resistivitiy during isochronal annealing, a reverse stage followed by normal stages. These constitute the Austerman and Hove peak. The reverse stage has previously been attributed to the separation of close Frenkel pairs of defects. In the present work the height of the peak has been measured as a function of bombarding electron energy between 0·4 and 2·0 Mev. The peak has been studied by measuring changes of electrical resistance of reactor grade A polycrystalline graphite. The measured peak height is approximately constant for electron energies from 2·0 Mev to just below 1·0 with a tendency to decrease as the electron energy decreases below 1·0 to 0·4 Mev. A method of analysis of the peak height in terms of the fraction (fiv) of close Frenkel pairs is given and from estimates of the energy dependence of fiv based on the theory of radiation damage it has not proved possible to account for the observed results. Other models are considered and analysed in a similar way. While it is not possible to provide a unique explanation it is concluded that the reverse stage must arise from the enhancement, rather than the diminution, of the number of close Frenkel pairs. The most likely model is one in which the free interstitial is mobile at ca. 90°K (energy of motion about 0.3 ev).