Irradiation of high-purity pyrolytic graphite samples was carried out, at temperatures of up to 1100K, by using α energies of between 5 and 28MeV, rates of between 8 x 1013 and 4 x 1015m2s, and doses of the order of 1017/m2. The He release, during and after irradiation, was monitored by means of mass spectrometry. It was found that the He release from graphite, during irradiation, depended strongly upon the specimen orientation. It was highest for specimens which were irradiated parallel to the basal plane. The corresponding diffusion coefficients could be described by:

D (m2/s) = 5 x 10-5 exp[-1.17(eV)/kT]

at temperatures of between 750 and 1050C. A fraction of the He was retained and then desorbed slowly. This amount increased with increasing implantation depth, and decreased with temperature.

P.Jung: Journal of Nuclear Materials, 1992, 191-194, 377-81