α-particles of constant energies (5-28MeV) and current densities from 0.8 to 40 x 1014 He/(m2s) were implanted to doses of typically 1017/m2 at up to 1100K into high-purity pyrolytic graphite. Helium release during and after implantation was monitored by mass spectrometry. Helium release from graphite during implantation strongly depended upon specimen orientation and was highest for specimens implanted parallel to the basal plane. The diffusivity at 750 to 1050K could be described by:

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

A fraction of the implanted helium, which increased with increasing implantation depth and decreased with temperature, was retained and desorbed slowly.

Diffusion and Retention of Helium in Graphite and Silicon Carbide. Jung, P.: Journal of Nuclear Materials, 1992, 191-194[A], 377-81