DIFFUSION AND RETENTION OF HELIUM IN GRAPHITE AND SILICON-CARBIDE

Alpha-particles of constant energies (5-28 MeV) and current densities from 0.8 to 40 X 10(14) He/(m2s) were implanted to doses of typically 10(17) m-2 at temperatures up to 1100 K into high purity pyrolytic graphite and silicon carbide. Helium release during and after implantation was monitored by mass spectrometry. Helium release from graphite during implantation strongly depends on specimen orientation and is highest for specimens implanted parallel to the basal plane. The corresponding diffusion coefficients are D[m2/s]=5 x 10(-5) exp(-1.17/kT) for 750 less-than-or-equal-to T [K] less-than-or-equal-to 1050. Helium release from SiC during implantation decreases strongly with increasing implantation depth. The diffusion coefficients are given by: D[m2/s]=1.1 x 10(-6) exp(-1.14/kT) for 800 less-than-or-equal-to T [K] less-than-or-equal-to 1050. In both materials a fraction of the implanted helium, which increases with increasing implantation depth and decreases with temperature, is retained and desorbs slowly.