THERMAL-DESORPTION OF HELIUM FROM HOMOGENEOUSLY IMPLANTED GRAPHITE

Super-fine grain graphite (FGG) and pyrolytic carbon (PYC) of thicknesses around 200-mu-m were homogeneously implanted at room temperature with alpha-particles (0.5 MeV less-than-or-equal-to E-alpha less-than-or-equal-to 22 MeV). Thermal helium desorption spectrometry in the temperature range from 400 to 1900 K can be described by diffusion kinetics with an activation energy of 1.1 eV in FGG and about 0.75 eV in PYC. Desorption during temperature ramping at a constant rate of 0.83 K/s also shows significant differences of the two graphite species. It is found that in FGG and especially in PYC a considerable fraction of the implanted helium is retained even beyond 1000 K. This is in contrast to results reported after low energy implantations to high doses for various graphite species.