Retention, re-emission and thermal desorption of He implanted into pyrolytic graphite

8 keV 3He ions have been implanted into edge-oriented pyrolytic graphite at temperatures of 120 K and 295 K. The retention behavior observed by nuclear reaction and residual gas analyses differs markedly from that observed in metals. Despite the negligible reflection coefficient, the mean re-emission coefficient at low fluence amounts to approx. equal to 30% at room temperature, with a relative maximum at a fluence of 1.5 x 1017 cm-2. Saturation is observed at fluences above 5 x 1017 cm-2. The implanted He is completely removed by annealing to 750 K. Reimplantation at room temperature shows an enhanced retention (approx. equal to 90%) at low fluences. The experimental results are discussed in terms of He diffusion, trapping at damage sites and possible structural changes of the material. (orig.)