Radiation damage and annealing in graphite implanted with H+2 and Mo+

Radiation damage due to H+2 and Mo+ implantation into hot-worked pyrolytic graphite and into single-crystal graphite flakes has been studied using 1-MeV 4He+ ion channeling. The implant energies used, 35 keV for H+2 and 120 keV for Mo+, yielded ranges of about 1600 and 600 A, respectively. For H2 implantation, a partially crystalline surface layer remains after a fluence of 2 x 1016 H/cm2; this layer is completely disordered at 2 x 1017 H/cm2. At higher hydrogen fluences a surface layer exfoliates. For Mo implantation the disorder introduced at a fluence of 5 x 1014 Mo/cm2 is sufficient to prevent channeling throughout the range. For both implant species, complete recrystallization of samples disordered up to the surface occurs for annealing only at temperatures above 2800 K. If a surface layer remains crystalline after implantation, recrystallization proceeds both from the bulk and from the surface, and crystallinity is restored at 2300 K. While hydrogen is known to be released at temperatures between 1100 and 1500 K, Mo remains within its original range distribution up to the temperature of complete recrystallization. At a temperature of 2300 K an ordering of the implanted Mo atoms with respect to the c axis is observed, indicating short-range migration within the range distribution. At 2800 K no more Mo could be found in the analyzed surface layer