Gas bubbles in beryllium implanted with He ions at temperatures ≤700 K and after post-implantation annealing

Helium bubbles in beryllium implanted with 5 and 15 keV He ions at temperatures, Tirr, from 300 to 700 K before and after post-implantation annealing at temperatures, Tann, up to 1100 K have been studied by TEM. Tiny overpressurized He bubbles are observed for Tirr ≤ 500 K. At Tirr = 700 K the bubbles are larger and mainly facetted. At higher fluences coarser and flattened-out gas cavities and labyrinth systems are developed. A qualitative explanation of He reemission data at different irradiation temperatures based on these observations is suggested. An estimate of the dissociation temperature of HeV complexes in Be gives a value of 900 K. The development of the gas porosity during annealing of Be implanted with He ions at Tirr ≤ 500 K demonstrates features similar to those of fcc-Ni and Cu, in which a part of He bubbles maintain their overpressure up to 70% of the melting point. Ion irradiation leads to an enhanced growth of a microcrystalline surface layer of cph-BeO which is suspected to act as a barrier for the permeation of thermal vacancies. Finally, the appearance and the parameters of helium bubbles are compared with those of deuterium filled cavities developed under equivalent conditions and the differences are discussed. (orig.)