Release of ion-implanted and transmutation-produced helium from nickel

Measurements of He release were performed on Ni samples doped by thermal neutron irradiation or ion implantation. Closed and open cylinders of Ni were neutron irradiated at 100 deg C. The amount of He released into the interior of the closed cylinders was equivalent to that produced in the volume of Ni within about 0.5 μm of the surface, indicating some mobility of He at room temperature. The open cylinders were heated in vacuum while the He release was measured. Below 1000 deg C, the amount of He released was equivalent to that produced within 0.02 μm of the surface. Surface evaporation could account for the He release at higher temperatures. Upon melting the Ni, all the remaining He was released. He release from Ni at temperatures between 18 and 600 K was measured after ion implantation of 35 keV He+ at 18 K. The only significant release was sharply peaked near 50 K. Results presented here on Ni and Cu indicate that the 50 K release does not originate from the metal, but from a carbon surface film which is formed during the implantation. Similar releases were observed following implantations of He into graphite and solid methane. (author)