Helium implantation in metals investigated by monoenergetic positrons

A new system has been developed to produce monoenergetic positrons of variable energy. The energy range available at present is between 150 eV and 28 keV. This positron beam can be used to investigate defects and defect structures produced below the surface in metals and alloys. The problems of extracting information about various defects in the near-surface region under the influence of the surface properties and the diffusion of the thermalized positrons will be discussed. Experimental results for neutron and helium-irradiated copper and nickel specimens as well as for amorphous alloys will be presented. The Doppler broadening technique of the annihilation radiation is applied for these measurements. (orig.)