Monoenergetic positron beams: Production and experiments

The production and application of monoenergetic positron beams of variable low energy for surface and interface studies are discussed. First, the basic concept of slow positron emission from surfaces, which enables a source of monoenergetic positrons in the thermal energy range, is given. The details of a compact magnetically guided positron beam facility recently commissioned in our laboratory are discussed. Experiments conducted using the facility are described. These include surface sensitive positronium fraction measurements as a function of incident positron energy on annealed Cu, oxygenated Cu and cold worked Cu. From these results, the effects of oxygen coverage and near-surface defects on positronium formation at the Cu surface are discussed. From an analysis of the experimental results, defect-sensitive positron diffusion length is deduced. Future plans using this facility are briefly outlined.