Study of Solids by Use of Nonthermalized Positrons

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Abstract
We have measured the energy distribution of positrons reemitter from the surfaces of solids after being implanted at low energy. It is shown that this provides a unique possibility to study energy losses of a charged particle down to near thermal energy. Such measurements are used to estimate the positron thermalization time in Al. A dramatic change in this energy distribution was observed after oxidation of the Al surface. We attribute this to the band gap of the oxide. Trapping of epithermal positrons with a remarkably high cross section was observed for both Al and Cu.