Positron dynamics in rare gas solids.

Using a beam of slow positrons in ultrahigh vacuum, we estimate the positron mean free path and energy loss rate and measure the inelastic thresholds due to exciton, electron-hole pair, and positronium formation in solid Ar, Kr, and Xe targets. The measurements are used to explain the large emission energies of positrons from the wide band gap solids in terms of a hot positron model. We are surprised to find that the solid rare gases make the most efficient moderators for producing "slow" positrons.