Absence of energy loss in positron emission from metal surfaces

Abstract
We have measured the energy distribution of positrons reemitted after implantation into W(110) and Ni(100) surfaces at room temperature. The previously reported broad distribution of the perpendicular component of the energy is now found to be due to a wide angular distribution of essentially monoenergetic positrons. The implied elasticity of the positron-emission process (e.g., less than 25 meV energy loss for 97% of the 1-eV positrons emitted from Ni) is in serious disagreement with present theories that require the emission of energetic electron-hole pairs for positron reemission from metals.