Positron Diffusion in Metals

From positron lifetime measurements in seven powders of Fe, Co, Ni, and W, the positron diffusion constant in metals at 300\ifmmode^\circ\else\textdegree\fi{}K is found to be ${D}_{+}=(1.0\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{2}$/sec. This agrees with a theory based on positron-electron scattering. Evidence is presented that positrons are trapped in metal surface states.