Slow positrons and thermal Ps production using thin foils

A one-dimensional diffusion model for positrons implanted in Cu and Al single crystal thin foils was solved. The fraction of thermalized positrons reaching the surface was obtained for various film thicknesses as a function of the incident positron energy in a transmission-mode geometry. The results indicate that for foil thicknesses of the order of the diffusion length (1000 Å) the reemission fraction is roughly half the fraction of a semi-infinite crystal. For thicker foils the annihilation of the positrons in the bulk and smearing effects due to a higher implantation energy effectively reduce the surface reemission. In this paper it is shown that thin foils can also be used to efficiently produce low-energy (from thermal to a few eV) Ps beams, physically separated from the primary positron beam.