Study of Mechanisms of Positronium Formation on Metal Surfaces

A magnetically guided variable-energy slow-positron beam was used to investigate the formation of positronium on untreated metal surfaces by measuring the annihilation ratio of 3γ to 2γ. The results indicated that positronium formation occurs exclusively at incident positron energies below 2 keV. Furthermore, no correlation was observed between the yield of positronium and the positron diffusion length within the sample. This suggests that the glancing angle scattering of low-energy positrons is the primary mechanism responsible for positronium formation in metal surfaces not cleaned in situ.