Positronium Diffusion in Solids

The strong dependence of positron annihilation spectra in solid powders on particle size is shown to be indicative of positronium (Ps) diffusion in solids. Ps diffusion constants and mean annihilation lengths are estimated from measurements on powders of Si${\mathrm{O}}_2$, ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$, and MgO. Because of Ps diffusion, positron emitters suspended in powders of very small particles constitute compact high-yield sources of free $o-\mathrm{Ps}$ and of gamma rays with continuous energy spectra.