Positron Diffusion in Solids

Theoretical positron-lifetime spectra are derived for targets with structures such that positrons can diffuse into interstices where they remain trapped until, after a distinct lifetime, they annihilate with an electron into $\gamma$ quanta. Exact expressions and convenient approximate formulas for different structural geometries are presented. The contribution of trapped positrons to the angular correlation between the two annihilation $\gamma$ quanta is derived. The propagation of errors in determining positron-diffusion constants or, conversely, parameters of the target microstructure from positron-annihilation characteristics is discussed.