TIME-DEPENDENT DIFFUSION AND ANNIHILATION OF POSITRONS IMPLANTED IN A SEMI-INFINITE MEDIUM

Annihilation spectra, resulting from the implantation of mono-energetic positrons in a perfect homogeneous solid, have been modelled allowing for the transport of both thermal and epithermal positrons. The resulting curves resemble a sum of two exponential components, although the intensities are not simply related to the positron back-diffusion probability. Because of the very short times involved, the effects of non-thermal positron transport are less significant than for steady-state diffusion models. At room temperature, internal reflection of thermal positrons at the surface has little effect on the solution to the diffusion equation for deeply implanted positrons.