Positron diffusion in solid and liquid metals

We made a systematic study of the diffusion of positrons in several liquid and solid metals with a vertical positron beam. Several interesting effects have been observed by studying the line shape parameter S, the fraction of positronium formed at the surface, and the diffusion length of positrons i n liquids. The S parameter and the fraction of positronium created and rele ased at the surface (F parameter) were measured as a function of temperatur e in the solid and liquid phases of Ga, Bi, Na, Sn, In, and Pb. An apprecia ble change just below or across the melting point in the S parameter was me asured in metals where positron trapping has not been observed (Ga, Bi, and Sn). In metals where positron trapping occurs in the solid phase. there is a small change or none in the S parameter (Pb and In), which indicates tha t there is only a small change in the nature of the traps. Positron trappin g was seen in all the liquid metals, indicating that they have a high densi ty of defect like sites that trap positrons. The diffusion length of positr ons in the Liquid was extracted from the experimental data. In Ga, Bi, and Sn, there is a large drop in the diffusion length upon melting, in Pb and I nt only a small change or none is seen. In all liquid metals the diffusion length increases as the temperature increases, suggesting that diffusion is related to trapping in temporary fluctuations in the liquid's structure. T he positron moves from fluctuation to fluctuation. The size of the traps st rongly depends on temperature, and so when it increases, the positron's hop ping motion increases. We also measured the reemitted energy spectra of pos itrons from liquid and solid surfaces. No appreciable change was seen with increasing temperature or when the metals melted.