Abstract

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 in liquids. The S parameter and the fraction of positronium created and released at the surface (F parameter) were measured as a function of temperature in the solid and liquid phases of Ga, Bi, Na, Sn, In, and Pb. An appreciable change just below or across the melting point in the S parameter was measured 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 that there is only a small change in the nature of the traps. Positron trapping was seen in all the liquid metals, indicating that they have a high density of defectlike sites that trap positrons. The diffusion length of positrons 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 In, 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. The positron moves from fluctuation to fluctuation. The size of the traps strongly depends on temperature, and so when it increases, the positron's hopping motion increases. We also measured the reemitted energy spectra of positrons from liquid and solid surfaces. No appreciable change was seen with increasing temperature or when the metals melted. © 1999 The American Physical Society.