A systematic study was made of positron diffusion, in liquid and solid metals, by using a vertical positron beam. Interesting effects were revealed by studying the line-shape parameter, the fraction of positronium which formed at the surface, and the diffusion-length of positrons in the liquids. The S-parameter, and the fraction of positronium created and released at the surface, were measured as a function of temperature in the solid and liquid phases. An appreciable change in the S-parameter just below, or across, the melting point was measured in metals (Ga, Bi, Sn) where positron trapping had not been observed. In metals (Pb, In) where positron trapping occurred in the solid phase, there was little or no change in the S-parameter. This indicated that there was only a small change in the nature of the traps. Positron-trapping was detected in all of the liquid metals; thus indicating that they had a high density of defect-like sites which trapped positrons. The diffusion length of positrons in the liquid was deduced from the experimental data. In the case of Ga, Bi and Sn, there was a large drop in the diffusion length upon melting. In Pb and In, little or no change was seen. In all liquid metals, the diffusion length increased as the temperature increased; thus suggesting that diffusion was related to trapping in temporary fluctuations of the liquid structure. The positron moved from fluctuation to fluctuation. The size of the traps depended strongly upon the temperature. When it increased, the positron-hopping motion increased. The re-emitted energy spectra of positrons from liquid and solid surfaces were also measured. No appreciable change was seen, with increasing temperature, or when the metals melted.

Positron Diffusion in Solid and Liquid Metals. E.Gramsch, K.G.Lynn, J.Throwe, I.Kanazawa: Physical Review B, 1999, 59[22], 14282-301