Liquid and solid Ga and Bi studies with a variable energy positron beam

The diffusion length of positrons has been measured in liquid and solid gallium and bismuth using a vertical positron beam. For Ga our results indicate a drop in the diffusion length from 1200 (100) to 30 (5) Angstrom as the sample melts, as the temperature is increased there is a surprising increase in the diffusion length. At the melting point the annihilation parameter S in the bulk increases sharply, and stays constant with temperature, which indicates that vacancy size trapping centers exist in the liquid. For Bi, the diffusion length drops in the liquid from 500 to 200 Angstrom, and increases slightly as the temperature is increased. This drop is interpreted in terms of increased scattering with Bi ions. A small change is observed in bulk S, indicating no trapping of positrons as the sample melts. This behavior, which is so different in these two metals, indicates the sensitivity of positrons to the liquid structure.