THE EFFECT OF THERMALIZATION LENGTH AND WORK FUNCTION ON EPITHERMAL POSITRON EMISSION FROM SOLIDS

Measurements have been made of the yield of slow positrons from Ag(100 ), polycrystalline Au, Pb and Sn, and 8 Angstrom SiO2/Si. The measured dependence of the total yields of re-emitted positrons upon incident positron energy indicates that only epithermal positron emission is ob served from these samples. By fitting the data using a procedure more commonly applied to the evaluation of thermal diffusion lengths by slo w-positron implantation spectroscopy the length L(T), over which posit rons are reduced in energy from similar to 10(1) eV to below the posit ron work function, has been determined. L(T) is related to the positro n thermalisation length. The largest value of L(T) of 24 +/- 3 Angstro m is for the SiO2/Si structure, attributed to the absence of a strong energy loss mechanism for positrons with energies below the SiO2/Si ba ndgap(s). By assuming that the most significant effect on the positron yield at the lowest incident energies is the size of the positive sur face potential step, work-function values have been deduced for the po lycrystalline metallic samples of 0.9 +/- 0.25 eV for Au and Pb, and 0 .8 +/- 0.25 eV for Sn.