The effect of thermalisation 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 Å, SiO2Si. The measured dependence of the total yields of re-emitted positrons upon incident positron energy indicates that only epithermal positron emission is observed from these samples. By fitting the data using a procedure more commonly applied to the evaluation of thermal diffusion lengths by slow-positron implantation spectroscopy the length LT, over which positrons are reduced in energy from ∼ 101 eV to below the positron work function, has been determined. LT is related to the positron thermalisation length. The largest value of LT of 24 ± 3 Å is for the SiO2Si structure, attributed to the absence of a strong energy loss mechanism for positrons with energies below the SiO2Si bandgap(s). By assuming that the most significant effect on the positron yield at the lowest incident energies is the size of the positive surface potential step, work-function values have been deduced for the polycrystalline metallic samples of 0.9 ± 0.25 eV for Au and Pb, and 0.8 ± 0.25 eV for Sn.