Measurement of coefficients for the back-scattering of 0.5-30 keV positrons from metallic surfaces

First measurements of the back-scattering coefficients for mono-energetic positrons of energies in the range 0.5-30 keV incident normally on Al, Cu, Ag and W samples are reported. The measured coefficients, which exhibit a small increase with energy in this energy range, and generally about 50% lower than reported values for electrons. Comparisons are made between the present results and earlier measurements at beta spectrum energies, and with recent Monte Carlo calculations.