SURFACE-BARRIER REFLECTION OF DIFFUSING POSITRONS

The reemitted positron yield from a 1500-angstrom-thick Ni(100) foil was measured as a function of incident positron energy. The magnitude of the positron internal surface reflection coefficient, R = 0.63 +/- 0.04, was determined by fitting calculated positron yields to the data. This value agrees with the value calculated using the one-dimensional, potential-step model advanced by Britton et al. [Phys. Rev. Lett. 62, 2413 (1989)]. The mean positron penetration depth varies with incident positron energy as zBAR = alpha-E(n), where alpha is fixed (alpha = 400/rho angstrom/keV(n), rho is the sample density in g cm-3) and n = 1.61 +/- 0.03, in good agreement with positron back-reemission measurements from thick single crystals.