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Energy spectroscopy of positrons re-emitted from polycrystalline tungsten

Energy spectra of positrons re-emitted from the surface of polycrystalline tungsten foil have been measured as a function of incident positron energy E in the range 100 eV-3 keV. The measurements were made using an electrostatic positron-beam apparatus incorporating a hemispherical energy analyser. At high E values the re-emitted spectra tend to a characteristic shape, dominated by a work-function peak at 2.7 eV with a low-energy shoulder comprising approximately 25% of the total spectrum. As E decreases a high-energy tail attributed to epithermal positrons increases steadily in relative intensity and extent. Below E approximately=500 eV the contribution of the low-energy distribution to the spectra increases significantly, and at 100 eV is dominant. These observations are interpreted in terms of elastic and inelastic processes at the tungsten surface.