Measurement of positron reemission from thin single-crystal W(100) films

Abstract

Epitaxial thin single-crystal (100) tungsten films 1000, 2500, and 5000 A\r{} thick have been fabricated by high-vacuum electron-beam evaporation. These films were subsequently used as thin-film moderators for the study of the positron-transmission-reemission process with a variable-energy (0--80 keV) monoenergetic positron beam in an ultrahigh-vacuum system. The films were shown to be routinely cleanable by heating first in oxygen (${10}^{\mathrm{\ensuremath{-}}6}$ Torr) and then in vacuum (${10}^{\mathrm{\ensuremath{-}}9}$ Torr). Transmission and back reemission of slow positrons from these surfaces was observed. The positron work function, ${\ensuremath{\varphi}}_{+}$ has been determined to be \ensuremath{\simeq}3.0 eV (\ifmmode\pm\else\textpm\fi{}0.3 eV). The transmission slow positrons were emitted in a narrow cone with a full width at half maximum of \ensuremath{\simeq}30\ifmmode^\circ\else\textdegree\fi{} consistent with the angular distribution of back-reemission positrons. The reemitted yields as a function of incident positron energy were found to be very different between forward reemission and back reemission. The maximum forward-reemission yields were 18% for 1000-A\r{}-thick W film and 12% for 2500-A\r{}-thick W film at 5 and 10 keV optimum incident positron energies, respectively. These results show that one can use thin single-crystal tungsten films as positron moderators or remoderators.