Epitaxial thin single-crystal (100) tungsten fibns 1000, 2500, and 5000 A 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 Torr) and then in vacuum (10 Torr).Transmission and back reemission of slow positrons from these surfaces was observed. The positron work function, P+ has been determined to be =3.0 eV (+0.3 eV). The transmission slow positrons were emitted in a narrow cone with a full width at half maximum of =30' 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 max-0~~. 0 imum forward-reemission yields were 18% for 1000-A-thick W film and 12% for 2500-A-thick % 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.

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