POSITRON REEMISSION FROM THIN W(100) FOIL IN 10(-6)-TORR PRESSURE

The production of low-energy positron beams for positron-gas scatterin g experiments has been tested using thin W(100) foils in high-vacuum p ressure (10(-6) Torr). The W foil was baked for cleaning and used as a positron moderator in Situ. The efficiency of positrons reemitted fro m the thin foil was 14 +/- 2% for a 1000-Angstrom-thick foil and 10 +/ - 2% for a 2000-Angstrom-thick one. The positron work function of the W(100) surface in 10(-6) Torr was -1.9 +/- 0.3 eV. The energy width in situ was 1.5 +/- 0.2 eV, narrower than that exposed to air. The feasi bility of the brightness-enhancement method in 10(-6) Torr for positro n-gas scattering experiments was discussed.