Positron and positronium emission from tungsten (111)

We present new experimental results on positron interactions with clean tungsten (111) single crystals. The measured value of the positron work function is ${\phi }_{+}=-2.59\mathrm{}\left(10\right)\mathrm{}$ eV, which implies a negative work function for positronium and its negative ion as well. The observed yield, which is defined as the fraction of slow positrons reemitted from the sample, is 41% for 1-keV incident beams, and high yields are expected for incident radioactive source ${\beta }^{+}$ because of the small ${\beta }^{+}$ penetration depth. The yield is limited by positronium formation and trapping into a surface state bound by $E_B=2.81\mathrm{}\left(10\right)\mathrm{}$ eV. Positron surface interactions for W(111) appear similar to other clean crystalline metals with the exception of the broad energy distribution of emitted positrons which appears not to be caused by surface contamination. Difficulties associated with this energy width can apparently be overcome by growing thin epitaxial Cu films on the W(111) surface, as suggested by Lynn and Lutz [Phys. Rev. B 22, 4143 (1980)], and may result in an efficient, bright, slow-positron source.