Generation of thermal muonium vacuum.

We find that thermal-energy muonium atoms are emitted from a clean hot tungsten foil in which positive muons are stopping near the surface. The temperature dependence of the thermal muonium signal yields a surprisingly low activation energy of 0.66(4) eV that suggests we are observing the thermionic emission of muonium from the solid. The total muonium yield is about 0.02 per muon. A number of new experiments should be possible using this unique source of thermal muonium in vacuum.