MUONIUM EMISSION TO A VACUUM FROM METALLIC AND OXIDIZED SURFACES OF HIGH-PURITY POLYCRYSTALLINE TUNGSTEN

A thermal-energy muonium (Mu) was observed in a vacuum from the surfac e of 99.9998%-pure tungsten foil at 1500 K with a yield of 8(1)% Mu pe r incident muon stopped in the foil. The yield was explained by the on e-dimensional diffusion constant of a positive muon in tungsten at 150 0 K, measured to be 1.3(2)x10(-2) cm(2)s(-1), about 70 times higher th an that of hydrogen. The desorption rate of Mu was estimated to be app roximate to 10(7) s(-1) from the time evolution of the spatial distrib ution of Mu from tungsten covered with oxide layers. X-ray photoelectr on spectroscopy verified that the surface was covered with approximate to 4x10(13) molecules cm(-2) of W2O6.