Muonium emission to a vacuum from metallic and oxidized surfaces of high-purity polycrystalline tungsten

Abstract

A thermal-energy muonium (Mu) was observed in a vacuum from the surface of 99.9998%-pure tungsten foil at 1500 K with a yield of 8(1)% Mu per incident muon stopped in the foil. The yield was explained by the one-dimensional diffusion constant of a positive muon in tungsten at 1500 K, measured to be 1.3(2)×10 −2 cm 2s −1, about 70 times higher than that of hydrogen. The desorption rate of Mu was estimated to be ≃10 7s −1 from the time evolution of the spatial distribution of Mu from tungsten covered with oxide layers. X-ray photoelectron spectroscopy verified that the surface was covered with ≃4×10 13 molecules cm −2 of W 2O 6.