THERMAL MUONIUM EMISSION FROM PLATINUM-GROUP METALS

The emission of thermal-energy muonium (Mu) was observed in vacuum fro m the surface of a platinum (Pt) foil at 1580 K with a yield of 4.7(6) % Mu per incident muon stopped in the foil, on the reasonable assumpti on of negligible Mu emission at T less than or equal to 1080 K. The yi eld at 1580 K was explained by one-dimensional diffusion of the positi ve muon inside bulk Pt, with a diffusion constant of 3.9(1)x10(-3) cm( 2)s(-1). Study of the temperature dependence of the Mu emission yielde d an activation energy of 2.9(1) eV, Above 1580 K a decrease in Mu emi ssion was caused by muon trapping in vacancies created thermally in Pt with a formation energy of 5.6(2) eV. In situ measurements of the sur face of Pt with X-ray photoelectron spectroscopy verified that at most one monolayer of atomic oxygen was adsorbed onto the surface at 1580 K. (C) 1998 Elsevier Science B.V.