Muonium Emission into Vacuum from Mesoporous Thin Films at Cryogenic Temperatures

We report on muonium (Mu) emission into vacuum following mu(+) implantation in mesoporous thin SiO2 films. We obtain a yield of Mu into vacuum of (38 +/- 4)% at 250 K and (20 +/- 4)% at 100 K for 5 keV mu(+) implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D-Mu(250K) = (1.6 +/- 0.1) x 10(-4) cm(2)/s and D-Mu(100K) = (4.2 +/- 0.5) 10(-5) cm(2)/s. Describing the diffusion process as quantum mechanical tunneling from pore to pore, we reproduce the measured temperature dependence similar to T-3/2 of the diffusion constant. We extract a potential barrier of (-0.3 +/- 0.1)eV which is consistent with our computed Mu work function in SiO2 of [-0.3, -0.9] eV. The high Mu vacuum yield, even at low temperatures, represents an important step toward next generation Mu spectroscopy experiments.