Surface interactions of muonium in oxide powders at low temperatures

Measurements of muonium (μ⁺e^-) spin relaxation in finely powdered samples of Al₂{O}₃$, MgO, and Si${O}₂$ indicate that the muonium atoms escape the powder grains with a high efficiency, regardless of the ambient temperature of the powder. In ${Al}₂O₃ powder in a pure He (or Ne) atmosphere at low temperature ($T<30$ K), the muonium spin relaxation rate is proportional to the fraction of the powder surface area not covered by adsorbed He (Ne); it is postulated that a He (Ne) film on the powder surfaces protects muonium against depolarizing collisions with the bare Al₂{O}₃$ surfaces.