Surface interactions of muonium in oxide powders at low temperatures

Measurements of muonium (${\mu }^{+}{e}^{-}$) spin relaxation in finely powdered samples of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$, MgO, and Si${\mathrm{O}}_2$ indicate that the muonium atoms escape the powder grains with a high efficiency, regardless of the ambient temperature of the powder. In ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ powder in a pure He (or Ne) atmosphere at low temperature ($T<\mathrm{}30\mathrm{}$ 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 ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ surfaces.