μ+charge exchange and muonium formation in low-pressure gases

Using the basic muon-spin-rotation technique, the fractions of energetic positive muons thermalizing in diamagnetic environments ($f_{\mu }$) or as the paramagnetic muonium atom ($f_{\mathrm{Mu}}$) have been measured in low-pressure pure gases (He, Ne, Ar, Kr, Xe, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, N${\mathrm{H}}_3$, and C${\mathrm{H}}_4$) as well as in several gas mixtures (Ne-Xe, Ne-Ar, Ne-N${\mathrm{H}}_3$, and Ne-C${\mathrm{H}}_4$). In the pure gases, the muonium fractions $f_{\mathrm{Mu}}$ are generally found to be smaller than expected from analogous proton-charge-exchange studies, particularly in the molecular gases. This is probably due to hot-atom reactions of muonium following the charge-exchange regime. Comparisons with muonium formation in condensed matter as well as positronium formation in gases are also presented. In the gas mixtures, the addition of only a few hundred parts per million of a dopant gas (e.g., Xe) which is exothermic for muonium formation gives rise to an $f_{\mathrm{Mu}}$ characteristic of the pure dopant gas itself, demonstrating the importance of the neutralization process right down to thermal energies. In all cases, the experimental signal amplitudes are found to be strongly pressure dependent, which is interpreted in terms of the time spent by the muon as neutral muonium in the charge-exchange regime, $t_n<0.2\mathrm{}$ ns. This time is generally shorter in the case of molecular gases than in rare gases.