Evidence of muonium formation using thin gold foils in vacuum

The production of thermal muonium in a vacuum region has been investigated using an array of 200 thin ({~}1000 {} thick) gold foils exposed to a stopping μ⁺ beam. By examining the observed time dependence of the μ⁺-decay spectra in various transverse magnetic fields, we estimate that the lower limit of the probability of muonium formation by these gold foils placed in vacuum was 0.28±{}0.05.