Thermal muonium in vacuo from silica aerogels

Muonium atoms have been observed emerging into vacuum from silica aerogel surfaces with a thermal velocity distribution corresponding to a temperature of 298(10) K after stopping positive muons from a subsurface muon beam. Yields up to 2.3(3)% could be achieved for aerogel samples of 160 mg/cm3 density. They represent an interesting alternative to the SiO2 powders previously used for muonium production in vacuum.