Muonium in sub- and supercritical water

Muonium has been studied in muon-irradiated water over a wide range of cond itions, from standard temperature and pressure (STP) up to 350 bar and up t o 420 degrees C, corresponding to water densities from 1.0 down to 0.1 g cm (-3). This is the first report of muonium in supercritical water. Muonium w as unambiguously identified from its spin precession frequencies in small t ransverse magnetic fields. The hyperfine constant was determined and found to be similar to the published values for muonium in water at STP and in va cuum. Muonium was found to be long-lived over the whole range of conditions studied. The fraction of muons which form muonium was found to vary marked ly over the density range studied. Correlation of the muonium fraction with the ionic product of water suggests a common cause, such as the rate of pr oton transfer between molecules involved in the radiolysis of water and the formation of MuOH, which competes with muonium formation.