Nonhomogeneous distribution of muonium and other paramagnetic products following positive muon radiolysis of water

Abstract

When energetic positive muons (μ+) stop in water roughly 40% form the hydrogen-like atom muonium (Mu = μ+e−) and the remainder are incorporated in diamagnetic compounds, such as MuH and MuOH. The two fractions can be distinguished experimentally by the precession of spin-polarized muons in a transverse magnetic field, but the muonium signal is only one-half the intensity expected. Previous studies have shown that spin-dependent interactions between Mu and hydrated electrons formed at the end of the muon track can result in the loss of muon polarization, via electron spin exchange or chemical reaction, to give MuH + OH−. These interactions are nonhomogeneous and the study of their kinetics should provide information on the distribution of radiolysis transients at the end of the track. Some relevant data have arisen from a study of the free radical products of the reaction between Mu and pyrogallol (1,2,3-trihydroxybenzene) in aqueous solution. For low concentrations of pyrogallol (< 3 mM) the radical signal amplitudes, measured by muon level-crossing resonance, are consistent with radical formation by addition of thermal Mu to one or other of the unsubstituted positions of the benzene ring. At higher concentrations this reaction competes with the muonium depolarization process and results in increased signal amplitude.