Magnetic field effects on positronium in liquids

Magnetic field quenching of positronium (Ps) has been studied in water and in benzene using in conjunction the lifetime spectroscopy and the Doppler broadening of the annihilation radiation lineshape techniques. The theoretical expressions related to the lifetime variations are well fitted to the data provided an additional parameter, η, descriptive of the expansion of the positronium wavefunction in matter is introduced, with η = 0.65 and 0.84 in water and in benzene, respectively. The interpretation of the Doppler data requires that the momentum distribution of p-Ps arising from magnetic quenching of triplet Ps is much narrower than that of p-Ps directly formed in the liquid. This is ascribed to the more complete thermalization of the quenched triplet at the moment of annihilation as compared to the latter species.