Positronium in a SrF2 single crystal: Temperature-induced transition from the localized to the delocalized state

Formation of positronium (Ps) in SrF2 single crystal is confirmed. Temperature dependence of the momentum distribution and the lifetime of the Ps have been investigated in the temperature range from 10 to 297 K. Temperature induced transition of the Ps from the localized to the delocalized state has been observed. At low temperatures, the Ps is in the localized state. When the temperature is above about 100 K, the Ps in the localized states coexists with that in the delocalized states. The intensity of the delocalized Ps component increases as the temperature rises. The effective mass of the delocalized Ps is found to be 6.3×2m, considerably larger than the free Ps mass (2m) in vacuum. The level of the bottom of the delocalized state is 0.012 eV higher than that of the localized state.