Abstract
Momentum distribution of annihilation γ rays from several alkali halide crystals has been measured using high resolution 1D angular correlation apparatuses. Formation of Bloch-type positronium (Ps) in NaBr and RbCl has been newly confirmed, while no evidence for it has been seen in LiF, KF, and CsI. Effective mass of the Ps in the Bloch state has been determined by analyzing the low temperature behavior of the Ps momentum distribution. The effective mass is larger than the free Ps mass in vacuum in all the alkali halides investigated. Detailed analysis of the temperature dependence of the Ps momentum distribution has been made on KI and KBr. Temperature induced transition to the self-trapped state as previously reported in NaF and KCl has been observed also in these materials.
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