The momentum distribution of annihilation photon pairs in LiF observed at 293 K was found to be slightly broader than that at 89 K. Assuming that the positrons are thermalized and annihilate mainly in the polaron states, the broadening was analyzed in terms of their effective mass. The positron polarons are very likely to have a mass considerably larger than those of the electron polarons. This result is consistent with the consequence of a band-mass calculation made in the present work. Irradiation effects due to the positron source on the angular correlation, i.e. a gradual increase in the peak height at 293 K and a decrease at 89 K, were also detected and attributed to the irradiation-induced changes in the density of positron traps. Holes play an important role in the decrease at 89 K.