Polarization effects for pair creation by photon in oriented crystals at high energy

Abstract

Pair creation by a photon in an oriented crystal is considered in the frame of the quasiclassical operator method, which includes processes with polarized particles. Under some quite generic assumptions the general expression is derived for the probability of pair creation of longitudinally polarized electron (positron) by circularly polarized photon in oriented crystal. In the particular cases ϑ 0 ≪ V 0/ m and ϑ 0 ≫ V 0/ m ( ϑ 0 is the angle of incidence, angle between the momentum of the initial photon and axis (plane) of crystal, V 0 is the scale of a potential of axis or a plane relative to which the angle ϑ 0 is defined) one has the constant field approximation and the coherent pair production theory correspondingly. Side by side with coherent process the probability of incoherent pair creation is calculated, which differs essentially from amorphous one. At high energy the pair creation in oriented crystal is strongly enhanced comparing with the amorphous medium. In the corresponding appendixes the integral polarization of positron is found in an external field and for the coherent and incoherent mechanisms.