Effects of polarization in electromagnetic processes in oriented   crystals at high energy
  Under quite generic assumptions the general expression is derived for the probability of circularly polarized photon emission from the longitudinally polarized electron and for the probability of pair creation of longitudinally polarized electron (positron) by circularly polarized photon in oriented crystal in a frame of the quasiclassical operator method. For small angle of incidence the expression turns into constant field limit with corrections due to inhomogeneous character of field in crystal. For relatively large angle of incidence the expression gets over into the theory of coherent radiation or pair creation. It is shown that the crystal is a very effective device for helicity transfer from an electron to photon and back from a photon to electron or positron. 
V. N. Baier and V. M. Katkov