  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.
