Propagation of polarized high-energy photons in crystals

The density matrix is obtained for short-wavelength photons passing through a medium. For sufficiently large thicknesses of a target this matrix is expressed by the forward scattering amplitude of a photon. For multi-GeV photons scattering via virtual electron-positron pairs is the most important process. Its amplitude in a crystal is calculated. The results obtained are used to find the optimal thicknesses, orientations and types of crystals for the circular - to - linear polarization conversion process.