  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.
