  It is shown that the diffraction on a polycrystal can be used for
investigation and diagnostics of X-ray radiation emitted in a forward direction
by relativistic charged particles moving in crystalline or other targets or
fields. Methods for measuring radiation spectral density, divergence, and
linear polarization at any requisite energy from a few units to tens of keV are
proposed. The explanation for the origination of experimentally observed and
earlier unidentified spectral peaks as a result of Bragg diffraction on a
polycrystal is described. The experiment for verification of the explanation is
suggested.
