||The angular distribution and energy width of PXR from a silicon
single crystal has been studied at a beam energy of 855 MeV. The
measurements of the angular distributions and photon fluxes are
in accord with the Feranchuk-Ivashin theory of PXR production . An
intrinsic energy width of less than 3 eV has been measured for
the (111) reflex with a critical absorption technique at the
K-absorption edge of titanium at 4.96 keV .
The energy width is limited by geometrical line broadening effects which can be
optimized to reach the ultimate limit given by the finite length of the wave
train. Experimental and Theoretical studies of the line shape have been performed in backward direction in which
geometrical line broading effects could be kept to a minimum. A silicon single crystal served as monochromator.
Line shapes have been measured for the (111) up to the (555) reflection showing that the natural linewidth th the order of a few
meV is significantly broaded due to small angle scattering of the electrons in crystal .