  Features of forward diffracted Parametric X-Radiation (PXR) were investigated
at experiments with the 855 MeV electron beam of the Mainz Microtron MAMI
employing a 410 micrometer thick tungsten single crystal. Virtual photons from
the electron field are diffracted by the (10-1) plane at a Bragg angle of 3.977
degree. Forward emitted radiation was analyzed at an energy of 40 keV with the
(111) lattice planes of a flat silicon single crystal in Bragg geometry. Clear
peak structures were observed in an angular scan of the tungsten single
crystal. The results were analyzed with a model which describes forward
diffracted PXR under real experimental conditions. The experiments show that
forward diffracted PXR may be employed to diagnose bending radii of lattice
planes in large area single crystals.
