Characterization of windows and filters for coherent X-ray beamlines

Beryllium windows and graphite heat absorbers used in coherent X-ray beamlines must be optically flat. If the optical path through the window varies by more than λ/4, the X-ray wave front will be distorted after passing through the window, and the divergence of the X-ray beam may be increased. This reduces the beam coherence. Beryllium, graphite and Kapton films have been tested using ultra-small-angle X-ray scattering. Wave-front distortion was also directly observed by means of phase-contrast X-ray microradiography. The measured increase of angular divergence is about 4 µrad. The wave-front distortion is larger than 2π (optical path difference of λ). These are serious problems for utilizing coherent X-ray beams.