Using crystal optics as a guard aperture in coherent diffraction imaging experiments*

A crucial issue in coherent x-ray diffraction imaging experiments is how to increase the signal-to-noise ratio when measuring relatively weak diffraction intensities from a nonperiodic object. To achieve such a goal, a guard aperture that can block the unwanted parasitic scattering from the beam-defining aperture is necessary. The conventional guard-edge-type aperture, however, is not easy to align and may produce secondary scattering from itself. In this presentation we present a novel crystal guard aperture concept, in which a pair of multiple-bounce crystal optics is employed [Xiao et al, Opt. Lett. 31, 3194(2006)]. Different from the guard-edge-type aperture, the crystal guard aperture does not produce secondary scattering and therefore guarantee super-clean incident beam. The effectiveness of the crystal guard aperture method has been verified by the theoretical analysis and simulations based on Fresnel propagations of a dynamically diffracted Bragg wave. Recent coherent diffraction experiment results also confirmed the validity of this new guarding scheme.