REGIMES OF X-RAY PHASE-CONTRAST IMAGING WITH PERFECT CRYSTALS

Perfect crystals have recently been used as X-ray wavefront analysers to help produce phase-contrast images of non-periodic objects. Such im ages are essentially the maps of the phase gradients introduced in a p lane X-ray wave upon passage through a weakly absorbing object. We sho w that the nature of the contrast in the images is determined by the P atio between the local wavefront curvature and the width of the crysta l rocking curve. Depending on this ratio being small or large, two qui te distinct regimes for image formation can be identified, namely the differential phase-contrast mode and the refractometric mode. We deriv e simple analytical formulae which can be used for the analysis of X-r ay images of phase objects obtained in these two regimes.