The successive studies of the propagation of x-rays in crystals distorted by a strain gradient whose magnitude is characterized by a parameter β are presented. It is shown how the new wavefield that appears when β Lambda (Lambda being the classical extinction length) is much larger than 1 pumps intensity out of the normal wavefield, which explains the transition from the dynamical to the kinematical theory. The quantity β Lambda can be increased either by increasing the distortion or by using smaller wavelengths or structure factors as checked experimentally on Si_1-xGe_x crystals.

Keywords: x-ray dynamical theory, kinematical theory, SiGe gradient crystal, curved crystals

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