Secondary extinction in rotating single-crystal slabs

Variation of the diffraction conditions for neutrons in a rotating crystal frame is discussed. It is shown that the angle of deviation from the Bragg position varies with a rate equal to double the angular velocity of the crystal. This makes it possible to calculate the diffraction power for type-I Zachariasen mosaic crystals and then to obtain a solution of the Darwin equation for a rotating crystal slab with zero absorption. Further considerations refer to the space distribution of diffracted beam intensity. It is shown that, if a suitable experimental configuration is adopted, the structure factor modules can be determined from the intensity dependence on the angular velocity, and the result is independent of the mosaic-spread distribution in the sample. Results of experiments are presented.