Aperture (slit) effects on the dynamical diffraction of X-rays and thermal neutrons

With increasing accuracy of perfect crystal methods, the effects of apertures (slits) on angular and spacial intensity distributions in dynamical perfect crystal diffraction have become quite important. We present an analytical treatment of such effects. The results are first applied to the diffraction of rays delivered by a source of given lateral extension, i.e. where the critical size of the source as such is essentially influencing the dynamical diffraction phenomena. Both the Laue and the Bragg geometry are considered. Next we calculate the change which a rocking curve measured in either geometry suffers from a slit with varying width and position in front of or between the crystals. Finally we report a pertinent experiment which we have performed in the Laue geometry. We find good agreement with the calculations. It follows that whenever dynamical diffraction is used in quantitative measurement the possible influence of diffraction from diaphragms requires special attention and analysis. For the latter the theory presented here is very well suited.