Anomalous extinction effects in X-ray reflexion by crystals

It is commonly supposed that the effect of the presence of perfect blocks produces a decrease in the intensity of X-ray reflexion from a mosaic crystal. This phenomenon is therefore known by the name of extinction (primary or secondary extinction, according to the conditions of the experiment). However, it appears that this is not necessarily so, and that, under certain conditions, a perfect crystal can give an integrated reflexion which is larger than that of a mosaic crystal. In this case, one would not have an extinction, but an enhancement in the reflected intensity due to the deviation of the crystal from the ideally mosaic state. These results follow from the dynamical theory, in particular from the formulæ developed by Ramachandran, and have actually been verified in this laboratory using perfect and milky crystals of calcite as representatives of the two extreme limits of perfection