Secondary extinction and neutron crystallography

Abstract

A detailed study of the effects of secondary extinction is made, with particular stress on the behaviour of the virtually non-absorbing crystals introduced by the advent of neutron diffraction. The penetration of a beam into a mosaic crystal of this kind will always be complete, and the relative importance of absorption and extinction is the reverse of that familiar in X-ray techniques. It is found that with increasing thickness the integrated reflexion of mosaic crystals falls away from proportionality to F2 and, for normal thicknesses, has values of the order of the mosaic spread of the crystal expressed in radians. There is a useful range where the integrated reflexion is proportional to F, as with the perfect crystal. The discussion leads to an illuminating comparison between primary and secondary extinction. Criteria for `thin', `thick', `non-absorbing' and `absorbing' crystals are given. The importance of these results for neutron crystallography is discussed; if a technique similar to X-ray goniometry is to be developed, then crystals even smaller than those conventional with X-ray technique will be required. For this work the best crystals are those of the greatest mosaic spread.