Processing oscillation diffraction data for very large unit cells with an automatic convolution technique and profile fitting

A method is described for the processing of X-ray diffraction data collected on an Arndt–Wonacott rotation camera particularly suitable for crystals with very large unit cells. Since often only single exposures can be obtained before the radiation has damaged such crystals, it is essential to select accurately those reflections which have fully penetrated the sphere of reflection. This requires not only a careful refinement of the crystal setting orientation but also a good knowledge of the mosaic spread plus beam divergence. The necessary parameters can be determined by convoluting the theoretically predicted diffraction pattern against the observed film in search of the maximum similarity. Films taken with the crystal misset by as much as 1° can readily be processed with rotational corrections reproducible to better than 0.01°. The many very weak intensities found in data derived from crystals with very large unit cells can be better measured by using a least-squares fit to a previously determined profile rather than by the usual integration process. Systematic variation of the profile across the surface of the film can be readily accommodated.