Section 9.1.8. Crystal-to-detector distance

The crystal-to-detector distance (CTDD) should be selected so that the whole area of the detector is usefully exploited. The shorter the CTDD, the higher the resolution of the indexed reflections at the edge of the image; but if the CTDD is too short, then the outer regions of the detector window record only indices with attached noise rather than intensities. A longer CTDD spreads the background radiation over a larger area of the detector as the background level diminishes in proportion to the square of the CTDD. In contrast, owing to collimation and focusing, the profiles of the Bragg reflections do not broaden so much, and the signal-to-noise ratio is enhanced at longer distances. It is advantageous to use the largest possible CTDD while ensuring that meaningful data are not lost beyond the active edge of the detector.

It is not straightforward to judge the resolution limit of meaningful diffraction. The most scientific approach involves recording, processing and merging a small number of images and making a decision on the basis of the resulting intensity statistics. However, this does require time, which should only pose a problem on ultra high intensity sources with very rapid data collection. A more pragmatic approach relies on visual inspection of the initial exposures using a graphical display at various contrast levels. Normally, if reflections are not visible by eye at the highest display contrast, their intensities are not meaningful. Some safety margin can be applied by setting the CTDD to a slightly shorter value than that estimated from visual inspection. Naturally, the resolution limit to which meaningful intensities extend depends on the exposure time, and the decision concerning the CTDD should follow the selection of the appropriate exposure (Section 9.1.11.2).

In addition to the significance of the reflection intensities, another important factor is the spatial resolution of spot profiles on the detector. If the crystal cell dimensions are large, the profiles may superimpose and the reflections may be impossible to integrate. At longer CTDD, the diffraction pattern spreads out and the profile overlap diminishes. If necessary, the detector can be offset from the central position to measure high-resolution data at long CTDD, but a larger total rotation is required to reach full data completeness. This applies only if the overlap of profiles belonging to the same lune results from a long axis lying parallel to the detector plane. The superposition of reflection profiles resulting from overlapping lunes will not be alleviated by increasing the CTDD; the only remedy for this is to reduce the rotation range Δφ per exposure.

In addition to the proper selection of the CTDD, attention should be given to the proper positioning of the beam stop. It should be centred with respect to the direct beam and cover the beam cross section completely. No part of the direct beam should reach the detector, and there should be no indirect scatter by the beam stop. The optimal reduction of air scatter is to have the smallest beam stop consistent with the dimensions of the beam, placed as close as possible to the crystal. For a given size of beam stop, the crystal-to-beam stop distance should be matched to the CTDD, sufficiently far from the crystal to minimize its shadow and concomitant obstruction of the valuable lowest-resolution reflections. If the beam stop is mounted on a metal wire, it is better to position the wire along the spindle axis where it will only interfere with those reflections around the blind region.