Section 19.5.5. Data collection

Fibre-diffraction data have generally been collected using laboratory X-ray sources and photographic film. However, synchrotron sources are increasingly being used (Shotton et al., 1998), taking advantage of reduced exposure time, the potential for time-resolved studies and the fact that many fibres (or the well oriented regions of fibres) are too small for laboratory data collection. Imaging-plate systems and charge-coupled device (CCD) cameras are replacing film as detectors (Yamashita et al., 1995; Okuyama et al., 1996; Shotton et al., 1998). Pinhole cameras, mirror-monochromator optics and double-mirror optics are used in different applications. Diffraction by most fibres is inherently weak, and very long repeat spacings often require long distances between the specimen and the detector, so fibre cameras are often flushed with helium to reduce air scatter. Constant relative humidity is often required and is achieved by bubbling the helium stream through a saturated salt solution followed by a salt trap.

The X-ray beam commonly strikes a stationary fibre perpendicular to the fibre axis. Because of the cylindrical averaging of the data, this procedure allows most of the diffraction pattern to be collected in a single exposure. There is, however, a `blind region' around the meridian, where the Ewald sphere does not intersect the diffraction pattern (Fraser et al., 1976). Data in this region are collected by tilting the fibre.