Section 19.3.3.2. Instrumentation for small-angle X-ray scattering

Kratky cameras, which are commercially available, have been used for many years in small-angle X-ray scattering studies of synthetic polymers and relatively large biological systems (Glatter & Kratky, 1982). These instruments record scattering in only one dimension, thus they are not always suitable for the study of weak X-ray scatterers, although excellent accessibility to small angles is often achieved. More recent small-angle X-ray scattering instruments have a pinhole collimation system similar to those used on synchrotron instruments described below. They allow isotropic scattering to be measured with a two-dimensional detector (Bu et al., 1998). Synthetic multilayered materials, such as Mo-B₄C, formed on a figured surface serve as a monochromator element as well as a focusing device and produce an X-ray beam with very small divergence (Schuster & Göbel, 1995). Many instruments on conventional sources could benefit from this new development in X-ray optics.

The needs for time-resolved capability and for measuring weak X-ray scattering have lead to the development of synchrotron small-angle X-ray scattering instruments. Most of these instruments employ the pinhole camera geometry and are suitable for fibre diffraction experiments as well. [For a review, see Koch (1988).] Major synchrotron facilities that produce radiation in the hard X-ray regime have at least one small-angle X-ray scattering instrument of this type. Gas proportional counters are common with these instruments, as they allow photon counting virtually without added noise (Petrascu et al., 1998). Gas proportional counters are equipped with very fast electronics modules, allowing more than 1000 time-resolved scattering patterns to be recorded successively every second (Boulin et al., 1988). The disadvantage of these detectors is the count-rate limit, i.e., counting efficiency drops to below 50% at about 10⁵ to 10⁶ counts per second. This is due to the fact that the position of photon arrival is converted to a time domain, which is then registered in a histogram memory module. The space-to-time conversion occurs first on the delay lines incorporated in the gas-chamber detector, then the time-to-space conversion is performed by time-to-digital converters. These processes take a fraction of a µs per X-ray photon using present technologies. The position of the X-ray photon cannot be recorded effectively when another photon is being processed, thus leading to the count-rate problem mentioned above. Cipriani et al. (1994) proposed a new delay-line technology to increase this limit by a factor of at least 10, making gas-chamber detectors useful at high count rates. Experimenters should, however, be aware of space-charge effects in gas proportional counters. Two experimental artifacts can be observed. First, intensity near the beam stop is reduced when macromolecular complexes are studied. Second, peaks with high intensity can be reduced at their maximum, giving the impression of two closely spaced weaker peaks with a trough at the location of the true peak. Arriving photons are not counted by the detector when the local count rate exceeds the recombination rate of a detector gas molecule. Modern integration detectors, such as the image plate and the charge-coupled-device (CCD)-based detectors, have virtually no count-rate limit and have been characterized for small-angle scattering. These new detectors have certain limitations, such as a relatively slow data-acquisition rate, a problem for time-resolved studies. In general, care should be taken to match the detector to the experiment.