Section 8.1.6. SR instrumentation

The divergent continuum of X-rays from the source must be intercepted by the sample cross-sectional area. The crystal sample acceptance, as seen above, is a good way to illustrate to the machine designer the sort of machine emittances required. Likewise, the beamline optics, mirrors and monochromators should not degrade the X-ray beam quality. Mirror surface and shape finish have improved a great deal in the last 20 years; slope errors of mirrors, even for difficult shapes like polished cylinders, which on bending give a toroidal reflecting surface, are now around 1 arc second (5.5 µrad) for a length of 1 m. Thus, over focusing distances of 10–20 m, say, the focal-spot smearing contribution from this is 55–110 µm, important for focusing onto small crystals. Choice of materials has evolved, too, from the relatively easy-to-work with and finish fused quartz to silicon; silicon having the advantageous property that at liquid-nitrogen temperature the expansion coefficient is zero (Bilderback, 1986). This has been of particular advantage in the cooling of silicon monochromators at the ESRF, where the heat loading on optics is very high. An alternative approach with the rather small X-ray beams from undulators is the use of transparent monochromator crystals made of diamond, which is a robust material with the additional advantage of transparency, thus allowing multiplexing of stations, one downstream from the other, fed by one straight section of one or more undulator designs. For a review of the ESRF beamline optics, see Freund (1996); for reviews of the macromolecular crystallography programmes at the ESRF, see Miller (1994), Branden (1994) and Lindley (1999), as well as the ESRF Foundation Phase Report (1987). See also Helliwell (1992), Chapter 5.

Detectors have been, and to a considerable extent are still, a major challenge. The early days of SR use saw considerable reliance on photographic film, as well as single-counter four-circle diffractometers. Evolution of area detectors, in particular, has been considerable and impressive, and in a variety of technologies. Gas detectors, i.e., the multiwire proportional chamber (MWPC), were invented and developed through various generations and types [Charpak (1970); for reviews of their use at SR sources, see e.g. Lewis (1994) and Fourme (1997)]. MWPCs have the best detector quantum efficiency (DQE) of the area detectors, but there are limitations on count rate (local and global) and their use at wavelengths greater than ~1 Å is restricted. The most popular devices and technologies for X-ray diffraction pattern data acquisition today are image plates (IPs), mainly, but not exclusively, with online scanners [Miyahara et al. (1986); for a recent review, see Amemiya (1997)], and charge coupled devices (CCDs) (Tate et al. 1995; Allinson, 1994; Westbrook & Naday, 1997). Image plates and CCDs are complementary in performance, especially with respect to size and duty cycle; image plates are larger, i.e., with many resolution elements possible, but are slower to read out than CCDs. Both are capable of imaging well at wavelengths shorter than 1 Å and with high count rates. Both have overcome the tedium of chemical development of film! Impressive performances for macromolecular crystallography are described for image plates (in a Weissenberg geometry) by Sakabe (1983, 1991) and Sakabe et al. (1995), and for CCDs by Gruner & Ealick (1995). Other detectors needed for crystallography include those for monitoring the beam intensity; these must not interfere with the beam collimation, and yet must monitor the beam downstream of the collimator (Bartunik et al., 1981); also needed are fluorescence detectors for setting the wavelength for optimized anomalous-scattering applications.

An area-detector development is the so-called pixel detector. This is made of silicon cells, each `bump bonded' onto associated individual electronic readout chains. Thus, extremely high count rates are possible, and large area arrays of resolution elements may be conceived at a cost. These devices can then combine the attributes of large image-plate sensitive areas with the fast readout of CCDs, along with high count-rate capability and so on. Devices and prototypes are being developed at Princeton/Cornell (Eikenberry et al., 1998), Berkeley/San Diego (Beuville et al. 1997), Imperial College, London (Hall, 1995), and by Oxford Instruments and the Rutherford Appleton Laboratory (`IMPACT' detector programme).