International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F. ch. 19.3, pp. 432-433   | 1 | 2 |

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

H. Tsurutaa and J. E. Johnsonb*

a SSRL/SLAC & Department of Chemistry, Stanford University, PO Box 4349, MS69, Stanford, California 94309-0210, USA, and bDepartment of Molecular Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, California 92037, USA
Correspondence e-mail:  jackj@scripps.edu

19.3.3.2. Instrumentation for small-angle X-ray scattering

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A small-angle X-ray scattering instrument typically consists of an X-ray source, a set of X-ray optics, a sample holder and a detector system (Fig. 19.3.3.5)[link]. The source can be an X-ray tube, a rotating-anode source or synchrotron radiation. The choice of optical components depends on the type of source, beam-flux requirement and angular range to be covered. The optics system includes a beam focusing device, such as a mirror, a monochromator, beam-collimation slits, vacuum paths and a beam stop. Owing to the very weak level of X-ray scattering from solutions of biological macromolecules, caution must be taken to minimize scattering from air and window materials used in the sample holder as well as to contain elements of the system under vacuum. The choice of detector depends primarily on the level of signal expected for typical samples at the detection plane. Other factors that are crucial in choosing a detector system include active area, spatial resolution and read-out speed. The level of noise generated within an entire detector system must always be constant and preferably much lower than the weakest level of signal to be measured.

[Figure 19.3.3.5]

Figure 19.3.3.5| top | pdf |

A small-angle X-ray scattering instrument on BL 4-2 at the Stanford Synchrotron Radiation Laboratory. (a) A diagram of the instrument composed of an eight-pole wiggler source (A), mirror slits (B), toroidal focusing mirror (C), monochromator slit (D), double-crystal monochromator (E), fast beam shutter (F), beam-defining slit (G), guard slit (H), ion chamber (I), crystal spindle axis (J), which would be replaced with a solution sample cell for solution X-ray scattering, beam stop (K) and an image plate detector (L), which would be replaced with a gas-chamber detector for solution scattering. Vacuum beam flight paths are drawn in dotted lines. (b) A view of the instrument as configured for small-angle single-crystal diffraction with a crystal-to-detector distance of 1.3 m. Some of the components in (a) are also seen in (b).

19.3.3.2.1. Instruments on conventional sources

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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[link]). 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[link]). 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[link]). Many instruments on conventional sources could benefit from this new development in X-ray optics.

19.3.3.2.2. Synchrotron instruments

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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)[link].] 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[link]). 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[link]). 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)[link] 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.

References

First citation Boulin, C. J., Kempf, A., Gabriel, A. & Koch, M. H. J. (1988). Data acquisition systems for linear and area X-ray detectors using delay line readout. Nucl. Instrum. Methods Phys. Res. A, 269, 312–320.Google Scholar
First citation Bu, Z., Perlo, A., Johnson, G. E., Olack, G., Engelman, D. M. & Wyckoff, H. W. (1998). A small-angle X-ray scattering apparatus for studying biological macromolecules in solution. J. Appl. Cryst. 31, 533–543.Google Scholar
First citation Cipriani, F., Gabriel, A. & Koch, M. H. J. (1994). Alternative approaches to delay line readout for multiwire proportional chambers. Nucl. Instrum. Methods A, 346, 286–291.Google Scholar
First citation Glatter, O. & Kratky, O. (1982). Editors. Small angle X-ray scattering. London: Academic Press.Google Scholar
First citation Koch, M. H. J. (1988). Instruments and methods for small-angle scattering with synchrotron radiation. Makromol. Chem. Macromol. Symp. 15, 79–90.Google Scholar
First citation Petrascu, A.-M., Koch, M. H. J. & Gabriel, A. (1998). A beginners' guide to gas-filled proportional detectors with delay line readout. J. Macromol. Sci. Phys. B37, 463–483.Google Scholar
First citation Schuster, M. & Göbel, H. (1995). Parallel-beam coupling into channel-cut monochromators using curved graded multilayers. J. Phys. D, 28, A270–A275.Google Scholar








































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