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. 6.1, pp. 131-132   | 1 | 2 |

Section 6.1.4.4. Crystal monochromators

U. W. Arndta

aLaboratory of Molecular Biology, Medical Research Council, Hills Road, Cambridge CB2 2QH, England

6.1.4.4. Crystal monochromators

| top | pdf |

When the X-rays from the tube target are specularly reflected by a mirror, the spectrum is cut off for X-rays below the shortest wavelength for which the critical angle is equal to the smallest angle of incidence on the mirror. For a typical mirror designed for Cu Kα radiation, this cutoff wavelength might be about 0.75 Å, and the harder X-rays can be further attenuated by a β-filter. Of course, the more nearly monochromatic the radiation falling on the sample, the lower the radiation damage and the higher the spot-to-background ratio in the recorded patterns.

White radiation is almost completely eliminated by reflecting the primary X-ray beam using a natural or artificial (multilayer) crystal. The most commonly used type of plane monochromator for macromolecular crystallography is a single crystal of graphite. This material (HOPG, or highly ordered pyrolytic graphite) has a relatively large mosaic spread, typically about 0.4°, and it cannot separate the Kα doublet. This separation is essential in most small-molecule investigations, but is unnecessary for macromolecular crystals, which rarely diffract beyond 1.5 Å, and disadvantageous where a high intensity of the beam reflected by the monochromator is the main consideration.

The intensity of the diffraction pattern obtained with a graphite monochromator is only about two or three times lower than that resulting from a β-filtered pinhole-collimated beam. The situation is different at synchrotron beam lines, which must incorporate a monochromator in order to select the desired X-ray energy band. Curved focusing crystals collect X-rays over a relatively large horizontal angular range and thus produce a beam with a horizontal convergence angle of up to several milliradians. Much more nearly parallel beams are produced by reflection at several crystals in tandem, often in the form of monolithic channel-cut monochromators. In present-day storage rings, the power density at the first optical element is of the order of 10 W mm−2 at wiggler and undulator beam lines. This amount of power can be dissipated by careful design of water-cooling channels (Quintana & Hart, 1995[link]; van Silfhout, 1998[link]). In addition, the monochromator crystal, usually of silicon or germanium, may be profiled to minimize distortions as a result of thermal stresses.

The next generation of insertion devices will subject the optical elements to loads of several hundred W mm−2. Possible engineering solutions to the very severe heat-loading problem include the use of diamond crystals as reflecting elements. This material has a very high thermal conductivity, especially at low temperatures.

References

Arndt, U. W., Duncumb, P., Long, J. V. P., Pina, L. & Inneman, A. (1998). Focusing mirrors for use with microfocus X-ray tubes. J. Appl. Cryst. 31, 733.Google Scholar
Bly, P. & Gibson, D. (1996). Polycapillary optics focus and collimate X-rays. Laser Focus World, March issue.Google Scholar
Buras, B. & Tazzari, S. (1984). Editors. European Synchrotron Radiation Facility. Geneva: ESRP.Google Scholar
Franks, A. (1995). An optically focusing X-ray diffraction camera. Proc. Phys. Soc. London Sect. B, 68, 1054–1069.Google Scholar
Kirkpatrick, P. & Baez, A. V. (1948). J. Opt. Soc. Am. 56, 1–13.Google Scholar
Lemonnier, M., Fourme, R., Rousseaux, F. & Kahn, R. (1978). X-ray curved-crystal monochromator system at the storage ring DCI. Nucl. Instrum. Methods, 152, 173–177.Google Scholar
Milch, J. R. (1983). A focusing X-ray camera for recording low-angle diffraction from small specimens. J. Appl. Cryst. 16, 198–203.Google Scholar
Montel, M. (1957). X-ray microscopy with catamegonic roof mirrors. In X-ray microscopy and microradiography, edited by V. E. Cosslett, A. Engstrom & H. H. Pattee Jr, pp. 177–185. New York: Academic Press.Google Scholar
Nagel, D. J. (1980). Comparison of X-ray sources. Ann. N. Y. Acad. Sci. 342, 235–247.Google Scholar
Quintana, J. P. & Hart, M. (1995). Adaptive silicon monochromators for high-power wigglers; design, finite-element analysis and laboratory tests. J. Synchrotron Rad. 2, 119–123.Google Scholar
Silfhout, R. G. van (1998). A new water-cooled monochromator at DORIS III. Synchrotron Radiat. News, 11, 11–13.Google Scholar








































to end of page
to top of page