Page 186 - Macromolecular Crystallography

HIGH-THROUGHPUT DATA COLLECTION AT SYNCHROTRONS 175

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Transport 2

31-ID-D Experiment station
31-ID-A (FOE)
Figure 12.1 SGX-CAT beamline schematic. The components of the beamline include: (1(not shown), 8) photon shutters; (2, 4) beam transport
tubes; (3, 5) collimators and vacuum pumps; (6) beam-defining slits; (7) monochromator; (9, 10) focusing and harmonic rejection mirrors;
and (12) CCD detector, supporting base, and sample robot.

enclosure contains optics for selection of the desired For high-throughput data collection, control of
X-ray energy. Usually this enclosure includes the the X-ray wavelength must be rapid and repro-
components to focus the beam into a small area to ducible. Current state-of-the-art monochromators
maximize brightness. However, at SGX-CAT these have mechanical precisions on the order of 0.0001
components are in the second hutch, which also angular degrees.
houses the equipment necessary to mount crystals Undulator insertion devices generate multiple
and perform the actual X-ray diffraction experiment. harmonics of their fundamental X-ray wavelength
Shutters control access of the X-ray photons to each at intensities comparable to that of the fundamen-
enclosure. tal. The unwanted wavelengths are usually removed
The selection of the X-ray wavelength or energy or ‘rejected’ using X-ray mirrors. A shallow angle
is achieved with a monochromator. At SGX-CAT, between the incoming beam and the mirror is cho-
this component is a double-crystal device from sensothatthedesiredX-raywavelengthisefficiently
Kohzu Precision Co. that contains two small dia- reflected, while most of the harmonics are absorbed.
mond crystals (111, 2d =∼4.1188 Å). Many protein In order to preserve the intensity of the reflected
crystallography beamlines use Si crystals (111, 2d X-ray beam, such mirrors are polished to near
∼6.2712 Å). Silicon crystals provide greater photon atomic smoothness. Typically, X-ray mirrors have
fluxes than diamond crystals at a given energy. more than one surface coating, so that harmonic
However, they do so at the expense of energy res- control can be exercised over the widest possible
olution and do not provide access to the shorter range of X-ray wavelengths. If only one surface type
X-ray wavelengths available from diamond-based were available, harmonic rejection at higher energies
devices. Because of the high levels of heat deposited could involve a significant deflection of the beam
into monochromator crystals by modern undula- and require a reconfiguration of the downstream
tor beamlines, they must be cooled. Otherwise the components of the beamline. The problem of har-
crystals tend to expand or contract, thereby subtly monics is greatest with insertion devices at the most
changing the wavelength of the transmitted X-rays. modern synchrotron sources. With earlier first and
In order to minimize the effects of monochroma- second-generation synchrotrons, the operating ener-
tor crystal heating, silicon must be maintained at gies of the rings were too low to generate significant
liquid nitrogen temperatures. For diamond, a low- harmonic content in the X-ray beam. Any harmon-
maintenance water-cooling system that keeps the ics could usually be eliminated by setting one of the
crystals at ∼25 C suffices. crystals in the monochromator slightly non-parallel
◦
X-ray energies are selected by rotating the crystal to the other.
pair relative to the incoming beam. The wavelength At SGX-CAT, the inherent size of the X-ray beam
delivered by a given crystal at a specific reflection at the position of the sample, if left unfocussed,
angle (2θ) is given by Bragg’s Law (Eq. 1). would be approximately 1500 µm×700 µm. Abeam
nλ = 2d sin θ (1) of this size would greatly exceed the cross-sections of