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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 19.1, p. 419
Section 19.1.2. Diffraction geometries
aDepartment of Biochemistry and Molecular Biology, CLSC 161A, University of Chicago, Chicago, IL 60637, USA |
The general experimental setup involves use of a monochromated beam, employing normal-beam (Caine et al., 1976![[link]](/graphics/greenarr.gif)
) or flat-cone geometry (Prince et al., 1978). Both approaches use flat detector surfaces, and thus there is a distortion inherent in all the diffraction phenomena that increases as a function of layer line along the axis of rotation. The extent of this effect can be calculated from the experimental parameters, but, in the case of a linear detector, there is only a moderate amount of flexibility available to make the necessary adjustments. The flat-cone geometry is well suited for a linear detector, since upper-level data fall on an undistorted plane. However, such a scheme requires that the detector be adjusted to different orientations with respect to the spectrometer axis (Prince et al., 1978). In the normal-beam configuration, the crystal is usually mounted on a four-circle goniometer, allowing independent rotations around the φ, χ and ω axes to cover a full sphere of reciprocal space. This method can be efficient when used with a two-dimensional area detector because of the distortion of the diffraction pattern.
A significant advance in neutron crystallography has been the development and use of modified Laue methods to collect data (Wilkinson & Lehmann, 1991; Wilkinson et al., 1992; Niimura et al., 1997). These methods greatly increase the available neutron flux by using the white neutron spectrum. The full white radiation cannot be used due to very high background scattering and overlap between the diffraction peaks. A reasonable compromise between maximizing intensity while minimizing the experimental problems is to limit the white radiation component to about a 20% wavelength band by employing Ti–Ni multiple-spacing multilayers (Niimura et al., 1997). In practice, the use of the Laue method in X-ray diffraction allows most of the reciprocal space to be recorded in one crystal setting. The quasi-Laue application requires several settings, depending on the neutron intensity distribution I(λ) and the crystal symmetry. Data processing can be done using Laue software modified for neutron data.
References
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