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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. 6.2, pp. 135-136
Section 6.2.1.3.3. Multilayer monochromators and supermirrors
a
Life Sciences Division M888, University of California, Los Alamos National Laboratory, Los Alamos, NM 8745, USA, and bSmall Angle Scattering Facility, Australian Nuclear Science & Technology Organisation, Physics Division, PMB 1 Menai NSW 2234, Australia |
Multilayers are especially useful for preparing a long-wavelength neutron beam from a cold source and for small-angle scattering experiments in which ![[\Delta \lambda /\lambda]](/teximages/fach6o2/fach6o2fi19.svg)
of about 0.1 is acceptable (Schneider & Schoenborn, 1984![[link]](/graphics/greenarr.gif)
). Multilayer monochromators are essentially one-dimensional crystals composed of alternating layers of neutron-different materials (e.g. Ni and Ti) deposited on a substrate of low surface roughness. In order to produce multilayers of excellent performance, uniform layers are required with low interface roughness, low interdiffusion between layers and high scattering contrast. Various modifications (e.g. carbonation, partial hydrogenation) to the pure Ni and Ti bilayers improve the performance significantly by fine tuning the layer uniformity and contrast (Mâaza et al., 1993). The minimum practical d-spacing is ~50 Å and a useful upper limit is ~150 Å. Multilayer monochromators have high neutron reflectivity (>0.95 is achievable), and their angular acceptance and bandwidth can be selected to produce a neutron beam of desired characteristics (Saxena & Schoenborn, 1977, 1988; Ebisawa et al., 1979; Sears, 1983; Schoenborn, 1992a).
The supermirror, a development of the multilayer monochromator concept, consists of a precise number of layers with graded d-spacing. Such a device enables the simultaneous satisfaction of the Bragg condition for a range of λ and, hence, the transmission of a broader bandwidth (Saxena & Schoenborn, 1988; Hayter & Mook, 1989; Böni, 1997).
Polarizing multilayers and supermirrors (Schärpf & Anderson, 1994) facilitate valuable experimental opportunities, such as nuclear spin contrast variation (Stuhrmann & Nierhaus, 1996) and polarized neutron reflectometry (Majkrzak, 1991; Krueger et al., 1996). Supermirrors consisting of Co and Ti bilayers display high contrast for neutrons with a magnetic moment parallel to the saturation magnetization and very low contrast for the remainder. With suitable modification of the substrate to absorb the antiparallel neutrons, a polarizing supermirror will produce a polarized neutron beam (polarization >90%) by reflection.
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