The Origin and Minimization of the Critical-Angle Reflection from a Thin-Film Multilayer Monochromator

A thin-film multilayer device, used as a monochromator for a white beam of neutrons, also reflects the long-wavelength component of the primary beam due to total reflection from the constituent materials of the multilayer and the substrate. This part of the spectrum consists of the primary beam's flux for all wavelengths above a critical value, which depends on the characteristics of the multilayer and its angular setting. The presence of this component in the neutron beam may interfere with high-resolution structure determinations. Although there is no unique way of eliminating this contamination, the following approaches may be taken to minimize it: (i) using a multilayer with the smallest achievable periodicity; (ii) appropriately selecting the materials of the bilayer, substrate, and the number of layers to minimize total reflection; (iii) coating the multilayer with a 50-100 Å, thin layer of gadolinium; and (iv) installing a nickel-coated silicon plate to reflect the undesirable wavelengths away from the main beam.