Bragg reflection transmission filters for variable resolution monochromators

Abstract

There are various methods for improving the angular and spectral resolution of monochromator and analyzer systems. The novel system described here, though limited to higher X-ray energies (> 20 keV), is based on a dynamic effect on the transmitted beam with a thin perfect crystal plate set in the Bragg reflection case. In the case of Bragg reflection from a perfect crystal, the incident beam intensity is reduced as it penetrates the crystal in the range of reflection. This extinction length is of the order of microns. The attenuation length, which determines the amount of normal transmission through the plate is generally much longer. Thus, in the case of the Bragg reflection, the attenuation of the transmitted beam can change by several orders of magnitude with a small change in energy or angle. A thin crystal plate cuts a notch in the transmitted beam with a width equal to its Darwin width, thus acting as a transmission filter. When used in a nondispersive mode with other monochromator crystals, the filter set at the Bragg angle will reflect the entire Darwin width of the incident beam and transmit the wings of the incident beam distribution. When the element is offset in angle by some fraction of the Darwin width, the filter becomes useful in adjusting the angular width of the transmitted beam and removing a wing. Used in pairs with symmetrical offset, the filters can be used to continuously adjust the intrinsic angular divergence of the beam with good wing reduction. Such filters may be useful in improving the angular resolution of a small-angle-scattering camera, for instance. These filters may be added to a Bonse-Hart camera with one pair on the incident beam to reduce the intrinsic beam divergence and a second pair on the analyzer arm to improve the analyzer resolution. Also, these filters could be used in dispersive monochromator systems to improve the energy resolution.