Asymmetric-cut monochromator with adjustable asymmetry

Abstract

A variable-incident-angle, asymmetric-cut, double-crystal monochromator with adjustable asymmetry was tested at Argonne National Laboratory to evaluate its possible use on beamlines at the Advanced Photon Source. For both undulator and wiggler beams, the monochromator can expand the area of the footprint of the beam on the surface of the crystals to 50 times the area of the incident beam. This increase in area will reduce the angular distortions of the crystalline planes by a factor of 2500. The asymmetric cut allows one to increase the acceptance angle for incident radiation and obtain a better match to the opening angle of the incident beam. This can increase the intensity of the diffracted beam by a factor of 2 to 5. The monochromator consists of two matched, asymmetric-cut silicon crystals mounted so that they can be rotated independently about three different axes. Rotation around the first axis controls the Bragg angle. Rotation around the second axis controls the angle between the surface of the crystal and the horizontal plane of the beam and can make the footprint a rectangle with minimum length for this footprint area. The third rotation axis is perpendicular to the diffraction planes and carries the asymmetric-cut crystal. Rotation around this axis controls the amount of asymmetry being used and thus the increase of the area of the footprint of the beam on the crystal surface. The asymmetric cut is 18° for the matched pair of crystals, which allows one to expand the footprint area by a factor of 50 for Bragg angles up to 19.15° (6 keV for Si[111] planes). This monochromator, with proper cooling, will be quite useful for analyzing the high intensity X-ray beams produced by both undulators and wigglers at the Advanced Photon Source.