A dual-bandwidth multilayer monochromator system

This paper reports on a planned upgrade to the capabilities of the Illinois Institute of Technology's BioCAT undulator beamline in an effort to increase the photon flux 10 to 100-fold. This is accomplished by the addition of a double multilayer monochromator (MLM) system complementing the existing cryogenically-cooled double crystal silicon monochromator system. The water-cooled MLMs are designed and fabricated with each having two coated stripes to deliver a 12 keV undulator beam with either 1.0% or 0.5% energy bandwidths (BW). To simplify the mechanical design and operation of the MLM system, the two coatings have been designed with the same period thickness so that the BW selection is accomplished (at the same Bragg angle) by a mere horizontal translation of the MLMs. The challenge has been to find a pair of coatings that provide the desired BWs with high reflectivity at the same incident angle. This was achieved using MoSi2/B4C and Mo/MoSi2/B4C/MoSi2 multilayers at an angle of 1.194 degrees. The bandwidths and reflectivities attained are 0.52 and 83.33% for the former and 0.86 and 75.9% for the latter. Details are provided.