A wide-bandpass multilayer monochromator for biological small-angle scattering and fiber diffraction studies

Many biological applications of small-angle X-ray scattering, in particular time-resolved studies, are often limited by the flux incident on the sample due to the smaller scattering cross section of biological specimens. The wider-energy bandpass of a monochromator that consists of a pair of synthetic multilayer microstructures can, in principle, provide a flux two orders of magnitude higher than that of an Si(lll) double-crystal monochromator. Two types of multilayers have been installed in the standard monochromator tank of beamline 4-2 at the Stanford Synchrotron Radiation Laboratory; the multilayer beam has been characterized for studies of small-angle X-ray scattering/diffraction from biological materials. Reflectivity and topography measurements indicate that the multilayers are quite adequate for these applications and a pair of Mo/B4C multilayers provided a 10-30 times increase in flux, compared with the flux level obtained with an Si(111) double-crystal monochromator. The increased flux level is very useful in time-resolved scattering studies as well as for recording weak scattering at higher angles. Having carried out many solution scattering and fiber diffraction experiments, we conclude that the use of multilayer does not result in significant broadening of diffraction peaks nor does it have appreciable effects on small-angle resolution. No significant increase in background is observed.