A WIDE-BANDPASS MULTILAYER MONOCHROMATOR FOR BIOLOGICAL SMALL-ANGLE SCATTERING AND FIBER DIFFRACTION STUDIES

Many biological applications of small-angle X-ray scattering, in parti cular 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, p rovide a flux two orders of magnitude higher than that of an Si(lll) d ouble-crystal monochromator. Two types of multilayers have been instal led in the standard monochromator tank of beamline 4-2 at the Stanford Synchrotron Radiation Laboratory; the multilayer beam has been charac terized for studies of small-angle X-ray scattering/diffraction from b iological 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, co mpared with the flux level obtained with an Si(111) double-crystal mon ochromator. The increased flux level is very useful in time-resolved s cattering studies as well as for recording weak scattering at higher a ngles. Having carried out many solution scattering and fiber diffracti on experiments, we conclude that the use of multilayer does not result in significant broadening of diffraction peaks nor does it have appre ciable effects on small-angle resolution. No significant increase in b ackground is observed.