CHARACERIZATION OF X-RAY WINDOW SCATTERING AND FLUORESCENCE FOR SAXS STUDIES

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. For the most challenging class of SAXS studies all sources of background scattering need to be eliminated or minimized, with the inevitable exception of water scattering. Sample cell x-ray windows for sample containment and vacuum path separation are primary sources of background. In order to improve the x-ray scattering facility, we have studied x-ray scattering from a variety of window materials, including silicon nitride foils, muscovite mica sheets, beryllium foils and three types of thin-wall x-ray capillaries on an absolute scale over a wide Q-range (0.01-1.2 A-1). Water and air scattering have also been recorded to re-evaluate the effectiveness of the BL4-2 in-vacuum capillary cell. The results indicate that Si3N4 is by far the best performer giving the least x-ray scattering in the entire Q range by two orders of magnitude compared to capillaries, likely due to its submicron thickness. We have verified that a 200-nm thick Si(3)N(4) window with 1 mm x1 mm area can withstand a 1 bar pressure difference, making it usable as a vacuum window. Borosilicate capillaries give slightly weaker x-ray scattering than quartz and glass capillaries, but all capillaries in particular quartz scatter relatively strongly at higher angles. X-ray scattering of 10 micron thick mica is comparable to capillaries at very small angles, but much weaker by a factor of 4 to 20 at intermediary to high scattering angles, thus making it a good choice for higher angle studies. Freshly cleaved mica scatters much less than untreated mica at small angles while sanded mica scatters orders of magnitude more strongly, indicating mica sheets should be cleaved to expose fresh surface prior to their use as an x-ray window. The use of energy-resolving Vortex detector allowed us to evaluate the contribution of x-ray fluorescence from contaminating metallic impurities within the window.