EVALUATION OF PARASITIC SCATTERING FROM WINDOW MATERIALS IN SAXS MEASUREMENTS

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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. We measured x-ray scattering intensities from a variety of commonly used materials for X-ray windows including silicon nitrite membranes, thin mica sheets and three types of thin-wall x-ray capillaries (glass, quartz and borosilicate). Silicon nitrite membranes give the lowest level of parasitic scattering over the entire Q-range primarily due to their extreme thinness (nominally 200 nm). Thin mica sheets (single crystal alumina silicate, 10-25 micron thick), especially those freshly cleaved to minimize surface scratches, exhibit a comparably low signal level in the small Q region as the silicon nitrite membranes, but scatter about an order of magnitude more at higher Q values. We observed unexpectedly large differences among the different capillaries due to the variations in wall thickness and different degree of amorphous structures. Quartz and borosilicate capillaries give a lower level of x-ray scattering than glass capillaries in the low Q-region, whereas glass capillaries give weaker and flatter scattering profile in the higher Q range. In general, the parasitic scattering signal at low Q values is dominated by the surface structures and depends on the surface quality of the window material as demonstrated by the significantly lower level of scattering from freshly cleaved mica sheets, whereas the signal level at higher Q values is primarily governed by the bulk material structures, thus depends on the thickness and composition of the window material. We conclude that one should take into account the range of momentum transfer of interest for the experiment for selecting the best window material.