Absolute scaling of small- and wide-angle X-ray scattering images recorded with short duration X-ray pulses on a large area fiber-taper X-ray detector

We have developed an X-ray scattering setup capable of capturing time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) images spanning q = 0.02 to over 5.2 Å^-1 on a single, large area Rayonix MX340-HS detector with time resolution as short as 120 ps. A key feature of this setup is a 0.51 mm-diameter partially transmissive beamstop that enables non-invasive, image-by-image recording of direct beam position and intensity during acquisition. This beamstop attenuates 12 keV undulator radiation by approximately eight orders of magnitude while suppressing off-axis second harmonic radiation. Continuous monitoring of the direct beam position facilitates long-term beam alignment and allows datasets acquired at different times to be placed on a common absolute scale prior to differencing. The accuracy of the difference scattering curves is ultimately limited by the performance of the large area, fiber-taper X-ray detector used in this study. To address accuracy issues, we present a detailed statistical characterization of the detector readout noise and responsivity and introduce a variance-per-count statistic that enables identification of zinger-free averages, generation of precise uniformity corrections, and statistically weighted conversion of two-dimensional scattering images into near shot-noise-limited one-dimensional scattering curves. The detector point-spread function and its effects on resolution and scaling are examined using scattering data from a fused silica plate and from apoferritin solution in a capillary. The ability to acquire high accuracy, high precision scattering curves over a broad range of q and temperatures provides a robust foundation for time-resolved studies of biomolecular structure and dynamics in solution.