Instrumental developments for anomalous small-angle X-ray scattering from soft matter systems

An optimized instrument for anomalous small-angle X-ray scattering from charged soft matter is described. The experimental setup takes special care for single-photon detection sensitivity, high energy resolution of the monochromator, in situ calibration of intensity and energy, and the avoidance of radiation damage. Measured intensities are normalized to an absolute scale online, which can be further decomposed to resonant and non-resonant contributions. The performance of the instrument is demonstrated by an example involving cationic surfactant micelles with bromide counter-ions. The counter-ion profile around the micelle is deduced from the analysis of anomalous scattering near the K-absorption edge of bromine. Two different approaches yield similar results for the radial profile of the counter-ions, showing strong condensation of the counter-ions on the micellar surface, in agreement with the inference from electrochemical methods.