Due to their interaction with matter, X-rays traversing a sample are deviated (scattered) from their path. The exit angle of the scattered photon with respect to its original path depends on the size and arrangement of the deviating structure, with larger structures deviating photons at smaller angles (this is described by Bragg's Law). Therefore, Small Angle X-ray Scattering (SAXS) provides information on long-range order, such as the shape and size distribution of nanoparticles and their possible interaction, the size and arrangement of nanometre-sized pores in a material, the diameter and orientation of fibers in tissues and composites or the outer envelope of a protein in solution, to name but just a few.
Because the beam footprint is typically much larger than the size of the structures it illuminates, SAXS analysis yields quantitative average values over the whole irradiated volume with high statistical significance.
Synchrotron radiation is highly brilliant (i.e. the X-ray beam is very parallel), and thus it is ideal for SAXS due to the low data smearing from beam divergence. Further, the tuneability of synchrotron radiation allows the resolution of the instrumental setup to be adapted and optimized to the sample under study. Finally, the high intensity of synchrotron X-rays makes time-resolved experiments a routine, following changes in the sample as they occur. Typical SAXS endstations can also accommodate a wide range of ancillary equipment to perform simultaneous measurements with complementary techniques and/or to study processes in situ.
SAXS is well established as a standard characterization tool in all fields of nanoscience. In biology, its ability to determine the low-resolution structure (outer envelope) of proteins in solution makes it an ideal investigation technique for difficult systems, especially in combination with complementary techniques. In grazing-incidence geometry, SAXS is a powerful tool to characterize thin films and buried interfaces in surface science.
Due to the flexibility in the sample environment, there are virtually no limits in the kinds of experiments one can do with SAXS (temperature, pH and pressure jumps or ramps, chemical reactions, mechanical tests, high magnetic or electrical fields, photoactivation...). It is also common to combine SAXS and other techniques (IR, UV/VIS or fluorescence spectroscopy, ellipsometry etc).
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