Small angle neutron scattering (SANS) uses elastic neutron scattering of a cold neutron beam (< 5 meV) at small scattering angles to investigate larger scale structures in substances, i.e. at a mesoscopic scale covering the range 10 – 1000 Å. This includes studies of polydispersity, structure and interactions in a wide range of disordered materials to study precipitations\defects structures in metals and ceramics, surfactants, polymers, liquid crystals, nanomaterials, biological molecules, lipids, fibres, etc. The objective of a SANS experiment is to determine the differential cross-section that contains all the information on the shape, size and interactions of the scattering bodies (assemblies of scattering centres) in the sample. By changing the sample detector distance, SANS instruments typically have a sensitivity larger than 3 decades of reciprocal space.
SANS instruments include conventional monochromatic elastic, or time-of-flight modes of operation as discussed below:
In conventional SANS the incoming cold neutron beam is monochromatic and the direction and spatial distribution of the coherently scattered beam measured with a large area detector. Incoherent scattering may be present due to hydrogen-containing samples, but this is mostly suppressed using deuteration, in particular in biological samples.
ToF-SANS: Time-of-flight small angle neutron scattering
SANS can also be conducted in time-of-flight (ToF) mode which enables controlling the wavelength resolution within a range (3.5% and 30%) to give a wide dynamic Q-range (in order of thousand) within a single measurement. By pulsing the neutron beam with mechanical choppers, neutrons scattered at various wavelengths as a function of their time-of-flight renders a complete data set in a single instrument setting.
USANS: Ultra Small Angle Neutron Scattering
The USANS technique extends the accessible Q-range to enable quantitatively characterisation of shape or structure of materials into the micrometer length scale, 0.1 to 50 micron, by using a diffraction approach. USANS achieves high angular resolution through the use of multiple reflections of the neutron beams respectively before and after the sample using perfect silicon channel-cut crystals set to 45⁰ in the beams. The incident crystal set selects the wavelength, whist the diffracted beam side is an analyser stage. USANS is useful for studies of pores and cracks in rocks, cement or engineering materials, very large biological or polymer molecules or macromolecular assemblies, and mesoscopic magnetic particles.