In a SANS experiment a beam of neutrons is directed at a sample, which can be a solution, a solid, a
powder, or a
crystal. The neutrons are elastically scattered by interaction with the nuclei or interaction with magnetic momentum of unpaired electrons. In X-ray scattering, photons interact with the electron cloud so the bigger the element, the bigger the effect. In neutron scattering, neutrons interact with nuclei and the interaction depends on the
isotope; some light elements like hydrogen show similar scattering cross section as heavy elements like
lead. In zero order
dynamical theory of diffraction the
refractive index is directly related to the
scattering length density and is a measure of the strength of the interaction of a neutron wave with a given nucleus. The following table shows the
neutron scattering length for a few chemical elements (in fm (femtometers)). The relative scale of the scattering lengths is the same. Also, the scattering from hydrogen H is distinct from that of
deuterium. Hydrogen is one of the few elements that has a negative scattering length, which means that neutrons deflected from hydrogen are 180° out of phase relative to those deflected by the other elements. These features are important for the technique of contrast variation (see below).
Related techniques SANS usually uses collimation of the neutron beam to determine the scattering angle of a neutron, which results in an ever lower signal-to-noise ratio for data that contains information on the properties of a sample at relatively long length scales, beyond ~1 μm. The traditional solution is to increase the brightness of the source, as in ultra-small-angle neutron scattering (USANS). As an alternative spin-echo small-angle neutron scattering (SESANS) was introduced, using
neutron spin echo to track the scattering angle, and expanding the range of length scales which can be studied by neutron scattering to well beyond 10 μm.
Grazing-incidence small-angle scattering (GISANS) combines ideas of SANS and of
neutron reflectometry. ==In biology==