Small-angle neutron scattering

Small-angle neutron scattering (SANS) is an experimental technique that uses elastic neutron scattering at small scattering angles to investigate the structure of various substances at a mesoscopic scale of about 1 - 100 nm.

Small angle neutron scattering is in many respects very similar to small-angle X-ray scattering (SAXS); both techniques are jointly referred to as small-angle scattering (SAS). Advantages of SANS over SAXS are its sensitivity to light elements, the possibility of isotope labelling, and the strong scattering by magnetic moments.

Technique

During a SANS experiment a beam of neutrons is directed at a sample, which can be an aqueous solution, a solid, a powder, or a crystal. The neutrons are elastically scattered by nuclear interaction with the nuclei or interaction with magnetic momentum of unpaired electrons. In X-ray scattering, photons interact with the electronic cloud so bigger the element, bigger is the effect but in neutron scattering, neutrons interact with nuclei and interaction depends on isotope; some light elements like deuterium show similar scattering cross section as heavy elements like Pb.

Neutron scattering

Neutron scattering, the scattering of free neutrons by matter, can refer to either the physical process or the experimental technique which uses this process for the investigation of materials. Neutron scattering as a physical process is of primordial importance in nuclear engineering. Neutron scattering as an experimental technique is used in crystallography, physics, physical chemistry, biophysics, and materials research. It is practiced at research reactors and spallation neutron sources that provide neutron radiation of sufficient intensity. Neutron diffraction (elastic scattering) is used for determining structures; Inelastic neutron scattering is used for the study of atomic vibrations and other excitations.

Scattering of fast neutrons

"Fast neutrons" (see neutron temperature) have a kinetic energy above 1 MeV. Their scattering by condensed matter (with nuclei having kinetic energies far below 1 eV) is in a good approximation an elastic collision with a particle at rest. At each collision the fast neutron transfers a significant part of its kinetic energy to the scattering nucleus; the more so the lighter the nucleus. In this way the neutron is slowed down until it reaches thermal equilibrium with the material in which it is scattered.