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.