branch of physics dealing with the theory and applications of the wave behaviour (wave-particle duality) of neutrons (neutron), the electrically neutral subatomic particles that are present in all atomic nuclei except those of ordinary hydrogen. Neutron optics involves studying the interactions of matter with a beam of free neutrons, much as spectroscopy represents the interaction of matter with electromagnetic radiation. There are two major sources of free neutrons for neutron-beam production: (1) the neutrons emitted in fission (nuclear fission) reactions at nuclear reactors (nuclear reactor) and (2) the neutrons released in particle-accelerator (particle accelerator) collisions of proton beams with targets of heavy atoms, such as tantalum. When a neutron beam is directed onto a sample of matter, the neutrons can be reflected, scattered, or diffracted, depending on the composition and structure of the sample and on the properties of the neutron beam. All three of these processes have been exploited in the development of analytic methods, with important applications in physics, chemistry, biology, and materials science. Among the diverse achievements in the field of neutron optics, neutron-scattering studies have yielded insight into the fundamental nature of magnetism, probed the detailed structure of proteins embedded in cell membranes, and provided a tool for examining stress and strain in jet engines.

In contrast to fast neutrons, which act more exclusively as particles when they strike materials, slow, or “thermal,” neutrons (slow neutron) have longer wavelengths—about 10⁻¹⁰ metre, comparable in scale to the distance between atoms in crystals—and thus exhibit wavelike behaviour in their interactions with matter. Slow neutrons scattered by the atoms in a solid undergo mutual interference (similar to the behaviour of X-rays and light) to form diffraction patterns from which details of crystal structure and magnetic properties of solids can be deduced. The American physicist Clifford G. Shull (Shull, Clifford G.) and the Canadian physicist Bertram N. Brockhouse (Brockhouse, Bertram N.) shared the 1994 Nobel Prize for Physics for their development of the complementary techniques and applications of neutron diffraction (elastic scattering) and neutron spectroscopy (inelastic scattering).