NEUTRON OPTICS

branch of physics dealing with the wave behaviour of neutrons, the electrically neutral subatomic particles that are present in all atomic nuclei except those of ordinary hydrogen. A beam of free neutrons, produced in nuclear reactors by ejecting neutrons from the core of atoms in reactions such as fission, is reflected, scattered, or diffracted by materials on which the beam is directed. In particular, in contrast to fast neutrons, which act more exclusively as particles when they strike materials, slow neutrons have longer wavelengths of about 10-10 metre (or about an angstrom unit), which corresponds to the distance between atoms in crystals. Slow neutrons scattered by the atoms in solids 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 are deduced.