The phenomenon associated with the interference processes which occur when neutrons are scattered by the atoms within solids, liquids, and gases. The use of neutron diffraction as an experimental technique is relatively new compared to electron and x-ray diffraction, since successful application requires high thermal-neutron fluxes, which can be obtained only from nuclear reactors. (A thermal neutron is defined as a neutron possessing a kinetic energy of about 0.025 eV.) These diffraction investigations are possible because thermal neutrons have energies with equivalent wavelengths near 0.1 nm and are therefore ideally suited for interatomic interference studies.