Neutron Scattering

Abstract

Neutron scattering, which includes diffraction (ND), inelastic (INS), quasi‐elastic (QENS), and transmission/absorption techniques, is one of the most direct ways to probe the microscopic structure, dynamics, and compositional distribution of bulk condensed matter. The properties of the neutron include: no charge; a mass, m n , equal to 1.675 × 10 −27 kg; and a magnetic moment, μ n , equal to 0.9662 × 10 −26 J T −1 . Although neutrons scatter weakly, but more or less uniformly, from the nucleus of most elements across the periodic table, there are several elements that scatter neutrons rather well. Neutrons are extremely good for investigating compounds bearing light elements, especially hydrogen, deuterium, carbon, and oxygen as well as magnetic compounds. A representative sample of the scattering properties of different nuclei (see Table 2) has been tabulated (including a graphic illustration). This is in stark contrast to X‐ray scattering, in which the principal interaction (and scattering) of the massless X‐ray photon is from the electron cloud; so the scattering amplitude increases with the atomic number. Hence, neutrons are a very versatile probe of condensed matter and have been used to characterize the atomic arrangement in crystalline and amorphous solids and liquids (magnetic materials in particular) via diffraction, lattice vibrations, molecular motion, and diffusion (both rotational and translational). Neutron beams with energies (1 meV–1 eV) and wavelengths (0.5–20 Å) are ideally suited for investigating condensed matter and can be produced in two ways. The first is from a reactor source, where a purposefully built, steady state or pulsed reactor produces neutrons as the result of nuclear fission. The second is the spallation source, where a steady or pulsed stream of energetic protons collides with a solid or liquid target; the resulting collision and thermalization process causes neutrons to be knocked out or spalled from the target. The energy and wavelength of these neutron beams can be adjusted using thermal moderators chosen to shift the peak in the energy distribution of the beam. Specially designed instrumentation that uses either crystal optics or time‐of‐flight (TOF) methods, is used to analyze the angular and energy distribution of the scattered neutrons as a result of the physical processes from the sample under study. This contribution will briefly review the production and instrumentation at both reactor and steady state sources. Because of the vast number of studies that have been performed using thermal neutrons, only a limited set of examples of how neutrons have been used to characterize crystal structure, dynamic modes in solids (phonons), diffusion and phase transformations of inorganic materials can be presented here. Finally, a brief introduction will be provided for the use of neutrons to investigate surface states of adsorbed species because, with the advent of new high intensity neutron sources and the production of nanometer scale materials with high surface to volume ratios, it is a topic that is undoubtedly going to receive significant attention in the near future.