Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of the structure of the material. The technique is similar to X-ray diffraction but, due to their different scattering properties, neutrons and X-rays provide complementary information: X-Rays are suited for superficial analysis, strong x-rays from synchrotron radiation are suited for shallow depths or thin specimens, while neutrons having high penetration depth are suited for bulk samples.Measurement of residual stress in materials using neutrons, IAEA, 2003
History
Discovery of the neutron
In 1921, American chemist and physicist William D. Harkins introduced the term "neutron" while studying atomic structure and nuclear reactions. He proposed the existence of a neutral particle within the atomic nucleus, though there was no experimental evidence for it at the time. In 1932, British physicist James Chadwick provided experimental proof of the neutron's existence. His discovery confirmed the presence of this neutral subatomic particle, earning him the Nobel Prize in Physics in 1935. Chadwick's research was influenced by earlier work from Irène and Frédéric Joliot-Curie, who had detected unexplained neutral radiation but had not recognized it as a distinct particle. Neutrons are subatomic particles that exist in the nucleus of the atom, they have higher mass than protons but zero [net] electrical charge.
In the 1930s Enrico Fermi and colleagues gave theoretical contributions establishing the foundation of neutron scattering. Fermi developed a framework to understand how neutrons interact with atomic nuclei.
Early diffraction work Diffraction was first observed in 1936 by two groups, von Halban and Preiswerk and Mitchell and Powers. In 1944, Ernest O. Wollan, with a background in X-ray scattering from his PhD work under Arthur Compton, recognized the potential for applying thermal neutrons from the newly operational X-10 nuclear reactor to crystallography. Joined by Clifford G. Shull they developed neutron diffraction throughout the 1940s.
Neutron diffraction experiments were carried out in 1945 by Ernest O. Wollan using the Graphite Reactor at Oak Ridge. He was joined shortly thereafter (June 1946) by Clifford Shull, and together they established the basic principles of the technique, and applied it successfully to many different materials, addressing problems like the structure of ice and the microscopic arrangements of magnetic moments in materials. For this achievement, Shull was awarded one half of the 1994 Nobel Prize in Physics. (Wollan died in 1984). (The other half of the 1994 Nobel Prize for Physics went to Bert Brockhouse for development of the inelastic scattering technique at the Chalk River facility of AECL. This also involved the invention of the triple axis spectrometer).
1950–60s The development of neutron sources such as nuclear reactors and spallation sources emerged. This allowed high-intensity neutron beams, enabling advanced scattering experiments. Notably, the High Flux Isotope Reactor (HFIR) at Oak Ridge and Institut Laue–Langevin (ILL) in Grenoble, France emerged as key institutions for neutron scattering studies.
1970–1980s This period saw major advancements in neutron scattering techniques by developing techniques to explore different aspects of material science: structure and behaviour.
Small-angle neutron scattering (SANS): Used to investigate large-scale structural features in materials. The works of Glatter and Kratky also helped in the advancements of this method, though it was primarily developed for X-rays.
Inelastic neutron scattering (INS): Provides insights into the dynamic process at the microscopic level. Majorly used to examine atomic and molecular motions.
1990-present Recent advancements focus on improved sources, using sophisticated detectors and enhanced computational techniques. Spallation sources have been developed at Spallation Neutron Source (SNS) in the U.S. and ISIS Neutron and Muon Source in the U.K., which can generate pulsed neutron beams for time-of-flight experiments. Neutron imaging and neutron reflectometry were also developed, which are powerful tools to analyse surfaces, interfaces, and thin film structures, thus providing valuable insights into the material properties.
Comparison of neutron scattering, XRD, and electron scattering +FeatureNeutron diffractionX-ray diffractionElectron scatteringPrincipleInteracts with atomic nuclei and magnetic moments enabling nuclear and magnetic scattering Scatters off electron cloud thus allowing probing of electron density.Scatters off electrostatic potential thus allowing probing of electron density.Penetration depthHigh (suitable to study bulk materials since neutrons penetrate deeply in)Moderate (good penetration but also absorption by heavy elements)Low (suitable for surface studies since electrons are strongly absorbed) or quite deep, depending upon the energy.Sensitivity to light elementsHigh (very sensitive to lighter elements like hydrogen or lithium)Low (poor sensitivity to lighter elements)High (can detect lighter elements).Magnetic studiesExcellent (can probe magnetic structure and spin dynamics)Limited (require specialized techniques like resonance magnetic scattering)Yields local informationResolutionHigh (depending on techniques and instrument)High (can yield very precise positions for crystal structure)Very high (can achieve high resolution)Sample environmentEfficient (used to study samples in different environment)EfficientLimited (requires vacuum and thin samples)ApplicationsStructure of materials and magnetic property of the material.Structure of materials, see x-ray diffractionUsed for bulk materials, surfaces, defects, see electron diffraction.
Principle
Processes Neutrons are produced through three major processes: fission, spallation, and low-energy nuclear reactions.
Fission In research reactors, fission takes place when a fissile nucleus, such as uranium-235 (235U), absorbs a neutron, and subsequently splits into two smaller fragments. This process releases energy along with additional neutrons. On average, each fission event produces about 2.5 neutrons. While one neutron is required to maintain the chain reaction, the surplus neutrons can be utilized for various experimental applications.
Spallation In spallation sources, high-energy protons (on the order of 1 GeV) bombard a heavy metal target (e.g., uranium (U), tungsten (W), tantalum (Ta), lead (Pb), or mercury (Hg)). This interaction causes the nuclei to spit out neutrons. Proton interactions result in around ten to thirty neutrons per event, of which the bulk are known as "evaporation neutrons" (~2 MeV), while a minority are known as "cascade neutrons" with energies reaching up to the GeV range. Although spallation is a very efficient technique of neutron production, the technique generates high-energy particles, therefore requiring shielding for safety.
Low-energy nuclear reactions Low-energy nuclear reactions are the basis of neutron production in accelerator-driven sources. The selected target materials are based on the energy levels; lighter metals such as lithium (Li) and beryllium (Be) can be used to achieve their maximum possible reaction rate under 30 MeV, while heavier elements such as tungsten (W) and carbon (C) provide better performance above 312 MeV. These Compact Accelerator-driven Neutron Sources (CANS) have matured and are now approaching the performance of fission and spallation sources.
De-Broglie relation Neutron scattering relies on the wave-particle dual nature of neutrons. The De-Broglie relation links the wavelength (λ) of a neutron to its energy (E)
where h is the Planck constant, p is the momentum of the neutron, m is the mass of the neutron, v is the velocity of the neutron.
Scattering Neutron scattering is used to detect the distance between atoms and study the dynamics of materials. It involves two major principles: elastic scattering and inelastic scattering.
Elastic scattering provides insight into the structural properties of materials by looking at the angles at which neutrons are scattered. The resulting pattern of the scattering provides information regarding the atomic structure of crystals, liquids, and amorphous materials.
Inelastic scattering focuses on material dynamics through the study of neutron energy and momentum changes during interactions. It is key to study phonons, magnons, and other excitations of solid materials.
Neutron–matter interaction X- rays interact with matter through electrostatic interaction by interacting with the electron cloud of atoms, this limits their application as they can be scattered strongly from electrons. While being neutral, neutrons primarily interact with matter through the short-range strong force with atomic nuclei. Nuclei are far smaller than the electron cloud, meaning most materials are transparent to neutrons and allow deeper penetration. The interaction between neutrons and nuclei is described by the Fermi pseudopotential, that is, neutrons are well above their meson mass threshold, and thus can be treated effectively as point-like scatterers. While most elements have a low tendency to absorb neutrons, certain ones such as cadmium (Cd), gadolinium (Gd), helium (3He), lithium (6Li), and boron (10B) exhibit strong neutron absorption due to nuclear resonance effects. The likelihood of absorption increases with neutron wavelength (σa ∝ λ), meaning slower neutrons are absorbed more readily than faster ones.
Instrumental and sample requirements The technique requires a source of neutrons. Neutrons are usually produced in a nuclear reactor or spallation source. At a research reactor, other components are needed, including a crystal monochromator (in the case of thermal neutrons), as well as filters to select the desired neutron wavelength. Some parts of the setup may also be movable. For the long-wavelength neutrons, crystals cannot be used, and gratings are used instead as diffractive optical components. At a spallation source, the time of flight technique is used to sort the energies of the incident neutrons (higher energy neutrons are faster), so no monochromator is needed, but rather a series of aperture elements synchronized to filter neutron pulses with the desired wavelength.
The technique is most commonly performed as powder diffraction, which only requires a polycrystalline powder. Single-crystal diffraction is also possible, but the crystals must be much larger than those that are used in single-crystal X-ray crystallography. It is common to use crystals that are about 1 mm3.Paula M. B. Piccoli, Thomas F. Koetzle, Arthur J. Schultz "Single Crystal Neutron Diffraction for the Inorganic Chemist—A Practical Guide" Comments on Inorganic Chemistry 2007, Volume 28, 3-38.
The technique also requires a device that can detect the neutrons after they have been scattered.
Summarizing, the main disadvantage to neutron diffraction is the requirement for a nuclear reactor. For single-crystal work, the technique requires relatively large crystals, which are usually challenging to grow. The advantages to the technique are many - sensitivity to light atoms, ability to distinguish isotopes, absence of radiation damage, and a penetration depth of several cm.
Nuclear scattering