Neutron diffraction

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.

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

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)

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