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.

|Scatters off electron cloud thus allowing probing of electron density.

|Scatters off electrostatic potential thus allowing probing of electron density.

|Penetration depth

|High (suitable to study bulk materials since neutrons penetrate deeply in)

|Sample environment

|Efficient (used to study samples in different environment)

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.

thumb|Three major fundamental processes generating neutrons for scattering experiments: nuclear fission (top), spallation (middle), low-energy reaction (bottom)

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)

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.

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,

Magnetic scattering does require an atomic form factor, as it is caused by the much larger electron cloud around the tiny nucleus. The intensity of the magnetic contribution to the diffraction peaks will therefore decrease toward higher angles.

Uses

Neutron diffraction can be used to determine the static structure factor of gases, liquids, or amorphous solids. Most experiments, however, aim at the structure of crystalline solids, making neutron diffraction an important tool of crystallography.

Neutron diffraction is closely related to X-ray powder diffraction. In fact, the single-crystal version of the technique is less commonly used because currently available neutron sources require relatively large samples and large single crystals are hard or impossible to come by for most materials. Future developments, however, may well change this picture. Because the data is typically a 1D powder diffractogram they are usually processed using Rietveld refinement. In fact the latter found its origin in neutron diffraction (at Petten in the Netherlands) and was later extended for use in X-ray diffraction.

One practical application of elastic neutron scattering/diffraction is that the lattice constant of metals and other crystalline materials can be very accurately measured. Together with an accurately aligned micropositioner a map of the lattice constant through the metal can be derived. This can easily be converted to the stress field experienced by the material.

Another use is for the determination of the solvation number of ion pairs in electrolytes solutions.

The magnetic scattering effect has been used since the establishment of the neutron diffraction technique to quantify magnetic moments in materials, and study the magnetic dipole orientation and structure. One of the earliest applications of neutron diffraction was in the study of magnetic dipole orientations in antiferromagnetic transition metal oxides such as manganese, iron, nickel, and cobalt oxides. These experiments, first performed by Clifford Shull, were the first to show the existence of the antiferromagnetic arrangement of magnetic dipoles in a material structure. Neutron diffraction continues to be used to characterize newly developed magnetic materials.

Hydrogen, null-scattering, and contrast variation

Neutron diffraction can be used to establish the structure of low atomic number materials like proteins and surfactants much more easily with lower flux than at a synchrotron radiation source. This is because some low atomic number materials have a higher cross section for neutron interaction than higher atomic weight materials.

One major advantage of neutron diffraction over X-ray diffraction is that the latter is rather insensitive to the presence of hydrogen (H) in a structure, whereas the nuclei 1H and 2H (i.e. deuterium, D) are strong scatterers for neutrons. The greater scattering power of protons and deuterons means that the position of hydrogen in a crystal and its thermal motions can be determined with greater precision by neutron diffraction. The structures of metal hydride complexes, e.g., Mg2FeH6 have been assessed by neutron diffraction.

The neutron scattering lengths bH = −3.7406(11) fm and bD = 6.671(4) fm,

Magnetic structure determination

Neutron diffraction is also a useful technique for determining magnetic structures in materials, as neutrons can interact with magnetic moments. It can be used to determine the antiferromagnetic structure of manganese oxide (MnO) using neutron diffraction. Neutron Diffraction Studies can be used to measure the magnetic moment. Orientation study demonstrates how neutron diffraction can detect the precise alignment of the magnetic moment in materials, something that is much more challenging with X-rays.

Phase transition in ferroelectrics

Neutron diffraction has been widely employed to understand phase transitions in materials including ferroelectrics, which show the transition of crystal structure with temperature or pressure. It can be utilised to study the ferroelectric phase transition in lead titanate (PbTiO3). It can be used to analyse atomic displacements and corresponding lattice distortions.

Residual stress analysis in engineering materials

Neutron diffraction can be used as a technique for the nondestructive assessment of residual stresses in engineering materials, including metals and alloys. Also used for measuring residual stresses in engineering materials.

Lithium-ion batteries

Neutron diffraction is especially useful for the investigation of lithium-ion battery materials, because lithium atoms are almost opaque to X-ray radiation. It can further be used to investigate the structural evolution of lithium-ion battery cathode materials during charge and discharge cycles.

High-temperature superconductors

Neutron diffraction has played an important role in revealing the crystal and magnetic structures in high-temperature superconductors. A neutron diffraction study of magnetic order in the high-temperature superconductor YBa2Cu3O6+x was done. The work of each of these scientific teams together with others across the globe has revealed the origins of the relationship between magnetic ordering and superconductivity, delivering crucial insights into the mechanism of high-temperature superconductivity.

Mechanical behaviour of alloys

Advancements in neutron diffraction have facilitated in situ investigations into the mechanical deformation of alloys under load, permitting observations on the mechanisms of deformation. The deformation behavior of titanium alloys under mechanical loads can be investigated using in situ neutron diffraction. This technique allows real-time monitoring of lattice strains and phase transformations throughout deformation.

thumb|Neutron diffraction, used along with molecular simulations, revealed that an ion channel's voltage sensing domain (red, yellow, and blue molecule at center) perturbs the two-layered cell membrane that surrounds it (yellow surfaces), causing the membrane to thin slightly.

Neutron diffraction for ion channels

Neutron diffraction can be used to study ion channels, highlighting how neutrons interact with biological structures to reveal atomic details. Neutron diffraction is particularly sensitive to light elements like hydrogen, making it ideal for mapping water molecules, ion positions, and hydrogen bonds within the channel. By analysing neutron scattering patterns, researchers can determine ion binding sites, hydration structures, and conformational changes essential for ion transport and selectivity.

Current developments in neutron diffraction

Advancements in neutron diffraction research

Neutron diffraction has made significant progress, particularly at Oak Ridge National Laboratory (ORNL), which operates a suite of 12 diffractometers—seven at the Spallation Neutron Source (SNS) and five at the High Flux Isotope Reactor (HFIR). These instruments are designed for different applications and are grouped into three categories: powder diffraction, single-crystal diffraction, and advanced diffraction techniques.

To further enhance neutron diffraction research, ORNL is undertaking several key projects:

Expansion of the SNS First Target Station: New beamlines equipped with state-of-the-art instruments are being installed to broaden the scope of scientific investigations.
Proton Power Upgrade: This initiative aims to double the proton power used for neutron production, which will enhance research efficiency, allow for the study of smaller and more complex samples, and support the eventual development of a next-generation neutron source at SNS.
Development of the SNS Second Target Station: A new facility is being constructed to house 22 beamlines, making it a leading source for cold neutron research, crucial for studying soft matter, biological systems, and quantum materials.
Enhancements at HFIR: Planned upgrades include optimizing the cold neutron guide hall to improve experimental capabilities, expanding isotope production (including plutonium-238 for space exploration), and enhancing the performance of existing instruments.

These advancements are set to significantly improve neutron diffraction techniques, allowing for more precise and detailed analysis of material structures. By expanding research capabilities and increasing neutron production efficiency, these developments will support a wide range of scientific fields, from materials science to energy research and quantum physics.

Modern trends in neutron scattering information technology

Neutron diffraction technology is evolving rapidly, with a focus on improving beam intensity and instrument efficiency. Modern instruments are designed to produce smaller, more intense beams, enabling high-precision studies of smaller samples, which is particularly beneficial for new material research. Advanced detectors, such as boron-based alternatives to helium-3, are being developed to address material shortages, while improved neutron spin manipulation enhances the study of magnetic and structural properties. Computational advancements, including simulations and virtual instruments, are optimizing neutron sources, streamlining experimental design, and integrating machine learning for data analysis. Multiplexing and event-based acquisition systems are enhancing data collection by capturing multiple datasets simultaneously. Additionally, next-generation spallation sources like the European Spallation Source (ESS) and Oak Ridge's Second Target Station (STS) are increasing neutron production efficiency. Lastly, the rise of remote-controlled experiments and automation is improving accessibility and precision in neutron diffraction research.

Current trends in structural biology

Modern advancements in neutron diffraction are enhancing data precision, broadening structural research applications, and refining experimental methodologies. A key focus is the improved visualization of hydrogen atoms in biological macromolecules, crucial for studying enzymatic activity and hydrogen bonding. The expansion of specialized diffractometers has increased accessibility in structural biology, with techniques like monochromatic, quasi-Laue, and time-of-flight methods being optimized for efficiency. Innovations in sample preparation, particularly protein deuteration, are minimizing background noise and reducing the need for large crystals. Additionally, computational tools, including quantum chemical modeling, are aiding in the interpretation of complex molecular interactions. Improved neutron sources, such as spallation facilities, along with advanced detectors, are further boosting measurement accuracy and structural resolution. These developments are solidifying neutron diffraction as a critical technique for exploring the molecular architecture of biological systems.

References

External links

National Institute of Standards and Technology Center for Neutron Research
From Bragg's law to neutron diffraction
Integrated Infrastructure Initiative for Neutron Scattering and Muon Spectroscopy (NMI3) - a European consortium of 18 partner organisations from 12 countries, including all major facilities in the fields of neutron scattering and muon spectroscopy
Frank Laboratory of Neutron Physics of Joint Institute for Nuclear Research (JINR)
IAEA neutron beam instrument database