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Neutron diffraction orelastic neutron scattering is the application ofneutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam ofthermal or coldneutrons to obtain a diffraction pattern that provides information of the structure of the material. The technique is similar toX-ray diffraction but due to their different scattering properties,neutrons andX-rays provide complementary information: X-Rays are suited for superficial analysis, strong x-rays fromsynchrotron radiation are suited for shallow depths or thin specimens, while neutrons having high penetration depth are suited for bulk samples.[1]
In 1921, American chemist and physicistWilliam D. Harkins introduced the term "neutron" while studyingatomic structure andnuclear reactions. He proposed the existence of a neutral particle within theatomic nucleus, though there was no experimental evidence for it at the time.[2] In 1932, British physicistJames Chadwick provided experimental proof of the neutron’s existence. His discovery confirmed the presence of this neutralsubatomic particle, earning him theNobel Prize in Physics in 1935. Chadwick’s research was influenced by earlier work fromIrène andFrédéric Joliot-Curie, who had detected unexplained neutralradiation but had not recognized it as a distinct particle.[3] Neutrons are subatomic particles that exist in the nucleus of the atom, it has higher mass than protons but no electrical charge.
In the 1930sEnrico Fermi and colleagues gave theoretical contributions establishing the foundation ofneutron scattering. Fermi developed a framework to understand how neutrons interact with atomic nuclei.[4]
Diffraction was first observed in 1936[5] by two groups, von Halban and Preiswerk[6] and by Mitchell and Powers.[7] In 1944,Ernest O. Wollan, with a background in X-ray scattering from his PhD work[8] underArthur Compton, recognized the potential for applying thermal neutrons from the newly operationalX-10 nuclear reactor tocrystallography. Joined byClifford G. Shull they developed[9] neutron diffraction throughout the 1940s.
Neutron diffraction experiments were carried out in 1945 byErnest O. Wollan using the Graphite Reactor atOak Ridge. He was joined shortly thereafter (June 1946)[10] byClifford 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 1994Nobel Prize in Physics. (Wollan died in 1984). (The other half of the 1994 Nobel Prize for Physics went toBert Brockhouse for development of the inelastic scattering technique at theChalk River facility ofAECL. This also involved the invention of the triple axis spectrometer).
The development of neutron sources such asreactors andspallation sources emerged. This allowed high-intensityneutron beams, enabling advanced scattering experiments. Notably, thehigh flux isotope reactor (HFIR) at Oak Ridge and Institut Laue Langevin (ILL) in Grenoble, France, emerged as key institutions for neutron scattering studies.[11]
This period saw major advancements in neutron scattering techniques by developing techniques to explore different aspects of material science, structure and behaviour.[12]
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 forX-rays.[12]
Inelastic neutron scattering (INS): Provides insights into the dynamic process at the microscopic level. Majorly used to examine atomic and molecular motions.[12]
Recent advancements focus on improved sources, using sophisticated detectors and enhanced computational techniques. Spallation sources have been developed at SNS (Spallation Neutron Source) in the U.S. andISIS Neutron and Muon Source in the U.K., which can generate pulsed neutron beams fortime-of-flight experiments.Neutron imaging andreflectometrywere also developed, which are powerful tools to analyse surfaces, interfaces and thin film structures, thus providing valuable insights into the material properties.
| Feature | Neutron diffraction | X-ray diffraction | Electron scattering |
|---|---|---|---|
| Principle | Interacts with atomic nuclei and magnetic moments enabling nuclear and magnetic scattering[13] | Scatter offelectron cloud thus allowing probing of electron density.[14][15] | Scatter off electrostatic potential thus allowing probing of electron density.[16] |
| Penetration depth | High (suitable to study bulk materials since neutrons penetrate deeply in)[13] | Moderate (good penetration but also absorption by heavy elements)[14][15] | Low (suitable for surface studies since electrons are strongly absorbed) or quite deep depending upon the energy.[16] |
| Sensitivity to light elements | High (very sensitive to lighter elements like hydrogen or lithium)[13] | Low (poor sensitvity to lighter elements)[14][15] | High (can detect lighter elements ).[16] |
| Magnetic studies | Excellent (can probemagnetic stucture and spin dynamics)[13] | Limited (require specialized techniques like resonance magnetic scattering)[14][15] | Yields local information[16] |
| Resolution | High (depending on techniques and instrument)[13] | High (can yield very precise positions for crystal structure)[14][15] | Very high (can achieve high resolution )[17] |
| Sample environment | Efficient (used to study samples in different environment)[13] | Efficient | Limited (requires vaccum and thin samples)[17] |
| Applications | structure of materials and magnetic property of the material.[13] | X-ray crystallography[14][15] | Used for bulk materials, surfaces, defects, seeelectron diffraction |
Neutrons are produced through three major processes, fission, spallation, and Low energy nuclear reactions.[citation needed]
In research reactors, fission takes place when a fissile nucleus, such asuranium-235 (235U), absorbs a neutron and subsequently splits into two smaller fragments. This process releases energy along with additional neutrons. On average, eachfission event produces about 2.5 neutrons. While one neutron is required to maintain thechain reaction, the surplus neutrons can be utilized for various experimental applications.[18]
In spallation sources, high-energy protons (on the order of 1GeV) bombard a heavy metal target (e.g.,uranium (U),tungsten (W),tantalum (Ta),lead (Pb), ormercury (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 identified 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.[19]
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 aslithium (Li) andberyllium (Be) can be used toachieve their maximum possible reaction rate under 30 MeV, while heavier elements such as tungsten (W) andcarbon (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.[21]
Neutron scattering relies on the wave-particle dual nature of neutrons. TheDe-Broglie relation links thewavelength (λ) of a neutron to its energy (E)[19]
where:h is Planck’s constant,p is themomentum of the neutron,m is the mass of the neutron,v is thevelocity of the neutron.
Neutron scattering is used to detect the distance between atoms and study the dynamics of materials. It involves two major principles:elastic scattering andinelastic 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.[13]
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.[22]
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 theFermi pseudopotential, that is, neutrons are well above theirmeson mass threshold, and thus can be treated effectively as point-likescatterers. While most elements have a low tendency to absorb neutrons, certain ones such ascadmium (Cd),gadolinium (Gd),helium (³He),lithium (⁶Li), andboron (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.[23][24]
The technique requires a source of neutrons. Neutrons are usually produced in anuclear reactor orspallation source. At aresearch reactor, other components are needed, including acrystal 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.[25] 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 aspowder diffraction, which only requires a polycrystalline powder. Single crystal work is also possible, but the crystals must be much larger than those that are used in single-crystalX-ray crystallography. It is common to use crystals that are about 1 mm3.[26]
The technique also requires a device that candetect 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,[26] as well as a penetration depth of several cm[1]
Like allquantumparticles, neutrons can exhibit wave phenomena typically associated with light or sound.Diffraction is one of these phenomena; it occurs when waves encounter obstacles whose size is comparable with thewavelength. If the wavelength of a quantum particle is short enough, atoms or their nuclei can serve as diffraction obstacles. When a beam of neutrons emanating from a reactor is slowed and selected properly by their speed, their wavelength lies near oneangstrom (0.1nanometer), the typical separation between atoms in a solid material. Such a beam can then be used to perform a diffraction experiment. Impinging on a crystalline sample, it will scatter under a limited number of well-defined angles, according to the sameBragg's law that describes X-ray diffraction.
Neutrons and X-rays interact with matter differently. X-rays interact primarily with theelectron cloud surrounding each atom. The contribution to the diffracted x-ray intensity is therefore larger for atoms with largeratomic number (Z). On the other hand, neutrons interact directly with thenucleus of the atom, and the contribution to the diffracted intensity depends on eachisotope; for example, regular hydrogen and deuterium contribute differently. It is also often the case that light (low Z) atoms contribute strongly to the diffracted intensity, even in the presence of large Z atoms. The scattering length varies from isotope to isotope rather than linearly with the atomic number. An element likevanadium strongly scatters X-rays, but its nuclei hardly scatters neutrons, which is why it is often used as a container material. Non-magnetic neutron diffraction is directly sensitive to the positions of the nuclei of the atoms.
The nuclei of atoms, from which neutrons scatter, are tiny. Furthermore, there is no need for anatomic form factor to describe the shape of the electron cloud of the atom and the scattering power of an atom does not fall off with the scattering angle as it does for X-rays.Diffractograms therefore can show strong, well-defined diffraction peaks even at high angles, particularly if the experiment is done at low temperatures. Many neutron sources are equipped with liquid helium cooling systems that allow data collection at temperatures down to 4.2 K. The superb high angle (i.e. highresolution) information means that the atomic positions in the structure can be determined with high precision. On the other hand,Fourier maps (and to a lesser extentdifference Fourier maps) derived from neutron data suffer from series termination errors, sometimes so much that the results are meaningless.
Although neutrons are uncharged, they carry amagnetic moment, and therefore interact with magnetic moments, including those arising from the electron cloud around an atom. Neutron diffraction can therefore reveal the microscopicmagnetic structure of a material.[27]
Magnetic scattering does require anatomic 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 towards higher angles.
Neutron diffraction can be used to determine thestatic structure factor ofgases,liquids oramorphous solids. Most experiments, however, aim at the structure of crystalline solids, making neutron diffraction an important tool ofcrystallography.
Neutron diffraction is closely related to X-raypowder diffraction.[28] 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 usingRietveld 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 thelattice constant ofmetals 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 thestress field experienced by the material.[1] This has been used to analyse stresses inaerospace andautomotive components to give just two examples. The high penetration depth permits measuring residual stresses in bulk components as crankshafts, pistons, rails, gears. This technique has led to the development of dedicated stress diffractometers, such as theENGIN-X instrument at theISIS neutron source.
Neutron diffraction can also be employed to give insight into the 3D structure any material that diffracts.[29][30]
Another use is for the determination of thesolvation 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 inantiferromagnetic 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.[31] Now, neutron diffraction continues to be used to characterize newly developed magnetic materials.
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 ofhydrogen (H) in a structure, whereas the nuclei1H and2H (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 ofmetal hydride complexes, e.g.,Mg2FeH6 have been assessed by neutron diffraction.[32]
The neutron scattering lengthsbH = −3.7406(11) fm[33] andbD = 6.671(4) fm,[33] for H and D respectively, have opposite sign, which allows the technique to distinguish them. In fact there is a particularisotope ratio for which the contribution of the element would cancel, this is called null-scattering.
It is undesirable to work with the relatively high concentration of H in a sample. The scattering intensity by H-nuclei has a large inelastic component, which creates a large continuous background that is more or less independent of scattering angle. The elastic pattern typically consists of sharpBragg reflections if the sample is crystalline. They tend to drown in the inelastic background. This is even more serious when the technique is used for the study of liquid structure. Nevertheless, by preparing samples with different isotope ratios, it is possible to vary the scattering contrast enough to highlight one element in an otherwise complicated structure. The variation of other elements is possible but usually rather expensive. Hydrogen is inexpensive and particularly interesting, because it plays an exceptionally large role in biochemical structures and is difficult to study structurally in other ways.
Since neutron diffraction is particularly sensitive to lighter elements likehydrogen, it can be used for its detection. It can play a role in determining thecrystal structure and hydrogen binding sites withinmetal hydrides, a class of materials of interest for hydrogen storage applications. The order of hydrogen atoms in thelattice reflects the storage capacity and kinetics of the material.[34]
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 theantiferromagnetic structure ofmanganese oxide (MnO) using neutron diffraction. Neutron Diffraction Studies can be used to measure themagnetic 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.[35]
Neutron diffraction has been widely employed to understand phase transitions in materials includingferroelectrics, which show the transition of crystal structure withtemperature orpressure. It can be utilised to study the ferroelectricphase transition inlead titanate (PbTiO₃). It can be used to analyseatomic displacements and corresponding lattice distortions. [36]
Neutron diffraction can be used as a technique for the nondestructive assessment of residual stresses in engineering materials, includingmetals andalloys. Also used for measuring residual stresses in engineering materials.[37]
Neutron diffraction is especially useful for the investigation oflithium-ion battery materials, because lithium atoms are almostopaque 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.[38]
Neutron diffraction has played an important role in revealing the crystal and magnetic structures in high-temperaturesuperconductors. 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 betweenmagnetic ordering andsuperconductivity, delivering crucial insights into the mechanism ofhigh-temperature superconductivity.[39]
Advancements in neutron diffraction have facilitated in situ investigations into the mechanical deformation of alloys under load, permitting observations on the mechanisms ofdeformation. The deformation behavior oftitanium 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.[40]
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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.
Neutron diffraction has made significant progress, particularly at Oak Ridge National Laboratory (ORNL), which operates a suite of 12 diffractometers—seven at theSpallation Neutron Source (SNS) and five at theHigh 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:
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 toenergy research andquantum physics.[41]
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 asboron-based alternatives tohelium-3, are being developed to address material shortages, while improved neutron spin manipulation enhances the study of magnetic and structural properties. Computational advancements, includingsimulations and virtual instruments, are optimizingneutron sources, streamlining experimental design, and integratingmachine 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.[42]
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 biologicalmacromolecules, crucial for studyingenzymatic activity andhydrogen bonding. The expansion of specializeddiffractometers has increased accessibility in structural biology, with techniques likemonochromatic, quasi-Laue, and time-of-flight methods being optimized for efficiency. Innovations in sample preparation, particularly protein deuteration, are minimizingbackground 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.[43]
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