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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 on 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.[1]

Article Draft

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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.[2] 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.[3] Neutrons are subatomic particles that exist in the nucleus of the atom, it has higher mass than protons but no electrical charge.

History and timeline

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1930-40: 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. [4]

1940s: First neutron scattering experiment carried out. At Oak Ridge Laboratory Ernest Wollan and Clifford Shull led groundbreaking neutron diffraction experiments which gave insights into atomic arrangements within materials.[5]

1950-60s: The development of neutron sources such as 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.[6]

1970-80s: This period saw major advancements in neutron scattering techniques by developing techniques to explore different aspects of material science, structure and behaviour.[7]

a)  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. [7]

b) Inelastic neutron scattering (INS): Provides insights into the dynamic process at the microscopic level. Majorly used to examine atomic and molecular motions.[7]

1990-present: 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. and ISIS Neutron and Muon Source in the U.K., which can generate pulsed neutron beams for time-of-flight experiments. [8] Neutron imaging and reflectometry were also developed, which are powerful tools to analyse surfaces, interfaces and thin film structures, thus providing valuable insights into the material properties.[9] The European Spallation Source (ESS), currently under development in Sweden, is set to enhance neutron scattering and thus development of an enhanced neutron facility to drive new scientific discoveries will be developed.[10]

In 1949, Ernest Wollan and Clifford Shull conducted experiments using a double-crystal neutron spectrometer positioned on the southern side of the ORNL graphite reactor to collect data.

Comparison of neutron scattering, XRD and electron scattering

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Feature Neutron diffraction X-ray diffraction Electron scattering
Principle Interacts with atomic nuclei and magnetic moments enabling nulear and magnetic scattering [11] Scatter off electron cloud thus allowing probing of electron density.[12] Scatter off electron cloud thus allowing probing of electron density.[13]
Penetration depth High (suitable to study bulk materials since neutrons penetrate deeply in)[11] Moderate (good penetration but also absorption by heavy elements)[12] Low (suitable for surface studies since electrons are strongly absorbed)[13]
Sensitivity to light elements High (very sensitive to lighter elements like hydrogen or lithium) [11] Low (poor sensitvity to lighter elements)[12] Moderate (can detect lighter elements but not as good as neutron)[13]
Magnetic studies Excellent (can probe magnetic stucture and spin dynamics) [11] Limited (require specialized techniques like resonance magnetic scattering )[12] Limited (not as efficient as neutron scattering)[13]
Resolution High (depending on techniques and instrument) [11] High (provides with spatial resolution for crystal structure)[12] Very high (can achieve high resolution )[13]
Sample environment Efficient (used to study samples in different environment) [11] Not efficient (gets disrupted) or absorbed by heavy elements)[12] Limited (requires vaccum and thin samples)[13]
Applications structure of materials and magnetic property of the material. [11] crystal structure determination[12] -  used in surface and interface studies

- can also be used in defect analysis[13]

Principle

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Processes

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Neutrons are produced through three major processes

Fission

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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.[14]

Spallation

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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 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.[15]

Illustration of three major fundamental processes generating neutrons for scattering experiments: Nuclear fission (Top), Spallation (middle), Low energy reaction (bottom).

Low energy nuclear reactions

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Low-energy nuclear reactions are the basis of neutron production in accelerator-driven sources. The selected targetmaterials are based on the energy levels; lighter metals such as lithium (Li) and beryllium (Be) can be used toachieve 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.[16]

De-Broglie relation

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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)[15]

where: h is Planck’s constant, p is the momentum of the neutron, m is the mass of the neutron, v is the velocity of the neutron.

Principles

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Neutron scattering is used to detect the distance between atoms and study the dynamics of materials. It involves two major principles.

Elastic scattering
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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.[11]

Inelastic scattering
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Physics of inelastic scattering, however, is centered around 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.[17]

Neutron matter interaction

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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.[18][19]

Applications

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Study of hydrogen storage materials

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Since neutron diffraction is particularly sensitive to lighter elements like hydrogen, it can be used for its detection. It can play a role in determining the crystal structure and hydrogen binding sites within metal hydrides, a class of materials of interest for hydrogen storage applications. The order of hydrogen atoms in the lattice reflects the storage capacity and kinetics of the material.[20]

Magnetic structure determination

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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.[21]

Phase transition in ferroelectrics

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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.  [22]

Residual stress analysis in engineering materials

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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.[23]

Lithium-ion batteries

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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.[24]

High temperature superconductors

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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.[25]

Mechanical behaviour of alloys

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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.[26]

Neutron diffraction for ion channels

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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 combined with molecular imaging could show the voltage sensing domain of ion channels (blue, red, yellow) disrupts the surrounding bilayer membrane (yellow). This interaction led to slight thinning of the membrane

Current developments in neutron diffraction

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Advancements in Neutron Diffraction Research

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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.[27]

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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.[28]

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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.[29]

References

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  1. ^ "The use of neutrons for materials characterization", Analysis of Residual Stress by Diffraction using Neutron and Synchrotron Radiation, CRC Press, pp. 15–39, 2003-02-06, ISBN 978-0-429-21190-4, retrieved 2025-02-21
  2. ^ Harkins, William D. (1917). "The Evolution of the Elements and the Stability of Complex Atoms". Journal of the American Chemical Society. 39 (5): 856–879. doi:10.1021/ja02250a002. ISSN 0002-7863.
  3. ^ "The existence of a neutron". Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character. 136 (830): 692–708. 1932. doi:10.1098/rspa.1932.0112. ISSN 0950-1207.
  4. ^ Amaldi, E.; Fermi, E. (1936). "On the Absorption and the Diffusion of Slow Neutrons". Physical Review. 50 (10): 899–928. doi:10.1103/PhysRev.50.899. ISSN 0031-899X.
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  13. ^ a b c d e f g Squires, Gordon Leslie. introduction to the Theory of Thermal Neutron Scattering.
  14. ^ LEMBO, MARY FRANCES (2006). Nuclear engineering. p. 15. ISBN 9780429224515.
  15. ^ a b Carpenter, John M. (2015). Elements of slow-neutron scattering: basics, techniques, and applications. Cambridge: Cambridge University Press. ISBN 978-1-139-02931-5.
  16. ^ Ashkar, Rana; Bilheux, Hassina Z.; Bordallo, Heliosa; Briber, Robert; Callaway, David J. E.; Cheng, Xiaolin; Chu, Xiang-Qiang; Curtis, Joseph E.; Dadmun, Mark; Fenimore, Paul; Fushman, David; Gabel, Frank; Gupta, Kushol; Herberle, Frederick; Heinrich, Frank (2018-12-01). "Neutron scattering in the biological sciences: progress and prospects". Acta Crystallographica Section D Structural Biology. 74 (12): 1129–1168. doi:10.1107/S2059798318017503. ISSN 2059-7983.
  17. ^ "Introduction to the Theory of Thermal Neutron Scattering; Dover Publications". Dover publications.
  18. ^ Bucknall, David (2012). "Introduction to the Theory of Thermal Neutron Scattering, 3rd edn., by G.L. Squires: Scope: textbook. Level: early career researchers, researchers, specialists, scientists". Contemporary Physics. 53 (6): 544–545. doi:10.1080/00107514.2012.745613. ISSN 0010-7514.
  19. ^ Dianoux, A. J., ed. (2003). Neutron data booklet (2 ed.). Institut Laue-Langevin Philadelphia, PA: Old City. ISBN 978-0-9704143-7-3.
  20. ^ Ravnsbæk, Dorthe B.; Filinchuk, Yaroslav; Cerný, Radovan; Jensen, Torben R. (2010). "Powder diffraction methods for studies of borohydride-based energy storage materials". Zeitschrift für Kristallographie. 225 (12): 557–569. doi:10.1524/zkri.2010.1357. ISSN 0044-2968.
  21. ^ Lines, M. E.; Jones, E. D. (1965). "Antiferromagnetism in the Face-Centered Cubic Lattice. II. Magnetic Properties of MnO". Physical Review. 139 (4A): A1313 – A1327. doi:10.1103/PhysRev.139.A1313. ISSN 0031-899X.
  22. ^ Jorio, A.; Currat, R.; Myles, D. A. A.; McIntyre, G. J.; Aleksandrova, I. P.; Kiat, J. M.; Saint-Grégoire, P. (2000). "Ferroelastic phase transition in Cs 3 Bi 2 I 9 : A neutron diffraction study". Physical Review B. 61 (6): 3857–3862. doi:10.1103/PhysRevB.61.3857. ISSN 0163-1829.
  23. ^ Jacob, Anais; Oliveira, Jeferson; Mehmanparast, Ali; Hosseinzadeh, Foroogh; Kelleher, Joe; Berto, Filippo (2018). "Residual stress measurements in offshore wind monopile weldments using neutron diffraction technique and contour method". Theoretical and Applied Fracture Mechanics. 96: 418–427. doi:10.1016/j.tafmec.2018.06.001.
  24. ^ Ziesche, Ralf F.; Kardjilov, Nikolay; Kockelmann, Winfried; Brett, Dan J.L.; Shearing, Paul R. (2022). "Neutron imaging of lithium batteries". Joule. 6 (1): 35–52. doi:10.1016/j.joule.2021.12.007.
  25. ^ Moodenbaugh, A. R.; Cox, D. E.; Vining, C. B.; Segre, C. U. (1984). "Neutron-diffraction study of magnetically ordered Er 2 Fe 3 Si 5". Physical Review B. 29 (1): 271–277. doi:10.1103/PhysRevB.29.271. ISSN 0163-1829.
  26. ^ Sun, C.; Brown, D.W.; Clausen, B.; Foley, D.C.; Yu, K.Y.; Chen, Y.; Maloy, S.A.; Hartwig, K.T.; Wang, H.; Zhang, X. (2014). "In situ neutron diffraction study on temperature dependent deformation mechanisms of ultrafine grained austenitic Fe–14Cr–16Ni alloy". International Journal of Plasticity. 53: 125–134. doi:10.1016/j.ijplas.2013.07.007.
  27. ^ "Future of Neutron Scattering at Oak Ridge National Laboratory: Three World Leading Neutron Scattering Facilities for Breakthrough Materials Science".
  28. ^ Ehlers, Georg; Crow, Morris L.; Diawara, Yacouba; Gallmeier, Franz X.; Geng, Xiaosong; Granroth, Garrett E.; Gregory, Raymond D.; Islam, Fahima F.; Knudson, Robert O.; Li, Fankang; Loyd, Matthew S.; Vacaliuc, Bogdan (2022). "Modern Trends in Neutron Scattering Instrument Technologies". Instruments. 6 (3): 22. doi:10.3390/instruments6030022. ISSN 2410-390X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  29. ^ Kono, Fumiaki; Kurihara, Kazuo; Tamada, Taro (2022). "Current status of neutron crystallography in structural biology". Biophysics and Physicobiology. 19 (0): n/a. doi:10.2142/biophysico.bppb-v19.0009. ISSN 2189-4779. PMC 9135615. PMID 35666700.{{cite journal}}: CS1 maint: PMC format (link)
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