Neutron imaging
Neutron imaging is a non-destructive radiographic technique that utilizes a collimated beam of neutrons to visualize the internal structure and composition of materials by detecting variations in neutron transmission through the sample.[1] Unlike X-ray imaging, which interacts primarily with electrons and is more effective for dense, high-atomic-number elements, neutron imaging relies on interactions between neutrons and atomic nuclei, enabling high sensitivity to light elements such as hydrogen, lithium, boron, and carbon, as well as superior penetration through heavy metals like lead and titanium.[2][3] This complementary capability makes it invaluable for applications where traditional methods fall short, such as imaging hydrogenous materials or dense metallic components.[4] The principles of neutron imaging are grounded in the Beer-Lambert law, which describes the exponential attenuation of the neutron beam due to absorption and scattering as it passes through the object, with the transmitted intensity captured by detectors to form spatially resolved images.[3] In practice, a thermal or cold neutron beam from sources like nuclear reactors or spallation facilities is directed through the sample, where it is converted into visible light using scintillation screens (e.g., lithium-6 doped zinc sulfide) and recorded by digital cameras such as CCD or CMOS sensors, achieving resolutions typically around 100–600 micrometers.[1][2] The technique encompasses both two-dimensional radiography for projection images and three-dimensional computed tomography, reconstructed from multiple angular projections to map internal density and elemental distributions.[5] Historically, neutron imaging emerged shortly after James Chadwick's discovery of the neutron in 1932, with the first successful radiographies demonstrated by Hartmut Kallmann and Ernst Kuhn in the late 1930s using early nuclear sources.[3] Over decades, advancements in neutron sources, detectors, and computational reconstruction have expanded its scope, from initial military and industrial uses to modern research in energy storage, geomechanics, and cultural heritage preservation.[3] Key applications include non-destructive evaluation of nuclear fuel, tracking lithium-ion diffusion in batteries, analyzing hydrogen embrittlement in metals, and studying fluid dynamics in fuel cells and geological samples, leveraging neutrons' unique isotope sensitivity and penetration power.[6][1] Ongoing developments aim for sub-micrometer spatial resolution and enhanced time-resolved imaging to address emerging challenges in materials science and engineering.[6]History
Early development
The discovery of the neutron by British physicist James Chadwick in 1932 revolutionized nuclear physics and paved the way for neutron imaging, as the particle's neutrality allowed it to penetrate materials opaque to X-rays, enabling unique contrast in radiographic applications.[7][8] Just three years later, in 1935, German physicists Hartmut Kallmann and Ernst Kuhn achieved the first successful neutron radiographs in Berlin, imaging simple objects such as pressure gauges, fire hydrants, and test tubes using a low-flux radium-beryllium isotopic neutron source supplemented by a small deuterium-tritium accelerator; their work, which continued through the late 1930s, earned a U.S. patent in 1940.[8][9] These pioneering experiments highlighted neutrons' potential for non-destructive testing but were constrained by extremely low neutron fluxes—on the order of 10^3 to 10^4 neutrons per second—necessitating exposure times of hours or days.[8] In the 1940s, progress stalled amid World War II, with early detection relying on indirect methods like nuclear emulsions sensitive to charged particles from neutron interactions and conversion screens (e.g., boron or lithium compounds) to produce visible tracks on photographic films; these techniques suffered from poor resolution, high background noise from gamma rays, and the scarcity of neutron sources beyond isotopic ones.[8][10] Following the war, the advent of nuclear reactors spurred significant expansion in the 1950s, with facilities like Argonne National Laboratory initiating systematic neutron radiography programs to leverage higher fluxes for materials inspection, exemplified by early experiments at reactors such as the BEPO at Harwell and the NRX at Chalk River, Canada—where the first dedicated imaging beamline was established in 1956 to support quantitative imaging advancements.[8][11][12]Key advancements and facilities
The introduction of scintillation screens and image intensifiers in the 1960s and 1970s marked a pivotal shift toward real-time neutron imaging capabilities. During this period, thermal neutron imaging gained traction with the availability of nuclear reactors, enabling the use of gadolinium oxysulfide scintillators coupled to electrostatic image intensifiers for dynamic observations, such as motion studies in materials. By the mid-1970s, commercial real-time systems based on these technologies became available, allowing for neutron television-like imaging with improved sensitivity over static film methods.[8][9] The 1980s and 1990s witnessed a transition to digital detectors, fundamentally enabling high-speed, real-time neutron imaging and expanding applications beyond static radiography. Advancements in charge-coupled device (CCD) cameras, paired with neutron-sensitive scintillator screens, allowed for low-light detection and digital data acquisition, initiating rapid growth in neutron tomography and radioscopy. This era's progress in computer processing and memory storage facilitated the widespread adoption of these systems, with micro-channel plate detectors emerging for time-resolved techniques.[13][8] The development of spallation neutron sources provided brighter, pulsed beams essential for high-flux imaging, overcoming limitations of reactor-based systems. The ISIS facility in the UK, operational since 1984, introduced pulsed neutron beams that supported time-resolved imaging experiments. Similarly, the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, commissioned in 2006, delivered megawatt-class proton power for enhanced neutron intensity, enabling ultrafast radiography and tomography with sub-millisecond temporal resolution. Key events in the 1990s included the optimization of CCD-based systems for neutron imaging, achieving spatial resolutions down to 50-100 micrometers and supporting 3D reconstruction via computed tomography. In the 2010s, integration of artificial intelligence revolutionized image processing, with machine learning algorithms applied for noise reduction, super-resolution enhancement, and automated feature extraction in neutron datasets. Pioneering works demonstrated deep learning models for reconstructing high-fidelity images from sparse projections, improving efficiency in data analysis for complex materials.[8][14] Global facilities continue to drive neutron imaging advancements through specialized beamlines. At the Paul Scherrer Institut (PSI) in Switzerland, the NEUTRA beamline utilizes a thermal neutron spectrum with fluxes exceeding 5 × 10^6 neutrons cm⁻² s⁻¹ mA⁻¹, supporting radiography and tomography of objects up to 30 cm × 30 cm field of view, including dual X-ray/neutron modalities and setups for highly radioactive samples via the NEURAP extension; upgrades under NEUTRA 2.0, approved post-2020, enhance resolution to below 50 μm. The Institut Laue-Langevin (ILL) in France features the NeXT beamline, optimized for high-speed imaging with fields of view from 4.1 mm² to 170 mm², enabling radiographic projections in 10 ms and full tomographies with 155 projections, incorporating polarized neutron capabilities for magnetic structure studies as of 2024. In the United States, the NIST Center for Neutron Research's Neutron Imaging Facility (NIF) offers a plug-and-play setup for in operando imaging of energy devices like fuel cells and batteries, with recent 2025 developments in curved Airy neutron beams improving scan resolution and reducing artifacts for industrial applications.[15][16][17][18][19]Fundamentals
Neutron interactions with matter
Neutrons interact with matter primarily through nuclear forces, as they carry no electric charge, allowing deep penetration compared to charged particles or electromagnetic radiation. The main types of interactions relevant to neutron imaging are transmission, where neutrons pass through without interacting; absorption, involving capture by atomic nuclei leading to reactions such as (n,γ) radiative capture or (n,α) charged particle emission; and scattering, which includes elastic scattering (conserving neutron kinetic energy while changing direction) and inelastic scattering (transferring energy to excite the nucleus). These interactions determine the attenuation and contrast in neutron images, with absorption and scattering reducing the beam intensity as it traverses the sample.[20] The probability of these interactions is quantified by cross-sections, with the microscopic cross-section σ (in barns, 1 barn = 10^{-28} m²) representing the effective interaction area per nucleus, and the macroscopic cross-section Σ (in cm^{-1}) describing the material's overall attenuation, given by Σ = ∑ n_i σ_i, where n_i is the number density of the i-th isotope; the linear attenuation coefficient μ is equivalent to Σ in this context. Unlike X-rays, which depend strongly on atomic number Z due to photoelectric effects, neutron interactions are isotope-specific and show little correlation with Z, enabling high contrast for light elements and specific isotopes. For example, boron-10 has a thermal neutron absorption cross-section of approximately 3840 barns, making it highly attenuating, while cadmium-113 exhibits a capture cross-section of about 20,000 barns, often used in shielding. Hydrogen, despite a low absorption cross-section (~0.33 barns), provides strong contrast through its high incoherent scattering cross-section of 80 barns, which scatters neutrons isotropically and reduces transmitted intensity effectively.[21][22] The transmitted neutron intensity I through a sample of thickness t follows the Beer-Lambert law:I = I_0 e^{-\mu t}
where I_0 is the incident intensity and μ incorporates both absorption and scattering contributions. This exponential attenuation forms the basis for radiographic imaging, with variations in μ across the sample creating spatial contrast. Neutron energy significantly influences these interactions: fast neutrons (energies >1 MeV) primarily undergo elastic scattering with relatively uniform cross-sections across elements, yielding lower contrast; thermal neutrons (energies ~0.025 eV), obtained via moderation, exhibit highly varied isotope-dependent cross-sections, enhancing imaging sensitivity to elemental composition.[23][24][25]