Neutron imaging (also known as neutron tomography or neutron radiography) is a useful and powerful tool for manufacturers in a wide range of industries, including aerospace, defense, and healthcare. By using neutron imaging, non-destructive testing professionals can help manufacturers in these industries design better, more efficient, and safer products, in addition to providing an invaluable resource for quality assurance and failure analysis.
However, though neutron imaging has existed for decades and the benefits it offers to the non-destructive testing community are well known, it is a widely underutilized non-destructive testing method due to its reliance on difficult-to-access nuclear research reactor facilities.
Neutron Imaging Services at Phoenix’s Imaging Facility
Our state-of-the-art radiographic imaging center is the first facility of its kind to offer commercial neutron imaging services without the use of a reactor alongside X-ray imaging. The imaging center’s compact, accelerator-based neutron imaging system produces a spatial resolution of the highest measurable image quality in a neutron picture by ASTM standards. With an ISO 9001:2015 and AS9100-2015 (Rev. D) certified Quality Management System, our imaging center guarantees exceptional service.
Located in Fitchburg, Wisconsin, the 10,000 square-foot neutron imaging and testing center provides neutron imaging services along with X-ray radiography and other neutron radiography solutions, like neutron activation analysis and radiation effects testing. Our imaging center and future facilities of its kind are one-stop, comprehensive radiography resources for the non-destructive testing community.
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How Neutron Imaging Works
In neutron imaging, a neutron source produces a beam of neutron radiation and directs the neutron beam at an object. Some neutrons pass through the object, while others do not, depending on a material’s neutron attenuation. Neutrons interact more heavily with certain light materials, like plastics and hydrogen, but have a greater penetration depth for dense materials like lead and steel. The neutrons that pass through the object produce an image, sometimes also called a neutron radiograph, showing the internal structure of the object.
Most non-destructive testing and inspection professionals rely on nuclear fission reactors instead of neutron beams to perform neutron imaging. The reactions within the reactor between free neutrons and rods of fissile uranium create even more neutrons, which can be repurposed for neutron radiography. However, only a few nuclear reactors (built for science research, not power generation) are available for use, and there are high logistical costs due to the size of the reactor, demand, security, and safety regulations associated with highly radioactive material, etc.
Our neutron imaging system features revolutionary accelerator-based technology that provides a high neutron yield without the size, safety, or security concerns associated with nuclear reactors. By using a compact particle accelerator, the system creates a beam of ions to collide with a target composed of isotopes of hydrogen. The collision causes nuclear fusion reactions which create neutron radiation – this radiation is the neutron beam used to generate neutron images.
Unlike reactor facilities, our system takes up relatively little space, is easy to operate, and does not produce the heavy radioactive elements found in nuclear reactors. Our neutron generators mitigate the risks and accessibility issues of relying on aging nuclear reactors. In addition to providing neutron radiography as a service, we also build neutron sources that are small enough to be installed onsite and user-friendly enough to be used with minimal training, helping clients in manufacturing and materials research and testing acquire N-ray imaging capabilities of their own – not to mention other applications of neutron radiography.
How is neutron imaging different from X-ray imaging?
The methodology behind neutron imaging is almost identical to X-ray imaging, but these imaging methods have a crucial difference: one uses neutron radiation and the other uses X-rays.
X-rays are photons more energetic than ultraviolet light but less energetic than gamma rays. Many materials, such as your skin, are transparent to X-rays, while denser materials, such as bone and metal, are opaque. X-rays interact with the electron clouds of other atoms. Since denser materials are made up of atoms with bigger electron clouds, X-rays cannot penetrate dense materials. This causes what is called X-ray attenuation, the reduction in the energy of X-rays as they pass through matter.
Neutrons are neutral particles found within the nuclei of atoms that can be stripped from the nucleus as a byproduct of nuclear fusion or fission reactions. Because neutrons have neither a positive nor a negative charge, the only way neutrons interact with other atoms is by colliding with the nucleus, which is astronomically small compared to the electron cloud. Because of this, neutron radiation is capable of easily passing through many dense materials that X-rays cannot. On the other hand, neutrons interact strongly with many low mass elements, such as hydrogen, resulting in a shallower penetration depth.
As a result, neutron imaging has many applications in the field of nondestructive testing where X-ray alone may come up short.
Neutron Imaging Applications
Applications of neutron imaging include quality assurance, materials research, prototyping, failure analysis, and many other areas of manufacturing.
Such applications include:
- Detecting internal flaws in cast parts
- Ensuring loading uniformity in munitions
- Showing defects in low-density and energetic materials
- Visualizing the internal structure of additively manufactured components
- Detecting the presence and position of liquid inside dense metal or enclosures
- Finding evidence of corrosion inside metal pipelines
- Detecting bonding flaws in adhesives
- Inspecting welds for structural integrity
- Detecting humidity and water contamination in electronic components
- Identifying the presence and position of o-rings, seals, lubricants, and adhesives inside complex assemblies
- Analyzing the internal distribution and movement of water in fuel cells
Neutron imaging has applications in many industries, including:
Our neutron radiography and X-ray radiography capabilities include:
- Thermal neutron imaging
- Fast neutron imaging
- X-ray imaging
- 3D X-ray CT
- 3D neutron CT
Internal flaws in cast parts
For components which are cast in molds, neutron imaging is a critical tool for quality assurance. For example, turbine blades require thorough QA testing as they rely on molded cooling vents to function properly in the engine without melting (since the operational environment of a jet engine exceeds the melting point of the material the turbines are made of). However, during the casting process, bits of ceramic material can get caught in the blade’s cooling channels, which will impede its ability to regulate its temperature. Neutron radiography with gadolinium tagging is the best way to detect trace amounts of ceramic blockage within turbine blades.
Loading uniformity in munitions and defects in energetic materials
Because neutrons can penetrate dense outer shells and visualize low-density, light materials within, neutron imaging has vital applications for quality assurance in munitions and energetic materials, including the devices used in aircraft ejection systems, spacecraft payload deployment, airbag modules, and so on.
Small defects in the chemical makeup of the energetic material in these devices can show up as gaps, bubbles, voids, or cracks. These defects can indicate a whole host of potential issues, including unforeseen chemical reactions that have lessened or increased the potency of the energetic material, which would lead to costly, hazardous, or even life-threatening misfires. These defects can only be visualized using a radiography method capable of penetrating the casing around the material – neutron radiography.
Many neutron radiography facilities have heavily restricted capabilities for imaging energetic material. Our imaging center has an energetic material handling program compatible with ATF and DoD Safety and Security manuals which allows up to 31 lbs of HD1.1 material.
The inner structure of additively manufactured parts
Additive manufacturing, or industrial 3D-printing, is a revolutionary new method for cheaply and easily fabricating components. Its benefits include making it much more convenient to rapidly design and redesign prototypes and to create components out of composites that would be difficult or unfeasible to create using traditional subtractive manufacturing methods.
Composite materials, which often combine denser substances with light materials to create new substances that can take advantage of the properties of both, drive innovation in manufacturing industries, especially in aerospace. More commonly used radiography methods, such as X-ray tomography, are not always capable of visualizing the composition of composites. Neutron radiography can be a useful tool in helping manufacturers with design and production of new composite materials using additive manufacturing.
Neutron Imaging Methods & Capabilities
Fast neutron imaging capabilities
Fast neutron imaging relies on high-energy neutron radiation (greater than 1 MeV). Of the neutrons produced by our neutron generators, a portion travel through a collimator, which directs them into a straight beam through the object to be imaged. The neutrons which pass through the object create an image depicting the object’s inner structure. Fast, high-energy neutrons are best suited for applications like imaging large or very dense objects.
- Neutron imaging of parts up to 14” x 17” x 12”
- Potential for longer parts with image indexing
Thermal neutron imaging capabilities
Thermal neutron imaging creates extremely low-energy neutron radiation by running the neutrons produced by our neutron source through a moderator filled with water. Since water does not readily absorb neutrons but rather scatters and deflects them, the neutrons scatter and collide with the water molecules, losing more and more energy with each collision until the neutrons reach thermal equilibrium, or room temperature (roughly 0.025 eV).
Because the neutrons have been slowed down, they pass through the object more slowly than fast neutrons, resulting in longer exposure times. Due to the longer exposure times required, they are much better suited for imaging small objects with a high amount of detail and clarity.
- ASTM Category 1 quality level
- Neutron imaging of parts up to 14” x 17” x 6”
While N-ray imaging has many benefits over X-ray imaging, the two are complementary technologies at heart. X-ray and N-ray images of the same object can yield very different information regarding its structure, and both sets of data can be of vital importance. We are developing methods to synthesize the data obtained in N-ray and X-ray imaging into detailed composite images, granting manufacturers and materials testing professionals never-before-seen information to truly understand the materials and components they’re working with.
Film neutron imaging vs. digital imaging
Most neutron imaging facilities use film in their imaging systems. When you take a photo or shoot video using film, the film stock reacts chemically to the light that hits it, producing an image. Film neutron imaging works the same way; however, neutrons do not interact directly with film. Instead, the neutrons pass through the film and collide with a conversion layer directly behind it composed of gadolinium, boron, lithium, or any other material with a high neutron cross-section. As the neutrons interact with the conversion layer, the particles given off by the reaction between the neutrons and the conversion layer create the image that appears on the film.
Just as you would do with film in any other imaging methods, the image then must be developed in a darkroom.
There are a wide variety of digital neutron imaging techniques as well as film. The digital neutron imaging method used at our imaging center is known as photostimulable luminescence (PSL). For PSL, as with film, the neutrons react with a conversion layer made from photostimulable phosphor. The atoms which interact with the neutrons gain extra energy, which is picked up by a sensor to create a digital image.
Digital neutron imaging is still in its infancy, but offers many benefits over film imaging. Among other things, digital imaging does not require a darkroom and chemical treatment to develop the image, nor does it require intimate contact between the conversion layer and film.
One downside to digital imaging is that unlike film, there are no universal quality standards set in place for digital neutron imaging. Most organizations which perform digital neutron imaging set their own internal standards. We are working closely with ASTM International, a materials testing organization that sets globally-adopted quality standards for materials, products, systems, and services, to codify rigorous universal standards for digital neutron imaging to ensure that all facilities which utilize such produce the same detailed, high-quality results and spatial resolution.
Standards and Qualifications
- Film neutron imaging compatible with ASTM E545-14 and ASTM E748-16
- Phoenix is actively working with ASTM to help develop a standard for digital neutron radiography
- ISO 9001 and AS9100 certified quality management system
- ASNT certification program-compliant with SNT-TC-1A and NAS-410
- Energetic materials handling programs compatible with ATF and DoD Safety and Security manuals
Training and Research
Phoenix is preparing training programs for certification of neutron generator system operators and neutron radiography NDT methods, including:
- Neutron activation analysis
- Neutron gauging
- Epoxy cements and potting material inspections
You can learn more about our currently offered neutron radiography interpretation course for NDT professionals here: