Neutron Radiography Expands Possibilities for Nondestructive Testing

During the late 19th century, Wilhelm Röntgen discovered X-rays and soon after discovered their properties for medical and industrial imaging when he created a radiograph of his wife’s hand. From this discovery, the powerful tool of X-ray radiography and tomography fell into the hands of medical professionals and industrial materials professionals.

Several decades later, during the 1930s, James Chadwick discovered the neutron, an electrically-neutral particle that resides in an atom’s nucleus. Soon afterward, the neutron was also recognized as a potential powerful tool for industrial radiography, just like X-rays.

As the technology behind X-ray imaging advanced and X-ray sources became more plentiful, X-radiography became more widely used in the field of nondestructive testing, and exhaustive quality standards were set in place to ensure that the use of this tool led to standardized and consistent results. The development of, and adherence to, these standards have helped push X-ray imaging along, leading to the development of both digital radiography, as opposed to film, and computed tomography as a powerful expansion of planar radiography into the third dimension.

Neutron radiography, or N-ray, is currently following the same trajectory, albeit several decades behind. Because neutron imaging is underutilized due to a dearth of accessible high-yield neutron sources, we’re still developing quality standards around it. For example, experts in the field of neutron imaging such as Phoenix LLC, a nuclear technology company in the greater Madison, Wisconsin, area, are currently working with ASTM committees to develop quality standards for digital neutron radiography akin to the standards that currently exist for digital X-radiography.

Despite being rarely used outside of a few manufacturing niches that rely heavily on it, N-ray has many potential applications, and recent developments in neutron sources are making it more accessible. In particular, the unique transmission properties of neutron radiation even provides planar N-ray with some advantages over X-ray CT.

What is neutron radiography?

Instead of the X-rays commonly used for industrial radiography and computed tomography, neutron radiography relies on neutron radiation, which has altogether different transmission properties. Although an X-ray’s ability to pass through a material varies linearly with the material’s density—with dense materials such as lead being excellent X-ray absorbers and lighter materials often all but invisible to it—neutrons work differently. While the science behind how neutrons interact with matter is complicated, neutrons tend to pass through some dense materials such as metals more easily than lighter materials such as plastics.

N-ray is not simply the inverse of X-ray, although at first glance it might seem so. While X-rays are transmitted very linearly with decreasing density, neutrons are more random in their behavior. If you take two elements in close proximity on the periodic table—for example, silver and cadmium—they will appear to be nearly identical when seen in an X-ray image because they are of such similar density. If you were to take a neutron image of the two elements, however, they would look completely different, and it would be easy to tell which was which. This phenomenon occurs for many other elements such as hydrogen, lithium, boron, cadmium, and gadolinium, among others. Neutrons, to put things simply, are complicated and strange things.

As a result, N-ray can provide a very different, and often very useful, perspective than planar X-ray radiography and CT. Although the methodology behind N-ray closely resembles planar X-ray imaging, the unique properties of neutron radiation also give it some advantages over 3D CT as well.

It is currently underutilized, though, due to lack of access to powerful neutron sources. Since the 1950s, only a handful of nuclear fission reactors intended for research purposes have had N-ray capabilities. However, advances in accelerator-based neutron generators, which rely on nuclear fusion and provide a high neutron output without some of the disadvantages associated with fission reactors, are shifting this paradigm. Phoenix LLC has recently opened a first-of-its-kind neutron imaging center which does not rely on a nuclear fission reactor as a neutron source.

N-ray vs. X-ray computed tomography

Compared to X-ray CT, two-dimensional neutron radiography can have several advantages, such as greater output due to higher throughput, more accurate results, and greater cost efficiency.

A typical industrial CT of one part can take up to several hours to produce, depending on the system and the user’s desired resolution. A planar neutron image will almost always take less time to produce, and will also provide much greater throughput than a CT system because many objects can be imaged in a single exposure, as opposed to CT, which can image only a single object at a time.

A Category I thermal neutron image, which meets the highest measurable threshold for quality according to ASTM standards, can provide just as much insight into a part as a 3D model produced via CT, or possibly more, depending on the part’s makeup. For example, light elements encased in metallic housings are very difficult to observe with industrial X-ray capabilities. At the higher energies required to penetrate the metallic housings, the X-rays will breeze past the light materials within, leaving the inside features completely invisible. For neutrons, however, the opposite can be true, as neutrons easily pass through many dense materials without penetrating other lighter materials. Therefore, in certain circumstances, a single 2D neutron can provide more accurate information than a CT. It all depends on what you’re inspecting and looking for.

For example, N-ray is extremely useful in aerospace manufacturing, particularly for manufacturers of jet engine turbine blades. Turbine blades are cast around ceramic molds with cooling channels to prevent them from breaking or melting due to the high temperatures of their operating environments. Due to a unique aspect of neutron radiography called gadolinium tagging, N-ray is especially suited to detecting bits of ceramic that could clog the blades’ cooling channels and lead to catastrophic failures. Although it is possible for CT to detect leftover ceramic material, CT is ill-suited to this application partly because each blade must be imaged individually; N-ray can image parts in bulk (up to 30 blades in a single image) and is in general much more reliable for finding leftover ceramic material.

N-ray is also particularly more useful than X-ray and CT for inspecting devices containing explosive material, such as parachute backups, ejection systems, rocket-stage and payload-fairing separation systems, airbags, munitions, and so on. Although CT’s 3D perspective can sometimes make up for the shortcomings of planar X-rays in penetrating the dense outer shells of these parts, it is absolutely unsuitable for large batches of very small energetic devices such as munitions because it can image only one part at a time. The unique properties of neutrons also make N-ray better for detecting oil buildup inside pipelines, metal corrosion, and adhesive bonding layers, among other use cases.

When considering whether N-ray or CT is more efficient and cost-effective, the matter gets even more complex, taking into account factors such as safety and the footprint of the system. Like X-rays, neutrons are a form of ionizing radiation, and therefore they can cause harm to living tissue. They are just about as dangerous as high-energy X-radiation, but because of their special transmission properties, neutron sources require not only roughly similar degrees of shielding to X-ray setups, but also special shielding. The typical lead door you’d find in an X-ray vault would do next to nothing to stop neutrons; they would pass through it as though it was barely there. For this reason, select materials that have high neutron-absorption cross sections, mainly hydrogen and boron, are used in sheets of plastic and high-density polyethylene to make shields against neutrons. This is a very common and effective method for radiation shielding.

Most neutron sources will require their own room due to the radiation present. At most facilities, the neutron-producing area has one room to itself, and the neutrons are transmitted to a second room that contains the radiographic instrumentation. Some small sources can be located in just one room or even out in the open environment so long as nobody is too close to them. Therefore, the footprint of an N-ray system compared to a CT system will vary depending on the size and strength of the neutron source. The question of footprint is especially relevant if you are considering installing a neutron generator onsite for convenient access to neutron imaging (which eliminates issues such as burdensome shipping logistics and some overhead costs), but not particularly relevant if you intend on relying on third-party neutron imaging vendors.

The cost-effectiveness of an N-ray system compared to a CT system, especially one installed onsite, comes into play when you consider the throughput. As mentioned above, a batch of high-volume components such as turbine blades that cannot be thoroughly inspected with planar X-ray alone would be extremely tedious to inspect using CT, while an N-ray system could inspect up to 30 blades in a single exposure. In this case, an N-ray system would recoup whatever it cost compared to a CT system rather quickly.

Complementary, not competitive

For decades, N-ray has been a little-used, costly nondestructive testing tool, since only a handful of nuclear reactor facilities existed that could provide neutron imaging to commercial clients, and nonreactor neutron sources were too small and weak to provide high-quality images. By designing a compact, powerful neutron source, Phoenix LLC has been working to bring expanded N-ray capabilities to the NDT community and help this powerful radiographic method achieve parity with X-radiography.

Phoenix created its own N-ray facility to assist industrial nondestructive testing professionals who have previously been unable to explore the possibilities of N-ray due to issues not only of cost but also accessibility. With future advancements and promotion of newer and more efficient neutron sources, as well as more neutron imaging facilities being developed, N-ray is expected to become more accessible and cost-effective. It’s following the same trend that X-ray radiography did during the decades after Röntgen’s discovery. As technology advanced, the tools became easier, more efficient, and more cost-effective to use, opening up more possibilities among industrial nondestructive testing specialists.

N-ray and X-ray should really be thought of not as competitors but as complementary methods, each with their own situational advantages and disadvantages, that can be used in harmony to better understand the composition of a part and detect possible flaws and discontinuities.

There will always be applications that planar X-ray imaging and 3D X-ray CT are better suited for, but there are some applications that cannot be done with any inspection technique other than neutron imaging. Today, there are finally accessible options for neutron sources and imaging detectors, and a burgeoning landscape of third-party vendors, that meet all users’ needs for the right applications.

Because neutron-based inspection techniques are becoming more accessible, we’re paving the way for new and advanced imaging techniques, such as combining the data from planar N-ray and X-ray into a hybrid image that shows both perspectives at once. It is also possible to perform computed tomography using neutrons instead of X-rays to produce 3D neutron images more directly analogous to the output of X-ray CT imaging.

Phoenix LLC is currently investigating the capabilities of neutron CT with great interest. These such advances in neutron-based inspection methods will give quality professionals even more tools to ensure safe, high-quality products and improve people’s lives around the world.

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