Neutron radiography is a non-destructive imaging method that reveals the internal structure of a sample by subjecting it to a neutron beam. Also known as neutron imaging or neutron tomography, neutron radiography was developed and codified as an imaging technique decades ago, but has yet to be fully adopted by the wider NDT community due to over-reliance on aging nuclear reactor facilities. Phoenix is changing the paradigm surrounding this powerful testing method by providing new avenues of accessibility to NDT professionals.
Phoenix provides neutron radiography services using revolutionary high-yield neutron generators. Our neutron CT system delivers the same high-resolution image quality as reactor-based facilities, without the heavy security, safety, and logistics overhead associated with reactor facilities that have until now made neutron CT prohibitively difficult to utilize.
What Is Neutron Radiography?
The field of NDT is comprised of a wide array of techniques used to evaluate the properties and structures of materials without causing damage to them. NDT is often performed for research and development, quality assurance, or failure analysis.
Common NDT techniques to discern the inner structure of a material or component include ultrasound, liquid penetrant testing, radiography, eddy current testing, and coherence interferometry. Each technique is unique and adapted to a specialized purpose, and each has its own advantages and disadvantages.
There are several methods of radiography used by NDT professionals: gamma ray, neutron and X-ray radiography. These methods all function roughly the same way; however, the differences between X-rays, gamma rays, and neutron radiation lead to very different results when you use them to image an object.
X-rays and gamma rays interact strongly with dense materials, but pass easily through lighter materials. However, neutron radiation passes easily through many dense materials, while interacting strongly with light elements such as hydrogen. As a result, many components that would be difficult to inspect using X-ray imaging or gamma rays are more easily analyzed using neutrons.
There are many applications in the non-destructive testing industry that neutron radiography (neutron radiography is also called N-ray radiography) is well suited for. Since neutrons can penetrate high-density material in a way that X-rays cannot, neutrons are widely used for critical aerospace components with thick outer shells, such as turbine blades and energetic fuses, which are difficult to inspect using X-ray radiography. N-ray can also be used to detect water and moisture within components, as well as archaeological specimens encapsulated in material that X-ray radiography cannot penetrate.
Neutron Radiography at a Glance
- Neutron radiography is a powerful, but underused, non-destructive inspection method
- Unlike X-ray radiography, neutron radiation can easily penetrate dense materials
- Neutron radiography has application in a wide range of industries, but especially in aerospace and defense
- The use of neutron radiography has been hindered by reliance on nuclear reactor facilities, which are difficult to access due to logistical issues
Neutron radiography and tomography are proven techniques for the nondestructive testing of manufactured components in the aerospace, energy and defense sectors, not to mention numerous research applications. Neutron tomography is presently underutilized in part because of a lack of accessible, high-intensity N-ray/neutron sources. Currently, these services are mainly supplied to the NDT community via a handful of neutron imaging-capable nuclear research facilities open for commercial use because only such facilities can supply enough neutrons for imaging. By developing a high-intensity neutron source that does not rely on a reactor, Phoenix is bringing newfound reliability and ease of access to this powerful testing method.
The Physics of Neutron Radiography
Radiography works by shooting a beam of radiation at an object. Some of the beam interacts with the sample and does not pass through; the portions of the beam which do pass through collide with an imaging agent and create an image of the object’s interior (called an attenuation pattern). In the case of neutron radiography, the beam is a neutron beam, resulting products are neutron radiographs or neutron images.
X-rays and gamma rays are high-energy photons. When a photon meets an atom, it interacts mainly with the atom’s electron cloud. If the atom is of a dense element (i.e. it has more electrons), the photon cannot pass through it – in the case of X-rays, this is called X-ray attenuation. This is why the X-rays you’d get in a hospital or dental clinic can easily pass through your flesh but not your bones, teeth, or any metal implants in your body. This is also why you can’t see through opaque objects: The light behind the object is blocked by the electron clouds and can’t keep going to reach your eyes.
Neutron radiation, though, is nothing like X-ray or gamma radiation. Neutron radiation is comprised of, as the name suggests, neutrons. Because neutrons are electrically neutral, they do not interact with the atomic electrons of elements in the same way that photons or electrons do. Instead, neutrons interact with the nuclei of elements.
The atomic nucleus is astronomically tiny compared to the electron cloud, so there is a much lower chance that a neutron will interact with heavier elements. This gives neutrons a unique ability to probe objects in a way that no other imaging method can.
X-Rays Vs. Neutrons
While X-ray and gamma radiography’s ability to provide information about low-density materials, in particular when in the presence of higher density materials, are very poor, neutron radiography does not suffer from this limitation.
Just like X-rays, when neutrons pass through an object, they provide information about the internal structure of that object. X-rays interact weakly with low atomic number elements (e.g. hydrogen) and strongly with high atomic number elements (e.g. metals). On the other hand, while X-rays follow nearly linear attenuation with density, neutrons follow no such trend. Neutrons interact strongly with some very light elements with a low atomic number, such as hydrogen, while easily passing through many of the dense high atomic number elements that would give X-rays pause.
X-ray radiography and neutron radiography are complementary, rather than competing NDT techniques. Since the two interact with materials in such different ways, both techniques can provide unique insights into a sample object’s composition and internal structure.
X-Ray and Neutron Radiography Fusion
Fusing the data from X-ray methods and neutron methods of radiography is especially beneficial for gaining a better understanding of the inner workings of large, complex components. Again, while neutron radiography has existed for decades, the practice of synthesizing neutron and X-ray images is woefully underexplored. After all, the average radiography facility has the capability for X-ray radiographs, but not neutron capabilities; likewise, the average reactor facility with neutron capabilities does not have X-ray capabilities.
In 2019, Phoenix secured funding from the US Army to demonstrate new neutron-based methods of non-destructive testing, including synthesizing the data from neutron radiography and X-ray imaging. Since our upcoming neutron radiography facility will have both neutron and X-ray imaging capabilities under the same roof, naturally, we’re in the best position to investigate and develop an X-ray/neutron radiograph fusion.
Neutron Radiography Metrics
In order to provide effective, consistent neutron radiography services to all our clients, it’s vitally important to adhere to rigorous standards when it comes to service, organization, and the quality of the product itself.
Phoenix has an ISO 9001 and AS9100 certified quality management system to ensure the highest quality of service across the board for each client we see, no matter how big or how small.
For neutron imaging using film, Phoenix adheres to the global neutron radiography metrics set in place by the American Society for Testing and Materials (ASTM), including ASTM E545-14 and ASTM E748-16. Adherence to these quality standards ensures that every neutron image our clients receive from us of their sample is of the highest caliber. These two standards set quality benchmarks for a fabrication process, test outline for image quality indicators, their setup, alignment, imaging parameters, and analysis, for qualifying an image to be at a certain minimum acceptable standard.
ISO 9001, AS9100, and NADCAP certified quality management system
Film neutron imaging compatible with ASTM E545-14 and ASTM E748-16
Phoenix is helping to develop comprehensive, universal digital neutron radiography standards
Phoenix is also a pioneer in digital neutron imaging. Since digital imaging is a relatively new and little-used form of neutron imaging, there are no global guidelines set in place to standardize radiography camera image quality through this method. Phoenix is an active member of the ASTM International Neutron Imaging Committee and is collaborating to set global standards for digital neutron radiography as rigorous as the standards set in place for film imaging.
Applications of Neutron Imaging
Neutron radiography is used to reveal the internal structure of manufactured components for non-destructive testing, particularly those in the aerospace, energy, and defense industries, where reliability of components is extremely important. The imaging technique can find critical flaws, defects, or damage in these components, potentially saving money and even lives.
Neutron imaging has applications for nondestructive inspection in many manufacturing industries, including the aerospace, defense, energy, automotive, medical, and construction industries. Sample applications include:
- Detecting internal flaws in cast parts
- Detecting defects in energetic materials, including munitions
- Inspecting the internal structure of additively-manufactured components
- Detecting the presence and position of liquids such as water inside dense metal and complex assemblies
- Finding evidence of corrosion inside metal pipelines
- Identifying bonding flaws in adhesives and disbonding of carbon fiber composites
- Inspecting concrete and welds for structural integrity
- Detecting corrosion, humidity, and water contamination in electronic components and mechanical structures
- Detecting and identifying the positions of o-rings, seals, lubricants, and adhesives inside complex assemblies
- Finding defects in silicon nitride ceramics
- Imaging the behavior of lubricating and cooling oils in refrigeration components
- Measuring the effectiveness of moisture repelling agents
- Imaging the propagation of shockwaves in gaseous bodies
- Imaging the patterns and dynamics of fluid sprays
- Measuring the concentrations of boron in radiation shielding materials
The Advantages of Neutron Radiography
Neutrons can pass easily through high-density metals and provide detailed information about internal, low density materials. This property is extremely important for several components that require nondestructive evaluation including engine turbine blades, munitions, spacecraft components, and composites such as wind turbine blades. For all these applications, neutron radiography provides definitive information that X-ray imaging cannot.
Neutrons and Contrast Agents
The physical properties of neutrons give neutron imaging other unique advantages over X-rays in nondestructive inspection.
For example, parts for inspection can be “tagged” with contrast agents such as gadolinium to better identify defects in the material that would have otherwise gone unnoticed.
Gadolinium, for example, has a high absorption rate for neutrons. When a neutron collides with an atom of gadolinium, it is absorbed by the nucleus, exciting the atom. When a gadolinium atom absorbs a stray neutron, it releases energy. By dousing a part such as a turbine blade in gadolinium before imaging it, small cracks, pores, and bits of foreign material will appear to glow in the resulting neutron image due to the gadolinium settling there. This makes some flaws that might be invisible both to neutrons as well as X-rays show up in stark contrast. This interaction between neutrons and elements with high neutron cross-sections not only has great potential in neutron tomography, but also in medical treatments such as neutron capture therapy.
The Aerospace Advantage
Contrast agent tagging plays a vital role in neutron imaging for materials such as jet engine turbine blades. Turbine blades are cast in a ceramic mold with cooling channels that prevent them from failing during operation. When the blades are removed from their molds, bits of ceramic may end up stuck in the cooling channels. If the cooling channels are blocked, the blades may melt from the high temperatures in their operational environment. These flaws can only be detected by tagging the turbines with a contrast agent and imaging them with a neutron source.
Neutron imaging has been used by the aerospace community for decades due to its utility in quality assurance for turbine blades and other vital components. Phoenix’s innovation in neutron generators will bring a new level of accessibility to this critical aerospace NDT technique.
The Explosives Advantage
In the realm of energetic material, neutron imaging has clear advantages over other forms of industrial radiography. Munitions, aircraft and spacecraft ejection mechanisms, and airbag modules all depend on delicately calibrated amounts of explosive chemicals to function properly. These kinds of devices all consist of a metal shell surrounding the energetic material. X-rays cannot penetrate the shell to show the condition of the lighter chemicals within; however, neutrons can. Using neutron tomography for quality assurance and failure analysis, manufacturers of energetic devices can root out products that are defective and harmful.
The Electronics Advantage
Because neutrons react strongly to hydrogen, neutron imaging is a useful tool in the electronics and semiconductor industry for picking up the presence of water or moisture in electronic components. It can be used in electronics research applications, such as design for weather-resistant electronics and failure analysis.
Neutron Radiography as a Service
The Phoenix system is the first commercially viable neutron generator for neutron radiography. It is a fraction of the size and cost of a nuclear reactor but strong enough to power a high-throughput neutron radiography system. Our system will allow for onsite, real-time neutron tomography of manufactured components in a professional factory setting.
The Phoenix Neutron Imaging Center, which will begin accepting new orders from clients in early 2020, utilizes a Phoenix neutron generator to provide neutron radiography services to industry and academia. PNIC and future facilities using a high-yield, non-reactor neutron source in lieu of a reactor will make this powerful non-destructive testing tool easier to utilize than ever before.