Industrial Radiography: Trends, Market Drivers, and Alternatives to Gamma-based Devices

Abstract

Industrial radiography (IR) is a nondestructive testing (NDT) modality used in industries such as oil and gas, automotive, and aerospace. The technique is commonly used to evaluate the structural integrity and uniformity of materials and components, similar to how medical x rays are used to identify breaks in bones. IR employs highly penetrating ionizing radiation (gamma rays, x rays, or neutrons) to produce a visible record of the internal conditions of an object, thus identifying any imperfections or defects. In applications where portability and accessibility are essential, such as the inspection of oil and gas pipelines, radiographic cameras (IR cameras) are commonly used. These devices usually enclose a radioactive isotope and are designed to be portable, making them easy to transport from one location to another. However, their portability combined with the radioactivity of the isotopic sources gives rise to security implications, including the risk of theft or loss followed by some type of malicious use. Due to this security risk, the replacement of gamma-based IR cameras with other non-isotopic alternatives has been an active area of interest. Here we present findings from a recent study led by Sandia National Laboratories aimed at investigating the most effective pathway toward reducing the use of gamma-based IR cameras through their replacement with other non-isotopic alternatives. We begin with an overview of industrial radiography and the industrial radiography market, followed by an analysis of various NDT modalities to identify which would be an adequate alternative to gamma-based IR cameras. Lastly, we discuss lessons learned regarding current trends and market drivers from an industry engagement that was performed as part of this study.

INTRODUCTION

nondestructive testing(NDT) is a set of techniques used to evaluate the integrity, assembly, and properties of materials and components without causing damage to or altering the tested object. These techniques are commonly employed for testing, certification, and inspection purposes. Industrial radiography (IR) is an NDT modality frequently used in industries such as oil and gas, automotive, and aerospace. This technique is commonly used in industry to evaluate the structural integrity and uniformity of materials and components, similarly to how medical x-rays are used to identify breaks in bones. IR employs highly penetrating ionizing radiation (gamma rays, x-rays, or neutrons) to produce a visible record of the internal conditions of an object, thus identifying any imperfections or defects. Nonetheless, in applications where portability and accessibility are essential, such as the inspection of oil and gas pipelines, radiographic cameras (IR cameras) are commonly used. These devices are designed in a portable fashion, making them easy to transport from one location to another, and they usually enclose a radioactive isotope that serves as the source of radiation. However, their portability combined with the radioactivity of the isotopic sources gives rise to security implications; hence, the replacement of gamma-based IR cameras with other non-isotopic alternatives has been an active area of interest.

In 2021, Sandia National Laboratories (Sandia) completed a study that examined the viability of increasing the use of non-radioisotopic alternative technologies available for NDT (Patrick et al. 2021). It was unclear at that time why the NDT industry had not fully adopted x-ray technology in lieu of gamma-based IR cameras, and thus the study aimed to characterize current applications of IR cameras, identify limitations preventing users from adopting x-ray technology, and examine the viability of non-radioisotopic alternative technologies available for NDT. It was concluded that the needs of IR end-users (customers) dictate which equipment is used in order to meet the cost and performance requirements, with customers often specifying the use of a particular technology when requesting radiography services. This conclusion suggested that it would be unlikely that an NDT provider would forfeit gamma-based radiography devices for a non-radioisotopic alternative, but it is possible to reduce the in-field deployment of such devices through an increased use of alternative technologies. These findings were consistent with those noted by the National Academies of Sciences, Engineering, and Medicine in the 2021 report Radioactive Sources: Applications and Alternative Technologies (NASEM 2021):

Little progress has been made domestically with adopting alternative technologies for some other commercial applications, particularly in some nondestructive testing applications and well logging. This is because there are currently no viable or cost-effective alternatives; the alternatives either compromise or do not offer enhancements in performance, or they produce data on material and structures that are not directly comparable to those produced by radioactive sources.

The 2021 Sandia-led study concluded with three principal recommendations based on the information learned throughout that study:

1. Continue working with x-ray manufacturers to identify requirements for the development of a more practical and physically smaller x-ray system;

2. Further investigate other non-isotopic options (ultrasound and ground- penetrating radar, in particular) that can be implemented to reduce the number of times a radioactive source is deployed for field application; and

3. Explore other avenues for outreach and incentives, particularly toward NDT customers relating to x-ray effectiveness and safety benefits.

This work presents a follow-on to the original study performed in 2021 and aims to address the recommendations made. Specifically, the objectives of the 2021 study were as follows:

• Survey existing NDT modalities and assess their applications, capabilities, and limitations, thus expanding the list of potential alternatives to gamma-based IR cameras;

• Engage with NDT stakeholders to gain a better understanding of the main drivers for choosing an NDT modality and identify any advancements in the use of x-ray for industrial radiography; and

• Identify areas of opportunity to promote the increased use of non-isotopic alternatives in lieu of gamma-based IR cameras.

NDT MODALITY OVERVIEW

NDT providers supply a wide range of techniques that are used in industry, but only a select few were considered in this analysis. Given the overarching objective of reducing the use of gamma-based IR cameras, selection criteria were defined such that the technologies being considered must be:

• Mature and fully developed

• Accessible with an established number of use cases

• Commercially adopted by the industry at large

Nascent NDT technologies, such as high-throughput terahertz and microwave imaging, were not included in this analysis as they are considered emerging technologies in the research and development phase with limited application (Brinker et al. 2020; Li et al. 2023). Applying the selection criteria defined above yielded a total of 12 NDT modalities outlined in Table 1.

Table 1.

List of NDT modalities considered for analysis.

Gamma Radiography	Ultrasonic Testing
X-ray Radiography	Acoustic Testing
Neutron Radiography	Ground Penetrating Radar
Process Compensated and Resonant Testing	Visual Inspection
Eddy Current Testing	Dye Penetrating Inspection
Magnetic Particle Testing	Leak Testing

A literature review was performed for each of the selected modalities to gain a better understanding of the technique, its advantages and limitations, industries where it is commonly used, and application use cases. A list of manufacturers, suppliers, and potential customers was also compiled during the literature review phase in preparation for the industry engagement phase to follow. A brief definition of each NDT modality is provided here for reference.

Radiographic testing

Radiographic testing (RT) refers to the use of radiation to inspect components and materials for defects similarly to how a medical x-ray is used to image bones. It can be in the form of either gamma radiography, x-ray radiography, or neutron radiography.

• Gamma radiography is an NDT modality that relies on gamma rays to illuminate any defects in the surface of a material or product. The gamma rays are produced by radioactive material sealed in a special container (IAEA 1996). The gamma source is placed on one side of the container while a photographic film is placed on the opposite side of the device. When the device is in use, the source is moved from its shielded position through a tube to the collimator, allowing the gamma rays to pass through the test object and the film to capture a radiograph of the test object.

• X-ray radiography is another NDT which uses x-rays to examine the inside of a product or structure in order to determine whether or not it has any critical flaws or defects. The main differing factor between x-ray and gamma radiography is the fact that x-ray radiography uses a power source rather than a radioisotope to generate the x-rays (TWI 2024). The test object is placed between the x-ray source and a sheet of x-ray film in order to capture the radiograph. Advances within this space tend to be related to the film used or the processing of observed signatures.

• Neutron radiography makes use of a neutron beam pointed at an object to form an image on photographic film sensitive to the neutron radiation. This form of NDT is known for its ability to reveal flaws or defects that may be invisible to x-ray radiography methods (Berger 1971; L’Annuziata 2007; Anderson et al. 2009).

Process compensated and resonant testing

Process compensated and resonant testing (PCRT) is an NDT technique that uses resonant ultrasound spectroscopy (RUS) in tandem with pattern recognition algorithms and statistical scoring to conduct process monitoring, material characterization, and life monitoring (Biedermann et al. 2017; Vibrant Corp 2023).

Eddy current testing

Eddy current testing (ECT) is an NDT that uses multiple electronic probes running through product tubes of varying lengths over or along the surface of a product in order to find flaws or defects in them. Any data gathered by the initial testing must be run through software in order to expose any defects (García-Martín et al. 2011; Mohamad et al. 2023).

Magnetic particle testing

Magnetic particle testing (MPT) is used on ferromagnetic materials to detect defects and flaws by running a magnetic current through said material surfaces. Flaws that can be detected (including those just beneath the surface) include cracks, pores, and lack of sidewall fusions for welds (Burke and Ditchburn 2013; Eisenmann et al. 2014; Singh et al. 2019).

Ultrasonic testing

Ultrasonic testing (UT) involves the use of soundwaves to find defects and measure the thickness of materials to those defects. This form of NDT is most effective on metals, as metal materials conduct the soundwaves better than other materials do (TEC-S 2023; Flyability 2024).

Acoustic testing

Acoustic testing (AT), or acoustic emission (AE), is an NDT method which employs ultrasonic stress waves being released onto the product in order to determine whether or not any defects or flaws are present. Unlike ultrasonic testing, the waves originate from within the material being inspected, as opposed to an external source (Pollock 2018; Desa et al. 2018).

Ground penetrating radar

Ground penetrating radar (GPR) is an NDT method that is largely used to scan concrete and other ground-type materials for defects and flaws. This scanning is accomplished through the use of a detection and imaging device that sends a pulse of energy down into the earth, revealing underground utilities, voids, rebar, conduit, post-tension cables, and any other structural elements hidden below the earth (GPRS 2024).

Visual inspection

Visual inspection (VI) is one of the most common forms of flaw inspection. It is simply a visual inspection of the product with only the naked eye. Though it does not require any special equipment, it does require special training on behalf of the operator to understand what to look for (AME 2024; ASNT 2024).

Dye penetrating inspection

Dye penetrant inspection (DPI) is another one of the most common forms of flaw inspection within the NDT field. DPI is an NDT method that works through the application of a special dye to a surface in order to identify any defects present. Materials including ceramics, metals, and plastics are tested for defects in welds, castings, forgings, plates, bars, and pipes (Eick 2003).

Leak testing

Leak testing (LT) is a form of NDT used to discover if any defects or flaws are present within an object or system beyond the established leak limit. Typically, LT is performed on systems that contain or move liquids or gases (ONESTOP NDT 2023).²

These testing modalities are used to inspect and detect various defects across a wide range of industries. No direct relationship was observed between industries and testing modalities as most industries employ several testing modalities, but Table 2 shows a partial list of industries that use NDT and examples of modalities used in each industry. It is noted that other industries exist where each modality is applied, and there are other modalities used in each industry beyond what is listed here.

Table 2.

Partial list of industries that utilize NDT services and examples modalities used in each industry.

Industry	RT	PCRT	ECT	MPT	UT	AT	GPR	VI	DPI	LT
Automotive	X	X	X	X	X				X	X
Construction	X		X		X		X	X	X
Oil and Gas	X				X	X		X	X
Pulp and Paper	X		X	X	X			X	X
Shipbuilding/Maritime	X		X	X	X			X	X
Power Generation	X		X	X	X			X	X
Chemical	X		X		X					X
Aerospace	X	X	X	X	X	X			X	X
Manufacturing	X			X	X			X	X
Food and Textile	X		X		X	X

INDUSTRY ENGAGEMENT

The NDT industry in itself is a service industry where NDT services are used across many other industries. Therefore, the distinction between an NDT supplier, a manufacturer, and a customer was central to this phase of the analysis. It was commonly perceived that the industrial radiography market consists of two parties: a manufacturer that develops NDT equipment, and a customer that acquires that equipment for daily use. This dynamic has been updated throughout the course of this study where it is now understood that the industry consists of three parties:

• Manufacturers: NDT stakeholders that develop equipment used for various NDT techniques. This category also includes those developing new and emerging NDT techniques and their related equipment.

• Suppliers: organizations that offer NDT as a service. These companies are usually capable of performing a wide-range of NDT techniques and can operate in a laboratory setting or deploy equipment for in-field operations.

• Customers: organizations that rely on industrial radiography as part of their day-to-day operations and either possess in-house equipment or subcontract such services through a third- party company.

A comprehensive list of NDT stakeholders was compiled over the course of this study in order to initiate an industry engagement. This list consisted of NDT service suppliers, equipment manufacturers, and suspected customers. It proved difficult to identify NDT customers with full certainty, so an attempt was made to identify companies that would potentially require NDT services. This attempt was consistent with findings from the 2021 study, where it was observed that NDT suppliers are highly sensitive to business-related information and any questions regarding their customer base. Potential NDT customers were identified as companies in fields that commonly use industrial radiography, such as the oil and gas industry. In addition, companies with job postings for NDT technicians were also considered as potential NDT customers. Nonetheless, gaining a better understanding of market drivers and NDT decision factors was most effectively achieved by considering the business model underlying the industrial radiography marker and engaging in discussions with NDT suppliers and customers. Manufacturers were not taken to be as important for this analysis, as they were considered a biproduct of the industry as opposed to a driving factor. It can be argued that manufacturers’ contribution to the supply and demand of NDT services plays a role in the context of developing a new product in response to customers’ demand for a given NDT technique or the development of a novel emerging technology that can advance the industry as a whole. In either case, this contribution did not align with the objectives of this work as it would pertain to an NDT technique that has yet to be commercially adopted.

Main takeaways from industry stakeholders

Various industry standards provide guidelines, procedures, and requirements for conducting inspections, interpreting results, and ensuring the safety and reliability of structures, components, and materials. The American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), the American Petroleum Institute (API), and other groups develop and publish international standards for materials, systems, and services (Flyability 2023). ASTM E1316 is the standard terminology guide for NDT with dozens of additional ASTM NDT standards covering specific applications for specific modalities (ASTM 2024). Since every component or product is unique and would require different forms of testing to examine surface and sub-surface discontinuities, NDT stakeholders refer to the relevant standards to identify the type of defect, material properties, object size, shape, cost, and efficiency to come up with a suitable method and solution. The NDT experts and engineers assess these factors along with the appropriate guides to recommend the most suitable modality for a particular application. Customers may have preferences based on their industry knowledge and experience. Still, ultimately, the selection is made with the help of professionals who understand the specific requirements and limitations of each NDT method.

ASME standards are updated every 2–3 y to incorporate new modalities, but the exact process in which that happens is not well understood by those not involved in the NDT community. Discussions with industry stakeholders suggested that changing a modality in such documents involves careful consideration of factors such as the number of radiographs that might need to shift, potential increased time requirements, and the impact on manufacturing processes. Manufacturer’s concerns are also considered when transitioning to newly introduced techniques.

When dealing with defects, there are numerous specific options available, and choosing the best method often depends on the expertise of a Level III practitioner and the established industry standards.³ Consequently, there is limited room for new suggestions. Factors such as orientation, thickness, geometry, and the percentage of defect concerning the material thickness are crucial. For x-ray testing, for instance, a defect should typically constitute 2% or more of the material’s thickness to be detected.

It was learned that different standards apply to different industries, with some industries being more regulated than others. In addition, some form of The Pareto Principle⁴ can also be observed in the NDT industry, where 80% of NDT services are performed in 20% of all industries that use NDT services; the oil and gas industry dominates that space. Hence, the other 80% of industries that do not use as much NDT services are those that have less stringent regulation.

It was also learned through SME interviews that the American Society for Nondestructive Testing (ASNT) organizes an annual conference event. This conference serves as a pivotal gathering for leading practitioners within the NDT industry. The SMEs highlighted the close-knit and community focused nature of the NDT industry as many NDT technicians are owner-operators. The interviewed stakeholders emphasized the critical importance of participating in this conference for entities aiming to influence modality preferences within the field. It is during this event that the "Standards Council" convenes to deliberate on potential modifications to the ASNT NDT testing standards.

Finally, it was noted that in practical applications, many customers opt for a combination of ultrasonic and x-ray modalities when considering an alternative to gamma radiography. In fact, it was stated that leveraging the strengths of both methods provides a more comprehensive inspection technique than gamma-based radiography alone. This statement was an interesting finding as x ray has always been at the forefront of discussion regarding the replacement of gamma-based sources, with little to no mention of ultrasonic testing. This oversight suggested that the combination of ultrasonic and x ray might be an adequate replacement for some applications that currently use gamma radiography.

ANALYSIS OF ALTERNATIVES

Recognizing the diverse range of defects encountered in the NDT industries, there is no direct substitute for gamma radiography. The analysis performed in this study showed that each NDT technique possesses unique strengths and limitations. The selection of a particular method hinges on factors such as material type, size and nature of the defect, and inspection conditions. The analysis also underscores the understanding that relying solely on a single NDT method may not be feasible to assess the defect due to the inherent limitations of various NDT techniques. Therefore, to ensure accurate evaluation of the defects or discontinuities and to comply with the industry norms, it is not uncommon for NDT suppliers and inspectors to employ a combination of NDT techniques. In addition, many standards allow inspectors to substitute one inspection method with another, as long as certain requirements are met.

The IR market is extremely complex and naturally exists in response to the need to identify various defects. However, there is no exclusive relationship between defects and NDT modalities. That is, a given defect could be detected using various NDT modalities, and the choice of modality would depend on factors such as cost, expertise, and required resolution. Similarly, there are no defects that are completely exclusive to specific industries; hence, NDT modalities are not exclusive to specific industries either. This highly entangled structure yields an inability to establish any direct links between defects, modalities, and industries. Inspecting a given defect with a specific modality is not straightforward as it involves intricate technical understanding, engineering, and industry-specific knowledge. Determining the most suitable NDT technique demands the expertise from a Level III-certified NDT technician. The process of acquiring NDT services usually commences by identifying the types of defects relevant to the industry and tailored to customer requirements while considering potential occurrences during manufacturing or in the product’s service stage. Adherence to codes and standards mandates periodic NDT inspections for safety and verification that no defects occur throughout the component’s service life. Nonetheless, structural defects can be broadly categorized into those commonly associated with manufactured products, such as castings, welding anomalies, deformations, laps, voids, inclusions, and delamination. Defects more prevalent during the service stage are due to overload, stress, corrosion, fatigue, cracks, voids, wear, and other design-related issues. For each defect identified and its likely cause, the most appropriate NDT technique is then selected through discussions between the customer and the supplier of the service.

Based on the literature review and input from the NDT industry, ultrasonic NDT appears to be the most reliable and widely used for internal defects such as porosity, laminations, and inclusions. While radiography can also be used for these internal defects, radiography is regarded as entirely impractical in some situations. This deficiency can be attributed to radiography being relatively much slower than ultrasonic and less economically feasible. Ultrasonic NDT also works well for various metals, such as aluminum, titanium, and alloys, and it is used extensively for internal bursts and cracks. Although ultrasonic NDT is well developed for internal defects such as cavities (or shrinkage type defects), various radiography techniques are seen as providing the optimum representation of the defect. Nonetheless, ultrasonic testing is proficient at detecting narrow horizontal cracks, delamination, and measuring wall thickness, in contrast to radiography. Radiography excels in detecting narrow vertical cracks, shallow surface defects, and porosity. However, it’s worth noting that ultrasound imaging is unsuitable for extremely cold temperatures due to the potential freezing of the sonic couplant fluid, rendering it ineffective.

For surface defects, NDT methods such as visual, magnetic particle, liquid penetrant, and eddy currents are more commonly used. Eddy current NDT may not be generally suitable for deep volume defects, but it is good at determining the depth and differences in microstructure, which manifest as differences in hardness. Additionally, eddy current NDT can be used for internal cracks that are closer to the surface. For hot cracking (a defect that occurs when shrinkage cracks form during solidification of weld metal) or tearing defects near inaccessible surfaces, visual, liquid penetrant, and magnetic particle NDT techniques are most helpful. Magnetic particle NDT is the most widely used method for steel materials. If the areas are inaccessible or cracks are subsurface, then ultrasonic NDT is used.

Details pertaining to the advantages, limitations, use cases, and applicable industries for the NDT modalities considered are discussed below for comparison.

Gamma radiography

Gamma source-based IR cameras use gamma radiation to image an object. In order to radiograph using gamma radiography, the gamma source is carefully positioned on one side of the specimen and the photographic film is placed on the other side of the specimen, remote from the source, minimizing the object film distance so that the image is not distorted. All radiographic isotope sources in gamma radiography cameras are encapsulated to prevent possible contamination. When the device is not in use, the radiation source is contained within the camera to avoid unnecessary exposure, and when in operation, the source is moved out of the shielded position through a tube to the collimator, allowing gamma rays to pass through the test object, exposing the film to form a radiograph. At the end of the exposure time, the source is cranked back into the exposure device. This style of gamma radiography is termed as projection gamma radiography. With the projection radiography arrangement, the radiographer can position the source away from themselves, thereby reducing the dose received and making it safe to use (Bossi et al. 2002).

Gamma radiography is used across various industries such as energy, aerospace, construction, nuclear power generation, and automotive. In these industries, it is used to inspect components for defects and issues in porosity, inclusions, weld cracks, corrosion, castings, and forging discontinuities. It is also used in quality control during manufacturing processes. The widespread adoption of this technique can be attributed to the advantages it contributes to the NDT industry. The high energy of gamma rays enables deep penetration, which is well-suited for inspecting thick-walled objects. The portability of gamma radiography equipment provides flexibility in field applications. This feature allows radiography to be carried out in remote locations where electric power may not be readily available. The simple apparatus and the compactness of gamma radiography equipment are practical in situations where accessing the interior of an object becomes challenging. Portable equipment is also advantageous in scenarios where testing is done within confined spaces. However, there are notable disadvantages to gamma radiography, including security risks, lower resolution, continuous emission, and radiation hazard.

X-ray radiography

X-ray radiography is similar to gamma radiography in functionality. The only distinguishing factor is the production of radiation (x-rays) using a power source as opposed to a radioisotope. The object to be inspected is placed between the x-ray source and a sheet of x-ray film. The x-rays pass through the inspected object onto the film, creating a latent image. The degree of exposure on the film is influenced by the x-ray attenuation properties, which vary based on the density and thickness of the inspected object. Unlike traditional x-ray film radiography, computed radiography (CR) replaces the film with a digital imaging plate, allowing for the capture and processing of x-ray images in a computerized format. CR employs a reusable imaging plate made of a photo-stimulate phosphor material. The phosphor can store x-ray energy when exposed. After exposure, a CR reader with a laser beam is used to convert the signal into a digital image. The digitized x-ray image is sent to a computer where it undergoes digital processing, including brightness, contrast, and image enhancements. The digital x-ray images can then be stored electronically, eliminating the need for physical storage of film (Quinn 1980). Digital radiography (DR) is another sub-type of x-ray NDT that is considered the latest advancement in radiography at the time of this writing. DR is similar to CR in principle; the only difference is that DR uses a solid-state detector, a flat panel detector, or a combination of a light-emitting phosphor and a digital converter, which produces a digital array and an image. The convenience of directly capturing images using digital sensors without the need for an intermediate storage medium, like in CR, makes DR a faster imaging technology (Bansal 2006).

In x-ray industrial radiography, x-ray machines operate at varying voltages depending on the specific requirements of the inspected object. The choice of the voltage is dependent on factors such as thickness and density of the material being inspected, the desired image quality, and the type of defects or features that need to be detected. Common types of x-ray voltages used are ≤150 kV, ≤350 kV, and < 500 kV. The first two categories are suitable for radiography on most common objects. Objects of extreme thickness, however, require an energy even higher than 500 kV.

X-ray radiography stands out as the most direct alternative to radionuclide radiography, offering a diverse range of energies. This substitution enables x-ray radiography to conduct equivalent inspections and assessments with comparable precision. X-rays have the advantage of being turned off and therefore do not emit radiation after exposure, contributing to a safer working environment. X-ray systems offer more precise exposure control, allowing for better manipulation of imaging parameters, and have the ability to provide real-time imaging for immediate analysis. X-ray radiography provides precise control over exposure, superior image quality, and enhanced contrast due to shorter wavelengths. Radiographic contrast refers to the degree of density difference between two areas on a radiograph. Contrast is a critical aspect of radiography, as it enhances the visibility of features of interest and helps differentiate between various abnormalities. Subject contrast (influenced by thickness and density variations) and film contrast (impacted by x-ray kilovoltage) contribute to overall contrast. Lower kilovoltage enhances contrast, evident in radiographs comparing high (220 kV) and low (120 kV) tube voltages (Prasad and Nair 2009; Rincon et al. 2014).

As a general guideline, x-ray sensitivity is typically around 2% of the material thickness. For example, in a steel piece with a thickness of 25 mm, the smallest void detectable would be approximately 0.5 mm in size. Due to this limitation, components are often radiographed from various angles. A narrow crack might not be visible unless the x rays are aligned parallel to the crack’s plane. The same principle applies to gamma radiation. In scenarios like this application, dye penetrant or ultrasonic testing would be a more practical option. X-ray radiography is widely used to inspect welds for defects such as lack of fusion or incomplete penetration. X-ray radiography is effective in detection of inclusions within a structure, cracks, fractures, or other discontinuities within materials and is used to measure the thickness of materials and identify variations (Quinn 1980).

X-ray radiography is used in many of the same industries as gamma radiography and in a similar capacity. The industries include energy, aerospace, construction, nuclear power generation, ship-building, and automotive. However, x ray is also used in the arts and cultural artifacts industry. X ray is used to examine the condition of paintings and sculptures and to identify the elemental composition of pigments, metals, and other materials used in artworks. X-ray examination is used to reveal hidden features or inconsistencies in artifacts and documents, aiding in detection of forgeries or counterfeit items. X-ray inspection is also employed in forensic investigations to examine objects related to criminal activities.

Ultrasonic testing

Ultrasonic NDT transmits ultrasonic high-frequency soundwaves through a material to assess its characteristics and structural integrity, as well as identify defects. Ultrasonic testing can be used to ascertain the size, position, geometry, and character of defects in a material. This technique operates by sending an acoustic pulse from a piezoelectric transducer into the object and receiving back echoes from the acoustic energy. A piezoelectric transducer is a device that converts electrical energy into acoustic energy (or acoustic energy into electric energy), producing data that can then be interpreted. The physical material changes shape when an electric field is applied; conversely, when a physical force is applied (changing the material’s shape), an electric charge is applied (Bond 2018). These transducers are normally connected to the material with gel, oil, or water, which is needed to transmit the acoustic energy into the object. The echoes reflect the boundaries and defects of the object.

The two main methods to perform ultrasonic testing are pulse-echo and through transmission. When using pulse-echo, one transducer is used to emit and receive the acoustic sound energy using the echoes’ signals reflecting off a boundary, such as the limits of the object or a defect. Echo signals are dispersed and reflected as they encounter these boundaries. The transmission method differs by having a separate receiver placed on the reverse side of the object to receive the acoustic signals. Flaws in the object will reduce the amount of energy received by the receiver on the other end, allowing for the location of the flaws to be found. Ultrasonic NDT can be used on many different material types but is typically used on dense, crystalline materials; metals are the most common material tested. Ultrasonic testing can also be used to test composites, ceramics, and plastics as well. Ultrasonic is not used on low-density, high-attenuation materials such as wood or paper, as they will create background noise leading to inaccurate results. For most materials, ultrasonic testing is typically effective at a thickness level of 6–8 mm (Carpentier and Rudlin 2023; Fujifilm 2022). Ultrasonic testing is used in a wide range of industries such as oil and gas, aerospace, medical, construction, manufacturing, and metallurgy. It is used for quality assurance purposes as well as monitoring known flaws to view changes over time. It can be used to test metal components and welds, concrete, pipelines, nuclear reactors, aircraft, and for archaeological uses.

Ultrasonic testing has several advantages. It provides higher precision in determining the depth of a flaw compared to other NDT methods, while also being able to detect minute flaws within an object. It can be used when only one side of the material is accessible using the pulse-echo method (Bond 2018). Ultrasonic testing can determine size, shape, and orientation of defects and characterize the alloy structure of a composite as they hold unique acoustic properties. Ultrasonic testing is not a hazardous testing method; it can be automated and provides portability. The testing provides instant results, leading to rapid decision making.

Ultrasonic testing has disadvantages as well. Technicians require a high degree of technical expertise and training to perform successful testing and interpret results. Ultrasonic testing is not well-suited to test small or coarse objects, or ones with irregular geometry. In ultrasonic testing, there is a dead zone that causes near-surface flaws to be missed (Bond 2018). The dead zone is more pronounced in thin objects. In most cases, ultrasonic testing requires coupling to the material with a bonding material. This coupling can produce false positives or spurious indicators in use, which can result from the shape or material proprieties themselves, or it may cause acceptable anomalies to be detected as defects, leading to false alarms.

Ultrasonic testing could be used to take the place of gamma-based testing in some circumstances, provided the defects are not near the surface, as ultrasonic testing is subject to a dead zone where defects are undetectable. This applicability could include welds, concrete, and metal castings. However, gamma radiography is not limited to material type, whereas ultrasonic requires high-density, low-attenuation materials. Ultrasonic provides advantages with thick materials, as the acoustic energy travels near instantaneously, whereas gamma testing requires longer exposure time.

OPPORTUNITIES FOR ALTERNATIVE TECHNOLOGIES

It is evident that the IR space is highly convoluted, with no direct link to how gamma-based IR cameras can be replaced with alternative modalities. Many industries use a variety of NDT techniques for a wide range of defects. Nonetheless, a targeted approach can be explored for future efforts by focusing on a single application of gamma-based IR cameras. By selecting an industry where gamma-based IR cameras are commonly used in situations where alternatives (such as the combination of ultrasonic and x ray) could be used, progress can be made reducing the deployment of radioactive sources for in-field operations. Although it is unclear which application is best, two areas of opportunity are presented below.

Small industry—relatively unregulated

The first area of opportunity that could potentially promote the use of alternatives to gamma-based radiography would be to incentivize industry stakeholders that are not subject to stringent codes in their day-to-day operations. For such industries, focusing on a single application where gamma-based radiography and alternative technologies are commonly used would be optimal. This approach would create an opportunity where incentives can be devised to promote the use of alternative technologies in place of gamma-based devices. Considering that a relatively unregulated industry would not be as active in the NDT space, this approach would have a slow and steady impact, but it would likely be the path of least resistance to support the reduction of gamma-based IR cameras. Interviews with NDT industry experts suggest that power generation and the construction industry would be ideal candidates for this approach.

Large industry—highly regulated

The second area of opportunity would be to identify widely used industry codes that specifically require the use of gamma-based devices. This identification could potentially warrant a comparison study to demonstrate the interchangeability of gamma-based IR cameras with an alternative modality. Such analysis would not only consider the modalities from a technical perspective but also from an economic feasibility perspective. This analysis would make the case for alternative technologies and create an opportunity for engagement with policymakers and industry stakeholders to implement changes to the codes. This approach would likely be a more difficult task, but it would have a higher impact on the reduction of gamma-based testing devices.

CONCLUSION

This study was performed as a follow-on to an original study performed by Sandia in 2021 to investigate the replacement of gamma-based IR cameras with non-isotopic alternative NDT techniques. An analysis of alternatives was performed between gamma radiography and other NDT modalities to identify if any could be an adequate replacement for gamma-based IR cameras. Modalities considered for analysis were selected based on a predefined set of inclusion criteria with consideration of the NDT market space as a whole. Interviews with industrial radiography stakeholders were also conducted as part of an industry engagement effort to better understand current market trends and deciding factors. The list of stakeholders considered for engagement included manufacturers and suspected customers but was heavily dominated by NDT service suppliers. This analysis was performed to identify areas of opportunity to effectively promote the increased use of non-isotopic alternatives in lieu of gamma-based IR cameras.

Results of this study suggest that several NDT modalities can be used in lieu of gamma radiography when considering a specific defect, but the combination of ultrasonic testing and x-ray radiography appears to be the most promising alternative overall. However, a wide range of NDT techniques are used across many industries for a variety of defects; hence, there is no exclusive application of gamma-based IR cameras to a specific industry or defect. Conclusions drawn from this analysis can be summarized as follows:

• The industrial radiography market is multifaceted, being comprised of three bodies: (1) NDT device manufacturers; (2) NDT suppliers that acquire devices and offer NDT as a service; and NDT customers who request NDT services from suppliers but could also perform NDT operations in-house.

• Many NDT techniques exist and are widely offered by NDT suppliers. Of these techniques, there are several that can provide comparable information regarding the testing, maintenance, and inspection of equipment and components to the same level as gamma radiography.

• There are no one-to-one alternatives for the replacement of gamma radiography, which suggests that gamma-based IR cameras will never be eliminated, but the reduced use of such devices is possible through increased use of non-isotopic alternatives.

• In current practice, the combination of ultrasonic NDT with x-ray radiography is the most commonly used alternative to gamma radiography. This duo is the most promising non-isotopic alternative for reducing the use of gamma-based IR cameras.

• The IR market is deeply intertwined where tens of industries use various NDT techniques for a wide range of defects. However, the 80/20 rule can be observed, where 80% of NDT services are requested by 20% of industries that require NDT services.

• NDT is more regulated in some industries than it is in others. Of note, the few industries that dominate the NDT market are governed by a specific set of codes and standards that enforce the choice of modality, while other industries are driven by customer request.

• The ASNT Annual Conference plays a crucial role in shaping modality preferences within the NDT industry. It is during this event that the Standards Council meets to discuss potential changes to ASNT NDT testing standards.

The information compiled throughout this study from open-source literature and interviews with industry stakeholders was used to identify areas of opportunity for the increased use of alternative technologies based on a more targeted approach. That is, a focused effort towards reducing or replacing the use of gamma-based IR cameras for a specific application within a specific industry with other non-isotopic alternatives. The choice of application and industry is arbitrary, but two potential opportunities were identified, each with its own pros and cons. The proposed paths forward were:

1. Incentivizing the use of alternatives to gamma radiography in relatively unregulated industries, such as the power generation or construction industries; and

2. Engaging with policymakers to amend codes and standards that specifically call out the use of gamma radiography in highly regulated industries, such as oil and gas.

Lastly, regarding the acceptability of non-gamma NDT modalities, findings from this study suggest that the common belief of gamma radiography being the best is simply a misconception. In fact, some industries, such as aerospace, exclusively use x-ray as it is superior to gamma radiography for their specific applications. Additionally, discussions with NDT suppliers uncovered that the combination of x-ray and ultrasonic testing is much more robust than standalone gamma radiography. These findings suggest that the NDT industry doesn’t have any reservations toward using non-gamma NDT modalities and that the main driving force for modality selection is the testing or inspection standards that the NDT industry abides by. Thus, it is important to acknowledge the difference between highly regulated industries and relatively less regulated ones, in addition to gaining a better understanding of the standards influencing modality selection.