What Type of Radiation is Used for NDT?

In the world of engineering, construction, aerospace, and many other high-risk industries, safety and reliability are paramount. Ensuring that equipment and materials meet strict quality standards is not just a matter of efficiency or cost-effectiveness; it’s about protecting lives and preventing catastrophic failures. One of the most effective ways to inspect materials without causing any damage is through a process known as Non-Destructive Testing, or NDT. Among the many NDT methods, the use of radiation plays a crucial role, particularly in detecting internal flaws that are not visible to the naked eye. But what type of radiation is used for NDT, and how does it all work? This blog post will take a deep dive into the types of radiation commonly used in NDT, explain how each type functions, and examine why certain radiation sources are more suited to particular applications. We’ll also explore the safety protocols surrounding these techniques, ensuring a well-rounded understanding of radiation-based non-destructive testing.

Understanding NDT and Its Role in Industry

Non-Destructive Testing refers to a group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. Unlike destructive testing, which may involve breaking, cutting or otherwise compromising the material, NDT allows for thorough inspection while maintaining the integrity of the item. In the UK and around the globe, industries such as oil and gas, power generation, aerospace, automotive, and manufacturing rely heavily on NDT to ensure safety compliance and maintain operational standards. The use of radiation in this context is particularly effective when assessing the internal structure of welds, castings, pipelines and pressure vessels, where surface-only inspections are not enough.

Radiographic Testing and Its Importance

Radiographic Testing (RT) is one of the most widely used forms of radiation-based NDT. The process works in much the same way as a medical X-ray. Radiation passes through the object being inspected and is captured on the other side by a film or digital detector. Denser materials absorb more radiation, while flaws such as cracks, voids or inclusions absorb less. This difference creates a contrast image that reveals internal imperfections. There are two main types of radiation used in radiographic testing: X-rays and gamma rays. Each has its own specific properties, benefits and limitations depending on the application.

X-Rays in Non-Destructive Testing

X-rays used in industrial radiography are similar in principle to those used in hospitals, though the equipment and exposure levels are tailored to the thickness and density of industrial materials. X-ray generators work by accelerating electrons and colliding them with a metal target, usually tungsten. This interaction produces X-rays which are directed toward the material being tested. X-rays are particularly effective when examining thinner materials or when high image resolution is required. For example, in the aerospace sector, where precision is crucial, X-rays are often used to inspect composite materials and small welds. The high clarity offered by X-ray imaging allows for accurate detection of minute defects that could compromise structural integrity over time. Another advantage of X-rays is their controllability. Since the radiation is generated electrically, operators can adjust the energy output and exposure time according to the material being examined. This flexibility reduces the risk of overexposure and improves image quality, especially in complex components.

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Gamma Rays and Their Industrial Applications

Gamma rays, unlike X-rays, are not generated electronically. Instead, they come from radioactive isotopes that naturally emit radiation. The most commonly used isotopes in industrial radiography are Iridium-192, Cobalt-60 and Selenium-75. Each isotope has a different half-life and emits gamma rays at different energy levels, making them suitable for various applications. Gamma ray sources are particularly useful in fieldwork and remote locations where electricity may not be available. Since these isotopes emit radiation continuously, the equipment required to use them is often more portable than X-ray generators, making them ideal for inspecting pipelines and structural welds in inaccessible areas. Iridium-192 is perhaps the most versatile of the gamma ray sources. It emits radiation with sufficient energy to penetrate moderate-thickness steel and is commonly used in weld inspection. Cobalt-60, on the other hand, emits higher-energy gamma rays and is better suited for inspecting thicker sections of metal or dense castings. Selenium-75 provides lower energy levels and is often chosen for its ability to produce higher contrast images on thinner materials, though its use is somewhat less common due to limitations in penetration depth.

The Physics Behind Radiation in NDT

To truly understand why radiation is used in NDT, it helps to consider the physics behind the technique. When radiation passes through a material, it interacts with the atoms in different ways depending on the material’s density and atomic structure. Denser materials block more radiation, creating a “shadow” effect on the detector. Any flaw or void within the material will allow more radiation to pass through, creating a lighter area on the radiograph. This fundamental principle allows technicians to detect not only surface cracks but also subsurface voids, inclusions, corrosion, and other internal anomalies that could compromise safety or performance.

Digital Radiography and the Shift from Film

Traditionally, radiographic testing was performed using photographic film. The film would capture the image created by radiation passing through the object, and after development, technicians could interpret the results. While still used today in some industries, film radiography is gradually being replaced by digital radiography systems. Digital systems offer several advantages. They allow for immediate image viewing and analysis, reducing downtime and improving workflow. Additionally, digital images can be enhanced, stored, and shared more easily than traditional film. These benefits are particularly valuable in sectors such as power generation or defence, where traceability and documentation are essential. Digital radiography also reduces the environmental impact of the process by eliminating the need for chemical film development and waste disposal. Moreover, it enhances safety, as technicians can make quicker decisions without needing to repeat exposures.

Radiation Safety and Regulation in the UK

Given that both X-rays and gamma rays are forms of ionising radiation, their use comes with strict safety requirements. Exposure to ionising radiation can cause serious health issues, including radiation sickness and an increased risk of cancer, so regulation and control are essential. In the United Kingdom, the use of radiation in industry is regulated by the Health and Safety Executive (HSE) under the Ionising Radiations Regulations 2017 (IRR17). These regulations require employers to assess and control the risks of working with ionising radiation, ensure proper training for operators, and maintain strict access controls in radiographic areas. Radiographers must wear personal dosimeters to monitor their exposure levels, and work areas must be appropriately shielded to prevent radiation from escaping into occupied zones. Additionally, radioactive sources used in gamma radiography must be securely stored and transported in accordance with national and international guidelines.

Choosing the Right Radiation Source for the Job

Selecting the appropriate type of radiation for an NDT application depends on several factors, including material thickness, object size, image resolution requirements, and location of the inspection. For example, inspecting a large pipeline weld in a remote area might call for a gamma ray source such as Iridium-192 due to its portability and depth of penetration. Conversely, examining a precision weld on an aircraft component in a controlled facility might be better suited to high-resolution X-ray imaging. Cost and availability also play a role. X-ray generators require a power source and regular maintenance, while gamma sources need to be replaced periodically due to radioactive decay. Each option comes with trade-offs, and experienced technicians must weigh these factors carefully to ensure both effective inspection and operational efficiency.

Innovations and the Future of Radiation-Based NDT

While traditional radiography remains a cornerstone of non-destructive testing, advances in technology continue to push the boundaries of what’s possible. One area of growth is in the use of computed radiography and digital detectors that offer even higher image resolution and improved data integration. Artificial intelligence is also beginning to make inroads in radiographic testing. By analysing thousands of images, machine learning algorithms can assist technicians in identifying defects more quickly and accurately, reducing human error and improving consistency. Moreover, advances in miniature X-ray and gamma ray devices are enabling more flexible inspection setups, particularly in confined or hazardous environments. These tools can be deployed via drones or robotic arms to inspect infrastructure such as bridges, nuclear facilities, and offshore rigs without putting human workers at risk.

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Environmental and Ethical Considerations

As with any industrial practice, the environmental and ethical implications of radiation use in NDT must be considered. While the benefits to safety and quality are undeniable, the handling and disposal of radioactive materials come with serious responsibilities. Companies must follow strict protocols to prevent contamination and ensure the long-term safety of personnel and the public. There is also a growing interest in reducing radiation use where possible, by complementing radiographic testing with other NDT methods such as ultrasonic testing, magnetic particle inspection, or eddy current testing. These techniques can often be used in conjunction with radiography to provide a more complete picture of material integrity without increasing radiation exposure.

Training and Certification for Radiographic Testing Professionals

Because radiographic testing involves the use of potentially hazardous ionising radiation, professionals working in this field must undergo rigorous training and certification. In the UK, certification typically follows guidelines set by the British Institute of Non-Destructive Testing (BINDT), and many technicians are qualified under the PCN (Personnel Certification in Non-Destructive Testing) scheme, which aligns with international standards such as ISO 9712. Becoming a certified radiographer involves both theoretical and practical training. Technicians must gain an in-depth understanding of radiation physics, safety procedures, image interpretation, and the use of both film and digital equipment. They also need hands-on experience under supervision before they can be considered competent to perform independent inspections. The training ensures that radiographers can accurately detect and evaluate defects without false positives or missed indications. Misinterpretation of a radiograph could lead to critical components being approved despite containing flaws—or conversely, unnecessary rework or rejection of perfectly sound materials. As such, a qualified radiographer plays a key role in the quality assurance chain. Ongoing professional development is also crucial. With advancements in digital imaging, AI-assisted evaluation, and more stringent safety regulations, radiographic professionals must stay up to date with current practices. This is not just a matter of career progression but a legal and ethical responsibility, especially when operating in safety-critical industries like nuclear power or aerospace.

Comparing Radiation-Based NDT to Alternative Techniques

While radiation-based methods offer exceptional advantages for detecting internal flaws, they are not always the most suitable choice for every scenario. It’s useful to understand how radiographic testing compares to other NDT methods that may be preferred depending on the specific application. Ultrasonic Testing (UT), for instance, is another popular method for detecting subsurface defects. Instead of radiation, it uses high-frequency sound waves to penetrate the material. Reflected waves indicate flaws or changes in material properties. UT is especially effective for inspecting thick components and is often used in pipeline inspection, weld assessment and corrosion monitoring. It has the added benefit of being safer, as it involves no ionising radiation, and inspections can often be conducted with minimal access from only one side of the component. Magnetic Particle Testing (MT) and Liquid Penetrant Testing (PT) are surface inspection methods used to detect cracks and other surface-breaking defects. They are highly sensitive and relatively simple to carry out but cannot detect subsurface issues the way radiography or ultrasonic testing can.

Final Thoughts: The Role of Radiation in Modern NDT

Radiation continues to play a vital role in modern non-destructive testing, offering unparalleled insight into the internal structure of materials and components. Whether through the use of X-rays or gamma rays, radiographic testing provides engineers and inspectors with the tools they need to ensure safety, reliability, and compliance in some of the most demanding industrial environments. While the technology continues to evolve, the fundamental principles of radiographic inspection remain the same: the need for clear, accurate images of internal defects and the importance of doing so without damaging the item being tested. With proper training, regulation, and the ongoing development of safer, more efficient equipment, radiation-based NDT will remain an essential tool for industries across the UK and beyond.

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