Radiographic testing (RT) plays a vital role in non-destructive testing (NDT), especially when inspecting welds for internal flaws. Among the many RT methods, X-ray and gamma ray inspections are the most widely used. Each has its advantages depending on the material type, thickness, and inspection environment.

In this guide, we’ll compare X-ray and gamma ray testing, answer common questions, and help you choose the right method for your next weld inspection. We’ll also share insights into how digital radiography (DR) and hybrid approaches can support more effective inspections.

What Are the 4 Types of Radiation?

The four primary types of radiation are:

1. Alpha (α) – heavy, low-penetration particles
   
2. Beta (β) – lighter particles with moderate penetration
   
3. Gamma (γ) – high-energy waves from inside the atomic nucleus
   
4. X-rays – electromagnetic waves from outside the atomic nucleus

X-rays and gamma rays are the only two relevant to weld inspection because they can penetrate solid metal and produce internal images.

 

What Are the 7 Types of Radiation?

When expanded, radiation types include:

• Alpha
  
• Beta
  
• Gamma
  
• X-rays
  
• Neutron
  
• Ultraviolet (UV)
  
• Infrared (IR)
  

Only gamma rays, X-rays, and neutron radiation are ionizing and used in NDT. Infrared is sometimes used in thermographic inspections, while UV aids fluorescent penetrant testing.

Understanding Radiographic Testing in NDT

X-Ray Testing

X-rays are produced using high-voltage tubes. The radiation originates outside the nucleus, and the energy level can be adjusted. This makes X-ray ideal for thin to moderately thick welds, especially when image detail is important.

Common Uses:

• • Aerospace components
    
  • Structural welds in manufacturing
    
  • Digital Radiography (DR) for real-time analysis
    

Benefits of X-ray and DR:

• • High-resolution images
    
  • Immediate results
    
  • Easier data storage and sharing
    
  • Lower overall inspection costs with digital integration
    

Gamma Ray Testing

Gamma rays are emitted during radioactive decay from isotopes like Iridium-192 and Cobalt-60. This radiation comes from inside the atomic nucleus, offering deep penetration through dense or thick materials.

Common Uses:

• • Pipeline welds
    
  • Thick steel structures
    
  • Field inspections with no access to electricity
    

Advantages:

• • No need for external power
    
  • High penetration
    
  • Compact and portable source devices
    

Film Radiography & Sensitometric Curves

Film-based inspection is still used where code compliance requires it. A sensitometric curve shows how film responds to different exposure levels, affecting image contrast and clarity.  Learn more about sensitometric calibration here.

Key terms:

• • Optical density: How dark the image appears
    
  • Gamma of the curve: The steepness, representing contrast
    
  • Proper calibration is essential for readable, accurate results
    

Which Ray Is More Powerful Than Gamma Ray?

 

Gamma rays typically have the highest natural energy, but X-rays produced by linear accelerators can sometimes exceed that energy level. However, “power” should be interpreted as penetration + control—and X-rays often win in image quality, while gamma rays excel in deep penetration.

What Is the Difference Between Gamma Rays and Infrared Rays?

• Gamma rays are high-energy and ionizing.
  
• Infrared rays are low-energy, non-ionizing, and used to detect heat (not structure).
  

Only gamma rays are used in RT for weld inspection. Infrared is more applicable in surface testing and thermal imaging.

Key Differences Between X-Ray and Gamma Ray

Feature	X-Ray	Gamma Ray
Source	Machine-generated (outside nucleus)	Radioactive isotope (inside nucleus)
Energy Control	Adjustable	Fixed by isotope
Penetration	Medium	High
Image Quality	High (especially with DR)	Moderate
Power Required	Yes	No
Best Use Case	Controlled environments	Field/remote inspections

 

Real-World Applications in Weld Inspection

TOFD vs Radiographic Testing

TOFD (Time of Flight Diffraction) is a high-accuracy method for flaw sizing and root defect detection, while RT (Radiography) is typically required for code compliance and documentation.

Use Cases:

• • Use TOFD when flaw measurement is key
    
  • Use RT when visual records or regulations demand it
    

Gamma Ray in Action

Gamma rays, especially from Cobalt-60, are often used to inspect:

• Rebar in thick concrete (5 ft+ deep)
  
• Heavy-walled pipes and pressure vessels
  
• Welds in power plants and refineries
  

X-Ray with Digital Radiography (DR)

Digital X-ray systems combine safety, speed, and flexibility for weld inspection:

• Eliminate film processing by capturing high-resolution images instantly—no darkroom required.
  
• Power on demand with an X-ray source you can turn on and off, minimizing radiation exposure and removing the need for special radioactive storage.
  
• Verify images immediately onscreen for faster decision-making in the field.
  
• Deploy anywhere with a modular, portable design built for rugged inspection environments.

Choosing the Right Tool for Weld Inspection

Consider the following when deciding between X-ray and gamma ray:

• Material thickness and density
  
• Power availability in the field
  
• Inspection standards (API, ASME, AWS)
  
• Image clarity needs
  
• Portability and safety requirements
  

In some cases, a hybrid approach may be best—pairing DR with TOFD, or combining gamma with UT for complete coverage.

X-Ray = Clarity. Gamma = Penetration.

Both X-ray and gamma ray testing have a place in weld inspection. If you need fine detail and fast image capture, X-ray with DR is the clear choice. If you need to inspect thick materials or work in the field without power, gamma ray is your best option.