Home - Education Resources - NDT Course Material - Radiography


Present State
Future Direction

Physics of Radiography
Nature of Penetrating Radiation
Gamma Rays
Decay Rate
  -Carbon 14 Dating
Inverse Square Law
Interaction of RT/Matter
Attenuation Coefficient
Half-Value Layer
Sources of Attenuation
  -Compton Scattering
Geometric Unsharpness
Filters in Radiography
Scatter/Radiation Control
Radiation Safety

Equipment & Materials
X-ray Generators
Radio Isotope Sources
Radiographic Film
Exposure Vaults

Techniques & Calibrations
Imaging Consideration
Radiographic Density
Characteristic Curves
Exposure Calculations
Controlling Quality

Film Processing
Viewing Radiographs
Radiograph Interp-Welds
Radiograph Interp - Castings

Advanced Techniques
Real-time Radiography
Computed Tomography



Interaction Between Penetrating Radiation
and Matter

When x-rays or gamma rays are directed into an object, some of the photons interact with the particles of the matter and their energy can be absorbed or scattered.  This absorption and scattering is called attenuation.  Other photons travel completely through the object without interacting with any of the material's particles.  The number of photons transmitted through a material depends on the thickness, density and atomic number of the material, and the energy of the individual photons.

Even when they have the same energy, photons travel different distances within a material simply based on the probability of their encounter with one or more of the particles of the matter and the type of encounter that occurs.  Since the probability of an encounter increases with the distance traveled, the number of photons reaching a specific point within the matter decreases exponentially with distance traveled.  As shown in the graphic to the right, if 1000 photons are aimed at ten 1 cm layers of a material and there is a 10% chance of a photon being attenuated in this layer, then there will be 100 photons attenuated.  This leave 900 photos to travel into the next layer where 10% of these photos will be attenuated.  By continuing this progression, the exponential shape of the curve becomes apparent. 

 The formula that describes this curve is:

The factor that indicates how much attenuation will take place per cm (10% in this example) is known as the linear attenuation coefficient, m.  The above equation and the linear attenuation coefficient will be discussed in more detail on the following page.