Interaction Between Penetrating Radiation and Matter

The number of photons that are aimed at a material will not usually equal the number of photons that transmit through the material. This is because photons get attenuated.The attenuation of photons depends on the properties and geometry of the material the photons try to penetrate.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:

I = I 0 e μ x I=I_{0}e^{-\mu x}

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