X-rays are attenuated as they pass through matter. That is, the intensity of an X-ray beam decreases the farther it penetrates into matter. Basically, each interaction of an X-ray photon with an atom of the material removes an X-ray from the beam, decreasing its intensity.
The amount of decrease in intensity of the X-ray beam depends upon two factors:
The intensity decreases exponentially with the distance traveled, orI = I 0exp (– Ax)
where I 0 is the initial X-ray beam intensity. Note that this exponential decay of photon intensity applies in the optical region of the electromagnetic spectrum as well. In this region, it is known as the Beer–Lambert law.
An interesting application of this equation is to determine the depth of penetration of X-rays. The attenuation length is defined as the depth into the material where the intensity of the X-rays has decreased to about 37% (1/e) of the value at the surface. That is, I = (1/e)I 0, or I/I 0 = 1/e. [Recall that e, sometimes called Euler's number or Napier's constant, is the base of natural logarithms, or e ≈ 2.7183.] Then, substituting into Equation 1, we get
0) = exp (–μρx)
This also is referred to as the "mean free path" of the X-rays.