Attenuation of X - Rays

The energy of X - Rays is absorbed by tissue - and absorption increases with the density of tissue - as X - Rays pass through human tissue. This is true even for soft tissue, though different tissue types absorb the energy to different degrees, which allows tumours to be identified. The relationship between the depth of penetration and the X - Ray energy unabsorbed is exponential, so that if the intensity of X - Rays entering the skin is  
  and the intensity of the X - Rays at a depth  
, then
\[I=I_0 e^{- \mu x}\]

  is the linear coefficient, different for different tissues.
The relationship implies that a constant fraction of the X - Ray intensity is absorbed per unit length of penetration of the X - Rays into the tissue. The attenuation coefficient is due to the interaction of X rays with the electrons surrounding atoms. Heavier atoms have more electrons so a higher attenuation coefficient. The attenuation coefficient varies roughly as the cube of the atomic number.