Since a linear attenuation coefficient is dependent on the density of a material, the
mass attenuation coefficient is often reported for convenience. Consider water for
example. The linear attenuation for water vapor is much lower than it is for ice
because the molecules are more spread out in vapor so the chance of a photon
encounter with a water particle is less. Normalizing by dividing it by the density of
the element or compound will produce a value that is constant for a particular element
or compound. This constant ( ) is known as the mass attenuation coefficient and
has units of cm2/gm. The mass attenuation coefficient can simply be converted to a
linear attenuation coefficient by multiplying it by the density ( ) of the material.