Mass attenuation coefficient
The mass attenuation coefficient or mass narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter. In addition to visible light, mass attenuation coefficients can be defined for other electromagnetic radiation (such as X-rays), sound, or any other beam that attenuates. The SI unit of mass attenuation coefficient is the square metre per kilogram (m2/kg). Other common units include cm2/g (the most common unit for X-ray mass attenuation coefficients) and mL⋅g−1⋅cm−1 (sometimes used in solution chemistry). "Mass extinction coefficient" is an old term for this quantity.
The mass attenuation coefficient can be thought of as a variant of absorption cross section where the effective area is defined per unit mass instead of per particle.
Mass attenuation coefficient is defined as
where
When a narrow (collimated) beam passes through a volume, the beam will lose intensity due to two processes: absorption and scattering.
Mass absorption coefficient, and mass scattering coefficient are defined as
where
In chemistry, mass attenuation coefficients are often used for a chemical species dissolved in a solution. In that case, the mass attenuation coefficient is defined by the same equation, except that the "density" is the density of only that one chemical species, and the "attenuation" is the attenuation due to only that one chemical species. The actual attenuation coefficient is computed by
where each term in the sum is the mass attenuation coefficient and density of a different component of the solution (the solvent must also be included). This is a convenient concept because the mass attenuation coefficient of a species is approximately independent of its concentration (as long as certain assumptions are fulfilled).
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