Mass attenuation coefficient

The mass attenuation coefficient is the ratio of the absorption coefficient and the density of each material. It expresses as the linear absorption coefficient as much electromagnetic radiation is attenuated depending on its photon energy when passing through a material. Its dimension is surface / mass, the usual unit cm2 / g

The exponential decrease of the initial intensity is described by the Lambert -Beer law:

Here, μ is the linear attenuation coefficient and d is the depth of penetration into the material.

The mass attenuation coefficient μ / (: density) is in practice often preferred over μ, because it is similar and almost constant for many materials in a big, important for gamma-ray energy range ( see figure for iron). Also it allows to take into account differing from the ordinary density. Above 2 MeV He is in the range of 0.05 cm ² / g Expressed using the mass attenuation coefficients must be in the Lambert -Beer law rather than the penetration depth of the mass density ρ · d be used. It is then

Tabulated values ​​of the mass attenuation coefficient ( engl. mass attenuation coefficient) are found for example in Ref

Involved physical effects

The slowdown comes above 1 MeV primarily driven by Compton scattering and pair production. At lower energies, however, the absorption is dominated by the photoelectric effect, weak superimposed by Rayleigh scattering.

The mass attenuation coefficient for the photoelectric effect depends not only on the material strongly on the energy or the wavelength of X- rays.

Z is the atomic number of the atomic nucleus, λ is the wavelength of X-rays and C is a parameter that depends on the material and wavelength. It changes at each of the absorption edges.