To describe the attenuation coefficient in a way independent of the number densities n i of the N attenuating species of the material sample, one introduces the attenuation cross section σ σ i = μ μ i ( z ) n i ( z ) . σ i has the dimension of an area; it expresses the likelihood of interaction between the particles of the beam and the particles of the species i in the material sample: T = exp ⁡ ⁡ ( − − ∑ ∑ i = 1 N σ σ i ∫ ∫ 0 ℓ ℓ n i ( z ) d z ) .