The effective atomic number revisited in the light of modern photon-interaction cross-section databases

The effective atomic number, Zeff, has been calculated for fatty acids and cysteine. It is shown that Zeff is a useful parameter for low-Z materials at any energy above 1 keV. Absorption edges of medium-Z elements may complicate the energy dependence of Zeff below 10 keV. The notion of Zeff is perhaps most useful at energies where Compton scattering is dominating, and where Zeff is equal to the mean atomic number, 〈Z〉, over a wide energy range around 1 MeV.