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For photon energies below 1 MeV the predominant phenomenon involved is photoelectric absorption, coherent scattering (Rayleigh), and incoherent scattering (Compton). Biological samples with small thickness attenuate the transmitted beam very little and the application of transmission technique is not possible, when atomic number variations are more significant. Indeed, the Rayleigh to Compton scattering ratio enables the material to be characterized only by its atomic number, independent of the material density or of the attenuation inside the tested object. Hence, evaluation of the parameter “effective atomic number” is still necessary to characterize the measurement sensibility and to establish a correlation with the estimated Rayleigh to Compton scattering ratio.