RAYLEIGH AND COMPTON-SCATTERING CONTRIBUTIONS TO X-RAY-FLUORESCENCE INTENSITY

The enhancement of XRF intensity due to Rayleigh and Compton scattering in the target (which may occur after or before the photoelectric interaction) is commonly neglected in XRF spectrometry. However, this contribution can modify the XRF intensities to an appreciable extent, becoming a source of uncertainty for the computational methods of quantitative analysis. Analytical expressions are presented to describe the intensities of the corresponding four double interactions (which mix the photoelectric effect with the dominant types of scattering) that enhance the XRF intensity: Rayleigh-photoelectric, Compton-photoelectric, photoelectric-Rayleigh and photoelectric-Compton. These relationships are obtained by using an iterative solution of the Boltzmann transport equation for photons, suitable for the study of multiple scattering processes. It is shown that the first three chains contribute with discrete enhancements of the same energy of the XRF line. The last, in contrast, contributes with a short-range continuous spectrum that overlaps the low-energy tail of the peaks. This contribution can modify the line shape in a non-symmetric way, making the unfolding of peaks from energy-dispersive spectra more imprecise. The corrective terms to the XRF intensity are computed for both pure and composite materials. A common property to all of them is their azimuthal symmetry, a consequence of the isotropic behaviour of the photoelectric effect. As a rule, the chains with Rayleigh scattering predominate in the low-energy regime and contribute between 1 and 10% of the primary XRF line. The chains with Compton scattering prevail at higher energy and their extent may exceed several tens percent. All the corrections show a broad range of variation depending on the incident energy, the composition of the target, which defines its attenuation properties for all the energies, and the incidence and take-off angles.