Scofield's theory confirmed

X-ray fluorescence (XRF) analysis is frequently used to analyze materials in the lab in order to measure the total content of certain elements or the mass per area. Hereby, the content of certain elements is deduced from the intensity of element-specific X-ray fluorescence curves. The quantitative indication of this content is done using Sherman's fundamental parameter approach which was drawn up in 1955. Here, atomic fundamental parameters – such as photoionization cross sections and fluorescence yields – describe the probabilities of X-ray excitation and of X-ray fluorescence decay in the interaction between X-radiation and matter.

The partial photoionization cross section indicates the probability of the excitation of an electron from a specific subshell to be excited. There are two different approaches to describe the way the cross sections depend on the energy of the exciting X-ray photons for different subshells. For simplification purposes, the approach stating that the ratio of the cross sections is assumed to be constant for different subshells is often selected. Model computations in nuclear physics realized by Scofield had, however, predicted as early as in the 1970s that this does not apply to subshells with orbitals of different symmetry and that the relation with the exciting photon energy changes in this case.

In PTB's laboratory at the electron storage ring BESSY II, this predicted energy dependence has now been successfully demonstrated experimentally for the first time by using radiometrically calibrated XRF instrumentation. The results clearly show that the different energy dependences must be taken into account for a reliable elemental analysis with small uncertainties.