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Atomic fundamental parameters verified

01.06.2015

Fig. 1: XRF facility in the PTB laboratory at the electron storage ring BESSY II.

Fig. 2: Comparison of different data for the partial photoionization cross sections of the Pd L1 subshell. The approach of an energy-independent ratio (green) shows deviations of up to a factor of 4.

By means of improved measurement procedures, PTB employees from the field of spectrometry with synchrotron radiation have succeeded – in cooperation with partners from industry – in experimentally confirming the different energy dependencies of partial photoionization cross sections which could so far be predicted only by theoretical calculations. Due to this, the quantification approach of an energy-independent ratio of the partial cross sections, which is often used in particular in X-ray fluorescence analysis (XRF), has been refuted. This makes X-ray-analytical laboratory investigations clearly more accurate.

In the case of XRF, the intensities of element-specific X-ray fluorescence lines can be used to quantify total elemental contents and mass depositions. The quantification is based on the fundamental parameter approach which Sherman established in 1955. Here, atomic fundamental parameters, such as photoionization cross sections and fluorescence yields, describe the interaction between X-rays and matter quantitatively. Partial photoionization cross sections indicate the probability for the excitation from different subshells. There are two different approaches for the dependence of the cross sections of different subshells on the energy of the exciting radiation: The approach that the ratio of the cross sections for different subshells is constant is more often used due to its simplicity. Quantum-mechanical calculations of Scofield showed already in the 1970s, however, that this ratio changes for orbitals with different symmetry with the energy of the exciting radiation. In the PTB laboratory at the electron storage ring BESSY II, this predicted energy dependence has now been confirmed by the use of radiometrically calibrated XRF instrumentation and an extended measurement procedure to provide experimental evidence for the first time. The results clearly show that the different energy dependencies must be taken into account for a reliable quantitative elemental analysis with small uncertainties.

Publication:

Opens external link in new windowP. Hönicke, M. Kolbe, M. Müller, M. Mantler, M. Krämer, B. Beckhoff, Experimental Verification of the Individual Energy Dependencies of the Partial L-Shell Photoionization Cross Sections of Pd and Mo, Physical Review Letters 113, 163001 (2014)

Contact:

P. Hönicke, 7.24, e-mail: Opens window for sending emailPhilipp.Hoenicke(at)ptb.de