The contribution of photo-and Auger electrons generated in analyte material to the x-ray fluorescence of elements with atomic numbers from Z -5 (boron) to 9 (fluorine) was evaluated. The calculations were performed for different x-ray tube anodes, window thicknesses and working regimes. This contribution to x-ray fluorescence excitation increases rapidly with decreasing atomic number of the measured element and depends greatly on the sample composition and experimental conditions. In some cases the contribution may exceed 90%. This high value results from the absorption in the window and in the x-ray anode tube material of the long-wavelength component of primary radiation which is most effective in photoionization of element atoms. It follows from the calculations that at shorter wavelengths, e.g. for Na and Cu, the contribution of photo-and Auger electrons to the excitation of their fluorescence seems to be much less significant than that obtained in otber work.
A mathematical model for an energy-dispersive X-ray spectrometer is proposed, which is based on the analysis of the radiation transport in a sample and in a detector, and an electron transport in the detector. The results obtained by using this model are compared with experimental data. The influence of parameters of sources of the primary X-rays, and the thickness and material of the detector on the signal-to-background ratios, was estimated. These ratios limited by electron and photon transport were compared. It is shown that the signal-to-background ratio can vary by several orders of magnitude at changing conditions of recording, and there are conditions when the use of Si or Ge detector is preferable.
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