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Efficiency and response functions were determined for a modern commercial energy-dispersive x-ray spectrometer in the photon energy range 0.1-5 keV using dispersed and undispersed synchrotron radiation. The spectrometer was equipped with an Si(Li) detector crystal, a thin-film window and a digital pulse processor. Monochromatized synchrotron radiation from the PTB VUV radiometry beamline at the electron storage ring BESSY was used for the absolute determination of the detection efficiency with a typical relative uncertainty of 1-2% in the photon energy range 0.1-1.5 keV by direct comparison with calibrated photodiodes. At higher photon energies, the efficiency was found by comparison of the measured and calculated undispersed synchrotron radiation spectrum. The absolute intensity of the synchrotron radiation was known with a relative uncertainty of less than 1%. In the overlapping region, the results from the two independent experiments are in full agreement. The energy dependence of the measured efficiency can be explained only with a detector model assuming that there is no dead layer. A simple model for the effect of incomplete charge collection (ICC) was applied to describe the measured response functions. Consequences of the ICC such as broadening and shift of low-energy peaks and redistribution of counts around 1.84 keV are explained with the model, in accordance with the experimental results.
A calibration procedure for the detection efficiency of energy dispersive X-ray spectrometers (EDS) used in combination with scanning electron microscopy (SEM) for standardless electron probe microanalysis (EPMA) is presented. The procedure is based on the comparison of X-ray spectra from a reference material (RM) measured with the EDS to be calibrated and a reference EDS. The RM is certified by the line intensities in the X-ray spectrum recorded with a reference EDS and by its composition. The calibration of the reference EDS is performed using synchrotron radiation at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt. Measurement of RM spectra and comparison of the specified line intensities enables a rapid efficiency calibration on most SEMs. The article reports on studies to prepare such a RM and on EDS calibration and proposes a methodology that could be implemented in current spectrometer software to enable the calibration with a minimum of operator assistance.
Energy dispersive X-ray spectrometry offers the opportunity for fast composition determination of specimens by X-ray fluorescence or electron probe microanalysis. For fundamental parameter based quantification, the knowledge of the detection efficiency of the spectrometer is essential. At low energies the efficiency is strongly influenced by X-ray absorption in the radiation entrance window. State-of-the-art windows consist of polymer foil containing C, N, and O, coated with Al and in some cases with a special B compound. The foil is supported by a Si grid to withstand the atmosphere pressure. The absorption of all these components must be known to describe the detection efficiency.
The transmittance of three types of widely used commercial windows has been measured. Transmittance curves have been fitted by analytical expressions using tabulated mass absorption coefficients. Because tabulated mass absorption coefficients do not consider near edge effects, there are strong deviations between measured and calculated transmittance below 0.6 keV. It is proposed to model the spectrometer efficiency by the measured window transmittance and calculated absorptions from front contact and possible contaminations. This reduces the number of unknown parameters drastically.