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The accurate knowledge of the properties of polycapillary X-ray semi-lenses has a significant influence on the quantitative results in a widespread field of applications involving microfocus X-ray beams. A routine procedure for the characterisation of a polycapillary X-ray semi-lens with a scanning electron microscope (SEM) having attached an energy dispersive spectrometer (EDS) is presented in this paper. A key issue of the procedure consists of fitting the semi-lens in front of the EDS for spectra acquisition. Relevant semi-lens parameters such as focal distance, full width at half maximum (FWHM) of the acceptance area, and transmission are determined in parallelising mode of the semi-lens. Special attention has been paid to the calculation of the transmission.
A calibrated instrumental setup was specially developed for the purposes of accurate measurements of absolute X-ray spectra emitted by modern micro-focus, low-power X-ray tubes. By knowing all the instrumental parameters the spectra measured with the 16 m long setup are recalculated into emitted X-ray tube spectra in photons per eV per msr per nA per s.
It is a latent wish of any SEM/EDS (scanning electron microscope with an energy dispersive spectrometer) analyst to “see more” of the analyzed specimen, i.e. to improve the existing analytical figures of merit.
One key issue are the relatively poor limits of detection (not below 0.1 mass-%) provided by energy dispersive X-ray spectrometry (EDX) with the conventional electron excitation (ED-EPMA). This is a consequence of relatively low peak-to-background ratios and reduced energy resolution when compared to wavelength dispersive spectrometry (WD-EPMA). Recent technological developments make possible to equip the SEM with a wavelength dispersive spectrometer (WDS), so that
significantly better energy resolution can be attained. Also a relative new product that can be easily attached to a SEM/EDS system is a micro-focus X-ray source. Hence, it is possible to perform (micro-focus) X-ray fluorescence spectrometry (μ-XRF) and take advantage of the enhanced peak-to-background ratios (well suited for trace analysis). However, there are also some disadvantages: an increased measurement time and excitation with a high current in the 10s of nA range are usually required for WDS. μ-XRF provides more bulk information and poor limits of detection for light elements. By combining the advantages of these analytical techniques “seeing more” becomes possible.
Popular X-ray tube models available in the literature, i.e. 'Pella', 'Ebel', and 'Finkelshtein and Pavlova', are systematically evaluated with the focus on the estimation of the associated uncertainties. Also taken in consideration and compared is our recent semi-empirical own approach already employed in our lab. This has been working for the common target elements rhodium, molybdenum and tungsten and was further extended in the present work for the target elements copper, chromium and vanadium. By using a modern scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) system this time, higher performances such as stability of the beam current and especially the better energy resolution of the EDS have enabled the reliable extension of our own X-ray tube spectrum approach into the low-energy range, due to increasing interest. Hence, also the more challenging X-ray lines of copper, chromium and vanadium L-series lying in the energy range below 12?keV are included into the model. Such low-energy L-lines or, e.g. M-lines of tungsten, are not treated explicitly by the other existing popular algorithms for the nowadays widely used geometries of side-window tubes, offering a unique virtue to our present, modern approach. With our own model, a measurement uncertainty of the X-ray tube spectra (considering the uncertainties associated with the SEM beam current, the detector acceptance solid angle and efficiency of the spectrometer) within 15% has been estimated. The validation of the approach is demonstrated with metrological measurements with a calibrated SEM/EDS system geometrically configured as a side-window X-ray tube.
Determination of the efficiency of an energy dispersive X-ray spectrometer up to 50 keV with a SEM
(2009)
Both electron and polychromatic photon excitations (micro-focus X-ray source) at a scanning electron microscope (SEM) are used to determine the efficiency of an energy dispersive X-ray spectrometer up to 50 keV by means of a calibrated X-ray spectrometer and reference materials (RM) specially selected for this purpose.
An inter-laboratory comparison organized as a pilot study (PS) has been performed under the umbrella of the Consultative Committee for Amount of Substance – Metrology in Chemistry (CCQM) / Surface Analysis Working Group (SAWG) with the objective to compare k-values measured with Electron Probe X-ray Microanalysis (EPMA) for copper-gold alloys. The k-value, defined as the ratio of the Xray photon intensity of an X-ray characteristic line of the unknown sample to the corresponding one in the reference material, is a rough estimate of the mass fraction of the respective element. Basically, the mass fractions can be calculated from the k-values after applying a matrix correction procedure, which includes empirical approaches, so that traceability gets lost. Due to its physical, measurable nature, the k-value has been selected in the present PS as the primary measurand. The mass fraction has been set as the secondary (derived) measurand. Both EDX and WDX have been employed. The WDX results and the overall comparability between the k-values obtained by EDX and WDX are still in progress.