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To compare the performance of different energy dispersive X-ray spectrometers (EDS), it is important to define characteristic spectrometer parameters. The ISO 15632 standard defines parameters like energy resolution as FWHM for the Kα lines of carbon, fluorine and manganese. The quantum efficiency, which is the ratio of the detected photons divided by the number of incoming photons for different energies, is another significant spectrometer property. It is important for the light element and low energy line detection sensitivity as well as for higher photon energies above 10 keV. A striking EDS feature, provided and marketed by many manufactures, is the active area of the detector, although actually, the solid angle available for photon collection is the more relevant geometrical parameter. It is defined as: Ω = A /r2 with A being the active area of a spherical detector and r being the distance between the point of the radiation origin and the center of the surface of the active detector chip. A more accurate calculation should be used for large flat detector areas. One should note that the solid angle Ω is not an intrinsic spectrometer property. It can only be defined for a specific detector in combination with a specific system (e.g. SEM, EPMA or TEM). Thus, the minimal possible distance r is determined by the particular geometry e.g. a possible interference with the pole piece or other detectors/components in the chamber of a microscope. New EDS technologies use e.g. integrated SDD chips or inclined chips in thinner detector fingers which can be placed closer to the sample with the final result of larger real solid angles. Therefore, the knowledge of the real solid angle is one of the crucial parameters of an EDS microscope combination. A straightforward way to estimate the real solid angle is to simply determine A and r. If respective data are not provided by the manufacturer, this approach can be difficult.
An energy dispersive X-ray spectrometer operating with a semiconductor detector should be specified in compliance with the ISO standard 15632 [1]. Requirements for specification are: a) a general description of the spectrometer to evaluate its performance, b) the energy resolution with corresponding dead time, c) the P/B ratio in the Fe55 spectrum and, finally, d) the L/K intensity Ratio in a Ni or Cu spectrum to estimate spectrometer efficiency at low energies. Items b) to d) can be easily checked by the user. Related procedures are recommended in the annexes of the standard.
The test material EDS-TM001 together with an accompanying software package, “EDX spectrometer check”, have been made available in 2009 by BAM to be employed by EDS (energy dispersive spectrometer) users to check the performance of an EDS attached to the SEM. Particularly for test laboratories operating under accreditation schemes like ISO/IEC 17025, a periodical control of the critical instrumental parameters in end-user laboratories is required. With EDS-TM001 or EDS-TM002 (second generation) test material, this periodical check is simplified to the acquisition of only one 10 kV spectrum. The software “EDX spectrometer check” is destined to evaluate automatically this spectrum and determine the performance of the EDS in terms of energy resolution and calibration as well as possible alteration of low-energy efficiency due to detector contamination. Energy resolution can be compared with the specified values according to the international ISO standard ISO 15632:2012. EDS-TM is a synthetic material consisting of a thick layer of C, Al, Mn, Cu and Zr in a well-defined composition, deposited on a steel (in case of EDS-TM001) or silicon (in case of EDS-TM002) substrate. Meanwhile, more than one hundred laboratories use the EDS-TM001 or EDS-TM002 test material for the periodical check of their EDS. A detailed description of the test material and software together with examples of application was published recently. New results and gained experiences will be presented as well.
When the FWHM of lines appearing in the EDS-TM spectrum are determined, the spectrum background must be subtracted accurately. The applied physical background subtraction procedure is robust and takes into account the transmission of the detector window. While the previous version considers only Moxtek AP windows, the new version includes selection of silicon nitride window and the case of windowless detector. Moreover, the new version allows importing of spectra in Bruker spx format and EMSA/MSA files from EDAX TEAM software.