How Sample Holder Geometries Influence the Quantification of X-ray Spectra
Judith Lammer
14:00 - 14:40 Friday 17 November 2017 Hörsaal Ventrex (NT03034)

Energy-dispersive X-ray spectrometry (EDXS) is a widely used method for the analysis of samples and their quantification in transmission electron microscopy. Although it is a fast and efficient way to do elemental analysis, it gets tricky, when it comes to measuring light elements. Light elements emit low energy X-rays, which are most probably partly absorbed by the sample itself. This leads to a reduced X-ray peak in the spectrum. A recently introduced quantification method, called the ζ-factor method [1], takes care of this problem and enables us to correct for absorption effects. To use this method in a proper way, the geometrical arrangement of the microscope and the EDX detector, as well as the influences of the sample holder must be considered.

We work with a multi-detector system at our microscope (FEI Titan³ 60–300), where 4 windowless SDD detectors are symmetrically placed around the optical axis. In this configuration, tilting the sample in one direction causes a severe shadowing of one or more detectors due to the sample holder. This means, that the detector areas are not fully illuminated, which causes wrong quantification results when using the ζ-factor method.
In this work, we focus on shadowing and show how different sample holders influence the quantification. We compare the results of two FEI low background double tilt high visibility holders and a tomography holder. We determined the exact position of the detectors by combining experimental measurements and geometrical simulations [2]. We noted that shadowing also depends on the X-ray energy due to the Beryllium specimen carrier inside the holder, which appears fully transparent for high energies, but partially blocks low-energy X-rays. Therefore, the amount of shadowing depends on not only tilt angles and specimen positions, but also on the X-ray energy, which can introduce significant errors upon quantification of both low- and high-energy X-ray lines.

This method provides a means to determine accurately the positions of different X-ray detectors in a multi-detector system. The knowledge about the geometric arrangement of sample, sample holder and EDX detectors is paramount for a better understanding of the EDX system setup, but also enables a post-experiment correction to quantitative results.

References
1. M. Watanabe, D. B. Williams, J Microsc. 221, 89–109, 2006.
2. J. Kraxner et al., Ultramicroscopy 172, 30–39, 2017.