Collaborative Research: Midwest Low KV Collaboration: Continuation of Development of Low KV High Spatial Resolution EPMA Technique to Sub-micron Geological Features

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This project involves development of new techniques in the use of the electron probe microanalyzer, to extend its usefulness in chemically analyzing smaller sized materials than previously possible. This instrument operates by focusing a beam of high energy electrons at a target sample inside a vacuum chamber, and then measuring the different X-rays inherently produced by this process. The rationale for this work is to better understand physical processes - both in geological and earth materials, but also in man-made materials at smaller and smaller scales - with a relatively simple instrument that is available in many universities, industrial and governmental research laboratories. Many times the very smallest features in a material can give critical information about how the material was created, and for manmade materials, how it will break or fail. In a just completed NSF-supported project (EAR-1554269), to develop a technique for extending the utility of the smaller electron beam field emission electron probe microanalyzer, we operated at lower accelerating voltage (i.e., 7 kV) vs traditional 15-20 kV. At these voltages, the higher energy K-series X-ray lines such those of the transition metals (e.g. Fe), are not generated - requiring that the lower energy L-series X-rays be measured and quantified. However, there are series difficulties with using these L-series X-rays, as they become complex entities due to spectral peak shift/shape changes, resulting from chemical bonding effects exacerbated by the adjacent absorption edge which adds more complexity. Thus, one single or simple reference standard is no longer a valid approach to quantification. In that project, we developed an innovative procedure to handle these iron L-alpha X-rays--and iron is an important element in many relevant geological materials. This research focused on the Fe-bearing mineral olivine and iron-silicides (found in meteorites and on the surface of the moon). This new project has several components to extend this technique: (1) extend this low kV Fe-L series work to other important important minerals such as sulfides, pyroxenes, garnets and amphiboles, (2) expand to look at L-lines of some Ni-bearing and Mn-rich minerals, (3) evaluate possible ways to improve measurements with "plasma" cleaning samples, (4) develop protocols using the innovative Soft X-ray Emission Spectrometer for the low energy X-rays generated at low kV, (5) investigate the feasibility of utilizing DFT (density functional theory) to simulate the complex spectral shapes of the L-series transition metals as well as the M-series of some rare earth elements (which have a similar set of conditions as the L-lines of Fe, etc.), and (6) investigate sample damage by the higher-electron-density field emission beam. This work will be done in a joint collaboration between the electron microprobe labs of the University of Wisconsin-Madison and the University of Minnesota-Minneapolis. The project will support a postdoctoral research fellow.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.