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A Study on the Change of the Fe La Mass Absorption Coefficients and Fluorescence Yields in Iron Silicide Samples by EPMA

The study of natural iron silicides is of importance because the extreme reducing environment required for their formation can give insight into the mechanism that created them. The presence of iron silicides has been reported in a variety of environments, from lightning-induced fulgarites to micro-meteoritic impact-formed lunar regolith and stardust samples. Natural iron silicide phases can be relatively small (few μm) and their quantification by EPMA can be difficult [1]. Indeed, despite the recent introduction of the field emission gun (FEG), which allows sub-micron resolution imaging with a very small electron beam, the X-ray lateral spatial analytical resolution remains too large at traditional accelerating voltages (e.g., 15 kV). To improve analytical resolution, the accelerating voltage can be decreased to 5–7 kV, reducing the typical size of the electron interaction volume from ~1–2 μm to ~0.2–0.7 μm (depending upon composition). However, by reducing the accelerating voltage, the Kα line of some elements of the periodic table cannot be excited. The L lines must then be used for these elements, but these L X-ray lines are not well known for the first-row transition metals and complex chemical and physical effects can occur. Indeed, the Lα X-ray line of those elements involves the partially filled 3d electron shell which is subject to strong bonding effects with the electrons of the neighboring atoms. As a result, the outer electronic shell energies are modified by these bonding effects and then are the associated transition probabilities. The different atomic parameter values (mass absorption coefficient (MAC), fluorescence yield, Coster-Kronig factor, ...) not only depend of the studied element but also of the composition and nature of the material, both of the unknown and of the standard used. This led Gopon et al. [1] to successfully demonstrate the utility of the Fe Ll line. While this is a useful alternative, there remains the issue of the low intensities of Fe Ll X-rays. This has led us to a further examination of the Fe Lα line (with its high count rate) for quantitative EPMA.