Soft X-Ray EPMA analyses of nanophase lunar Fe-Si compounds

Abstract

Conventional electron-probe microanalysis (EPMA) has an X-ray analytical spatial resolution on the order of 1-4 μm width/depth. Many of the naturally occurring Fe-Si compounds analyzed in this study are smaller than 1 μm in size, requiring the use of lower accelerating potentials and non-standard X-ray lines for analysis. The problems with the use of low energy X-ray lines (soft X-rays) of iron for quantitative analyses are discussed and a review is given of the alternative X- ray lines that may be used for iron at or below 5 keV (i.e., accelerating voltage that allows analysis of areas of interest smaller than 1 μm). Problems include the increased sensitivity to surface effects for soft X-rays, peak shifts (induced by chemical bonding, differential self- absorption, and/or buildup of carbon contamination), uncertainties in the mass attenuation coefficient (MAC) for X-ray lines near absorption edges, and issues with spectral resolution and count rates from the available Bragg diffractors. In addition to the results from the traditionally used Fe Lα line, alternative approaches, utilizing Fe Lβ, and Fe Ll-η lines, are discussed.