Mike Jercinovic  Sensitivity in EPMA
Improved Analytical Resolution and Sensitivity in EPMA Some Initial Results from the Ultrachron Development Project
Mike Jercinovic 

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Abstract
Monte Carlo modeling of electron scattering and Xray emission for PbMα in monazite (REE+actinidephosphate) can be used to estimate expected analytical spatial resolution (quadrature sum of the error functions associated with beam diameter and scattering volume, and Xray emission volume). Such highZ phases allow analytical resolution in the submicron realm if beam brightness is sufficient, and if the accelerating potential is kept below 15kV. At 10kV, the analytical resolution will remain below 500nm for currents up through 200nA if a beam diameter of 200nm or less is realized. Improving the counting efficiency allows an increase in precision at lower voltage and/or beam currents for the more beam sensitive materials. To this end, new VLPET spectrometers were developed which are approximately 5× the collection area of the standard PET, and are equipped with accordingly large detectors. The improved collection efficiency substantially improves count rates above previously existing monochromators. Software integration of multiple spectrometers counting the same analytical line and statisticallybased sampling allow further improvement in precision. The application of large and very large crystals, along with count integration results in sensitivity (detection limit) improvement by nearly 4× for PbMα in monazite for single point analysis. For 6point analysis, the age precision (comparing standard error of the mean) is improved approximately 3× using VLPET and 2spectrometer integration relative to standard PET on the SX50. When beam damage and/or contamination issues are critical, lengthy count times are not preferable, as in speleothem analysis. The Ultrachron has been applied to Sr analysis in CaCO_{3} speleothems using 5spectrometer integration involving PET, LPET, and VLPET spectrometers. Using 15kV and 100nA, 100 sec analyses were performed giving a precision of 29 ppm (2σ) at a concentration of 200 ppm (single point detection limit = 50 ppm). Similar 5spectrometer integration for analysis of Zr in rutile at 20kV and 200nA (600 s per point) results in a single point detection limit of 14 ppm, and 3 ppm for a 15 point acquisition.
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