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Ultrachron

 

 

Goals for analytical precision:

Develop hardware and software improvements in order to expand EPMA into the trace element realm, fundamentally shifting the basic precision well beyond existing EPMA technology. The immediate goal is to improve element count precision by 3x, with a resulting age precision improvement on the order of 2x. This means improving X-ray counting by a factor of 10x (right).

There are two basic methods for improving precision in EPMA: 1) generating more X-ray counts; and 2) enhancing X-ray detection efficiency. The Ultrachron project aggressively proceeds on both fronts.

Limitations in the approach: 1) Generation of greater counts must not appreciably sacrifice spatial resolution (continue to use 15kV or less, and beam diameter should not degrade beyond 200nm at high current at the applied potential); and 2) improvement in X-ray collection efficiency must not sacrifice spectral resolution.

Generate more counts: High brightness source provides greater current density. Improved current regulation provides stability for lengthy count times (10min-60min in general).

Enhance X-ray collection efficiency: New spectrometers developed for ultra=high intensity. Maintains 160mm Rowland Circle radius for high spectral resolution. Collection efficiency improved 5x by development of VLPET monochromators and associated VL counters. These are the largest fully-focussing monochromators ever developed for EPMA. Also - software modified to integrate counts from two or more spectrometers to further improve effiency in a given count time.

 

Following Poisson statistics, X-ray count precision is estimated by the square-root of the raw counts. Upper: 1% precision is improved to 0.3% by increasing from 10000 to 100000 counts. Lower: Measured Pb values in monazite ( UMass Cameca SX50, 15kV, 200nA, 600 sec.) gives the upper curve for a range of Pb concentrations. Increasing counts improves statistics following the successive curves.
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