Ultrachron Precision

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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-focusing monochromators ever developed for EPMA. Also – software modified to integrate counts from two or more spectrometers to further improve efficiency in a given count time.

Following Poisson statistics, X-ray count precision is estimated by the square-root of the raw counts. Left: 1% precision is improved to 0.3% by increasing from 10000 to 100000 counts. Right: 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.

Testing

Precision is dramatically improved via the developments realized in the Ultrachron project. The VL spectrometers improve count rates b y about 4X above standard PET (right), without sacrificing spectral resolution. The 160mm Rowland Circle radius is maintained. Measurables relevant to EPMA geochronology are shown in the table below, which compares an analysis of the GSC SHRIMP monazite standard GSC 8153 performed on the Cameca SX50 at UMass with an analysis performed using the SX-Ultrachron at UMass using the same analytical parameters (voltage, current, count time). Precision is more than doubled in the age analysis. For individual element analysis using the VLPETs (see Pb below), precision is improved by a factor of 4. More precise analyses of 3 ppm Pb, 4 ppm U are achieved in 14 pt analyses (15min/pt.) still using 15kV and 200nA.

Note that higher collection efficiency allows not only improved precision, but also permits precise analysis at reduced voltage or current density in beam-sensitive materials (carbonates, phosphates, etc.).

Comparison of PbMa intensity PET (SX50) vs VLPET (SX-Ultrachron) over a range of beam accelerating potential. PbMa, analysis could be performed at 8kV using VLPET, attaining the same count rates as PET at 15kV. Similarly, current or count time could be lowered if either beam exposure damage or analysis time were important issues.

 

GSC 8153 monazite analsyis (ca. 505). U (UMb)and Pb (PbMa) were analyzed using PET on the SX50. U was analyzed using LPET, and Pb with VLPET on the Ultrachron.

 

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