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Electron optical goals:

High brightness gun and high performance column optics to achieve optimum beam size through range of voltage (5-30kV) at very high current (200 nA to 1 uA). Beam current regulation must operate effectively up to 1 uA, and throughout the lengthy count times necessary for trace element analysis. Gun must be compatible with anticontamination system. Essentially need lower voltage and higher current to optimize both spatial resolution(minimus scattering volume) and counting for improved precision.

The high average atomic number of accessory phases such as monazite confine electron scattering to a point where beam size optimization becomes significant (right) . Small beam size must be achieved with high current (>200nA) and optimal overvoltage paticularly for Pb, Th, and U X-ray production. Electron scattering in the specimen dominates the analytical resolution as beam voltage exceeds 15kV. At 20kV, the resolution of analysis is at least 1.5um independant of the achieved beam diameter. At 15kV or less, significant improvements can be made in analytical resolution, particularly if the beam diameter is kept to 200nm or less. Note that the minimum voltage is about 8kV for efficient (2-3x overvoltage) actinides and Pb count rates. Any improvement in beam diameter below 100nm will only result in an improvement in analytical resolution of about 20nm for Z=38 at 8kV. Note that a complete evauation of the analysis must address possible secondary fluorescence ranges (both characteristic and continuum).

Two essential modes are required:

1) Full compositional analysis of Pb, actinides, REEs, and other major and minor elements at >10kV.

2) Analysis of Pb, U, and Th with assumed matrix composition (for lowest voltage, highest analytical resolution operation) at 8-10kV.

Analytical resolution vs. accelerating potential at different beam diameters (D Beam in nanometers) for Moacyr monazite. Upper: 0-30kV based on radius containing 99.5% of total emitted intensity of Pb Mα. Lower: 4-11kV comparing φ(ρZ) depth (red) containing 99.5% with radial distribution (yellow). For details, see Jercinovic et al. (2008).
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