Spatial Resolution Sensitivity Accuracy Ultrachron Home
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(minimum 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 particularly 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 independent 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 evaluation 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.
Testing
Analytical resolution relevant to geochronologic analysis can be evaluated by boundary testing of sharply sector zoned monazite (right). These tests were performed with an optimized LaB6 cathode after approximately 3000 hrs. of service life. Tests were done at 10 and 15kV, and 40, 100 and 200 nA beam currents. Beam motion is normal to interface, while ThMa intensity is acquired (below). Scan data can be evaluated by second-derivative analysis to determine resolution (lower-right). Resulting resolution measurements are summarized in figure upper right. See Jercinovic et al. (2008) for details.
