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A field -emission scanning electron microscope (FE -SEM) with energy -dispersive X -ray spectrometer (EDS) detector of a superconducting transition -edge sensor (TES) microcalorimeter is a new system for electron -microprobe chemical analyses. FE -SEM with TES was used for qualitative and semi -quantitative analyses of rareearth elements (REE) at a low accelerating voltage of 5 kV. Four characteristic M -lines were detected in the LaB 6 spectrum: LaMζ at 640, LaMαβ at 841, LaMγ at 1021, and a weak line (M 2 N 4 transition) at 1100 eV. The spectra of other rare -earth borides, rare -earth phosphates, and monazite were assigned in the same way as the La M -lines were. For quantitative analyses, we used a calibration curve method, using standard specimens of known chemical compositions. Linear calibration curves for plots of P, Ca, La, Ce, Pr, and Nd intensities versus each weight percentage were obtained. Semi -quantitative analyses of rare -earth minerals should be carried out at low accelerating voltages using a calibration curve method. In a TES -EDS system, a low accelerating voltage can be used to improve the spatial resolution, without the sensitivity disadvantages of low -energy X -ray emissions. Moreover, a strong increase in the Mαβ intensity with increasing atomic number Z was seen, so the detection limits of heavy REE was much lower than those of light REEs. These results suggest that the TES -EDS system could be a useful analytical tool in rare -earth mineralogy.

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