Recording lead Mα line in rare-earth phosphates using a JEOL JXA-8100 microanalyser

Abstract

Conditions of recording the Pb Mα line and the background in monazite age dating are studied on an example of simple synthetic rare-earth phosphates. By selecting cutoff thresholds, pulses were discriminated from the radiation superimposing on the analytical signal in the highest orders of reflection. The interfering lines are Y Lγ2,3, Th Mζ1, and Th Mζ2. High-aperture and standard spectrometers of the microanalyser do not ensure the complete resolution of Th radiation with the Pb Mα line, and its software allows the analyst to take into account the superposition of only one element. The contribution of interfering radiation is insignificant: the presence of 1% Y2O3 or ThO2 results in the overestimation of PbO by ∼0.009 and 0.002%, respectively. The nearest free regions for measuring background are at ±4.5 mm from the Pb Mα line. Using the background measured in the short-wave region systematically underestimates the amount of PbO by 0.098 wt %. Measurements on the other side lead to overestimation by 0.061%. Calculations of the average background value are more preferable and underestimate the results by approximately 0.019%. To decrease the error more, one should record a spectrum for the determination of its bremsstrahlung component under the lead line.