The determination of low concentrations of U, Th and K by XRF spectrometry

Abstract

A rapid easily-implemented method is presented for determining low concentrations of the major heat-producing elements, U, Th and K, in rocks of varying composition, by X-ray fluorescence spectrometry, using single pressed powder pellets. For U and Th the Compton scatter peak intensity is measured in order to obtain relative mass absorption coefficients. The 2σ lower detection limits obtained are 0.2 ppm for U, 1.0 ppm for Th and 0.001% for K, using analytical times of 4800, 480 and 72 s, respectively. A good correlation is achieved with results from gamma-ray spectrometry for most samples used in this study, with a total machine time per sample for XRF spectrometry 9–27% of that necessary for analysis by gamma-ray spectrometry. Disequilibrium between U and its daughter-products is likely in samples for which values from the two methods are poorly correlated. In one case, a Southern Alpine Hercynian granite, analysis by gamma-ray spectrometry alone would yield a value of present-day heat production more than 25% too large.