Investigation of a Correction Procedure for Surface Irregularity Effects Based on Scatter Peak Intensities in the Field Analysis of Geological and Archaeological Rock Samples by Portable X-ray Fluorescence Spectrometry

Abstract

Discrepancies arise in the analysis by portable XRF of geological and archaeological rock samples that have irregular shaped surfaces, because the instrument is calibrated for the quantitative analysis of flat samples. A simple correction procedure was investigated to overcome these discrepancies in which the measured intensity is normalised by the ratio of the scatter peak intensity from a compositionally similar flat reference sample to the scatter peak intensity measured from the sample itself. The scatter peak data were obtained from the ⁵⁵ Fe, ¹⁰⁹ Cd and ²⁴¹ Am sources incorporated in the instrument used for this investigation. Under controlled conditions, this correction has proved to be successful in compensating for effective air gaps of up to 3 mm in the analysis of the K lines of higher atomic number elements (Rb, Sr, Y, Zr, Nb, Ba) and up to 1 mm for the Fe K line. Low energy K lines are affected by air attenuation in the air gap, which is not accounted for in this simple model. The scatter peak from the ⁵⁵ Fe source is preferred for the correction because its intensity is least dependent on sample composition, but the ¹⁰⁹ Cd scatter peak can be used instead with more careful matching of the composition of the flat sample used to derive the reference scatter peak intensity. Apart from additional air attenuation, the principle limitations to the application of this method to larger air gaps were ( i ) the change in scatter angle and, therefore, relative scatter intensity as the air gap is increased and ( ii ) the increasing contribution from scatter in air, particularly to the measured ⁵⁵ Fe scatter peak at larger air gaps between sample and analyser.