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

Discrepancies arise in the analysis by portable XRF of geological and archaeological rock samples that have irregular shaped surfaces, becau se the instrument is calibrated for the quantitative analysis of flat samples. A simple correction procedure was investigated to overcome th ese discrepancies in which the measured intensity is normalised by the ratio of the scatter peak intensity from a compositionally similar ha t reference sample to the scatter peak intensity measured from the sam ple itself, The scatter peak data were obtained from the Fe-55, Cd-109 and Am-241 sources incorporated:,in the instrument used for this inve stigation, Under Controlled conditions, this correction has proved to be successful in compensating far effective air gaps of up to 3 mm in the analysis of the K lines of higher atomic number elements (Ph, ST, Y, Zr, Nb, Ba) and up to 1 mm for the Fe K line. Low energy It lines a re affected by air attenuation in the air gap, which is not accounted for in this simple model, The scatter peak from the Fe-55 source is pr eferred for the correction because its intensity is least dependent on sample composition, but the Cd-109 scatter peak can be used instead w ith more careful matching of the composition of the fiat sample used t o derive the reference scatter peak intensity, Apart from additional a ir attenuation, the principle limitations to the application of this m ethod to larger air gaps were (i) the change in scatter angle and, the refore, relative:scatter intensity as the air gap is increased and (ii ) the increasing contribution from scatter in air, particularly to the measured Fe-55 scatter peak at larger air gaps between sample and ana lyser.