Measurement of 7Be in soils and sediments by gamma spectroscopy

Measurement of cosmogenic 7Be in soils and sediments by gamma spectroscopy has been routine for three decades. However, the analytical constraints of this method have yet to be adequately described. Natural abundance of 7Be in bulk soils and sediments is typically low (106 to 108 atoms kg− 1), resulting in very low signal-to-noise ratio in the gamma spectrum. 7Be quantification is further complicated by numerous interferences from ubiquitous 238U- and 232Th-series radionuclides. We investigated the environmental gamma spectrum in the region of 7Be emission using spectra obtained from hundreds of natural stream sediment samples, one well-characterized sediment naturally-enriched in 7Be, and pure standard materials enriched in both 238U-series and 232Th-series decay chains. In typical samples no fewer than three discrete photopeaks directly interfere with 7Be measurement, one of which, the 228Ac 478.3 keV photon, yields significant positive bias. Risks of 7Be false-positives due to this overlapping photopeak approach 50% for typical soils or sediments. Detection limits for 7Be in these materials using conventional high-purity germanium instruments are as low as 0.1 Bq, but increase by up to a factor of four in the presence of modest activities of 238U- and 232Th-series nuclides. Here we demonstrate a methodology for the measurement of low 7Be abundances in soils and sediments that is unbiased and that optimizes precision.