Measurement of 7Be in soils and sediments by gamma spectroscopy

Abstract     Direct measurement of the fallout radionuclide Be in soils and sediments by high-resolution gamma spectroscopy has been routine for three decades.  However, the analytical constraints of this method have yet to be adequately described.  Natural abundances of 7Be in soils and sediments are low (106 to 108 atoms kg-1), resulting in very low signal-to-noise ratios.  Numerous, previously uncharacterized interferences from ubiquitous 238U and 232Th series radionuclides further complicate 7Be quantification.  We investigate the 7Be gamma emission region in spectra obtained from hundreds of natural sediment samples, one well-characterized sediment naturally-enriched in 7Be, and pure standard materials enriched in both 238U-series and 232Th-series nuclides.  In typical samples no fewer than three discrete photopeaks yield direct interference to 7Be measurement, one of which, the 228Ac 478.3 keV photon, yields significant positive bias.  Detection limits for 7Be in sediments using conventional HPGe instruments are as low as 0.1 Bq in the absence of 238U and 232Th series nuclides, but increase by up to a factor of four in their presence.  Here we demonstrate a methodology for the measurement of low 7Be abundances in soils and sediments that optimizes precision and is unbiased by common interferences.

Landis, J.D., Renshaw, C.E., Kaste, J.M., Measurement of 7Be in soils and sediments by gamma spectroscopy, Chemical Geology, in press, 2012

© Dartmouth College 2012