An atom counter for measuring Kr-81 and Kr-85 in environmental samples
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Due to its simple production and transport processes in the terrestrial environment, the long-lived noble-gas isotope Kr-81 is the ideal tracer for old water and ice in the age range of 10(5)-10(6) years, a range beyond the reach of C-14. Kr-81-dating, a concept pursued in the past four decades by numerous laboratories employing a variety of techniques, is now available for the first time to the earth science community at large. This is made possible by the development of ATTA-3, an efficient and selective atom counter based on the Atom Trap Trace Analysis method and capable of measuring both Kr-81/Kr and Kr-85/Kr ratios of environmental samples in the range of 10(-14)-10(-10). The instrument was calibrated with 12 samples whose Kr-85/Kr ratios were independently measured using Low Level Decay Counting, including six samples that were measured in a blind arrangement. Compared to the previously reported ATTA-2 instrument, the counting rates of ATTA-3 are higher by two orders of magnitude and the required sample size lower by one order of magnitude. For Kr-81-dating in the age range of 150-1500 kyr, the required sample size is 5-10 mu L STP of krypton gas, which can be extracted from approximately 100-200 kg of water or 40-80 kg of ice. Moreover, a laser-induced quenching scheme was developed to enable measurements of both the rare Kr-81,Kr-85 and the abundant Kr-83, whose isotopic abundances differ by 11 orders of magnitude. This scheme allows ATTA-3 to directly determine Kr-81/Kr and Kr-85/Kr ratios without other supplemental measurements. Combining the significant reduction in sample size with numerous advances in the measurement procedure, ATTA-3 represents the state-of-the-art instrument for routine analysis of these rare noble gas tracers in a wide range of earth science applications. © 2012 Elsevier Ltd. All rights reserved.