An atom counter for measuring 81Kr and 85Kr in environmental samples

Due to its simple production and transport processes in the terrestrial environment, the long-lived noble-gas isotope 81Kr is the ideal tracer for old water and ice in the age range of 105–106 years, a range beyond the reach of 14C. 81Kr-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 81Kr/Kr and 85Kr/Kr ratios of environmental samples in the range of 10−14–10−10. The instrument was calibrated with 12 samples whose 85Kr/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 81Kr-dating in the age range of 150–1500 kyr, the required sample size is 5–10 μL 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 81,85Kr and the abundant 83Kr, whose isotopic abundances differ by 11 orders of magnitude. This scheme allows ATTA-3 to directly determine 81Kr/Kr and 85Kr/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.