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

摘要

Due to its simple production and transport processes in the terrestrial environment, the long-lived noble-gas isotope ~(81)Kr 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 ~(14)C. ~(81)Kr-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 ~(81)Kr/Kr and ~(85)Kr/Kr ratios of environmental samples in the range of 10 ~(-14)-10 ~(-10). The instrument was calibrated with 12 samples whose ~(85)Kr/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 ~(81)Kr-dating in the age range of 150-1500kyr, the required sample size is 5-10μL STP of krypton gas, which can be extracted from approximately 100-200kg of water or 40-80kg of ice. Moreover, a laser-induced quenching scheme was developed to enable measurements of both the rare ~(81,85)Kr and the abundant ~(83)Kr, whose isotopic abundances differ by 11 orders of magnitude. This scheme allows ATTA-3 to directly determine ~(81)Kr/Kr and ~(85)Kr/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.