Detection and quantification capabilities for 85Kr with the NIST low-level gas counting system:: Impacts of instrumental and environmental backgrounds

Among the most important, conservative geochemical tracers are the long-lived isotopes of krypton, Kr-81 and Kr-85. Following a brief review of the metrology and applications of these radionuclides, we focus on the low-level measurement of the 10.8 year fission product Kr-85, in connection with its use for studying atmospheric transport and short term (decadal) atmosphere-ocean exchange and mixing. Special challenges for the study of 85Kr in the environment are: (1) reducing detection/quantification limits to mBq levels, to minimize the need for costly (large) atmospheric and marine samples, where current levels are ca. 1.4 Bq.m(-3) (northern troposphere) and 0.08 mBq.kg(-1) (surface ocean water); and (2) designing a protocol that accounts for the influence of both instrumental and environmental backgrounds and their variability. The capability of the NIST enhanced low-level gas counting system to meet these challenges was demonstrated theoretically and experimentally, showing that ambient Kr-85 can be quantified in just 1.5 1 air or 25 1 seawater with 2000 min paired counting periods.