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

Abstract

Among the most important, conservative geochemical tracers are the long-lived isotopes of krypton, ⁸¹Kr and ⁸⁵Kr. 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 ⁸⁵Kr, 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 ⁸⁵Kr can be quantified in just 1.5 l air or 25 l seawater with 2000 min paired counting periods.