Analysis of 85Kr: a comparison at the 10-14 level using micro-liter samples

The isotopic abundance of Kr-85 in the atmosphere, currently at the level of 10(-11), has increased by orders of magnitude since the dawn of nuclear age. With a half-life of 10.76 years, Kr-85 is of great interest as tracers for environmental samples such as air, groundwater and ice. Atom Trap Trace Analysis (ATTA) is an emerging method for the analysis of rare krypton isotopes at isotopic abundance levels as low as 10(-14) using krypton gas samples of a few micro-liters. Both the reliability and reproducibility of the method are examined in the present study by an inter-comparison among different instruments. The Kr-85/Kr ratios of 12 samples, in the range of 10(-13) to 10(-10), are measured independently in three laboratories: a low-level counting laboratory in Bern, Switzerland, and two ATTA laboratories, one in Hefei, China, and the other in Argonne, USA. The results are in agreement at the precision level of 5%.