Enhanced detection limits for radiokrypton analysis
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Enhanced detection limits for radiokrypton analysis Jake C. Zappala1 · Derek McLain2 · Peter Mueller1 · Jennifer L. Steeb2 Received: 11 May 2020 / Published online: 21 September 2020 © UChicago Argonne, LLC 2020
Abstract We present a method for improving detection limits of Atom Trap Trace Analysis for the krypton radioisotopes 85Kr and 81 Kr. For the case of 85Kr this work demonstrates that systematic use of isotopically depleted gas for calibration and extended conditioning of the instrument results in a detection limit of 900 85Kr atoms per 11 µl of Kr gas, equivalent to a 85Kr/Kr isotopic abundance of 3 × 10–15. This improvement of roughly two orders of magnitude over previously reported limits will help to expand the reach of radiokrypton dating towards longer age ranges where most of the radioisotopes have decayed. Additionally, the method offers an opportunity to investigate radiokrypton production via spontaneous fission within naturally occurring minerals to understand potential underground production of these isotopes. Keywords Radiokrypton · Atom trap trace analysis · Groundwater dating
Introduction Radiokrypton dating is a rapidly growing method applied by the geoscience community [1] that uses the isotopic abundance of the radioisotopes 81Kr (half-life = 229 ± 11 kyr) [2] and 85Kr (half-life = 10.739 ± 0.014 year) [3] as indicators to determine “ages”, i.e. mean residence times, for groundwater and ancient ice. This technique has already found applications in evaluating groundwater flow dynamics [4], paleoclimate investigations [5, 6], groundwater management [7], and the calibration of other isotope tracers [8–10]. The growth of this field was made possible by the development of Atom Trap Trace Analysis (ATTA) into a routine tool for radiokrypton analysis [11–14]. ATTA is a laser-based atom counting technique free of interferences from other isotopes, isobars, isomers, atomic, or molecular species. One consistent limitation of the ATTA technique, however, has been an instrumental “memory effect”, i.e. a crosscontamination between samples that is caused by the radiofrequency (RF) driven plasma discharge which functions as the atomic beam source of the analytical instrument. This * Jake C. Zappala [email protected] 1
Physics Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, IL 60439, USA
Strategic Security Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, IL 60439, USA
2
memory effect negatively impacts both sample throughput rates and detection limits. As a direct result of this memory effect, Kr radioisotope abundance measurements via ATTA have detection limits of ∼1 × 10− 13 and ∼1 × 10− 14 for 85Kr/ Kr and 81Kr/Kr, respectively. These detection limits constrain the ability to measure samples at the “older” end of the radioisotopes’ respective age ranges as well as to conduct high-sensitivity nuclear data measurements, both of which require high detection efficiency on a very small number of atoms (order 103). The latter type of experiment is of
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