Hyperfine spectroscopy of the 1s(5)-2p(9) transition of 39Ar

An atom trap system for 39Ar dating with improved precision

Cosmogenic ³⁹Ar dating is an emerging technique in dating mountain glacier ice, mapping ocean circulation, and tracing groundwater flow. We have realized an atom-trap system for the analysis of the radioactive isotope ³⁹Ar (half-life = 269 years) in environmental samples. The system is capable of analyzing small (1-5 kg) environmental water or ice samples and achieves a count rate of 10 atoms/h for ³⁹Ar at the modern isotopic abundance level of 8 × 10^-16. By switching frequently between counting ³⁹Ar atoms and measuring the stable and abundant isotope ³⁸Ar, drift effects in the trapping efficiency are largely suppressed, leading to a more precise measurement of the isotope ratio ³⁹Ar/³⁸Ar. Moreover, cleaning techniques are developed to alleviate cross-sample contamination, reducing the background ³⁹Ar count rate down to <0.5 atoms/h. These advances allow us to determine the ³⁹Ar age in the range of 250-1300 years with precisions of <20%.

39Ar detection at the 10(-16) isotopic abundance level with atom trap trace analysis

Atom trap trace analysis, a laser-based atom counting method, has been applied to analyze atmospheric 39Ar (half-life=269  yr), a cosmogenic isotope with an isotopic abundance of 8×10(-16). In addition to the superior selectivity demonstrated in this work, the counting rate and efficiency of atom trap trace analysis have been improved by 2 orders of magnitude over prior results. The significant applications of this new analytical capability lie in radioisotope dating of ice and water samples and in the development of dark matter detectors.

Dating of 'young' groundwaters using environmental tracers: advantages, applications, and research needs

Many problems related to groundwater supply and quality, as well as groundwater-dependent ecosystems require some understanding of the timescales of flow and transport. For example, increased concern about the vulnerabilities of 'young' groundwaters (less than ~1000 years) to overexploitation, contamination, and land use/climate change effects are driving the need to understand flow and transport processes that occur over decadal, annual, or shorter timescales. Over the last few decades, a powerful suite of environmental tracers has emerged that can be used to interrogate a wide variety of young groundwater systems and provide information about groundwater ages/residence times appropriate to the timescales over which these systems respond. These tracer methods have distinct advantages over traditional approaches providing information about groundwater systems that would likely not be obtainable otherwise. The objective of this paper is to discuss how environmental tracers are used to characterise young groundwater systems so that more researchers, water managers, and policy-makers are aware of the value of environmental tracer approaches and can apply them in appropriate ways. We also discuss areas where additional research is required to improve ease of use and extend quantitative interpretations of tracer results.