An atom trap system for Ar dating with improved precision.

Cosmogenic Ar 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 Ar (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 Ar at the modern isotopic abundance level of 8 × 10. By switching frequently between counting Ar atoms and measuring the stable and abundant isotope Ar, drift effects in the trapping efficiency are largely suppressed, leading to a more precise measurement of the isotope ratio Ar/Ar. Moreover, cleaning techniques are developed to alleviate cross-sample contamination, reducing the background Ar count rate down to <0.5 atoms/h. These advances allow us to determine the Ar age in the range of 250-1300 years with precisions of <20%.