Development of a low-level 39Ar calibration standard – Analysis by absolute gas counting measurements augmented with simulation

Measurements of Argon-39 from locations near historic underground nuclear explosions

Measurement of radioactive gas seepage from an underground nuclear explosion is one of the primary methods to confirm whether an event was nuclear in nature. Radioactive noble gas indicators that are commonly targeted by such measurements (e.g. ¹³³Xe, ³⁷Ar) have half-lives of 35 days or less. Argon-39, an activation product similar to ³⁷Ar, is produced by the interaction between neutrons and potassium in the surrounding geology and has a half-life of 269 years. Measurements taken at three sites near three historic underground nuclear test locations at the Nevada National Security Site have all shown highly elevated levels of ³⁹Ar in soil gas decades after the test events. Elevated levels of ³⁹Ar were also detected in atmospheric air collected near two of these sites, and outside the entrance of the one tunnel site. These measurements demonstrate that ³⁹Ar has the potential to be a long-term signature of an underground nuclear event which can be reliably detected at the surface or in the shallow subsurface. This radionuclide detection of an underground nuclear event decades after the event takes place is in contrast to the commonly held assumption that detecting underground nuclear events via radionuclides at the surface needs to be done in a matter of months. Depending upon what further studies show about the robustness of this signature in a variety of geological settings, it may in fact be easy to detect underground nuclear events at the surface for a very long time post-detonation.

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%.