Background concentrations of Argon-39 in shallow soil gas
Introduction
Radioisotopes of noble gases are important indicators of underground nuclear explosions (UNE). Several studies have explored the formation and transport of xenon, krypton and argon radioisotopes associated with UNEs (Schoengold et al., 1996; Riedmann and Purtschert 2011; Hebel 2010; Johnson et al., 2015; Wilson et al., 2015). These studies have primarily focused on radioxenon and 37Ar. One exception to this is McIntyre et al. (2017), who also identified 39Ar at former UNE sites at the Nevada National Security Site (NNSS).
While 39Ar research associated with dark matter detection (Acosta-Kane et al., 2008; Mei et al., 2010; Sramek et al., 2017), groundwater age-dating (Loosli 1983; Andrews et al., 1991; Lehmann et al., 1993; Mace et al., 2017), and geothermal gas (Yokochi et al., 2013) have previously been published, McIntyre et al. (2017) was the first to report the presence of 39Ar in association with UNEs. McIntyre et al. (2017) reported the separation and measurement method of 39Ar and provided some initial estimates of concentrations measured in soil gas above a UNE.
The specific activity of 39Ar in the atmosphere has been reported by Benetti et al. (2007) as 1.01 ± 0.02 (stat) ± 0.08(syst) Bq per kg of natAr (or 16.6 mBq/m3 whole-air equivalent). The production of 39Ar in the atmosphere is dominated by the cosmic ray neutron-induced 40Ar(n,2n)39Ar reaction (Loosli and Oeschger 1968). In the shallow (solid) subsurface, natural production of 39Ar occurs primarily through two cosmic ray reactions: negative muon capture on 39K and through the 39K(n,p)39Ar reaction, similar to the formation of 37Ar from 40Ca (Fabryka-Martin 1988; Mei et al., 2010; Sramek et al., 2017). As cosmic-ray induced processes, a correlation with latitude would be expected (Johnson et al., 2015). As with 37Ar, the use of 39Ar as an indicator of a UNE requires an understanding of the natural concentrations of 39Ar present in soil gas. Without knowing the expected range of naturally occurring concentrations of 39Ar, it is difficult to determine what could constitute an elevated concentration that could be related to a UNE.
This paper presents the results from 16 soil gas samples and three atmospheric air samples collected from various locations across the western United States. These locations were targeted as background locations far from UNE sites. A review of installation, collection and analysis methods are provided followed by site characterization information and discussion of the measured concentrations of 39Ar at each background location.
Section snippets
Installation and characterization
Samples were collected in four general areas of the United States of America (Fig. 1): Chinook Pass through the Cascade Range (Ch, 46.9°N 121.0°W); Richland, Washington (R, 46.3°N 119.3°W); Death Valley National Park, California (DV, 36.7°N 116.9°W); Tule Springs National Monument, Nevada (TS, 36.3°N 115.3°W). At most areas, samples were collected from several different locations, often at multiple depths. Installation was accomplished using Geoprobe® PRT soil vapor equipment (1-in drive rods,
Results and discussion
The concentrations of 39Ar measured in shallow soil gas samples varied from atmospheric air concentrations to about 3.5 times atmospheric air concentrations (Table 1, Fig. 2). One sample (Ch7-2m) was quantified using both the narrow spectrum (5–15 keV) and the full spectrum (5–400 keV) method. The result was within the analytical error for both approaches. This provides confidence that the narrow spectrum approach used to quantify 8 other samples provides results accurate to within the
Conclusion
The results presented here represent the first published natural background 39Ar concentrations in shallow soil gas, which are necessary if 39Ar is to be used as an indicator of UNE. In order to differentiate between signal (UNE produced 39Ar) and background (naturally-produced 39Ar) requires an understanding of the range of naturally occurring concentrations, and the factors that influence the naturally occurring concentrations. While previously reported concentrations of 39Ar in soil gas (
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The U.S. Department of Energy, National Nuclear Security Administration, Office of Nonproliferation and Arms Control, and the U.S. Department of State, Bureau of Arms Control, Verification and Compliance funded the sample collection and analysis of this work. The U.S. Department of Energy, National Nuclear Security Administration, Office of Nonproliferation and Arms Control, and the U.S. Department of State, Office of the Nonproliferation and Disarmament Fund funded the development of the Argon
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Measurements of Argon-39 from locations near historic underground nuclear explosions
2021, Journal of Environmental RadioactivityCitation Excerpt :Natural production of 39Ar in the subsurface is dominated by negative muon capture on 39K, and at deeper levels by the 39K(n,p)39Ar reactions (Johnson et al., 2015; Lehmann et al., 1993; Mei et al., 2010; Sramek et al., 2017). Recent measurements have also been made by Fritz et al. (2021) of the natural 39Ar background at various locations in the western United States. Measured atmospheric 39Ar concentrations at three locations were in general agreement with the 0.016 Bq/m3 air reported by Benetti et al. (2007) while the concentrations in the shallow subsurface (1–4 m deep) ranged from atmospheric levels to 3.5 times the global atmospheric background (Fritz et al., 2021).