Natural 37Ar Concentrations in Soil Air: Implications for Monitoring Underground Nuclear Explosions

Riedmann, Robin; Purtschert, Roland (2011). Natural 37Ar Concentrations in Soil Air: Implications for Monitoring Underground Nuclear Explosions. Environmental science & technology, 45(20), pp. 8656-8664. Washington, D.C.: ACS Publications 10.1021/es201192u

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For on-site inspections (OSI) under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) measurement of the noble gas 37Ar is considered an important technique. 37Ar is produced underground by neutron activation of Calcium by the reaction 40Ca(n,α)37Ar. The naturally occurring equilibrium 37Ar concentration balance in soil air is a function of an exponentially decreasing production rate from cosmic ray neutrons with increasing soil depth, diffusive transport in the soil air, and radioactive decay (T1/2: 35 days). In this paper for the first time, measurements of natural 37Ar activities in soil air are presented. The highest activities of ∼100 mBq m–3 air are 2 orders of magnitude larger than in the atmosphere and are found in 1.5–2.5 m depth. At depths >8 m 37Ar activities are <20 mBq m–3 air. After identifying the main 37Ar production and gas transport factors the expected global activity range distribution of 37Ar in shallow subsoil (0.7 m below the surface) was estimated. In high altitude soils, with large amounts of Calcium and with low gas permeability, 37Ar activities may reach values up to 1 Bq m–3.

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