title: Natural ³⁷Ar concentrations in soil air: implications for monitoring underground nuclear explosions.
abstract: 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 (T(1/2): 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.
authors: Climate- & Environmental Physics, Physics Institute, University of Berne, Switzerland, CH-3012 Berne, Switzerland. riedmann@climate.unibe.ch
authors: Robin A
authors: RA
authors: Riedmann
authors: Roland
authors: R
authors: Purtschert
mesh_terms: D000388:Air / Q000032:analysis*; D001128:Argon / Q000032:analysis* / Q000737:chemistry*; D004784:Environmental Monitoring; D012987:Soil / Q000032:analysis*
chemical_list: D012987:Soil; D001128:Argon