First experimental determination of the40Ar((Formula presented))39Ar reaction cross section and39Ar production in Earth’s atmosphere

The cosmogenic 39Ar(t1/2 = 268 years) isotope of argon is used for geophysical dating and tracing of underground and ocean water, as well as ice owing to its appropriate half-life and chemical inertness as a noble gas; 39Ar serves also in nuclear weapon test monitoring. We measured for the first time the total cross section of the main 39Ar cosmogenic production reaction in the atmosphere, namely 40Ar(n,2n)39[jls-end-space/]Ar, using 14.8 ± 0.3 MeV neutrons. The neutrons, produced by a deuterium-tritium generator, impinged on a stainless steel sphere filled with Ar gas highly enriched in the 40Ar isotope and were monitored by a stack of fast-neutron activation foils. The reaction yield was measured by atom counting of long-lived 39Ar with noble gas accelerator mass spectrometry and, independently, by decay counting relative to atmospheric argon (39Ar/Ar= (Formula presented)). A total 40Ar(n,2n)39[jls-end-space/]Ar cross section of 610 ± 100 mb was determined at 14.8 ± 0.3 MeV incident neutron energy. This result serves as a benchmark for recent theoretical calculations and evaluations, found to reproduce well the experimental total cross section. We use these energy-dependent theoretical cross sections together with experimental spectra of cosmogenic neutrons at different altitudes to calculate the global average rate of neutron-induced 39Ar atmospheric production, resulting in (Formula presented) 39Ar atoms/cm2[jls-end-space/]/day. The secular equilibrium between the 39Ar calculated production rate and radioactive decay rate leads to a partial isotopic abundance 39Ar/Ar(Formula presented), showing that ≈[jls-end-space/]73% of atmospheric 39Ar is produced by cosmogenic neutrons, the remaining part believed to be induced by muons and high-energy γ rays. The 40Ar((Formula presented))39Ar cross section at 14 MeV is also a key parameter for quantifying the anthropogenic contribution to atmospheric 39Ar produced during the thermonuclear tests of the 1960s. We estimate that anthropogenic 39Ar accounts for roughly 20% of the present atmospheric inventory.