First experimental determination of the ${}^{40}$Ar($n,2n$)${}^{39}$Ar reaction cross section and ${}^{39}$Ar production in Earth's atmosphere

Abstract

The cosmogenic ${}^{39}$Ar(t${}_{1/2}$= 268 years) isotope of argon is used for geophysical dating and tracing owing to its appropriate half-life and chemical inertness as a noble gas; ${}^{39}$Ar serves also in nuclear weapon test monitoring. We measured for the first time the total cross section of the main ${}^{39}$Ar cosmogenic production reaction in the atmosphere, namely ${}^{40}$Ar$(n,2n{)}^{39}$Ar, using 14.8$\pm 0.3$ MeV neutrons. The neutrons, produced by a deuterium-tritium generator, impinged on a stainless steel sphere filled with Ar gas highly enriched in the ${}^{40}$Ar isotope. The reaction yield was measured by atom counting of ${}^{39}$Ar with noble gas accelerator mass spectrometry and, independently, by decay counting relative to atmospheric argon. A total ${}^{40}$Ar$(n,2n{)}^{39}$Ar cross section of 610$\pm 100$ mb was determined. 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 ${}^{39}$Ar atmospheric production, resulting in $770\pm 240$ ${}^{39}$Ar atoms/cm${}^2$/day. The secular equilibrium between the ${}^{39}$Ar calculated production rate and radioactive decay rate leads to a partial isotopic abundance ${}^{39}$Ar/Ar$=(5.9\pm 1.8)\times 10^{-16}$, showing that $\approx$73% of atmospheric ${}^{39}$Ar is produced by cosmogenic neutrons. The ${}^{40}$Ar($n,2n$)${}^{39}$Ar cross section at 14 MeV is also a key parameter for quantifying the anthropogenic contribution to atmospheric ${}^{39}$Ar produced during the thermonuclear tests of the 1960s. We estimate that anthropogenic ${}^{39}$Ar accounts for roughly 20% of the present atmospheric inventory.

35 pages, 6 figures, Geochimica et Cosmochimica Acta, in press,