First experimental determination of the $^{40}$Ar($n,2n$)$^{39}$Ar reaction cross section and $^{39}$Ar production in Earth's atmosphere
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$\pm0.3$ 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$\pm100$ 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 $770\pm240$ $^{39}$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.