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.
Categories: nucl-ex