First experimental determination of the 40Ar(n,2n)39Ar reaction cross section and 39Ar production in Earth’s atmosphere

Le résumé fourni par la source

The cosmogenic 39 Ar(t = 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; 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 Ar, using 14.8 ± 0.3 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 and were monitored by a stack of fast-neutron activation foils. The reaction yield was measured by atom counting of long-lived 39 Ar with noble gas accelerator mass spectrometry and, independently, by decay counting relative to atmospheric argon ( 39 Ar/Ar= ). A total 40 Ar Ar cross section of 610 ± 100 mb was determined at 14.8 ± 0.3 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 39 Ar atmospheric production, resulting in 39 Ar atoms/cm /day. The secular equilibrium between the 39 Ar calculated production rate and radioactive decay rate leads to a partial isotopic abundance 39 Ar/Ar , showing that 73% of atmospheric 39 Ar is produced by cosmogenic neutrons, the remaining part believed to be induced by muons and high-energy rays. The 40 Ar( ) 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.

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