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Abstract: We investigated the production cross section of two long-lived isotopes of argon, $^{39}$Ar (268 y) and 42Ar (33 y), produced respectively by the $^{40}$Ar(n,2n)$^{39}$Ar and the $^{40}$Ar(n,$\gamma$)$^{41}$Ar(n,$\gamma$)42Ar reactions. The radioactive isotope $^{39}$Ar is widely used for dating and tracing groundwater, ocean water, and ice. It is also important for monitoring nuclear weapons tests. In this work, the total cross section of the main atmospheric production reaction, $^{40}$Ar(n,2n)$^{39}$Ar, was measured for the first time using 14.8 MeV neutrons. The neutrons, generated by a deuterium–tritium neutron source at Helmholtz Zentrum Dresden Rossendorf, irradiated a stainless-steel sphere filled with enriched $^{40}$Ar gas, and the neutron flux was monitored using fast-neutron activation foils. The reaction yield was determined by noble-gas accelerator mass spectrometry at Argonne National Laboratory and independently by low-level decay counting (LLC) relative to the $^{39}$Ar activity of atmospheric argon. The measured total cross section of the $^{40}$Ar(n,2n) reaction was found to be 610±100 mb [1]. Using energy-dependent cross sections from independent theoretical calculations together with measured cosmogenic-neutron spectra at different altitudes, the global atmospheric production areal rate of $^{39}$Ar is estimated to be 770±240 atoms/cm2/day. The $^{42}$Ar nuclide was produced by a 6 days irradiation of ampoules filled with enriched $^{42}$Ar gas at the high-flux reactor of Institut Laue-Langevin, together with selected monitors for neutron fluence monitoring. After appropriate radioactive cooling, the irradiated gas was transferred to separate cylinders. The $\gamma$-activity growth curve of $^{42}$K (12 hr), $\beta$-daughter of $^{42}$Ar was followed and the $^{42}$Ar-$^{42}$K secular equilibrium activity was established. Results of the $^{41}$Ar(n,$\gamma$)$^{42}$Ar thermal neutron cross section, determined from this activity and the measured neutron fluence will be presented.
[1] S. Bhattacharya et. al., Geochimica et Cosmochimica Acta. 415 196 (2026).
| Session | Decay Data and Delayed Neutrons |
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