First experimental determination of the $^{40}$Ar($n,2n$)$^{39}$Ar reaction cross section and $^{39}$Ar production in Earth's atmosphere
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arXiv
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| Natura: | Preprint |
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2025
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| author | Bhattacharya, S. Paul, M. Sahoo, R. N. Purtschert, R. Hoffmann, H. F. R. Pichotta, M. Zuber, K. Bemmerer, D. Döring, T. Schwengner, R. Avila, M. L. Lopez-Saavedra, E. Dickerson, J. C. Fougères, C. McLain, J. Pardo, R. C. Rehm, K. E. Scott, R. Tolstukhin, I. Vondrasek, R. Bailey, T. Callahan, L. Clark, A. M. Collon, P. Kashiv, Y. Nelson, A. Robertson, D. Neto, D. Ugalde, C. Tessler, M. Vaintraub, S. |
| author_facet | Bhattacharya, S. Paul, M. Sahoo, R. N. Purtschert, R. Hoffmann, H. F. R. Pichotta, M. Zuber, K. Bemmerer, D. Döring, T. Schwengner, R. Avila, M. L. Lopez-Saavedra, E. Dickerson, J. C. Fougères, C. McLain, J. Pardo, R. C. Rehm, K. E. Scott, R. Tolstukhin, I. Vondrasek, R. Bailey, T. Callahan, L. Clark, A. M. Collon, P. Kashiv, Y. Nelson, A. Robertson, D. Neto, D. Ugalde, C. Tessler, M. Vaintraub, S. |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2512_18433 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | First experimental determination of the $^{40}$Ar($n,2n$)$^{39}$Ar reaction cross section and $^{39}$Ar production in Earth's atmosphere Bhattacharya, S. Paul, M. Sahoo, R. N. Purtschert, R. Hoffmann, H. F. R. Pichotta, M. Zuber, K. Bemmerer, D. Döring, T. Schwengner, R. Avila, M. L. Lopez-Saavedra, E. Dickerson, J. C. Fougères, C. McLain, J. Pardo, R. C. Rehm, K. E. Scott, R. Tolstukhin, I. Vondrasek, R. Bailey, T. Callahan, L. Clark, A. M. Collon, P. Kashiv, Y. Nelson, A. Robertson, D. Neto, D. Ugalde, C. Tessler, M. Vaintraub, S. Nuclear Experiment 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. |
| title | First experimental determination of the $^{40}$Ar($n,2n$)$^{39}$Ar reaction cross section and $^{39}$Ar production in Earth's atmosphere |
| topic | Nuclear Experiment |
| url | https://arxiv.org/abs/2512.18433 |