Determination of the cross section of the 39 K( n , p ) 39 Ar reaction induced by D-D neutrons with neutron activation and noble gas mass spectrometry techniques*

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Abstract The cross-sections of 39 K( n , p ) 39 Ar at an energy of 2–3 MeV play an important role in nuclear structure research and 40 Ar/ 39 Ar geochronology application. Due to the limitations of n - 3 He coincidence technology and counting instruments, the data in literature are from before 1967, and existing data are scarce and significantly diverge. Meanwhile, there are large discrepancies between the measured and evaluation results. By taking advantage of the high sensitivity and resolution of the noble gas mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), the cross-sections of 39 K( n , p ) 39 Ar were measured by combining neutron activity analysis and noble gas mass spectrometry, and the uncertainties are discussed in detail. The cross-sections of 39 K( n , p ) 39 Ar were measured as 103.84 ± 16.33, 109.76 ± 15.88, and 150.27 ± 24.19 mb at 2.56 ± 0.08, 2.69 ± 0.08, and 2.96 ± 0.12 MeV energies, respectively. The measured data filled the data gaps and provided more accurate data support for 40 Ar/ 39 Ar dating. Furthermore, the theoretical excitation function of 39 K(n, p) 39 Ar was calculated using TALYS-1.97 computer codes. Then, the experimentally determined cross-sections were analyzed by comparing them with the data from the EXFOR database and evaluated nuclear data in ENDF/B-VIII.0, JEFF-3.2, TENDL-2021, BROND-3.1, and JENDL-5 databases. According to the comparative results, the measured cross-sections of 39 K( n , p ) 39 Ar exhibit a rapid energy-dependent increase between 2–3 MeV, aligning with higher literature values and resolving previous discrepancies. Compared with the previously reported data, the precision of the determined cross-sections in this study showed considerable improvement. The comparison of measured data indicates that the combined detection method of neutron activity analysis and noble gas mass spectrometry techniques is suitable for measuring the cross-sections of nuclear reactions with long-lived product nuclei and the application of 40 Ar/ 39 Ar geochronology with a D-D neutron source.

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