Stellar Ar 36,38 (n,γ) Ar 37,39 Reactions and Their Effect on Light Neutron-Rich Nuclide Synthesis

The Ar36(n,γ)Ar37 (t1/2=35 d) and Ar38(n,γ)Ar39 (269 yr) reactions were studied for the first time with a quasi-Maxwellian (kT∼47 keV) neutron flux for Maxwellian average cross section (MACS) measurements at stellar energies. Gas samples were irradiated at the high-intensity Soreq applied research accelerator facility-liquid-lithium target neutron source and the Ar37/Ar36 and Ar39/Ar38 ratios in the activated samples were determined by accelerator mass spectrometry at the ATLAS facility (Argonne National Laboratory). The Ar37 activity was also measured by low-level counting at the University of Bern. Experimental MACS of Ar36 and Ar38, corrected to the standard 30 keV thermal energy, are 1.9(3) and 1.3(2) mb, respectively, differing from the theoretical and evaluated values published to date by up to an order of magnitude. The neutron-capture cross sections of Ar36,38 are relevant to the stellar nucleosynthesis of light neutron-rich nuclides; the two experimental values are shown to affect the calculated mass fraction of nuclides in the region A=36-48 during the weak s process. The new production cross sections have implications also for the use of Ar37 and Ar39 as environmental tracers in the atmosphere and hydrosphere.