Direct measurement of the 13C(α,n)16O cross section into the s-process Gamow peak

Abstract

One of the main neutron sources for the astrophysical s-process is the reaction 13C(α,n)16O, taking place in thermally pulsing Asymptotic Giant Branch stars at temperatures around 90 MK. To model the nucleosynthesis during this process the reaction cross section needs to be known in the 150-230keV energy window (Gamow peak). At these sub-Coulomb energies cross section direct measurements are severely affected by the low event rate, making us rely on input from indirect methods and extrapolations from higher-energy direct data. This leads to an uncertainty in the cross section at the relevant energies too high to reliably constrain the nuclear physics input to s-process calculations. We present the results from a new deep-underground measurement of 13C(α,n)16O, covering the energy range 230-300keV, with drastically reduced uncertainties over previous measurements and for the first time providing data directly inside the s-process Gamow peak. Selected stellar models have been computed to estimate the impact of our revised reaction rate. For stars of nearly solar composition, we find sizeable variations of some isotopes, whose production is influenced by the activation of close-by branching points that are sensitive to the neutron density, in particular the two radioactive nuclei 60Fe and 205Pb, as well as 152Gd

arXiv comment7 pages, 4 figures, accepted on PRL on 17th August 2021

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• The s Process and Beyond
• Horizons: Nuclear Astrophysics in the 2020s and Beyond
• Deep underground laboratory measurement of ${}^{13}$C($\alpha$,$n$)${}^{16}$O in the Gamow windows of the $s$- and $i$-processes
• Advances in Radiative Capture Studies at LUNA with a Segmented BGO Detector
• Exploring Stars in Underground Laboratories: Challenges and Solutions
• Improved $S$-factor of the ${}^{12}$C(p,$\gamma$)${}^{13}$N reaction at $E=$320-620~keV and the 422~keV resonance
• Underground Measurements of Nuclear Reaction Cross-Sections Relevant to AGB Stars
• SHADES -- ${}^{22}$Ne($\alpha$,n)${}^{25}$Mg reaction rate in the Gamow window
• Status and future directions for direct cross-section measurements of the 13C(a,n)16O reaction for astrophysics
• Detectors and Shieldings: Past and Future at LUNA
• Underground nuclear astrophysics: Status and recent results from Felsenkeller laboratory
• A prototype neutron-detector array for future deep-underground s-process studies
• Response of a Liquid ${}^3$He Neutron Detector