Direct measurement of the 13C(α,n)16O reaction in the Gamow window of the s-process nucleosynthesis

One of the main neutron sources for the astrophysical s process is the C-13(alpha,n)O-16 reaction, which takes place in thermally pulsing asymptotic giant branch (TP-AGB) stars at environmental temperature around 90 MK. To model the nucleosynthesis process connected with the reaction, it is important to know with high accuracy the cross section reaction in the energy window 240-150 keV, the so called Gamow window. At these sub-Coulomb energies, direct cross section measurements are severely affected by the low event rate and low signalto-noise ratio. In this work, a new study of the astrophysical S(E)-factor for the C-13(alpha,n)O-16 reaction is presented. In the framework of the LUNA scientific programme, a direct measurement of the absolute cross section of the C-13(alpha,n)O-16 reaction in an energy window from 300 keV down to 230 keV, significantly closer to the Gamow peak, has been performed. Lower uncertainties with respect to literature values are obtained allowing to reduce overall uncertainties on reaction rates calculation. 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 Fe-60, Pb-205 and and Gd-52.

Automated summary

By directly measuring the absolute cross section of the C-13(alpha,n)O-16 reaction, the reaction rate can be accurately calculated, allowing for the prediction of the production of certain isotopes in stars.