Speaker
Description
More than ∼ 50% of nuclei with A ≥ 56 are produced by the s-process [1], a succes-sion of neutron captures slower than β-decay rates, constraining the nucleosynthesis path close to the stability valley. The neutron source feeding the s-process has been identified in the 13C(α, n)16O reaction taking place in asymptotic giant branch (AGB) stars. In this contribution, I will focus on the indirect measurement of its S-factor by means of the Trojan Horse Method (THM), and in particular on the concordance scenario we have reached by the concurrent application of the THM and the ANC and the reanalysis of the available direct data.
Indeed, due to its astrophysical importance, many direct and indirect determina-tions of the 13C(α, n)16O S-factor have been carried out (see, e.g., [2] for a review). The low center-of-mass energies of astrophysical relevance (≤ 230 keV) make direct measurements extremely challenging due to the small cross sections, and especially due to the interplay between the rise in the S-factor linked to the 17O 6.356 MeV 1/2+ threshold level and the enhancement produced by the electron screening. An additional problem is the scatter of the absolute normalizations of existing data, as large as a factor of 2 [2], which could be attributed to systematic errors on neutrons detection efficiency.
The indirect ANC approach lead to several coherent measurements of the ANC of 6.356 MeV 17O threshold level (see [3] for a list). Therefore, we decided to change the paradigm usually adopted in THM applications and normalized the THM S-factor to the ANC of the 6.356 MeV state. This approach lead to a concordance scenario for the 13C(α, n)16O S-factor, where both direct and indirect data accurately agree.
The results, extensively discussed in [3], have been recently confirmed by [4], where an accurate measurement down to 230 keV (upper edge of the Gamow window) has been reported. This shows the importance of the interplay between direct (espe-cially underground) and indirect measurements to reach accurate reactions rates for astrophysical applications.
