ON THE MEASUREMENT OF THE 13C(α, n)16O S-FACTOR AT NEGATIVE ENERGIES AND ITS INFLUENCE ON THE s-PROCESS

The 13C(α, n)16O reaction is the neutron source for the main component of the s-process, responsible for the production of most of the nuclei in the mass range 90 ∼< A ∼< 208. This reaction takes place inside the helium-burning shell of asymptotic giant branch stars, at temperatures ∼< 108 K, corresponding to an energy interval where the 13C(α, n)16O reaction is effective in the range of 140-230 keV. In this regime, the astrophysical S(E)-factor is dominated by the –3 keV sub-threshold resonance due to the 6.356 MeV level in 17O, giving rise to a steep increase in the S-factor. Its contribution is still controversial as extrapolations, e.g., through the R-matrix and indirect techniques such as the asymptotic normalization coefficient (ANC), yield inconsistent results. The discrepancy amounts to a factor of three or more precisely at astrophysical energies. To provide a more accurate S-factor at these energies, we have applied the Trojan horse method (THM) to the 13C(6Li, n 16O)d quasi-free reaction. The ANC for the 6.356 MeV level has been deduced through the THM as well as the n-partial width, allowing us to attain unprecedented accuracy for the 13C(α, n)16O astrophysical factor. A larger ANC for the 6.356 MeV level is measured with respect to the ones in the literature, (C-tildeα13C17O(1/2+)2 = 7.7 ± 0.3stat-1.5+1.|6 norm fm–1, yet in agreement with the preliminary result given in our preceding letter, indicating an increase of the 13C(α, n)16O reaction rate below about 8 × 107 K if compared with the recommended values. At ∼108 K, our reaction rate agrees with most of the results in the literature and the accuracy is greatly enhanced thanks to this innovative approach