The 13 C(α, n) 16 O 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 heliumburning shell of asymptotic giant branch stars, at temperatures 10 8 K, corresponding to an energy interval where the 13 C(α, n) 16 O 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 17 O, 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 13 C( 6 Li, n 16 O)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 13 C(α, n) 16 O astrophysical factor. A larger ANC for the 6.356 MeV level is measured with respect to the ones in the literature, (C 17 O(1/2 + ) α 13 C ) 2 = 7.7 ± 0.3 stat +1.6 −1.5 norm fm −1 , yet in agreement with the preliminary result given in our preceding letter, indicating an increase of the 13 C(α, n) 16 O reaction rate below about 8 × 10 7 K if compared with the recommended values. At ∼10 8 K, our reaction rate agrees with most of the results in the literature and the accuracy is greatly enhanced thanks to this innovative approach.

Nuclear reaction rates are among the most important input for understanding the primordial nucleosynthesis and therefore for a quantitative description of the early Universe. An up-to-date compilation of direct cross sections of 2 H(d,p) 3 H, 2 H(d,n) 3 He, 7 Li(p,α) 4 He and 3 He(d,p) 4 He reactions is given. These are among the most uncertain cross sections used and input for Big Bang nucleosynthesis calculations. Their measurements through the Trojan Horse Method (THM) are also reviewed and compared with direct data. The reaction rates and the corresponding recommended errors in this work were used as input for primordial nucleosynthesis calculations to evaluate their impact on the 2 H, 3,4 He and 7 Li primordial abundances, which are then compared with observations.