Measurement of the ¹³C(α,n)¹⁶O reaction at astrophysical energies using the Trojan Horse Method. Focus on the -3 keV subthreshold resonance

Most of the nuclei in the mass range 90 ≤ A ≤ 208 are produced through the so-called s-process, namely through a series of neutron capture reactions on seed nuclei followed by β-decays. The ¹³C(α,n)¹⁶O reaction is the neutron source for the main component of the s-process. It is active inside the helium-burning shell of asymptotic giant branch stars, at temperatures ≤ 10⁸ K, corresponding to an energy interval of 140 - 230 keV. In this region, the astrophysical S (E)-factor is dominated by the -3 keV sub-threshold resonance due to the 6.356 MeV level in ¹⁷O. Direct measurements could not soundly establish its contribution owing to the cross section suppression at astrophysical energies determined by the Coulomb barrier between interacting nuclei. Indirect measurements and extrapolations yielded inconsistent results, calling for further investigations. The Trojan Horse Method turns out to be very suited for the study of the ¹³C(α,n)¹⁶O reaction as it allows us to access the low as well as the negative energy region, in particular in the case of resonance reactions. We have applied the Trojan Horse Method to the ¹³C(⁶Li; n¹⁶O)d quasi-free reaction. By using the modified R-matrix approach, the asymptotic normalization coefficient (C(O(1/2+),α¹³C)]² of the 6.356 MeV level has been deduced as well as the n-partial width, allowing to attain an unprecedented accuracy for the ¹³C(α,n)¹⁶O astrophysical factor. A preliminary analysis of a partial data set has lead to (C(O(1/2+),α¹³C)]² = (6.7-0.6+0.9) fm^-1, slightly larger than the values in the literature, determining a ¹³C(α,n)¹⁶O reaction rate in agreement with the most results in the literature at ∼ 10⁸ K, with enhanced accuracy thanks to this innovative approach. (authors)