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

• Published in: The Astrophysical journal 2013-11, Vol.777 (2)
• Main Authors: La Cognata, M., Spitaleri, C., Trippella, O., Kiss, G. G., Guardo, G. L., Puglia, S. M. R., Romano, S., Spartà, R., Rogachev, G. V., Avila, M., Koshchiy, E., Kuchera, A., Santiago, D., Mukhamedzhanov, A. M., Lamia, L.
• Format: Article
• Language: English
• Subjects: ALPHA REACTIONS; ASTROPHYSICS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASYMPTOTIC SOLUTIONS; CARBON 13 TARGET; CARBON 17; COMPARATIVE EVALUATIONS; HELIUM BURNING; KEV RANGE; LITHIUM 6 REACTIONS; MEV RANGE; NUCLEAR PHYSICS AND RADIATION PHYSICS; NUCLEOSYNTHESIS; OXYGEN 16; OXYGEN 17; QUASI-FREE REACTIONS; R MATRIX; S PROCESS; STARS
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/777/2/143

The {sup 13}C(α, n){sup 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 helium-burning shell of asymptotic giant branch stars, at temperatures ∼< 10{sup 8} K, corresponding to an energy interval where the {sup 13}C(α, n){sup 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 {sup 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 {sup 13}C({sup 6}Li, n {sup 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 {sup 13}C(α, n){sup 16}O astrophysical factor. A larger ANC for the 6.356 MeV level is measured with respect to the ones in the literature, (C-tilde{sub α{sup 1}{sup 3}C}{sup 17O(1/2+)2} = 7.7 ± 0.3{sub stat} {sub -1.5}{sup +1.}|6 norm fm{sup –1}, yet in agreement with the preliminary result given in our preceding letter, indicating an increase of the {sup 13}C(α, n){sup 16}O reaction rate below about 8 × 10{sup 7} K if compared with the recommended values. At ∼10{sup 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.