Determination of the Subthreshold State Contribution inC13(α,n)O16

Abstract

The reaction rate of the stellar reaction 13C(alpha,n)16O, which is currently considered to be the main neutron source for the slow (s) process at low energies, has been rederived using the direct alpha-transfer reaction 13C(6Li,d)17O leading to the subthreshold state at 6.356 MeV in 17O. The contribution of the subthreshold state is found to be much smaller than the currently accepted predictions for the main neutron source of the s process, indicating less of a role of this reaction as the neutron source for the s-process scenario in low-mass stars at the asymptotic giant branch.

Nucleosynthesis of s-elements in rotating AGB stars

“…The results (Fig. 9) confirm that the slower rates proposed by Kubono et al (2003) slightly decreases the efficiency of the s-process. The effects remain however relatively weak and do not affect our previous conclusions.…”

“…The rate of the reaction 13 C(α, n) 16 O has been recently reexamined experimentally by Kubono et al (2003) who find a smaller contribution to the subthresold state at 6.356 MeV compared to the NACRE rate adopted here. This small contribution leads to a rate smaller by a factor of about 4 at T 8 = 1.…”

Measurement of the−3 keVResonance in the ReactionC13(α,Cognata1, Spitaleri2, Trippella3 et al. 2012Phys. Rev. Lett.473350View full textAdd to dashboardCite

“…The 13 Cð; nÞ 16 O reaction is the neutron source for the main component of the s-process, responsible for the production of most nuclei in the mass range 90 & A & 204. It is active inside the helium-burning shell in asymptotic giant branch stars, at temperatures & 10 8 K, corresponding to an energy interval where the 13 Cð; nÞ 16 O is effective from 140 to 230 keV.…”

“…It is active inside the helium-burning shell in asymptotic giant branch stars, at temperatures & 10 8 K, corresponding to an energy interval where the 13 Cð; nÞ 16 O is effective from 140 to 230 keV. In this region, the astrophysical SðEÞ-factor is dominated by the À3 keV subthreshold resonance due to the 6.356 MeV level in 17 O, giving rise to a steep increase of the SðEÞ-factor.…”

“…The discrepancy amounts to a factor of 3 or more right at astrophysical energies. Therefore, we have applied the Trojan horse method to the 13 Cð 6 Li; n 16 OÞd quasifree reaction to achieve an experimental estimate of such contribution. For the first time, the ANC for the 6.356 MeV level has been deduced through the Trojan horse method as well as the n-partial width, allowing to attain an unprecedented accuracy in the 13 Cð; nÞ 16 O study.…”