Transfer reactions as tool for nuclear astrophysics : the 13 C ( α , n ) 16 O case

The cross section of the nuclear reactions involved in stellar nucleosynthesis (e.g. AGB stars) are often very difficult to measure directly at stellar energies because of their very small value. Moreover, this situation can be complicated by the existence of very low energy resonances and/or subthreshold resonances. Indirect methods such as transfer reactions and the ANC method offer the possibility to overcome these difficulties. In this context, recent indirect measurement of the reaction 13 C(α,n) 16 O is presented. The 13 C(α,n) 16 O reaction is considered as the main neutron source for s-process in low-mass asymptotic giant branch stars. At low energies of astrophysical interest, the contribution of the subthreshold state 6.356 MeV of 17 O to the 13 C(α,n) 16 O cross section should be taken into account. However, the results of previous studies of this contribution lead to different conclusions. Hence, we investigated the effect of this resonance on the astrophysical S-factor through a new precise measurement of the alpha spectroscopic factor, S α , and the corresponding ANC of the 6.356 MeV state using the transfer reaction 13 C(7 Li,t) 17 O at two different incident energies. The measured angular distributions and the obtained spectroscopic factor and the asymptotic normal-ization constant (ANC) are presented as well as their impact on 13 C(α,n) 16 O cross section and reaction rate. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.