Indirect study of the 12C(α,γ)16O reaction via the 12C(7Li,t)16O transfer reaction

The 12C(α,γ)16O reaction plays a crucial role in stellar evolution. The rate of this reaction determines directly the 12C-to-16O abundance ratio at the end of the helium burning phase of stars and consequently has a big effect on the subsequent nucleosynthesis and even on the evolution of massive stars. However, despite many experimental studies, the low-energy cross section of 12C(α,γ)16O remains uncertain. The extrapolation of the measured cross sections to stellar energies (E∼300 keV) is made particularly difficult by the presence of the 2+ (Ex=6.92 MeV) and 1- (Ex=7.12 MeV) subthreshold states of 16O. To further investigate the contribution of these two subthreshold resonances to the 12C(α,γ)16O cross section, we determine their α-reduced widths via a measurement of the transfer reaction 12C(7Li,t)16O at two incident energies, 28 and 34 MeV. The uncertainties on the determined α-spectroscopic factors and the α-reduced widths were reduced thanks to a detailed distorted-wave Born approximation analysis of the transfer angular distributions measured at the two incident energies. The R-matrix calculations of 12C(α,γ)16O cross section using our obtained α-reduced widths for the 2+ and 1- subthreshold resonances lead to an E1 S factor at 300 keV of 100±28 keVb, which is consistent with values obtained in most of the direct and indirect measurements as well as the NACRE collaboration compilation while the result for the E2 component SE2(300keV)=50±19 keVb disagrees with the NACRE adopted value. © 2012 American Physical Society.