Sub-Coulomb α Transfers on ¹²C and the ¹²C(α,γ)¹⁶O S Factor

The ¹²C(α,γ)¹⁶O reaction is crucial for the understanding of He burning in massive stars, but the low-energy cross section is highly uncertain. To address this problem we have measured at sub-Coulomb energies total cross sections for the ¹²C(⁶Li,d)¹⁶O and ¹²C(⁷Li,t)¹⁶O reactions to the bound 2⁺ and 1^- states of ¹⁶O. The data are analyzed to obtain the reduced α widths of these states. Together with capture and phase-shift data, these results provide for a more accurate determination of the low-energy 12C(α,γ)¹⁶O S factor: S_E1(0.3 MeV)  =  101±17 keV b and S_E2(0.3 MeV)  =  42_-23⁺¹⁶ keV b for the E1 and E2 multipole components of the reaction.