The ^(12)C(α, γ)^(16)O reaction and stellar helium burning

The cross section for the reaction ^(12)C(α, γ)^(16)O has been measured for a range of c.m. energies extending from 1.41 MeV to 2.94 MeV, by using ^(12)C targets of high isotopic purity, large NaI(T1) crystals, and the time-of-flight technique for the suppression of prompt neutron background and time-independent background. Gamma-ray angular distributions were measured at c.m. energies of 2.18, 2.42, 2.56 and 2.83 MeV. By means of theoretical fits, which include the coherent effects of the 1^− states of ^(16)O at 7.12 MeV, 9.60 MeV, and those at higher energies, the electric-dipole portion of the cross section at astrophysically relevant energies has been determined. A three-level R-matrix parametrization of the data yields an S-factor at E_(c.m.) = 0.3 MeV, S(0.3 MeV) = 0.14^(+0.14)_(−0.12) MeV · b. A “hybrid” R-matrix optical-m parameterization yields S(0.3 MeV) = 0.08^(+0.05)_(−0.04) MeV · b. This S-factor is of crucial importance in determining the abundances of ^(12)C and ^(16)O at the end of helium burning in stars