Measurements of branching ratios from the 7.12-MeV state in 16O and the 12C(α, γ)16O reaction cross section

Gamma-ray branching ratios of the 7.12-MeV state in O-16 have been measured using the gamma-gamma coincidence technique. The extrapolations of several components of the important C-12(alpha, gamma)O-16 cross section to helium-burning energies are dependent upon the 7.12 -> 6.92 MeV branching ratio. An experimental upper limit has been placed on this transition; the 7.12 -> 6.13 MeV branching ratio has also been measured. Relative cross sections and gamma-ray angular distributions have been measured for the F-19(p, a alpha gamma) reaction at several energies near E-p = 2 MeV, leading to the 6.13-, 6.92-, and 7.12-MeV states of O-16. Monte Carlo simulations of the various detectors have been developed and the simulated detector efficiencies have been compared and adjusted to experimental measurements. The available experimental data relating to the C-12(alpha, gamma)O-16 cross section have been fitted with the R-matrix formalism. The new upper limit on the 7.12. 6.92 MeV branching ratio impacts the extrapolation of both the E2 ground-state cross section and the cascade cross section through the 6.92- MeV state.