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 ¹⁶O have been measured using the γ-γ coincidence technique. The extrapolations of several components of the important ¹²C(α,γ)¹⁶O 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 γ-ray angular distributions have been measured for the ¹⁹F(p,αγ) reaction at several energies near E_p=2 MeV, leading to the 6.13-, 6.92-, and 7.12-MeV states of ¹⁶O. 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 ¹²C(α,γ)¹⁶O 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.