Global R-Matrix Analysis of the 12C(α, γ)16O reaction

The 12C(α,γ)16O reaction, in combination with the triple-alpha process, determines the 12C/16O fraction at the end of stellar helium-burning. This fraction has been shown to strongly influence any subsequent stellar evolution and, due to imprecise knowledge of 12C(α, γ)16O reaction rate, complicates precision tests of stellar models. The 12C(α, γ)16O reaction rate at stellar energies is determined via the R-matrix analysis of experimental data. These data are discussed, and new measurements of the E1-E2 interference sign between the Ec.m. = 2.68-MeV E2 resonance and an underlying E1 strength are described. An E1-E2 asymmetry parameter of a = 0.07 ± 0.05 was extracted from the thick-target γ-ray yields of the narrow resonance at angles of 45° and 135°. The positive sign of a corresponded to constructive interference at forward angles and, further, allowed the interference between the resonance and an E2 background to be identified as constructive below the resonance energy. The E2-E2 interference was then used to evaluate the global SE2 data within the vicinity of the resonance 2.5 < Ec.m. ≤ 3.0 MeV. A global analysis of SE2 data that agreed with the interference scenario has determined the E2-E2 interference scheme of the 4.34-MeV resonance and background, resulting in a value of SE2(300) = 62+9−6keV b.