Oxygen Formation in the Light of Gamma-Beams: [Precision Measurements of the 12C(α, γ) Reaction with Gamma-Beams and a Time Projection Chamber]

The carbon/oxygen (C/O) ratio at the end of stellar helium burning is not known with sufficient accuracy due to the large uncertainty in the cross-section of the 12C(α, γ)16O reaction. We developed a new method to measure this reaction that is significantly different from the experimental efforts of the past four decades. Data were measured with vanishingly small background, inside one detector, that also serves as an active target (AT-TPC). Angular distributions of the 12C(α, γ)16O reaction were obtained by measuring the inverse 16O(γ, α)12C reaction with gamma-beams (from the HIγS facility) and an optical readout time projection chamber (O-TPC) detector. We agree with current world data on the measured total reaction cross-section. We further evidence the strength of our method with angular distributions measured over the 1–resonance at Ecm ~ 2.4 MeV. We extract the E1-E2 mixing phase angle (ϕ12) over this resonance and obtained values that follow the trend of prediction based on unitarity of the scattering amplitude. Our technique promises to yield results that will surpass the quality of the currently available data. We continue these measurements to lower energies at the HIγS facility of TUNL at Duke University, and anticipate measurements down to Ecm = 1.1 MeV at the ELI-NP, the EU facility in Romania..