Precision measurements on oxygen formation in stellar helium burning with gamma-ray beams and a Time Projection Chamber.

The carbon/oxygen (C/O) ratio at the end of stellar helium burning is the single most important nuclear input to stellar evolution theory. However, it is not known with sufficient accuracy, due to large uncertainties in the cross-section for the fusion of helium with C to form O, denoted as C(α, γ)O. Here we present results based on a method that is significantly different from the experimental efforts of the past four decades. With data measured inside one detector and with vanishingly small background, angular distributions of the C(α, γ)O reaction were obtained by measuring the inverse O(γ, α)C reaction with gamma-beams and a Time Projection Chamber (TPC) detector. We agree with current world data for the total reaction cross-section and further evidence the strength of our method with accurate angular distributions measured over the 1 resonance at E ~ 2.4 MeV. Our technique promises to yield results that will surpass the quality of the currently available data.