Background-free 12C(α, γ) angular distribution measurements with a time projection chamber operating in Gamma beams

The carbon oxygen ratio (C/O) at the end of stellar helium burning is a crucial nuclear input to stellar evolution theory. Knowledge of the C/O ratio with sufficient accuracy has eluded measurement over the past five decades. It is determined by the rate of oxygen formation in the fusion of helium with 12C, denoted as 12C(α, γ)16O. Even though recent methods employing a time projection chamber can measure the time-reverse photo-dissociation reaction, the results still do not show unambiguous agreement with the predictions of quantum scattering theory. Here, we improve this method using a N2O gas target. This improvement allows us to eliminate the background caused by 12C photo-dissociation events, obtain complete angular distributions (0∘−180∘), and measure the cross sections over the 1− resonance in 16O at Ecm ~ 2.4 MeV. These measurements resolve the discrepancy that was previously observed between the measured E1−E2 mixing phase angle (ϕ12) of 12C(α, γ)16O and the predictions of quantum scattering theory. This newfound agreement demonstrates the viability of our method for conducting measurements at lower energies.