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 E _cm ~ 2.4 MeV. These measurements resolve the discrepancy that was previously observed between the measured E1-E2 mixing phase angle (ϕ ₁₂) 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.