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 ¹² C, denoted as ¹² C( α ,  γ ) ¹⁶ O. 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 N ₂ O gas target. This improvement allows us to eliminate the background caused by ¹² C photo-dissociation events, obtain complete angular distributions (0 ^∘ −180 ^∘ ), and measure the cross sections over the 1 ⁻ resonance in ¹⁶ O at E _cm  ~ 2.4 MeV. These measurements resolve the discrepancy that was previously observed between the measured E 1− E 2 mixing phase angle ( ϕ ₁₂ ) of ¹² C( α ,  γ ) ¹⁶ O and the predictions of quantum scattering theory. This newfound agreement demonstrates the viability of our method for conducting measurements at lower energies.