Precision 12C(α,γ) Angular Distribution Measurements: First Significant Physics Result 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. 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. Knowledge of the C/O ratio with sufficient accuracy has eluded measurements over the past 51 years. We previously demonstrated the feasibility of a new method using gamma beams to measure the time-reverse photo-dissociation reaction, ¹⁶ O(γ,α) ¹² C, inside a Time Projection Chamber detector. Here we report the first significant ``Physics Result'' obtained using our new method. We used an N ₂ O gas target that, unlike our previous measurement, allowed us to eliminate ¹² C, photo-dissociation events - a significant background that can lead to systematic errors. In this current, essentially background-free measurement, we obtained detailed complete angular distributions (0°-180°) with higher statistics. Cross sections were measured over the 1 ^- resonance in ¹⁶ O at E ^cm ≈2.4 MeV, where previous measurements of ¹² C(α,γ) ¹⁶ O exhibit a disturbing disagreement of the measured E1-E2 mixing phase angle (φ ¹² ) with an elementary prediction of quantum mechanics. The agreement presented in this work demonstrates the viability of our new method and provides a strong impetus for conducting measurements at lower energies.