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

## TL;DR

Scientists measured a key nuclear reaction in stars using a new method that eliminates background noise, resolving a long-standing discrepancy with theoretical predictions.

## Contribution

The study introduces a background-free method using a N2O gas target to measure the 12C(α, γ)16O reaction with improved accuracy.

## Key findings

- The new method eliminates background from 12C photo-dissociation events.
- Complete angular distributions and cross sections were measured for the 1− resonance in 16O.
- The E1−E2 mixing phase angle now agrees with quantum scattering theory predictions.

## Abstract

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.

The capture of helium by carbon to form oxygen is a key astrophysical reaction. The authors show that measuring this reaction using high-intensity photons and a Time Projection Chamber has significant advantages over conventional techniques, providing data with measured E1-E2 mixing angles that agree with quantum mechanical predictions.