Gant South Building

Stellar evolution and the synthesis of the elements are governed by key nuclear reactions, among which the fusion of 12C with an alpha particle to form 16O, denoted as the 12C(α,γ)16O reaction, is "of paramount importance". The ratio of carbon to oxygen produced during stellar helium burning, which is determined by the 12C(α,γ)16O reaction, allow us for example to predict the fate of massive stars, whether they end up as neutron stars or black holes. Despite five decades of study, this reaction's cross section remains poorly constrained at the astrophysically relevant energies. This thesis presents the development and implementation of a new method to measure the cross-section of the 12C(α,γ)16O reaction by measuring the time-reverse process – the 16O(γ,α)12C reaction – using a Time Projection Chamber (TPC) operated in intense γ-ray beams. The first-generation optical readout TPC (O–TPC) was constructed at UConn and used at the High Intensity γ source (HIγS) facility at Duke University. Building on these results, a next-generation electronic readout TPC (eTPC) was constructed and commissioned at the University of Warsaw, incorporating a fully digital electronic readout system for high-rate data acquisition. The eTPC was exposed to quasi-monoenergetic γ-rays from 8.51–13.9 MeV, corresponding to Ecm=1.4-4.8 MeV of the 12C(α,γ)16O reaction. A comprehensive analysis framework was developed to identify the 16O(γ,α)12C events and reconstruct their kinematics. This permitted angular distributions of the photo-dissociation events to be examined. The analyzed angular distributions yield results which are consistent with a fundamental prediction of quantum mechanics, a feat not seen in earlier data sets. The results demonstrate that this method can achieve accurate event reconstruction, clean event separation, accurate energy calibration, and angular resolution sufficient for astrophysical studies. This work establishes the validity of our new method for precision measurement of the 12C(α,γ)16O reaction through its time reverse process. This paves the way toward future measurements at lower energies with reduced uncertainty and improved extrapolation to stellar conditions.