Gant South Building

Time projection chamber (TPC) detectors, operating in gamma beams, were used to study the structure of 12C and 16O at energies above 10 MeV. In this energy region, the algebraic cluster model (ACM), which models 12C as an equilateral triangle of alpha particles, predicts new states in 12C, which are relevant for the structure of excitations of the Hoyle state (E = 7.6542 MeV). The Hoyle state is responsible for the production of 12C in the universe, and its structure is central to stellar evolution. Specifically, the current experiment searched for a predicted 2+ state at around 12 MeV, belonging to a rotational band corresponding to the exotic bending-mode vibration. The structure of 16O is directly relevant for oxygen formation during stellar helium burning. An electronic TPC detector (eTPC) was prepared at the University of Warsaw with assistance from UConn students, when possible during the pandemic. It was used at the High-Intensity γ-Source (HIγS) at Duke University. The HIγS facility is ideally suited for ℓ = 1 (E1) and ℓ = 2 (E2) nuclear excitations. Data were collected at gamma energies ranging from 8.51 to 13.9 MeV, with low background and substantial statistics. The high-energy results, above 10 MeV, were the focus of this thesis. Algorithms were developed to analyze tracks recorded in the eTPC. It records in three dimensions, with high efficiency and low background, all relevant kinematic variables characterizing the reactions. These yield a complete, detailed view of the interaction. Complete and detailed angular distributions of 12C and 16O photodissociation spanning the entire angular range (0 to 180◦) were measured and analyzed using a partial wave decomposition. The results are compared with fundamental predictions of quantum mechanics, i.e. unitarity, and to specific nuclear structure models. No new states were found for 12C below 12.5 MeV. The 16O data improve our ability to extrapolate measurements made at laboratory energies to stellar conditions.