Photo-dissociation for studying the astrophysical ¹²C(α, γ) ¹⁶O reaction and the structure of ¹²C

This thesis uses Time Projection Chambers (TPCs) operating in γ beams to study nuclear reactions critical to stellar helium burning and to elucidate the structure of 12C. Two photo-dissociation reactions were investigated —16O(γ, α0) 12C and 12C(γ, α0) 8Be —both time-reversed analogues of reactions that occur in stars. The first determines the uncertainty in the carbon-to-oxygen ratio during helium burning, emphasised by W. Fowler in his Nobel Prize speech to be of “paramount importance” in nuclear astrophysics. The second was used to probe the clustered structure of 12C, and to provide insight into the Hoyle state, without which life would not exist. Two experiments were performed at the High Intensity Gamma-ray Source (HIγS) facility at Duke University. The first used an Optical Time Projection Chamber (OTPC) with an N2O + N2 active gas target, to measure four angular distributions from the 16O(γ, α0) 12C reaction between the broad J π (1−) resonance at Ex = 9.59 MeV and narrow J π (2+) resonance at Ex = 9.84 MeV. New methods were developed to extract reliable values of the E1–E2 mixing phase angle (ϕ12) in agreement with elastic scattering data, addressing a long-standing inconsistency in the field. The second experiment used the Warsaw electronic Time Projection Chamber (eTPC) with a CO2 active gas target, measuring angular distributions from Ex = 8.51−13.9 MeV. This provided higher statistics both at lower astrophysically relevant energies and at higher energies for the 16O(γ, α0) 12C reaction, where the latter provided data able to constrain the background terms in an Rmatrix analysis. Angular distributions for 12C(γ, α0) 8Be were also investigated. Resonance parameters for the Algebraic Cluster Model-predicted J π n (2+ 2 ) Hoyle band excitation and the J π n (1− 1 ) bending band excitation in 12C were extracted. No evidence was found to confirm the J π n (2+ 3 ) state predicted by the same model.