Studies in Nuclear Astrophysics with an Optical Readout TPC (O-TPC) at HIγS

An Optical Readout Time Projection Chamber (O-TPC) operating with the gas mixture of CO₂(80%) + N₂(20%) at 100 torr with gamma beams from the HIγS facility of TUNL at Duke University were used to study the formation of carbon and oxygen during helium burning. Measurements were carried out with circularly polarized gamma-ray beams at energies: E_γ = 9.08, 9.38, 9.58, 9.78, 10.08, 10.38 and 10.68 MeV. We have begun the process of extracting complete angular distributions for the ¹⁶O(γ, α)¹²C reaction and the ¹²C(γ, 3α) reaction (eventually with energy bins of approximately 100 keV) in order to determine the values of S_E1, S_E2 and the mixing phase ϕ₁₂ of the ¹⁶O(γ, α)¹²C reaction. The rate of carbon formation at high temperatures (T > 3 GK) was suggested to increase due to contributions from a higher lying 2⁺ state. We have measured an angular distribution of (essentially) pure E2 transition at E_γ = 9.78 MeV of the ¹²C(γ, 3α) reaction, providing conclusive evidence for the elusive 2⁺₂ state in ¹²C.