Nuclear Physics with Gamma Beams and TPC Detectors

Abstract:  State-of-the-art gamma beam facilities such as HIyS at Duke University and ELI-NP in Romania, along with emerging detector technologies, are permitting key advances nuclear structure and astrophysics. Here, I present two examples; elucidating the structure of the Hoyle state in carbon-12 and measuring the cross section for the 12C(a,y)16O reaction using an optical TPC detector at HIyS. The carbon/oxygen ratio at the end of stellar helium burning is a hugely important nuclear input to stellar evolution calculations. However, it is not known accurately, due to significant uncertainties in the 12C(a,y)16O cross section. In our new study [1], angular distributions of the 12C(a,y)16O reaction were obtained by measuring the inverse 16O(y,a)12C reaction with gamma-beams and a Time Projection Chamber (TPC) detector. Data for the total reaction cross section and angular distributions from Ecm = 2 - 3.3 MeV are presented. Secondly, the structure of carbon-12 was explored by populating the 2+ excitation of the Hoyle state at 10 MeV using gamma beams [2]. An optical TPC detector allowed separation of decays through the 8Be ground state and those through excited states in 8Be, or indeed, direct decays. By placing an upper limit on the direct decay branching ratio (BR) of this 2+ state, a theoretical extrapolation permitted the direct decay BR for the Hoyle state to be determined. [1] R. Smith, M. Gai, et al. "Precision measurements on oxygen formation in stellar helium burning with gamma-ray beams and a Time Projection Chamber." Preprint, Research Square, (2021). [2] R. Smith, M. Gai, et al. "Stringent upper limit on the direct 3a decay of the Hoyle state in C 12." Physical Review C 101, 021302 (2020).