Carbon-12 + Carbon-12 Fusion: Measurement and Advances Toward the Gamow Energy

Abstract

Knowledge of carbon fusion reaction rates at stellar temperatures is necessary for a complete picture of stellar evolution. The minimum stellar mass necessary for quiescent carbon burning as well as the conditions necessary for type Ia supernovae depend on them. The astrophysically relevant region of reaction rates is contained within a narrow energy band (the Gamow window), which lies on the fax low energy end of the cross section curve. Due to the difficulty in measurements of cross sections at low energies, current rates in this regime rely heavily on extrapolated values from higher energies and ignore the effects of possible resonances in or near the Gamow window. The challenge in obtaining data at low energy primarily stems from observing very low counting rates, due to the exponentially falling cross section, in the presence of background that is insignificant at higher energies. Historically the measurement of the total fusion cross section has been halted at the high energy edge of the Gamow window by background radiation from hydrogen isotopes. Recent advances in target preparation have extended the data near the region of stellar interest, but the emergence of a highly improbable two-step reaction has again limited the lowest attained energy. However, improvements in target quality and vacuum composition have yielded promising results. In the present work the identification and reduction of this background will be discussed along with carbon fusion data, implications, and future techniques. The 12 – (¹²C, p)²³ Na fusion reaction has been studied by charged particle spectroscopy from E_cm = 2.1 – 4 MeV. Thick targets were used in an ultra low hydrogen environment in order to suppress beam induced background. Confirmation of previously measured resonances below E_cm = 4 MeV as well as new structure down to E_cm= 2.25 MeV have been studied. Furthermore a Bragg ionization chamber has been developed for the purpose of future charged particle measurements of the 12C(¹² C,p) ²³Na and ¹²C( ¹²C, α)²⁰Ne reactions simultaneously.