Abstract
Background: The fusion excitation function of C 12+ C 12 contains many resonances from above the Coulomb barrier towards sub-barrier energies. These oscillations might be linked to molecular states in the compound nucleus C 12 with drastic consequences for stellar carbon burning. Moreover, these structures render the common extrapolations from measurements into the astrophysics region of interest at deep sub-barrier energy extremely uncertain.
Purpose: We have investigated the cross section around the lowest direct-coincident gamma-particle measurements, where previously only limits could be established with the aim of obtaining a detailed description of the excitation function. We have furthermore analyzed the ratio of extreme decay branching into the first excited state of daughter nuclei with alpha or proton emission at relative kinetic energies where previous measurements are in disagreement with each other.
Methods: The experiments were carried out at the Andromède accelerator, IJCLab Orsay (France), with carbon-beam intensities of up to 3 p µ A on thin rotating carbon-target foils. Light charged particles and gamma decays were detected using silicon strip detectors and LaBr3(Ce) crystals, respectively, in coincidence measurements with nanoseconds precision.
Results: We establish data points with highly improved accuracy at the high-energy tail of the lowest resonance detected so far in direct measurements, right in the astrophysics region of interest (RoI). The findings are in agreement with an earlier interpretation of the fusion excitation function composed of resonances on top of a global trend of empirically determined fusion hindrance behavior. At a relative energy between 3.1 MeV and 3.3 MeV, an oscillating behavior of the branching ratio into the first excited final state with alpha or proton emission is observed, with extreme values significantly deviating from data at higher energy.
Conclusions: Our findings in the astrophysics RoI support reaction-rate models with a lower average S-factor trend, that deviates significantly from standard extrapolations between 2.2 MeV and 2.6 MeV, for stellar carbon-burning simulations of up to 25 M⊙ stars. Based on our data, an overall increase of the S-factor at deep sub-barrier energy cannot be confirmed. The extremely low ratio of the branching into the first excited state with proton over alpha emission of ≈2% at 3.23 MeV might indicate the presence of alpha-cluster compound states in C 12. This highly favors α emission with fundamental consequences in possible stellar carbon-burning sites.