Astrophysical Rates for Explosive Nucleosynthesis: Stellar and Laboratory Rates for Exotic Nuclei

Nuclear theory is important in the determination of reaction cross sections and rates for astrophysics in several respects. Firstly, a large number of reactions in astrophysical environments involve highly unstable nuclei which are unaccessible in laboratory measurements. This is especially true for the high temperature plasmas of stellar explosions. Secondly, despite of high plasma temperatures, the relevant particle interaction energy is low by nuclear physics standards. This is a challenge for experimentalists, especially when studying charged-particle reactions at or below the Coulomb barrier. Very small cross sections at low energy may even prevent a measurement and have to be predicted in this case. Thirdly, the nuclei occur in excited states because they are in thermal equilibrium with the stellar plasma. This modifies the reaction cross sections and has important consequences for the determination of stellar reaction rates. Laboratory measurements only account for a fraction of the possible transitions and the actual plasma corrections (stellar enhancement factors, SEF) can only be provided by theory. (Electron screening is also important but will not be discussed here.) Large-scale predictions of reaction cross sections across the chart of nuclei are required to provide the rates needed for reaction networks applied to nucleosynthesis. In astrophysical applications usually different aspects are emphasized than in pure nuclear physics investigations. Reliable predictions of the nuclear properties and optical model ingredients, like optical potentials for particle and α transmission coefficients [ 1, 2, 3, 4, 5, 6], nuclear level densities (NLD) [ 7, 8], resonance energies, and γ-transition strengths [ 9, 10], are needed far from stability. Even at stability, there are still considerable uncertainties, especially in the optical potential for charged particles which is not well constrained at low energy. In addition to the well-known difficulty in predicting α-potentials [ 3, 11, 12, 13, 14], it was recently found that also the optical potential for protons may require special attention [ 1, 2, 15, 16].