EUROCORES_EuroGenesis_1.Call_Mikroskopic optical potentials for nucleosynthesis

Stellar nucleosynthesis occurs via a network of nuclear reactions in various stages of stellar evolution. In this proposal focus is given to nuclear reactions involving alpha-particles which are relevant for nucleosynthesis in explosive scenaria. In particular we are interested in the synthesis of so-called p-nuclei, which refers to 35 stable proton-rich nuclei between Se-74 to Hg-196. These p-nuclei, apart from Mo-92, are typically 10-100 times less abundant than the corresponding more neutron rich isotopes. From our current understanding the synthesis of p- nuclei occurs in stellar environments at high temperatures (T9=3-6), e.g. in Type II supernova explosions. At present the set of measured cross sections for the relevant nuclear reactions is limited and one relies on theoretical estimates. One of the key quantities for reaction calculations are optical potentials. At present they are not well known at astrophysically relevant energies and lead to a substantial uncertainties of the p-process reaction rates in particular for nuclear reactions involving alpha-particles. The proposed project aims at a consistent microscopically based determination of the photodisintegration and capture cross sections relevant for p-process nuclear synthesis. One key goal is the determination of microscopically determined optical potentials for nucleon-nucleus and alpha-nucleus scattering based on an extended nuclear structure approach, which seems best suited for astrophysically relevant energies. Particular emphasis will be given to the consistent use of realistic interactions and to proper treatment of the Pauli principle. Photodisintegration as well as capture cross section calculations within the statistical model will be performed taking fully into account the non-locality of the derived optical potentials. These calculations will be performed for a series of p-nuclei as well as for specific reactions measured in the EXNUC project. There is good hope that with these developments we can further elucidate the quantitative understanding of the p-process.

The central focus of the project was the development of microscopic models for the determination of effective interactions between alpha-particles and nuclei, the so-called optical potentials, for the description of scattering processes at low energies. These developments are of particular importance for nuclear astrophysics because alpha-nucleus optical potentials play a crucial role in the calculation of reactions relevant for nucleosynthesis, but are not experimentally accessible at the low energies of interest.Starting from a basic nucleon-nucleon interaction two microscopic approaches for optical potentials were considered. In the first approach an extension of the nuclear matter approach to alpha-nucleus optical potential has been performed. This approach was successfully applied for nucleon-nucleus scattering and considers the nucleus on the basis of an infinitely extended nuclear matter. The second development addresses the nuclear structure approach which accounts for the most important excitations of the nucleus by means of many-body techniques. This approach is rather involved and its use was rather limited in recent years. The achieved improvements concerns consistency in the formulation and the use of modern nuclear structure tools exploiting the capabilities of modern computers. Both microscopic approaches were numerically implemented. The determination of optical potential for spherical nuclei and their application to elastic scattering was performed. The results clearly indicate that the nuclear structure approach, albeit improved consistency and essentially increased completeness, accounts only for 50-60% of the expected absorption. Hence it is only slightly better than previous studies suffering of significant limitations. On the hand the nuclear matter approach yields a fair description of elastic alpha-nucleus scattering over the whole energy range. This represents an essential progress with regard to a microscopically based calculation of low-energy alpha-nucleus reactions for the nucleosynthesis of elements.The extension of the nuclear matter approach to deformed nuclei seems possible which would allow the application of the method to most reactions of interest in nucleosynthesis. Futhermore the developed alpha-nucleus optical potentials are of interest for materials research. They represent an important ingredient in the calculations of gas producing reactions which are responsible for the embrittlement of irradiated materials.