Consequences of nuclear structure for thermo-nuclear reactions
Many nuclear reactions of practical relevance go via a compound nuclear stage. Examples are the reaction chains in the synthesis of elements in stellar events, fission reactions in nuclear power generation, reactions used for transmutation of radioactive waste, reactions that cause radiation damage, etc. For various practical reason, many of these reactions cannot be measured directly. Hence nuclear theory must be used for estimating the reaction rates of interest. Compound reactions rates are determined by the binding energies, level densities, radiative strength functions, and capture cross sections. Nuclear structure has an impact on all of these ingredients. At present, the following problems are studied:
- How does deformation and Landau fragmentation change the E1 strength function in the energy range of few MeV up to the GDR resonance energy. Landau fragmentation is taken into account by means of Random Phase Approximation for separable interactions. Deformation is treated in adiabatic approximation. Deformation probability distributions are generated by means of the Interacting Boson Model. The approach promises the description of transitional nuclei between well deformed and spherical nuclei described by theory so far.
- The Wigner term (linear in T, the isospin) in the binding energies of proton rich nuclei near N=Z is attributed to the proton-neutron pairing. A model for calculating the pair correlations exactly (i. e. avoiding the BCS mean field approximation) is developed, which couples pair-vibrations (calculated in RPA) with a small shell model space comprising few levels near the Fermi surface. Improvement of predicting the binding energies of nuclei near and above 100 Sn is expected.
- The constant-temperature regime of the level density at low energy, which is an experimentally well known phenomenon, is explained as the transition from the paired to the normal state in a small isolated system, in analogy to the transition of a superconductor to the normal state when the temperature increases. The pair correlations are treated going beyond the mean-field BCS approximation (diagonalization in a particle number projected basis) and studying the micro canonical ensemble. Understanding the constant-temperature regime and the transition to the Fermi-gas regime will clarify the physics behind the commonly used phenomenological level-density expression as well as allow us to better predict the parameters of these expressions.