Microscopic description of the low lying and high lying electric dipole strength in stable Ca isotopes

The properties of the low lying and high lying electric dipole strength in the stable 40Ca, 44Ca and 48Ca isotopes have been calculated within the Extended Theory of Finite Fermi Systems (ETFFS). This approach is based on the random phase approximation (RPA) and includes the single particle continuum as well as the coupling to low lying collective states which are considered in a consistent microscopic way. For 44Ca we also include pairing correlations. We obtain good agreement with the existing experimental data for the gross properties of the low lying and high lying strength. It is demonstrated that the recently measured A-dependence of the electric dipole strength below 10 MeV is well understood in our model: due to the phonon coupling some of the strength in 48Ca is simply shifted beyond 10 MeV. The predicted fragmentation of the strength can be investigated in (e,e′) and (γ,γ′) experiments. The isovector dipole strength below 10 MeV is small in all Ca isotopes. Surprisingly, the proton and neutron transition densities of these low lying electric dipole states are in phase, which indicate isoscalar structure. We conclude that for the detailed understanding of the structure of excited nuclei e.g. the low lying and high lying electric dipole strength an approach like the present one is absolutely necessary.