Pygmy and giant electric dipole responses of medium-heavy nuclei in a self-consistent Random Phase Approximation approach with finite-range interaction

摘要

The pygmy dipole resonance (PDR) is studied in various medium-heavy nuclei byusing a Gogny interaction in a self-consistent Hartree-Fock plus Random PhaseApproximation method. We compare the details of the PDR structure with those ofthe giant dipole resonance (GDR). In the PDR protons and neutrons vibrate inphase, and the main contributions are given by particle-hole excitationsinvolving the neutrons in excess. On the contrary, in the GDR protons andneutrons vibrate out of phase, and all the nucleons are involved in theexcitation. The values of the parameters we used to define the collectivity ofan excitation indicate that the PDR is less collective than the GDR. We alsoinvestigate the role of the residual interaction in the appearance of the PDRand we find a subtle interplay with the shell structure of the nucleus.