Nuclear and Coulomb excitations of low-lying dipole states in exotic and stable nuclei

Nuclei with a consistent neutron excess show the presence of dipole strength at low excitation energy. This strength, carrying few per cent of the isovector EWSR, has been often associated to the so called Pygmy Dipole Resonance (PDR). In this mode the isoscalar and isovector components are strongly mixed and therefore there is a possibility to study these low-lying dipole states by using both isoscalar and isovector probes. We show that valuable informations on the nature of the PDR can be obtained by excitation processes involving also the nuclear part of the interaction. The use of different bombarding energies, of different combinations of colliding nuclei involving different mixture of isoscalar/isovector components, together with the mandatory use of microscopically constructed formfactors, can provide the clue to reveal the characteristic features of these states. The relative population of the PDR with respect to the GDR may change by changing the parameters of the reactions. In particular, at low incident energy the excitation probability of the PDR state is sensibly higher than the GDR one. Our conclusion is that the best conditions to reveal the PDR can be achieved at relatively low incident energies (few tens of MeV per nucleon), where the interplay between the Coulomb and nuclear interactions plays a fundamental role in singling out these states.