The Nuclear Giant Dipole Resonance under Extreme Conditions

The isovector giant dipole resonance (GDR) represents a well-established collective motion of finite nuclei extensively studied for more than fifty years with fundamental contributions from the Dubna theory group led by V.G. Soloviev. The dependence on the nuclear structure of the reference state, on top of which the collective mode is built, has already suggested its use in order to study nuclei far from the ground state. The time structure of the GDR mode actually allows the possibility of using it as a probe of nuclear systems very far from normal conditions. Here we report on the properties of the GDR built on very exotic nuclear systems: (i) high temperature, at the limit of the nuclear stability; (ii) during charge equilibration in fusion dynamics. The isovector density wave propagation in symmetric nuclear matter at finite temperature is studied within the Landau-Vlasov theory. In cold nuclear matter, a zero sound is found, which changes to a first sound with increasing temperature. This transition, at variance to the case of a one-component Fermi liquid, does not result in the appearance of a maximum in the attenuation of the collective motion as a function of temperature. The damping width of a volume isovector mode is always monotonically increasing with temperature due to the presence of the collisional friction between neutron and proton liquids. However, a clear effect of the transition is expected on the structure of the dipole response function. A transition temperature of about 4.5 MeV is deduced in heavy nuclei. Features of direct GDR excitation in intermediate dinuclear systems of very exotic shape or charge distributions formed in particular fusion entrance channels (mass asymmetry vs. charge asymmetry) are described. The beam energy dependence of the effect due to the coupling between the preequilibrium dipole mode and other collective dynamical contribution is discussed. Certain optimum entrance channel conditions ensuring this important preequilibrium cooling mechanism are derived.