Pb by the high resolution measurement of the following gamma decay are also presented in their preliminary form.
IntroductionThe use of heavy ions inelastic scattering at approximately 20 MeV/u to study highly excited states (up to the region of the Giant Quadrupole Resonance) is a good tool when the measurement of the subsequent gamma decay is also performed with high resolution. Some partial results of the most recent experiments of this type, performed to investigate the electric-dipole (E1) response of nuclei at energies around the particle threshold, are reported in this contribution. The understanding of the electric-dipole response at energy around the binding energy is presently attracting considerable interest since the dipole strength distribution in that region affects considerably the reaction rates in astrophysical scenarios [1,2], where photodisintegration reactions are important. In addition the E1 strength is also interesting because it is expected to provide information on the neutron skin and thus on the symmetry energy of the equation of state [3][4][5][6][7]. The first evidence of an accumulation of low-lying E1 strength in heavy nuclei, larger than that due to the tail of the giant dipole resonance (GDR), dates back to early 70's [8]. However, only in recent years, experimental and theoretical investigations, on both stable and radioactive nuclei, revealed that this is a common phenomenon in most atomic nuclei [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The accumulation of E1strength around the particle separation energy is commonly denoted as pygmy dipole resonance (PDR) (see e.g.[5]) due to the much smaller size of its strength in comparison with the giant dipole resonance (GDR). The hydrodynamical model describes this pygmy strength as associated to the vibration of the neutron skin. An interesting feature in the region of the pygmy resonance has been observed [25][26][27][28][29] in a number of different stable nuclei, by comparing results of photonscattering and scattering experiments. In particular, it has been found that one group of states is excited in both type of reactions, while another group of states at higher energies is only excited in the ( ') case. These experimental findings are in qualitative agreement with different phonon models which predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying group of states with a stronger isovector character associated to the tail of the giant dipole resonance. The use of an additional probe as the inelastic scattering of 17 O at 20 MeV/u which has, similarly to alpha particles, a rather strong isoscalar character is expected to add valuable information on the quest of the nature of these low-lying E1 states. In addition, with the same experiment we intended to study of the gamma and neutron decay of the Isoscalar Giant Quadrupole Resonance (ISGQR) in the 10-13 MeV range. In the past, inelastic scattering of 17 O ions at the energy of 22 MeV/u was used to study the gamma deca...