5th International Symposium on Nuclear Symmetry Energy NuSYM15, June 29 - July 2, 2015 Kraków, POLAND

Fabio Crespi
University of Milan / INFN Milano, ITALY

Isospin Character of Low-Lying Pygmy Dipole States via Inelastic Scattering of 17O

The electric dipole (E1) strength in atomic nuclei is almost completely concentrated in the extensively studied isovector giant dipole resonance (IVGDR). Particular attention has been given in the last few years to detailed investigations of the properties of the dipole strength around the particle emission threshold. This E1 strength is denoted as Pygmy Dipole Resonance (PDR) and is found to be particularly sizable in neutron rich nuclei. To investigate this one needs high-resolution measurements and comparison of data obtained with different probes. In particular, nuclei with sizable neutron skin, such as the doubly magic $^{208}$Pb, are very interesting. The $\gamma$ decay from the low-lying dipole states was measured in different nuclei ($^{208}$Pb, $^{124}$Sn, $^{90}$Zr) using the inelastic scattering of $^{17}$O at 340 MeV. The emitted $\gamma$ rays were detected with high resolution with the AGATA demonstrator array and the scattered ions were detected in two segmented $\Delta E-E$ silicon telescopes. The multipolarity of the observed gamma transitions was determined with remarkable sensitivity thanks to angular distribution measurements. Cross sections and angular distributions of the $\gamma$ rays and of the scattered particles were measured. The results are compared with $(\gamma, \gamma)$ and $(p, p)$ data. The data analysis with the distorted wave Born approximation approach gives a good description of the elastic scattering and of the inelastic excitation of the $2^{+}$ and $3^{−}$ states. For the dipole transitions, a form factor obtained by folding a microscopically calculated transition density was used. This has allowed us to extract the isoscalar component of the low-lying $1^−$ excited states.