Abstract
The electric dipole (E1) response is one of the basic properties of atomic nucleus. Its mayor part is exhausted by the Isovector Giant Dipole Resonance (IVGDR). Such resonances can be excited with different probes as for example: photons, charged particles or heavy ions, followed by subsequent decays by emission of particles and γ’s. Below IVGDR, around particle separation energy, a small fraction of fragmented dipole states are also observed, and associated to so called Pygmy Dipole Resonance (PDR), which is connected with a collective oscillation of number of neutrons at the nuclear surface against the inert proton-neutron core. Many microscopic calculations have shown the relationship between the PDR strength and the neutron-skin thickness, which is determined by the symmetry energy of the equation of state (EOS) and directly linked to its parameters.

This presentation reports on the study on soft dipole modes in $^{140}$Ce using ($^{17}$O,$^{17}$O’γ) reaction. The experiment was performed at Laboratori Nazionali di Legnaro, Italy. Inelastic scattering of $^{17}$O ion beam at 20 MeV/A was used to excite the resonance modes in the 140Ce target. Gamma rays were registered by 5 triple clusters of AGATA-Demonstrator and 8 large volume scintillators (LaBr3). The scattered $^{17}$O ions were identified by two ΔE-E Si telescopes of the TRACE array mounted inside the scattering chamber. The telescopes consisted of 2 segmented Si-pad detectors, each made of 60 pixels.

General aim was to investigate the structure of these states, in particular its isospin mixing by comparison to previous data from (γ, γ’) and (α, α’) experiments. The more specific goal was to confront the experimental data with theoretical calculations using distorted wave Born approximation (DWBA) based on form factor obtained by folding the microscopically calculated transition density.

During the presentation, progress in complex data analysis will be discussed and preliminary results of the experiment will be presented and discussed.

*This work has been partly supported by the stipend from Marian Smoluchowski Krakow Research Consortium ‘Matter-Energy-Future’ as a Leading National Research Center (KNOW) and also by the Polish National Science Centre under contract No. 2013/09/N/ST2/04093