Pygmy dipole resonance in 140Ce using the (n,n'ɣ) reaction at NFS

Giant resonances are collective vibrations of the nucleus in which the majority of the nucleons that compose the nucleus are involved in the excitation.
Among the different giant resonances, the well-known isovector giant dipole resonance (IVGDR), which corresponds to the oscillation of the neutron fluid against the proton fluid, is a broad resonance with a mean energy between 12 and 24 MeV and a width in the range of 2.5 to 6 MeV which exhausts almost 100% of the isovector electric dipole strength. Additional E1 strength has been observed at lower energy in neutron-rich nuclei, near the neutron separation threshold.
This small-size structure, in comparison to the IVGDR, is commonly known as the pygmy dipole resonance (PDR) and can be described as the oscillation of a neutron skin against a symmetric proton/neutron core.

In the e833 experiment, we used a new probe, namely the (n,n'ɣ) reaction at NFS, to characterize the structure of the PDR in 140Ce.
Neutron scattering, in addition to be charge free, offer the possibility to address the PDR structure problem with a new approach, complementary to the usual study that consists in comparing isoscalar and isovector excitations. Indeed, it is well established that for hadron scattering at energies of the order of a few tens of MeV per nucleon, the proton-neutron interaction is three times larger than the one between alike nucleons. Thus, while proton scattering mainly probes the neutron components of a transition, neutron scattering is sensitive to the protons of the nucleus. By using the (n,n'ɣ) probe at NFS we aim to reveal the nature of the PDR at nuclear surface and highlight the role of the protons in the PDR excitation in the 140Ce.