Evolution of the Dipole Response for Neutron-Rich Tin Isotopes 124Sn to 132Sn

The Isovector Giant Dipole Resonance (IVGDR) is a collective nuclear excitation mode characterized by an out-of-phase motion of protons and neutrons against each other. As such, it creates a local isospin imbalance. The nuclear matter property that describes the stiffness towards changes in isospin symmetry is called the symmetry energy. Since it acts as the restoring force for the IVGDR, its parameters, namely the value at saturation density (J) and the slope of the density dependence (L), will affect observables related to the collective mode. These parameters, especially L, are still poorly constrained. The dipole polarizability, calculated as the inverse-energy-weighted sum of the transition strength, is one such observable that can be used to place limits on the symmetry energy. However, experiments investigating the IVGDR do not measure it directly, but rather the cross-section distribution over the excitation energy region, from which it is derived. The prospects of placing constraints on the symmetry energy by measuring the total Coulomb excitation cross-section were investigated in this work. The experiment was carried out with the R3B-LAND setup at the GSI Helmholzzentrum für Schwerionenforschung in Darmstadt, Germany. The dipole response of tin isotopes using beams of Sn-124, Sn-128, Sn-130, and Sn-132 was induced via Coulomb excitation in the electromagnetic field of lead target nuclei. The fragment nucleus, neutrons, and gamma-rays resulting from the de-excitation were detected, and the cross-sections for the 1-, 2-, and 3-neutron emission were determined. The measured Coulomb excitation cross-sections were tentatively compared to the ones calculated with families of Skyrme and DDME energy density functionals spanning the range from 30 to 110.8 MeV for L and 26.83 to 38 MeV for J. For the collection of functionals considered, the Coulomb excitation cross-section followed an approximately linear relationship with the polarizability. At this stage, the measurements strongly favor the functionals with lower L values (L