Isoscalar Dipole Excitation in Tin Isotopes

The investigation of the nuclear structure in stable and unstable nuclei revealed a concentration of dipole strength well below the Giant Dipole Resonance. In the low-lying dipole excitation a separation of the isoscalar and isovector part occurred. One of the questions to answer is the evolution of the excitation mode with increasing neutronexcess. The mode is probed with the tin isotopes Sn-128 and Sn-132 with inelastic isoscalar excitation in inverse kinematics. The experiment was performed at RIKEN RIBF in Japan, Wako. With the aid of the fragment separator BigRIPS and the spectrometer ZeroDegree Spectrometer were the reaction channel Sn(α ,α’γ ) selected. The target chamber CRYPTA embodied liquid helium as the isoscalar target. It was surrounded by two scintillating crystal arrays. They consists out of NaJ crystals (DALI2) and large volume LaBr3:Ce crystals (HECTOR) placed in forward direction. The detected γ-rays contain the information of the excitation energy. For both isotopes known transition energies, mainly decays of 2+ states to the ground state 0+ , were measured and their peak-centroid and cross section determined. The measured transition energies match the literature values. In addition, for both isotopes two new transition energies and their cross section were determined. The tran- sition energies in Sn-128 are 5.45(15) MeV and 6.35(15) MeV with the cross sections 112(29) μb and 79(29) μb, respectively, and in Sn-132 they are 6.25(10) MeV and 7.10(10) MeV with the cross sections 3.82(30) mb and 0.258(16) mb, respectively. The nature of the transitions is not clarified and theoretical alculations are performed. It is remarkable that no transition strength was bserved above the neutron separation-threshold. Under the assumption that the transitions are E1-type, the extracted results are com- pared to theoretical calculations performed for isoscalar low-lying dipole excitation using Quasiparticle Random Phase Approximation. Within the uncertainties they agree with each other. Furthermore, the results are compared to other experiments performed with stable isotopes and isoscalar probes. A clear trend of the excitation energy with the asymmetry N−Z was not observed. In addition the result for Sn-132 is compared to the isovector mode in the same isotope. The excitation energy is about 9.8(7) MeV. If the low-lying dipole mode was observed in the experiments, then the low-lying dipole modes separates in energy the isoscalar and isovector part