Experimental studies of proton-neutron mixed symmetry states in the mass A ∼ 130 region

Considerable progress has been achieved recently in the experimental investigation of quadrupole-collective isovector excitations in the valence shell, the so called mixed-symmetry states (MSSs), in the mass A ∼ 130 region. This is due to a new experimental technique for study MSSs which is based on the observation of low-multiplicity γ-ray events from inverse kinematics Coulomb excitation with the large 4π spectrometer, such as Gammasphere. The obtained experimental information for the MSSs of stable N 80 isotones indicates that for low-collective vibrational nuclei the underlying single-particle structure can be the most important factor for preserving or fragmenting the MSSs through the mechanism of shell stabilization. The evolution of the MSSs from 134Xe to 138Ce is also used to determine the local strength of the proton-neutron interaction derived for first time from states with symmetric and antisymmetric nature.