A new collective excitation: The F-spin-vector, gamma-vibration

We apply the Tamm-Dancoff approximation in order to investigate vibrational excitations in the F-spin-symmetric O(6) limit of the proton-neutron interacting boson model. In addition to the well known K=1 scissors mode, we find a second F-spin-vector, K=2, vibrational mode. The K=2 surface distortions of the proton and neutron fluids, respectively, oscillate against each other. Since the proton-neutron interaction restrains this motion, the new collective mode is expected to lie at sizeable excitation energies of about 2 MeV, as opposed to the soft, in-phase γ-motion of the fluids, which is a spurious, zero-energy mode. (orig.)