Isovector quadrupole excitations in the valence shell of the vibrator nucleus136Ba:Evidence from photon scattering experiments

Abstract

Photon scattering experiments have been performed on the nucleus ${}^{136}$Ba with photon energies of ${E}_{\ensuremath{\gamma}}<~4.1$ MeV and ${E}_{\ensuremath{\gamma}}<~2.8$ MeV. At 2.1 MeV clear evidence for the ${2}_{\mathrm{ms}}^{+}$ state has been found. From the measured lifetime we extract signatures for the isovector quadrupole excitation in the valence shell: a weakly collective $E2$ decay to the ground state and a strong $M1$ decay to the ${2}_{1}^{+}$ state. As the resonant photon scattering with bremsstrahlung is a complete reaction, we can conclude that the ${2}^{+}$ state at 2129 keV is a rather pure ${2}_{\mathrm{ms}}^{+}$ state with little fragmentation. This is in contrast to the ${1}^{+}$ scissors state. A comparison with an IBM-2 calculation is given.