The evolution of the one-quadrupole phonon mixed-symmetry state is important for understanding the role of the quadrupole proton-neutron interaction in the valence shell. To study the evolution of the $2_{1,\mathrm{ms}}^{+}$ mixed-symmetry state in the $N=78$ isotones above $Z=50$, a Coulomb excitation measurement was performed to identify the $2_{1,\mathrm{ms}}^{+}$ state in ${}^{136}$Ce by measuring absolute transition strengths. The $2_{1,\mathrm{ms}}^{+}$ state was found to be predominantly concentrated in the $2_4^{+}$ state of this nucleus. The simple picture of shell stabilization given to account for the fragmentation of the strength observed in ${}^{138}$Ce does not seem to apply to ${}^{136}$Ce.

• Received 23 April 2012

DOI:https://doi.org/10.1103/PhysRevC.86.014303