Excited states in ${}^{138}\mathrm{Ce}$ have been studied via the ${}^{12}\mathrm{C}({}^{138}\mathrm{Ce},{}^{138}\mathrm{Ce}^{*})$ Coulomb excitation reaction at 480 MeV. Relative cross sections have been determined from the $\gamma$-ray yields observed with Gammasphere. The $E2$ and $M1$ strength distributions between the lowest six $2^{+}$ states up to 2.7 MeV enables us to identify the $2_4^{+}$ state in ${}^{138}\mathrm{Ce}$ as the dominant fragment of the one-phonon $2_{1,\mathrm{ms}}^{+}$ mixed-symmetry state. Mixing between this level and a nearby isoscalar state is observed and is more than 4 times larger than in the neighboring isotone ${}^{136}\mathrm{Ba}$. This is direct evidence that the stability of mixed-symmetry states strongly depends on the underlying subshell structure.

• Received 27 November 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.122501