Mixed symmetry in the vibrational nucleus 142Ce.

Low-lying levels of the nucleus $^{142}\mathrm{Ce}$ have been studied by Coulomb excitation with $^{4}\mathrm{He}$, $^{12}\mathrm{C}$, and $^{16}\mathrm{O}$ projectiles to determine various $B(E\ensuremath{\lambda})$ values and the static quadrupole moment $Q({2}_{1}^{+})$. Excellent agreement is found between the observed properties and predictions made using the U(5) limit of the proton-neutron interacting boson model. The results strongly support the contention that the ${2}_{3}^{+}$, 2.004-MeV level has mixed proton-neutron symmetry. Combining the present measurement for $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{3}^{+})$ of $0.070\ifmmode\pm\else\textpm\fi{}0.011 {e}^{2}{b}^{2}$ with known branching and mixing ratios gives a substantial $M1$ strength of $(0.26\ifmmode\pm\else\textpm\fi{}0.05){\ensuremath{\mu}}_{N}^{2}$ for the ${2}_{3}^{+}$ \ensuremath{\rightarrow} ${2}_{1}^{+}$ transition.