The mixed symmetry $2^{+}$ states in the $N=84$ isotones ${}^{142}\mathrm{Ce}$ and ${}^{144}\mathrm{Nd}$ are investigated within the quasiparticle-phonon model, with special attention to their $M1$ decay to the lowest symmetric $2^{+}$ state. The observed pronounced splitting of the $M1$ strength is shown to be caused by the phonon coupling, made quite effective by the peculiar shell structure of the two-valence neutrons above the $N=82$ shell closure.

• Received 28 May 2009

DOI:https://doi.org/10.1103/PhysRevC.80.024311