The ground and low-lying states of neutron-rich exotic Te and Sn isotopes are studied in terms of the nuclear shell model by the same Hamiltonian used for the spherical-deformed shape phase transition of Ba isotopes, without any adjustment. An anomalously small value is obtained for $B\left(E2;0_1^{+}\to 2_1^{+}\right)$ in ${}^{136}\mathrm{Te}$, consistent with a recent experiment. The levels of ${}^{136}\mathrm{Te}$ up to yrast ${12}^{+}$ are shown to be in agreement with observed ones. It is pointed out that ${}^{136}\mathrm{Te}$ can be an exceptionally suitable case for studying mixed-symmetry $1^{+}$, $2^{+}$, and $3^{+}$ states, and predictions are made for energies and $M1$ and $E2$ properties. Systematic trends of structure of heavier and more exotic Sn and Te isotopes beyond ${}^{136}\mathrm{Te}$ are studied by the Monte Carlo shell model, presenting an unusual and very slow evolution of collectivity/deformation.

• Received 23 August 2004

DOI:https://doi.org/10.1103/PhysRevC.70.054313