High-spin states populated in the decay of microsecond isomers in the transitional nuclei ${}^{121}\mathrm{Sb}$ and ${}^{123}\mathrm{Sb}$ have been investigated in detail in several experiments using $\gamma$-ray and electron spectroscopy. The nuclei were formed using multinucleon transfer and fusion-fission reactions with ${}^{136}\mathrm{Xe}$ beams and also using the ${}^{120}\mathrm{Sn}({}^7\mathrm{Li},\alpha 2n){}^{121}\mathrm{Sb}$ and ${}^{122}\mathrm{Sn}({}^7\mathrm{Li},\alpha 2n){}^{123}\mathrm{Sb}$ incomplete-fusion reactions. Isomeric half-lives ranging from several nanoseconds to a few hundred microseconds were determined by means of conventional decay curve analyses, whereas very short-lived isomers ($T_{1/2}~1$ ns) were identified using the generalized centroid-shift method. A number of new transitions were observed, including a branch through spherical states from the $19/2^{+}$ member of the $9/2^{+}$ deformed band in ${}^{121}\mathrm{Sb}$, in competition with the main decay path through the rotational band. This is attributed to mixing between the $19/2^{+}$ band member and a $19/2^{+}$ spherical state. Both levels are predicted to coincide approximately in energy in ${}^{121}\mathrm{Sb}$. The fact that a $25/2^{+}$ isomer occurs for $A=121$ and the lighter isotopes, while a $23/2^{+}$ isomer is observed for $A=123\text{−}131$ is explained through a multistate mixing calculation, taking into account the gradual shift of the $2d_{5/2}$ and $1g_{7/2}$ proton orbitals and the change in proton-neutron effective interactions from an attractive particle-particle type in the lower part of the shell to a repulsive particle-hole type with increasing the neutron number toward the $N=82$ shell closure. The observed enhancement of the $B(E2;19/2^{-}\to 15/2^{-})$ values in ${}^{121}\mathrm{Sb}$ and ${}^{123}\mathrm{Sb}$ over the $B(E2;7^{-}\to 5^{-})$ values in the corresponding Sn cores is discussed in terms of configuration mixing between spherical and deformed states.

• Received 17 November 2008

DOI:https://doi.org/10.1103/PhysRevC.79.024306