A multinucleon transfer reaction between a thin self-supporting ${}_{\mathrm{\hspace{0.5em}78}}{}^{198}\mathrm{Pt}$ target and an $850\text{MeV}$ ${}_{\mathrm{\hspace{0.5em}54}}{}^{136}\mathrm{Xe}$ beam has been used to populate and study the structure of the $N=80$ isotone ${}_{\mathrm{\hspace{0.5em}56}}{}^{136}\mathrm{Ba}$. Making use of time-correlated $\gamma$-ray spectroscopy, evidence for an $I^{\pi }=\left({10}^{+}\right)$ isomeric state has been found with a measured half-life of $91\pm 2\text{ns}$. Prompt-delayed correlations have also enabled the tentative measurement of the near-yrast states which lie above the isomer. Shell-model calculations suggest that the isomer has a structure which can be assigned predominantly as ${\left(\nu h_{11∕2}\right)}_{{10}^{+}}^{-2}$. The results are discussed in terms of standard and pair-truncated shell-model calculations, and compared to the even-$Z$ $N=80$ isotones ranging from ${}_{\mathrm{\hspace{0.5em}50}}{}^{130}\mathrm{Sn}$ to ${}_{\mathrm{\hspace{0.5em}68}}{}^{148}\mathrm{Er}$. A qualitative explanation of the observed dramatic decrease in the $B(E2:{10}^{+}\to 8^{+})$ value for the $N=80$ isotones at ${}^{136}\mathrm{Ba}$ is given in terms of the increasing single-hole energy of the $h_{11∕2}$ neutron configuration as the proton subshell is filled. The angular momentum transfer to the binary fragments in the reaction has also been investigated in terms of the average total $\gamma$-ray fold versus the scattering angle of the recoils.

• Received 26 September 2003

DOI:https://doi.org/10.1103/PhysRevC.69.024316