The level structures of the very neutron-rich nuclei ${}^{128}\mathrm{Pd}$ and ${}^{126}\mathrm{Pd}$ have been investigated for the first time. In the $r$-process waiting-point nucleus ${}^{128}\mathrm{Pd}$, a new isomer with a half-life of $5.8(8)\mu \mathrm{s}$ is proposed to have a spin and parity of $8^{+}$ and is associated with a maximally aligned configuration arising from the $g_{9/2}$ proton subshell with seniority $\upsilon =2$. For ${}^{126}\mathrm{Pd}$, two new isomers have been identified with half-lives of 0.33(4) and $0.44(3)\mu \mathrm{s}$. The yrast $2^{+}$ energy is much higher in ${}^{128}\mathrm{Pd}$ than in ${}^{126}\mathrm{Pd}$, while the level sequence below the $8^{+}$ isomer in ${}^{128}\mathrm{Pd}$ is similar to that in the $N=82$ isotone ${}^{130}\mathrm{Cd}$. The electric quadrupole transition that depopulates the $8^{+}$ isomer in ${}^{128}\mathrm{Pd}$ is more hindered than the corresponding transition in ${}^{130}\mathrm{Cd}$, as expected in the seniority scheme for a semimagic, spherical nucleus. These experimental findings indicate that the shell closure at the neutron number $N=82$ is fairly robust in the neutron-rich Pd isotopes.

• Received 9 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.152501