The nuclear structure of neutron-rich $N>126$ nuclei has been investigated following their production via relativistic projectile fragmentation of a $E/A=1$ GeV ${}^{238}\mathrm{U}$ beam. Metastable states in the $N=128$ isotones ${}^{208}\mathrm{Hg}$ and ${}^{209}\mathrm{Tl}$ have been identified. Delayed $\gamma$-ray transitions are interpreted as arising from the decay of $I^{\pi }=(8^{+})$ and ($17/2^{+}$) isomers, respectively. The data allow for the so far most comprehensive verification of the shell-model approach in the region determined by magic numbers $Z<82$ and $N>126$.

• Received 1 October 2009

DOI:https://doi.org/10.1103/PhysRevC.80.061302