An isomeric state in ${}^{255}\mathrm{Lr}$ with a half-life of $t_{1/2}\hspace{0.5em}=\hspace{0.5em}1.4(1)$ ms and $E_x>720$-keV has been observed for the first time using the GABRIELA setup at the focal plane of the VASSILISSA separator. Based on its $K$-forbiddeness, the configuration of the state is most probably formed by coupling the valence proton to a two quasiparticle neutron excitation. Possible three quasiparticle configurations are discussed.

• Received 31 December 2007

DOI:https://doi.org/10.1103/PhysRevC.78.021302