Decay of a microsecond seniority 3 isomeric state in Hf155

Excited states in the neutron-deficient nuclide ${}^{155}\mathrm{Hf}$ have been investigated in experiments performed at the Accelerator Laboratory of the University of Jyväskylä. The ${}^{155}\mathrm{Hf}$ nuclei were produced in fusion-evaporation reactions induced by beams of 295 and 315 MeV ${}^{58}\mathrm{Ni}$ ions bombarding an isotopically enriched ${}^{102}\mathrm{Pd}$ target and separated using the recoil mass separator MARA. An isomeric state having a half-life of 510(30) ns was discovered and is interpreted as a seniority $\upsilon =3, (\pi h_{11/2}^2\otimes \nu f_{7/2})27/2^{-}$ configuration. The $\gamma$-ray transitions emitted in the deexcitation of the isomeric state to the ground state were identified and a level scheme was constructed, from which the excitation energy of the isomer was determined to be 2581.5(10) keV. A $B\left(E2\right)$ value of 0.45(3) W.u. was deduced for the 105.4 keV transition depopulating the isomeric state. The deduced level scheme and $B\left(E2\right)$ value are compared with systematics and shell-model calculations.