A decay spectroscopic study of the neutron-rich isotopes has been performed using fragmentation of a ${}^{86}\mathrm{Kr}$ primary beam. Fragments from this reaction have been selected by the LISE2000 spectrometer at the Grand Accélérateur National d’Ions Lourds (GANIL). Half-lives of 29 isotopes, including the first ones identified for ${}^{61}\mathrm{Ti}$ ($15\pm 4$ ms), ${}^{64}\mathrm{V}$ ($19\pm 8$ ms), and ${}^{71}\mathrm{Fe}$ ($28\pm 5$ ms), have been determined and compared with model predictions. ${}^{67,68}\mathrm{Mn}$ $\beta$-delayed $\gamma$ rays were observed for the first time. The branching for the $\beta$-delayed neutron emission was measured to be greater than $10(5)\%$ in the ${}^{67}\mathrm{Mn}$ decay. The ${}^{67}\mathrm{Fe}$ isomeric level is firmly determined at higher energy than assigned in previous works. The excitation energies of the first ($2^{+}$) and ($4^{+}$) states of ${}^{68}\mathrm{Fe}$ are suggested to lie at $522(1)$ and $1389(1)$ keV, respectively, thus bringing confirmation of assignments based on in-beam $\gamma$-ray spectroscopy. Beyond-mean-field calculations with the Gogny D1S force have been performed for even-mass nuclei through the Fe isotopic chain. Not only ${}^{68}\mathrm{Fe}$ but most of the neutron-rich Fe isotopes with neutron numbers below $N=50$ are interpreted as soft rotors. The calculated mean occupancy of the neutron $g_{9/2}$ and $d_{5/2}$ orbitals in correlated ground states is steadily growing with increasing neutron number throughout the isotopic chain. Interpretation of ${}^{67}\mathrm{Fe}$ data is based upon the present calculations for the ${}^{66}\mathrm{Fe}$ and ${}^{68}\mathrm{Fe}$ even cores.

• Received 23 April 2010

DOI:https://doi.org/10.1103/PhysRevC.83.054312