Decay of Neutron-Rich Mn Nuclides and Deformation of Heavy Fe Isotopes

The use of chemically selective laser ionization combined with $\beta$-delayed neutron counting at CERN/ISOLDE has permitted identification and half-life measurements for 623-ms ${}^{61}\mathrm{Mn}$ up through 14-ms ${}^{69}\mathrm{Mn}$. The measured half-lives are found to be significantly longer near $N=40\mathrm{}$ than the values calculated with a quasiparticle random-phase-approximation shell model. Gamma-ray singles and coincidence spectroscopy has been performed for ${}^{64,66}\mathrm{Mn}$ decays to levels of ${}^{64,66}\mathrm{Fe}$, revealing a significant drop in the energy of the first $2^{+}$ state in these nuclides that suggests an unanticipated increase in collectivity near $N=40\mathrm{}$.