β-Decay measurements for N>40 Mn nuclei and inference of collectivity for neutron-rich Fe isotopes

A decay spectroscopic study of the neutron-rich isotopes has been performed using fragmentation of a Kr86 primary beam. Fragments from this reaction have been selected by the LISE2000 spectrometer at the Grand Accélé rateur National d'Ions Lourds (GANIL). Half-lives of 29 isotopes, including the first ones identified for Ti61 (15±4 ms), V64 (19±8 ms), and Fe71 (28±5 ms), have been determined and compared with model predictions. Mn67,68 β-delayed γ rays were observed for the first time. The branching for the β-delayed neutron emission was measured to be greater than 10(5)% in the Mn67 decay. The Fe67 isomeric level is firmly determined at higher energy than assigned in previous works. The excitation energies of the first (2⁺) and (4⁺) states of Fe68 are suggested to lie at 522(1) and 1389(1) keV, respectively, thus bringing confirmation of assignments based on in-beam γ-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 Fe68 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 Fe67 data is based upon the present calculations for the Fe66 and Fe68 even cores.