$\beta$-decay measurements for N>40 Mn nuclei and inference of collectivity for neutron-rich Fe isotopes

Expérience GANIL E243b (décembre 2003), spectromètre LISE2000; A decay spectroscopic study of the neutron-rich isotopes, has been performed using fragmentation of a 86Kr 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 61Ti (15 ± 4 ms), 64V (19 ± 8 ms), and 71Fe (28 ± 5 ms), have been determined and compared with model predictions. 67,68Mn $\beta$ -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 67Mn decay. The 67Fe isomeric level is firmly determined at higher energy than assigned in previous works. The excitation energies of the first (2+) and (4+) states of 68Fe 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 68Fe 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 g9/2 and d5/2 orbitals in correlated ground states are steadily growing with increasing neutron number throughout the isotopic chain. Interpretation of 67Fe data is based upon present calculations for the 66Fe and 68Fe even cores.