β-delayed neutron spectroscopy of Co70,72 ground-state and isomeric-state decays

The β-decaying states of Co70,72 were studied at the National Superconducting Cyclotron Laboratory using the VANDLE neutron time-of-flight array. The (6-,7-)β-decaying state in Co70 is near-spherical with a lifetime of 113±7 ms, and the low-spin (1+,2+)β-decaying state is postulated to be the prolate deformed ground state with a lifetime of 508±7 ms. Both decay predominantly to the bound states of Ni70. For the first time neutron-emissions from neutron unbound states from both the (6-,7-) and (1+,2+)β decays were measured. Even with the low statistics data, we were able to disentangle the neutron emission from both decays, which enabled a determination of β-decay strength above the neutron separation energy of Ni70. Neutron emission probabilities were measured to be 7.1±1.5% and 9.4±1.7%, respectively, for the (6-,7-) and (1+,2+) decays. The decay pattern of the Co70 is driven by neutron f5/2 to proton f7/2 Gamow-Teller transformation. The observed population of neutron unbound states is attributed to the conversion of p1/2 and p3/2 neutrons to p3/2 and p1/2 protons excited across the Z=28 closed shell.