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

Abstract

The 𝛽-decaying states of 70,72Co were studied at the National Superconducting Cyclotron Laboratory using the VANDLE neutron time-of-flight array. The (6−,7−)⁢𝛽-decaying state in 70Co 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 70Ni. 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 70Ni. 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 70Co 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.