Half-lives and branchings for β-delayed neutron emission for neutron-rich Co–Cu isotopes in the r-process

The beta decays of very neutron-rich nuclides in the Co-Zn region were studied experimentally at the National Superconducting Cyclotron Laboratory using the NSCL beta-counting station in conjunction with the neutron detector NERO. We measured the branchings for beta-delayed neutron emission (P-n values) for Co-74 (18 +/- 15%) and Ni75-77 (10 +/- 2.8%, 14 +/- 3.6%, and 30 +/- 24%, respectively) for the first time, and remeasured the P-n values of Cu77-79, Zn-79,Zn-81, and Ga-82. For Cu77-79 and for Zn-81 we obtain significantly larger P-n values compared to previous work. While the new half-lives for the Ni isotopes from this experiment had been reported before, we present here in addition the first half-life measurements of Co-75 (30 +/- 11 ms) and Cu-80 (170(-50)(+110) ms). Our results are compared with theoretical predictions, and their impact on various types of models for the astrophysical rapid neutron-capture process (r-process) is explored. We find that with our new data, the classical r-process model is better able to reproduce the A = 78-80 abundance pattern inferred from the solar abundances. The new data also influence r-process models based on the neutrino-driven high-entropy winds in core collapse supernovae.