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 ${}^{74}\mathrm{Co}$ ($18\pm 15\%$) and ${}^{75-77}\mathrm{Ni}$ ($10\pm 2.8\%$, $14\pm 3.6\%$, and $30\pm 24\%$, respectively) for the first time, and remeasured the $P_n$ values of ${}^{77-79}\mathrm{Cu}$, ${}^{79,81}\mathrm{Zn}$, and ${}^{82}\mathrm{Ga}$. For ${}^{77-79}\mathrm{Cu}$ and for ${}^{81}\mathrm{Zn}$ 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 ${}^{75}\mathrm{Co}$ ($30\pm 11$ ms) and ${}^{80}\mathrm{Cu}$ (${170}_{-50}^{+110}$ 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.

• Received 8 March 2010

DOI:https://doi.org/10.1103/PhysRevC.82.025806