Nuclei with magic numbers serve as important benchmarks in nuclear theory. In addition, neutronrich nuclei play an important role in the astrophysical rapid neutron-capture process (r-process). 78 Ni is the only doubly-magic nucleus that is also an important waiting point in the r-process, and serves as a major bottleneck in the synthesis of heavier elements. The half-life of 78 Ni has been experimentally deduced for the first time at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University, and was found to be 110 +100 −60 ms. In the same experiment, a first half-life was deduced for 77 Ni of 128 +27 −33 ms, and more precise half-lives were deduced for 75 Ni and 76 Ni of 344 +20 −24 ms and 238 +15 −18 ms respectively.Doubly-magic nuclei with completely filled proton and neutron shells are of fundamental interest in nuclear physics. The simplified structure of these nuclei and their direct neighbors allows one to benchmark key ingredients in nuclear structure theories such as single-particle energies and effective interactions. Doubly-magic nuclei also serve as cores for shell model calculations, dramatically truncating the model space, thus rendering feasible shell model calculations in heavy nuclei. All this is of particular importance for nuclei far from stability, where doubly-magic nuclei serve as beachheads in the unknown territory of the chart of nuclides [1,2].When considering the classic nuclear shell gaps and excluding superheavy nuclei, there are only 10 doublymagic nuclei, and only four of these are far from stability: 48 Ni, 78 Ni, 100 Sn, and 132 Sn. Of these, 48 Ni and 78 Ni are the most exotic ones, and the last ones with experimentally unknown properties. 78 Ni therefore represents a unique stepping stone towards the physics of extremely neutron-rich nuclei. In a pioneering experiment, Engelmann et al. Very neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process (r-process) [5,6]. The r-process is responsible for the origin of about half of the heavy elements beyond iron in nature, yet its site and exact mechanism are still unknown. 78 Ni is the only doubly-magic nucleus that represents an important waiting point in the path of the r-process, where the reaction sequence halts to wait for the decay of the nucleus [7].One popular astrophysical site for the r-process is the neutrino driven wind off a hot, newborn neutron star in a core-collapse supernova explosion [8]. In this case the rprocess begins around mass number A = 90, with lighter nuclei being produced as less neutron-rich species in an α-rich freeze-out. For such a scenario 78 Ni would not be directly relevant. However, the α-rich freezeout fails to accurately reproduce the observed abundances for nuclei with A = 80−90 [9], and the associated r-process does not produce sufficient amounts of the heaviest r-process nuclei around A =195 [10].78 Ni is among the important r-process waiting points in models that try to address these issues. Examples ...

The β-decays of very neutron rich nuclides in the Co-Zn region were studied experimentally at the National Superconducting Cyclotron Laboratory using the NSCL β-counting station in conjunction with the neutron detector NERO. We measured the branchings for β-delayed neutron emission (Pn values) for 74 Co (18±15%), and 75−77 Ni (10±2.8%, 14±3.6%, and 30±24%, respectively) for the first time, and remeasured the Pn values of 77−79 Cu, 79,81 Zn, and 82 Ga. For 77−79 Cu and for 81 Zn we obtain significantly larger Pn 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 Co (30±11 ms) and 80 Cu (170 +110 −50 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 rprocess 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.

The neutron long counter NERO was built at the National Superconducting Cyclotron Laboratory (NSCL), Michigan State University, for measuring β-delayed neutron-emission probabilities. The detector was designed to work * Corresponding author. Tel.: +1 517 9087428; fax: +1 517 3535967Email address: pereira@nscl.msu.edu (J. Pereira) 1 Present address: Navy Nuclear Power School, Goose Creek, South Carolina, USA 2 Present address: Los Alamos National Laboratory, MS E540, Los Alamos, New Mexico, USA Preprint submitted to ElsevierMay 28, 2018 in conjunction with a β-decay implantation station, so that β decays and β-delayed neutrons emitted from implanted nuclei can be measured simultaneously. The high efficiency of about 40%, for the range of energies of interest, along with the small background, are crucial for measuring β-delayed neutron emission branchings for neutron-rich r-process nuclei produced as low intensity fragmentation beams in in-flight separator facilities.