This is an exciting time for the study of r-process nucleosynthesis. Recently, a neutron star merger GW170817 was observed in extraordinary detail with gravitational waves and electromagnetic radiation from radio to γ rays. The very red color of the associated kilonova suggests that neutron star mergers are an important r-process site. Astrophysical simulations of neutron star mergers and core collapse supernovae are making rapid progress. Detection of both, electron neutrinos and antineutrinos from the next galactic supernova will constrain the composition of neutrino-driven winds and provide unique nucleosynthesis information. Finally FRIB and other rare-isotope beam facilities will soon have dramatic new capabilities to synthesize many neutron-rich nuclei that are involved in the r-process. The new capabilities can significantly improve our understanding of the r-process and likely resolve one of the main outstanding problems in classical nuclear astrophysics.However, to make best use of the new experimental capabilities and to fully interpret the results, a great deal of infrastructure is needed in many related areas of astrophysics, astronomy, and nuclear theory. We will place these experiments in context by discussing astrophysical simulations and observations of r-process sites, observations of stellar abundances, galactic chemical evolution, and nuclear theory for the structure and reactions of very neutron-rich nuclei. This review paper was initiated at a three-week International Collaborations in Nuclear Theory program in June 2016 where we explored promising r-process experiments and discussed their likely impact, and their astrophysical, astronomical, and nuclear theory context.

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 ...

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The complete three-body correlation pictures are experimentally reconstructed for the two-proton decays of the 6 Be and 45 Fe ground states. We are able to see qualitative similarities and differences between these decays. They demonstrate very good agreement with the predictions of a theoretical three-body cluster model. Validity of the theoretical methods for treatment of the three-body Coulombic decays of this class is thus established by the broad range of lifetimes and nuclear masses spanned by these cases. Implementations for decay dynamics and nuclear structure of 2p emitters are discussed.

By studying the (109)Xe→(105)Te→(101)Sn superallowed α-decay chain, we observe low-lying states in (101)Sn, the one-neutron system outside doubly magic (100)Sn. We find that the spins of the ground state (J=7/2) and first excited state (J=5/2) in (101)Sn are reversed with respect to the traditional level ordering postulated for (103)Sn and the heavier tin isotopes. Through simple arguments and state-of-the-art shell-model calculations we explain this unexpected switch in terms of a transition from the single-particle regime to the collective mode in which orbital-dependent pairing correlations dominate.

The internal-conversion and internal-pair-production decays of the first excited 0 + state in 68 Ni are studied following the β decay of 68 Co. A novel experimental technique, in which the ions of 68 Co were implanted into a planar germanium double-sided strip detector and which required digital pulse processing, is developed. The values for the energy of the first excited 0 + state and the electric monopole transition strength from the first excited 0 + state to the ground state in 68 Ni are determined to be 1605(3) keV and 7.6(4) × 10 −3 , respectively. Comparisons of the experimental results to Monte Carlo shell-model calculations suggest the coexistence between a spherical ground state and an oblate first excited 0 + state in 68 Ni.

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.

Two new alpha emitters 109Xe and 105Te were identified through the observation of the 109Xe --> 105Te --> 101Sn alpha-decay chain. The 109Xe nuclei were produced in the fusion-evaporation reaction 54Fe(58Ni,3n)109Xe and studied using the Recoil Mass Spectrometer at the Holifield Radioactive Ion Beam Facility. Two transitions at Ealpha = 4062 +/- 7 keV and Ealpha = 3918 +/- 9 keV were interpreted as the l = 2 and l = 0 transitions from the 7/2+ ground state in 109Xe (T1/2 = 13 +/- 2 ms) to the 5/2+ ground state and a 7/2+ excited state, located at 150 +/- 13 keV in 105Te. The observation of the subsequent decay of 105Te marks the discovery of the lightest known alpha-decaying nucleus. The measured transition energy Ealpha = 4703 +/- 5 keV and half-life T1/2 = 620 +/- 70 ns were used to determine the reduced alpha-decay width delta2. The ratio delta105Te(2)/delta213Po(2) of approximately 3 indicates a superallowed character of the alpha emission from 105Te.