Structure of nuclei far from stability

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Low-energy nuclear physics is on the verge of a significant paradigm shift in the basic understanding of its objects of study. In light nuclei it is now possible to provide ab-initio predictions of nuclear structure inspired by the Quantum Chromodynamics (QCD) theory of strong interactions, breaking a decades-long reign of phenomenological effective interactions derived from global or local fits to experimental data. These methods are used for analysis of light-ion induced reactions taking into account their structure within a unified framework; this defies the traditional separation between nuclear reaction and nuclear structure theories. At TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics, sustained development in accelerators and sophisticated gamma-ray detection systems permits the study of nuclei very far from stability that tests understanding developed for stable and near-stable nuclei. A striking example is the change of nuclear shell ordering prompted by the neutron or proton excess that rearranges shell structure, often in unexpected ways, and breaks trends of observables such as spins/parities, masses, binding, and excitation energies established near stability. Configuration Interaction methods using two- and three-body interactions inspired by QCD allow the many-body nuclear problem to be solved for medium mass nuclei, including those far from stability, while density functional theories provide a further extension for investigating the validity of these interactions up to the heaviest nuclei. Current theoretical methods track errors imposed by approximations systematically, predicting trends together with uncertainties. The goal of the proposal is to examine experimentally the dependence of nuclear interactions on the proton/neutron asymmetry as a means of validating the state-of-the-art nuclear structure theories. The research plan focuses on three areas: 1) electromagnetic transition rate studies in nuclei with equal number of protons and neutrons, in particular the benchmark 68Se nucleus, and in nuclei with extreme neutron-excess in the region of rapidly changing shape deformation, in particular 94Sr, 2) structure studies of the most neutron-rich stable nuclei near the double-beta decay candidates, in particular near 130Te, 150Nd, 154Sm and 160Gd, as well as  3) studies of neutron-induced reactions in light nuclei with nearly equal proton and neutron number, in particular elastic neutron scattering on hydrogen and deuterium. Nuclear models validated based on the provided data will be used to examine nuclear shape evolution and coexistence far from stability, nuclear matrix elements of interest to the neutrino mass measurement from neutrino-less double beta decay and Fermi constant measurement from superallowed beta decay, and presence of two- and three-body effects in neutron interactions with the lightest elements.