Electromagnentic probe of nuclear interactions

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Low-energy nuclear physics recently underwent a significant paradigm shift in the basic understanding of its objects of study. In light and medium mass 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. At TRIUMF, Canada's particle accelerator centre, sustained development in accelerators and gamma-ray detection systems permits the study of nuclei which are very far from stability, testing theories and understanding developed based on data obtained near stability.  A striking example is the change of nuclear shell ordering prompted by the neutron or proton excess that rearranges shell structure 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 and heavy mass nuclei, including those far from stability, while density functional theories provide an extension for application of these interactions up to the heaviest nuclei. Current theoretical methods track approximation errors systematically, predicting trends together with uncertainties, thus enabling direct tests of QCD-inspired effective interactions through comparison to the experimental data. The long-term goal of the proposed program is to improve quantitative understanding of the interactions between nucleons (protons and neutrons) inside atomic nuclei. The short-term objective is to examine experimentally the dependence of nuclear interactions on the asymmetry between the proton number (Z) and the neutron number (N). In particular, it is proposed to execute a program of precision measurements of electromagnetic transition rates using heavy radioactive beams provided by TRIUMF while taking advantage of developments of the experimental infrastructure at Simon Fraser University (SFU). Experiments awaiting execution at TRIUMF are measurements of electromagnetic transition rates in 55Co and 55Ni which are proton- and a neutron-hole nuclei with respect to closed-shell 56Ni, and in 20Mg, the most neutron deficient isotope at Z=12. It is also proposed to study products of reactions induced by fast neutrons, as well as, fission fragments and dynamics, taking advantage of the neutron generator and development of gamma-ray spectroscopy techniques at SFU. In particular, it is proposed to deploy the 8p spectrometer in measurements of energy and angular momenta distributions of fission fragments. The program will benefit researchers in nuclear science providing information crucial for validation of state-of-the-art nuclear structure and fission theories while providing excellent training of Highly Qualified Personnel which is of direct benefit to Canada.