Graduate Student, University of Notre Dame
Ab initio calculations are capable of describing nuclear properties in low mass nuclei from the basic building blocks of nucleons and their interactions. Different nuclear interactions and techniques are used in the ab initio framework to calculate experimental observables, which when verified by experiment, can guide these calculations. Measuring electromagnetic transition strengths can provide stringent tests of these ab initio calculations, but many are limited to low mass nuclei (A<20). In this region, the number of electromagnetic transition strength measurements for unstable nuclei is severely lacking. In order to provide more tests of ab initio calculations, Coulomb excitation experiments were performed to measure the E2 transition strengths of the first excited state transition in 7Be and 8Li. The 7Be and 8Li were produced and separated with TwinSol at the Notre Dame Nuclear Science Laboratory and the excitation cross sections were measured using γ-ray yields produced via Coulomb excitation in coincidence with the scattered nuclei. The B(E2; 3/2- → 1/2-) of 7Be and the B(E2; 2+ → 1+) of 8Li were deduced from these experiments, then compared to No-Core Shell-Model and Green’s Function Monte Carlo calculations with several different interactions. In the comparison to the ab initio calculations, it was found that taking a ratio of the transition strengths in 7Be and its mirror nucleus, 7Li, yielded a robust and converged value across all the considered ab initio calculations. Further, the ab initio calculations for this ratio showed no dependence on the choice of interaction and were in good agreement with each other and the experimental results. In a similar way, a ratio was constructed for 8Li by taking a ratio of the B(E2) with the square of the electric quadrupole moment of the 8Li ground state. However, the ab initio results for this ratio did vary with the interaction choice, not displaying the same interaction independence seen in the 7Be case, and only one interaction was in good agreement with the experimental result. This discrepancy between the two comparisons indicates a substantial difference in the structure of the first excited state in 7Be compared to 8Li. Additionally, the sensitivity of the calculated 8Li transition strength to the choice of interaction makes it useful for gaining insight into the interactions used by ab initio calculations
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