B(E2) measurements in light radioactive nuclei for guiding ​ab initio ​calculations

Samuel Henderson
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 ​7​Be and ​8​Li. The ​7​Be and ​8​Li 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 ​7​Be and the B(E2; 2​+​ → 1​+​) of ​8​Li 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 ​7​Be and its mirror nucleus, ​7​Li, 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 ​8​Li by taking a ratio of the B(E2) with the square of the electric quadrupole moment of the ​8​Li 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 ​7​Be 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 ​7​Be compared to ​8​Li. Additionally, the sensitivity of the calculated ​8​Li 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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