Studies of Nuclear Structure via B(E2) Measurements in ⁷Be and ⁸Li

Light nuclei represent some of the simplest systems for study in nuclear physics. Despite this, there are structures in light nuclei, such as halo nuclei and cluster structures, that are difficult to predict with current nuclear models. One of the most promising theories making progress to explain these structures in light nuclei are ab initio calculations. These calculations require values to benchmark their results and test their predictive power. Two measurements were made to support this end, the B(E2; 3/2⁻ →1/2⁻) transition strength of ⁷Be and the B(E2; 2⁺ → 1⁺) transition strength of ⁸Li. These results were compared to a variety of ab initio calculations, as were the ratios of the ⁷Be and ⁷Li B(E2) transition strength and the ratio of the Li B(E2) transition strength and the square of the Li ground-state electric-quadrupole moment. The ratios were a new approach that did not suffer from convergence problems seen when calculating individual B(E2) values. The ab initio B(E2) ratio calculations in the A=7 nuclei were consistent with each other within 20% for a variety of interactions and ab initio approaches and all agreed with the ratio of the measured transition strengths. The ab initio calculated ratios in ⁸Li differ for different interactions in contrast to ⁷Be, and are all an order of magnitude smaller the experimental measurement. This discrepancy with experiment cannot currently be explained and highlights a need for the structure of this nucleus to be more thoroughly explored. The discrepancy among the different ab initio calculations seems related to the particular sensitivity of the 1⁺ → 2⁺ transition to the details of the orbital angular momentum and spin angular momentum of the excited state. These studies helped to determine which nuclei are good candidates for the B(E2) comparisons in this region and have helped expand our knowledge of the structure of the A=7 and A=8 nuclei.