A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus 42 Si-going beyond earlier comparisons of excited-state energies-is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying 42 Si(2 + 1) energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich 42 Si with a one-proton removal reaction from 43 P projectiles at 81 MeV/nucleon. The measured cross sections to the individual 42 Si final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited 0 + states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13) keV in 42 Si is proposed to be the (0 + 2) level.

Background: Selenium and germanium nuclei are associated with both triaxiality and shape coexistence. The relative influence of these deformation effects on the low-lying nuclear structure remains the subject of much discussion, with additional attention drawn to 76 Se and 76 Ge due to the potential for the observation of neutrinoless double-beta decay.Purpose: Experimental observables related to the deformation of 76 Se are lacking in precision. The purpose of the present work is to provide electric quadrupole matrix elements with improved precision in order to determine the deformation of low-lying states in a model-independent manner.Methods: Sub-barrier Coulomb excitation was employed at the reaccelerated beam facility of the National Superconducting Cyclotron Laboratory using the JANUS setup. Using this method nineteen E2 matrix elements were extracted.Results: Extracted matrix elements agree within uncertainties with those in the literature but with improved precision. Through both a comparison with geometric models and a model-independent evaluation of E2 matrix elements using rotational invariants the ground state of 76 Se is best described as having a significant triaxial component, while not being maximally triaxially deformed.Conclusions: Selenium-76 exhibits a significant degree of triaxiality in its ground state. A detailed comparison with configuration interaction calculations indicates that this can be well reproduced theoretically.

The lifetimes of the first excited 2 þ states in the N ¼ Z nuclei 80 Zr, 78 Y, and 76 Sr have been measured using the γ-ray line shape method following population via nucleon-knockout reactions from intermediateenergy rare-isotope beams. The extracted reduced electromagnetic transition strengths yield new information on where the collectivity is maximized and provide evidence for a significant, and as yet unexplained, odd-odd vs even-even staggering in the observed values. The experimental results are analyzed in the context of state-of-the-art nuclear density-functional model calculations.