Collapse of Nuclear Collectivity along the N = Z Line

The lifetime of the Jπ=2⁺ state in the self-conjugate ⁸⁸Ru nucleus has been determined in an experiment performed using rare-isotope beams provided by the new Facility for Rare Isotope Beams. This is the heaviest N=Z nucleus for which such a measurement has been achieved. ⁸⁸Ru was populated by both one-neutron knockout and charge-exchange reactions, and the lifetime of 14.3-3.4+2.5 ps was determined using the triple-foil plunger technique. The extracted electromagnetic transition strength shows that the quadrupole collectivity has dropped significantly compared with the highly deformed N=Z region around A∼80. These results are compared with state-of-the-art large-scale shell-model and discrete nonorthogonal shell-model calculations. The theoretical calculations indicate a moderate triaxial deformation and suggest that low-lying states in this nucleus are no longer dominated by strong many-particle many-hole excitations, unlike the lighter, highly deformed N=Z nuclei nearby.