Nature of seniority symmetry breaking in the semimagic nucleus ⁹⁴Ru

Direct lifetime measurements via γ-γ coincidences using a fast timing detector array consisting of LaBr₃(Ce) scintillators has been applied to determine the lifetime of low-lying states in the semimagic (N=50) nucleus ⁹⁴Ru. The experiment was carried out as the first in a series of “FAIR-0” experiments with the DESPEC experimental setup at the Facility for Antiproton and Ion Research (FAIR). Excited states in ⁹⁴Ru were populated primarily via the β-delayed proton emission of ⁹⁵Pd nuclei, produced in the projectile fragmentation of an 850 MeV/nucleon ¹²⁴Xe beam impinging on a 4 g/cm² ⁹Be target. While the deduced E2 strength for the 2⁺→0⁺ transition in the yrast cascade follows the expected behavior for conserved seniority symmetry, the intermediate 4⁺→2⁺ transition exhibits a drastic enhancement of transition strength in comparison with pure-seniority model predictions as well as standard shell model predictions in the fpg proton hole space with respect to doubly magic ¹⁰⁰Sn. The anomalous behavior is ascribed to a subtle interference between the wave function of the lowest seniority ν=2, Iπ=4⁺ state and that of a close-lying ν=4 state that exhibits partial dynamic symmetry. In addition, the observed strongly prohibitive 6⁺→4⁺ transition can be attributed to the same mechanism but with a destructive interference. It is noted that such effects may provide stringent tests of the nucleon-nucleon interactions employed in state-of-the-art theoretical model calculations