One-proton and one-neutron knockout reactions from N=Z=28Ni56 to the A=55 mirror pair Co55 and Ni55

Author(s): Spieker, M; Gade, A; Weisshaar, D; Brown, BA; Tostevin, JA; Longfellow, B; Adrich, P; Bazin, D; Bentley, MA; Brown, JR; Campbell, CM; Diget, C Aa; Elman, B; Glasmacher, T; Hill, M; Pritychenko, B; Ratkiewicz, A; Rhodes, D | Abstract: © 2019 American Physical Society. We present a high-resolution in-beam γ-ray spectroscopy study of excited states in the mirror nuclei Co55 and Ni55 following one-nucleon knockout from a projectile beam of Ni56. The newly determined partial cross sections and the γ-decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. The new experimental data are compared to large-scale shell-model calculations in the full pf space which include charge-dependent contributions. A mirror asymmetry for the partial cross sections leading to the two lowest 3/2- states in the A=55 mirror pair was identified as well as a significant difference in the E1 decays from the 1/21+ state to the same two 3/2- states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the E1 decay pattern, however, points at stronger mixing between the two lowest 3/2- states in Co55 than in its mirror Ni55.

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