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The delicate balance between like-nucleon (proton-proton and neutron-neutron, pp and nn and proton–neutron pn pairing correlations plays a crucial role in shaping the structure, stability, and collective behavior of atomic nuclei. Proton--neutron interactions are particularly strong when both nucleons occupy the same orbitals, owing to the large overlap of their spatial wave functions, having a mayor a major impact on nuclei with $N\!\approx\!Z$. Although reduced in neutron-rich systems, these correlations can be microscopically enhanced in regions close to doubly magic nuclei
High-spin nuclear states provide a sensitive probe $pn$ interaction . At high angular momentum, the Coriolis force breaks nucleon pairs, promoting proton and neutron alignment. Signatures of such strong $pn$ interaction are isomeric $\beta$-decaying states in $N = Z$ nuclei that arise from fully aligned configurations of a proton pair and a neutron pair, such as the 12$^+$ ($\nu 0f_{7/2}^{-2}\times\pi0f_{7/2}^{-2}$) state in $^{52}$Fe~ and the 16$^+$ ($\nu0h_{9/2}^{-2}\times\pi0g_{9/2}^{-2}$) isomer in $^{96}$Cd.
In this contribution, we present the results of a high-statistics $\beta$-decay experiment conducted at the ISOLDE facility at CERN, utilizing pure Cd beams. Strong evidence for a $\beta$-decaying high-spin isomer has been identified in $^{128}$Cd, the two-neutron–two-proton hole nucleus relative to doubly magic $^{132}$Sn. Based on the experimental information and the comparison to calculations, the state is assigned as the so-far unobserved $18^+$ spin-gap isomer in $^{128}$Cd arising from the $\nu(0h_{11/2})^{-2}\pi(0g_{9/2})^{-2}$. An interpretation of the $^{128}$Cd isomeric state will be provided within the framework of symmetry-conserving configuration-mixing calculations as well as shell-model calculations.