Structure of high-lying levels populated in the $^{96}$Y $rightarrow ^{96}$Zr $beta$ decay

The nature of $J^{pi}=1^-$ levels of $^{96}$Zr below the $beta$-decay $Q_{beta}$ value of $^{96}$Y has been investigated in high-resolution $gamma$-ray spectroscopy following the $beta$ decay as well as in a campaign of inelastic photon scattering experiments. Branching ratios extracted from $beta$ decay allow the absolute $E1$ excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of $1^-$ levels below the $Q_{beta}$ value, which are investigated in the theoretical approach of the Quasiparticle Phonon Model. This study clarifies the nuclear structure properties associated with the enhanced population of high-lying levels in the $^{96}$Y$_{gs}$ $beta$ decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum.