Structure of high-lying levels populated in the ⁹⁶Y →⁹⁶Zr β decay

The nature of the high-lying final levels of the ⁹⁶Y_gs β decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum, has been investigated in high-resolution γ-ray spectroscopy following the β decay as well as in a campaign of inelastic photon scattering experiments. The comprehensive approach establishes 1⁻ levels associated with the Pygmy Dipole Resonance as high-lying final levels in the β decay. Branching ratios extracted from β decay complement photon scattering and 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 the 1⁻ levels below the Q_β value, which are investigated in the Quasiparticle Phonon Model. The calculations reveal that the components populated in β decay contribute only with small amplitudes to the complex wavefunction of these 1⁻ levels. A comparison of the β decay results to data from total absorption γ-ray spectroscopy demonstrates a good agreement between both measurements.