Revealing the nuclides that shape the high-energy Daya Bay antineutrino spectrum

The Daya Bay collaboration just released their reactor antineutrino spectrum in the 8 to 11 MeV region. While the Huber model for ²³⁵U and ^239,241Pu could be valid for antineutrino energies up to 8.5 MeV, its reliability above 7 MeV rapidly declines due to high uncertainty and dispersion in the corresponding electron spectra. Thus, our only tool to study this newly-published spectrum is the summation model, which adds the antineutrino spectra of every fission product weighted by their corresponding cumulative fission yield. But since few fission products may significantly contribute to the summation model at such high energies, many of which are neutron-rich with high β^- Q-values and very short-half lives, this high-energy data is of great analytical interest. In this work, using nuclear databases available at the National Nuclear Data Center at Brookhaven National Lab, we identified the main fission products contributing to the antineutrino spectrum above 8 MeV, including a subset for which only theoretical spectra are available. We then devised a parametrization for each fission product’s decay scheme to produce the most energetic possible antineutrino spectrum for a sensitivity study, which could guide future experiments (using, for instance, the Total Absorption Spectroscopy technique).