LAUSR

We calculate the inverse-$\ensuremath{\beta}$-decay (IBD) antineutrino spectrum generated by nuclear reactors using the summation method to understand deviations from the smooth Huber-Mueller model due to the decay of individual fission products, showing that plotting the ratio of two adjacent spectra points can effectively reveal these deviations. We obtain that, for binning energies of 0.1 MeV or lower, abrupt changes in the spectra due to the jagged nature of the individual antineutrino spectra could be observed for highly precise experiments. Surprisingly, our calculations also reveal a peaklike feature in the adjacent points ratio plot at 4.5 MeV even with a 0.25-MeV binning interval, which we find is present in the IBD spectrum published by the Daya Bay Collaboration in 2016. We show that this 4.5-MeV feature is caused by the contributions of just four fission products, $^{95}\mathrm{Y}, ^{98,101}\mathrm{Nb}$, and $^{102}\mathrm{Tc}$. This would be the first evidence of the decay of a few fission products in an IBD antineutrino spectrum from a nuclear reactor. This result is supported by applying the same numerical technique to the measured aggregate electron spectra.