Effects of Recent Fission Product Yield Measurements on Reactor Antineutrino Spectra

The Daya Bay, RENO, and Double Chooz collaborations have all made precision measurements of the reactor antineutrino spectra. When compared to reactor models, these measurements show a ≈5% smaller flux than anticipated, as well as a difference in the spectral shape, referred to as the “5-MeV bump.” These two deviations comprise the “reactor antineutrino anomaly.” Reactor antineutrino spectra are calculated using two techniques: the conversion method, which relies on converting measured fission β-decay spectra into antineutrino spectra, and the summation method, which sums the hundreds of β decays present in a reactor weighted by their cumulative fission product yields (FPYs). The summation approach gives powerful insights into the individual contributions of the antineutrino spectra, but relies on accurate nuclear decay data and FPYs. The TUNL-LLNL-LANL collaboration employed a fast target transfer system to measure cumulative FPYs of the actinides ²³⁵U, ²³⁸U, and ²³⁹Pu, with half-lives ranging from 0.5 s to 100 s, from neutron-induced fission at 2 MeV. The cyclic activation technique is used with a variety of irradiation and counting times. Using γ-ray intensity values from the literature, we determined approximately 50 short-lived cumulative FPYs for each of the three actinides. These include most of the key fission products expected to contribute to the “5-MeV bump.” Our newly measured FPY data are compared with the presently accepted evaluations ENDF/B-VII and JEFF-3.3, then substituted into summation method models to produce updated predictions of the reactor antineutrino spectra. In general, good agreement with the evaluations are found; however, differences in some cumulative FPYs are found to play a key role. This talk will give an overview of the reactor antineutrino anomaly, the measurements performed at TUNL, and their impact to reactor antineutrino spectra.