Joint Determination of Reactor Antineutrino Spectra from ²³⁵U and ²³⁹Pu Fission by Daya Bay and PROSPECT

A joint determination of the reactor antineutrino spectra resulting from the fission of ²³⁵U and ²³⁹Pu has been carried out by the Daya Bay and PROSPECT Collaborations. This Letter reports the level of consistency of ²³⁵U spectrum measurements from the two experiments and presents new results from a joint analysis of both data sets. The measurements are found to be consistent. The combined analysis reduces the degeneracy between the dominant ²³⁵U and ²³⁹Pu isotopes and improves the uncertainty of the ²³⁵U spectral shape to about 3%. The ²³⁵U and ²³⁹Pu antineutrino energy spectra are unfolded from the jointly deconvolved reactor spectra using the Wiener-SVD unfolding method, providing a data-based reference for other reactor antineutrino experiments and other applications. This is the first measurement of the ²³⁵U and ²³⁹Pu spectra based on the combination of experiments at low- and highly enriched uranium reactors.