Measurement of the Antineutrino Spectrum from U 235 Fission at HFIR with PROSPECT

This Letter reports the first measurement of the U235 νe energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9 m from the 85 MWth highly enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678±304(stat) νe-induced inverse beta decays, the largest sample from HEU fission to date, 99% of which are attributed to U235. Despite broad agreement, comparison of the Huber U235 model to the measured spectrum produces a χ2/ndf=51.4/31, driven primarily by deviations in two localized energy regions. The measured U235 spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the νe energy region of 5-7 MeV.