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

Abstract

This Letter reports the first measurement of the 235U ¯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 235U. Despite broad agreement, comparison of the Huber 235U model to the measured spectrum produces a χ2/ndf=51.4/31, driven primarily by deviations in two localized energy regions. The measured 235U 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.