Measurement of the Antineutrino Spectrum from ${}^{235}$U Fission at HFIR with PROSPECT

Abstract

This Letter reports the first measurement of the ${}^{235}$U $\overline{\hat{I}{½}_e}$ energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9m from the 85MW${}_{\mathrm{th}}$ highly-enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678$\pm$304 (stat.) $\overline{\hat{I}{½}_e}$-induced inverse beta decays (IBD), the largest sample from HEU fission to date, 99% of which are attributed to ${}^{235}$U. Despite broad agreement, comparison of the Huber ${}^{235}$U model to the measured spectrum produces a $\ddot{I}{‡}^2/ndf=51.4/31$, driven primarily by deviations in two localized energy regions. The measured ${}^{235}$U spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the $\overline{\hat{I}{½}_e}$ energy region of 5-7MeV.