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

• Received 31 December 2018
• Revised 22 March 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.251801