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

## TL;DR

This paper presents the first precise measurement of the $^{235}$U antineutrino energy spectrum at a highly-enriched uranium reactor using PROSPECT, revealing deviations from theoretical models in specific energy regions.

## Contribution

It provides the first large-scale measurement of the $^{235}$U antineutrino spectrum at a HEU reactor, highlighting discrepancies with existing models.

## Key findings

- Observed 31,678 antineutrino events, the largest from HEU fission to date.
- Detected deviations in the measured spectrum compared to the Huber model.
- Found the spectrum shape consistent with low-enriched uranium reactor anomalies.

## Abstract

This Letter reports the first measurement of the $^{235}$U $\overline{\nu_{e}}$ 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.) $\overline{\nu_{e}}$-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 $\chi^2/ndf = 51.4/31$, 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 $\overline{\nu_{e}}$ energy region of 5-7MeV.