First antineutrino energy spectrum from $^{235}$U fissions with the STEREO detector at ILL

TL;DR

The STEREO experiment measured the $^{235}$U antineutrino spectrum at ILL, revealing a local distortion around 5.3 MeV not predicted by existing models, with high statistical significance.

Contribution

First direct measurement of the $^{235}$U antineutrino spectrum shape using the STEREO detector, including a detailed unfolding and validation process.

Findings

Detected 43,000 antineutrinos at 10 m from reactor core.

Identified a significant local excess at 5.3 MeV in the spectrum.

No major biases found in the unfolding method.

Abstract

This article reports the measurement of the $^{235}$U-induced antineutrino spectrum shape by the STEREO experiment. 43'000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure $^{235}$U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around $E_\nu \simeq 5.3$ MeV. A gaussian fit of this local excess leads to an amplitude of $A = 12.1 \pm 3.4\%$ (3.5$\sigma$).