First Antineutrino Energy Spectrum From U-235 Fissions With The Stereo Detector At Ill

This article reports the measurement of the U-235-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 U-235 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 similar or equal to 5.3 MeV. A Gaussian fit of this local excess leads to an amplitude of A = 12.1 +/- 3.4% (3.5 sigma).