First antineutrino energy spectrum from <sup>235</sup> U fissions with the S <scp>TEREO</scp> detector at ILL <sup>*</sup>
H. Almazán, L. Bernard, Adrien Blanchet, A. Bonhomme, C. Buck, P. del Amo Sanchez, I. El Atmani, L. Labit, J. Lamblin, A. Letourneau, D. Lhuillier, M. Licciardi, M. Lindner, T. Materna, H. Pessard, J. S. Réal, J. S. Ricol, C. Roca, R. Rogly, T. Salagnac, V. Savu, S. Schoppmann, V. Sergeyeva, T. Söldner, A. Stutz, M. Vialat
Abstract
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 ν ≃ 5.3 MeV. A Gaussian fit of this local excess leads to an amplitude of A = 12.1 ± 3.4% (3.5 σ ).