Accurate Measurement of the Electron Antineutrino Yield of U 235 Fissions from the STEREO Experiment with 119 Days of Reactor-On Data

We report a measurement of the antineutrino rate from the fission of U235 with the STEREO detector using 119 days of reactor turned on. In our analysis, we perform several detailed corrections and achieve the most precise single measurement at reactors with highly enriched U235 fuel. We measure an IBD cross section per fission of σf=(6.34±0.06[stat]±0.15[sys]±0.15[model])×10-43 cm2/fission and observe a rate deficit of (5.2±0.8[stat]±2.3[sys]±2.3[model])% compared to the model, consistent with the deficit of the world average. Testing U235 as the sole source of the deficit, we find a tension between the results of lowly and highly enriched U235 fuel of 2.1 standard deviations.