Observation of antineutrinos from 235U fission by the PROSPECT experiment at HFIR
Mark A. Tyra, Denis E. Bergeron, Jerome J. LaRosa, Hans P. Mumm, Svetlana Nour
This letter reports the first observation of antineutrinos from the fission products of highly enriched 235U at the High Flux Isotope Reactor. PROSPECT, the Precision Reactor Oscillation and Spectrum Experiment, consists of a segmented 4 ton 6Li-doped liquid scintillator detector covering a baseline range of 7-9 m from the reactor and operating under less than 1 meter water equivalent overburden. Data collected during 33 days of reactor operation at a nominal power of 85 MW yields 24792 +/ 276 inverse beta decay detections. Observation of reactor antineutrinos from HFIR is achieved in PROSPECT at 5 statistical significance within three hours of on-surface reactor-on data-taking. A reactor-model-independent analysis of the prompt energy spectrum distortion as a function of baseline sets new limits on neutrino oscillations in the high (>1eV2) Dm2 regime, and disfavors the best fit of the Reactor Antineutrino Anomaly at >95% confidence level.