Neutrino Studies at the High Flux Isotope Reactor – Results and Perspectives*

I will present the latest scientific results from the PROSPECT (Precision Oscillation and Spectrum) experiment; describe plans for the PROSPECT-II detector upgrade; and describe the impact of next-generation, reactor-based neutrino sources in enabling fundamental and applied reactor neutrino science. The PROSPECT collaboration, which includes both universities and U.S. National Laboratories, has been successful in making world-leading short-baseline measurements detecting antineutrinos from the High Flux Isotope Reactor (HFIR). HFIR is an 85 MW HEU research reactor located at ORNL. PROSPECT is a ton scale liquid scintillator detector with minimum overburden that features efficient optical segmentation and a ⁶Li-doped liquid scintillator with good light yield and pulse-shape discrimination properties enabling excellent energy reconstruction and background rejection. The PROSPECT physics program focuses on the search for eV-scale sterile neutrino oscillation at short baselines and a precise measurement of the U-235 reactor antineutrino energy spectrum. The first phase of this experiment has established the strongest limits on the existence of eV-scale sterile neutrinos in the high mass range. Furthermore, HFIR provides a unique compact neutrino source where over 99% of the antineutrino flux comes from the decay of U-235 fission products, enabling an exclusive measurement of the U-235 antineutrino energy spectrum. The initial results have motivated an expanded scientific and technical scope, including joint data analyses with other neutrino experiments, applications of Machine Learning to neutrino data, optimized data analysis with single ended event reconstruction, boosted dark matter searches, and potential reactor monitoring applications. Furthermore, they have enabled the development of more robust detectors with improved scintillators, new concepts for the energy scale determination, and detailed background characterization for future reactor experiments.