This Letter reports the first scientific results from the observation of antineutrinos emitted by fission products of 235 U at the High Flux Isotope Reactor. PROSPECT, the Precision Reactor Oscillation and Spectrum Experiment, consists of a segmented 4 ton 6 Li-doped liquid scintillator detector covering a baseline range of 7-9 m from the reactor and operating under less than 1 m water equivalent overburden. Data collected during 33 live days of reactor operation at a nominal power of 85 MW yield a detection of 25 461 AE 283 ðstatÞ inverse beta decays. Observation of reactor antineutrinos can be achieved in PROSPECT at 5σ statistical significance within 2 h of on-surface reactor-on data taking. A reactor model independent analysis of the inverse beta decay prompt energy spectrum as a function of baseline constrains significant portions of the previously allowed sterile neutrino oscillation parameter space at 95% confidence level and disfavors the best fit of the reactor antineutrino anomaly at 2.2σ confidence level.

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This Letter reports the first measurement of the 235 U νe energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9 m from the 85 MW th highly-enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678 ± 304 (stat.) νe-induced inverse beta decays (IBD), the largest sample from HEU fission to date, 99 % of which are attributed to 235 U. Despite broad agreement, comparison of the Huber 235 U model to the measured spectrum produces a χ 2 /ndf = 51.4/31, driven primarily by deviations in two localized energy regions. The measured 235 U spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the νe energy region of 5-7 MeV.Reactor ν e experiments have been central to the understanding of neutrinos, including the first observation of ν e [1], the discovery of ν e oscillations [2], observation of ν e produced within the Earth [3], and the measurement of the neutrino mixing angle θ 13 [4][5][6]. Most of these experiments were located at low-enriched uranium (LEU) nuclear power reactors where more than 99 % of emitted ν e come from the beta decay of fission products of four isotopes ( 235 U, 238 U, 239 Pu, and 241 Pu). At power reactors, the emitted ν e flux and spectrum evolve over time as the isotopic composition changes in the fuel cycle. Comparisons between theoretical predictions and experimental results reveal a ∼6 % global flux deficit [7-10] and disagreement of the energy spectrum [11][12][13][14] and flux-evolution [15,16]. Explanations for these possibly independent phenomena may lie in the complex nuclear physics of reactors [17][18][19][20][21][22][23][24], physics beyond the Standard Model such as eV-scale sterile neutrinos [8], or both [25-27]. New experiments at compact-core, highly enriched uranium (HEU) research reactors enable short baseline searches for sterile neutrino oscillations and the measurement of the nearly time-independent emission of ν e from 235 U fission [28][29][30]. PROSPECT has recently reported a search for sterile neutrinos at the High Flux Isotope Reactor (HFIR) [31]. This Letter reports the first measurement of the ν e energy spectrum from HFIR by the PROSPECT experiment and the higheststatistics 235 U spectral measurement since the ILL experiment observed ∼5000 ν e candidates in 1981 [32].Located at Oak Ridge National Laboratory, HFIR is an 85 megawatt thermal (MW th ) HEU research reactor. The cylindrical reactor core (diameter: 0.435 m, height: 0.508 m) contains 93 % 235 U enriched fuel, leading to a ∼99 % 235 U fission fraction. Each 24-day operating cycle uses fresh fuel, minimizing 239 Pu and 241 Pu production. The PROSPECT detector is deployed in a ground-level room at a center-to-center distance of (7.9 ± 0.1) m from the reactor core. The core center is located 40°below the horizontal and the surrounding building provides less than one meter-water-equivalent of concrete overburden.

Preprint submitted to Nuclear Instruments and MethodsThe Precision Reactor Oscillation and Spectrum Experiment, PROSPECT, is designed to make both a precise measurement of the antineutrino spectrum from a highly-enriched uranium reactor and to probe eV-scale sterile neutrinos by searching for neutrino oscillations over meter-long baselines. PROSPECT utilizes a segmented 6 Li-doped liquid scintillator detector for both efficient detection of reactor antineutrinos through the inverse beta decay reaction and excellent background discrimination. PROSPECT is a movable 4-ton antineutrino detector covering distances of 7 m to 13 m from the HFIR reactor core. It will probe the best-fit point of theν e disappearance experiments at 4 σ in 1 year and the favored regions of the sterile neutrino parameter space at more than 3 σ in 3 years. PROSPECT will test the origin of spectral deviations observed in recent Theta13 experiments, search for sterile neutrinos, and address the hypothesis of sterile neutrinos as an explanation of the reactor anomaly.This paper describes the design, construction, and commissioning of PROSPECT and reports first data characterizing the performance of the PROSPECT antineutrino detector.

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