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.

This paper describes the design and performance of a 50 liter, two-segment 6 Li-loaded liquid scintillator detector that was designed and operated as prototype for the PROSPECT (Precision Reactor Oscillation and Spectrum) Experiment. The two-segment detector was constructed according to the design specifications of the experiment. It features low-mass optical separators, an integrated source and optical calibration system, and materials that are compatible with the 6 Li-doped scintillator developed by PROSPECT. We demonstrate a high light collection of 850±20 PE/MeV, an energy resolution of σ = 4.0±0.2% at 1 MeV, and efficient pulse-shape discrimination of low dE/dx (electronic recoil) and high dE/dx (nuclear recoil) energy depositions. An effective scintillation attenuation length of 85±3 cm is measured in each segment. The 0.1% by mass concentration of 6 Li in the scintillator results in a measured neutron capture time of τ = 42.8±0.2 µs. The long-term stability of the scintillator is also discussed. The detector response meets the criteria necessary for achieving the PROSPECT physics goals and demonstrates features that may find application in fast neutron detection.

A : This work reports the production and characterization of lithium-loaded liquid scintillator (LiLS) for the Precision Reactor Oscillation and Spectrum Experiment (PROSPECT). Fifty-nine 90 liter batches of LiLS ( 6 Li mass fraction 0.082%±0.001%) were produced and samples from all batches were characterized by measuring their optical absorbance relative to air, light yield relative to a pure liquid scintillator reference, and pulse shape discrimination capability. Fifty-seven batches passed the quality assurance criteria and were used for the PROSPECT experiment.

A 1-meter-long trapezoidal Triple-GEM detector with wide readout strips was tested in hadron beams at the Fermilab Test Beam Facility in October 2013. The readout strips have a special zigzag geometry and run along the radial direction with an azimuthal pitch of 1.37 mrad to measure the azimuthal φ-coordinate of incident particles. The zigzag geometry of the readout reduces the required number of electronic channels by a factor of three compared to conventional straight readout strips while preserving good angular resolution.The average crosstalk between zigzag strips is measured to be an acceptable 5.5%. The detection efficiency of the detector is (98.4 ± 0.2)%. When the non-linearity of the zigzag-strip response is corrected with track information, the angular resolution is measured to be (193 ± 3) µrad, which corresponds to 14% of the angular strip pitch. Multiple Coulomb scattering effects are fully taken into account in the data analysis with the help of a stand-alone Geant4 simulation that estimates interpolated track errors.