Danielle Norcini

Reactor experiments have been devoted to establishing the properties of the weakly-interacting neutrino. Recent neutrino oscillation experiments at low-enriched uranium (LEU) reactors suggest a disagreement between the observed electron antineutrino flux and energy spectrum when compared to leading model predictions. The ~6% flux deficit, known as the Reactor Antineutrino Anomaly, measured by detectors with baselines <500 m can be explained by the addition of a beyond-the-Standard-Model particle, an eV-scale sterile neutrino. This new type of matter would have a profound impact on fundamental physics and cosmology. The spectral deviations may be attributed to an incomplete understanding of antineutrino emission from fissile material. However, the insufficiencies in the nuclear models and the specific isotopic contributions are not yet clear.

PROSPECT is a newly constructed, short-baseline experiment observing antineutrinos from the 85 MW_th highly-enriched uranium High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Positioned 7-9 m from the HFIR core, the main science goals are to unambiguously search for eV-scale sterile neutrino oscillations and to precisely measure the antineutrino emission from ²³⁵U fission. PROSPECT uses the inverse beta decay interaction to identify neutrino events, but must do so in a near-surface environment with harsh reactor-related and cosmogenically-induced backgrounds. To overcome this challenge, PROSPECT developed a novel, compact ⁶Li-loaded scintillator detector with efficient neutron tagging and particle identification capabilities. These features, combined with event topology reconstruction made possible by optical segmentation, allow an unprecedented level of background suppression to observe antineutrinos with high energy resolution within a few hours of reactor-on operation.

This thesis details the detector design, instrumentation development, and construction of the PROSPECT experiment at the Yale Wright Laboratory from 2014–2018. The detector calibration, event reconstruction, and performance are also discussed. With 33 (28) days of reactor-on (off) data at HFIR, the first search for eV-scale sterile neutrinos is presented. No sterile neutrino oscillations are observed within the sensitive (Δm₄₁², sin²2ϴ₁₄) parameter space and PROSPECT excludes the best-fit sterile neutrino hypothesis to the Reactor Antineutrino Anomaly at 2.2σ confidence level. Using a slightly larger data set containing >31,000 antineutrino candidates, PROSPECT has produced the world-leading measurement of the ²³⁵U spectrum with a signal-to-background of 1.7 and 5%/E(MeV) energy resolution. The experimental data shows discrepancies with the leading Huber reactor model, with further tests disfavoring ²³⁵U as the sole isotope responsible for the spectral deviations observed by LEU experiments at 2.1σ confidence level.