  The 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 7m to 13m from the High Flux
Isotope Reactor core. It will probe the best-fit point of the $\bar\nu_e$
disappearance experiments at 4$\sigma$ in 1 year and the favored regions of the
sterile neutrino parameter space at more than 3$\sigma$ in 3 years. PROSPECT
will test the origin of spectral deviations observed in recent $\theta_{13}$
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.
