  We report on the technical design and expected performance of a 592 kg
heavy-water-Cherenkov detector to measure the absolute neutrino flux from the
pion-decay-at-rest neutrino source at the Spallation Neutron Source (SNS) at
Oak Ridge National Laboratory (ORNL). The detector will be located roughly 20 m
from the SNS target and will measure the neutrino flux with better than 5%
statistical uncertainty in 2 years. This heavy-water detector will serve as the
first module of a two-module detector system to ultimately measure the neutrino
flux to 2-3% at both the First Target Station and the planned Second Target
Station of the SNS. This detector will significantly reduce a dominant
systematic uncertainty for neutrino cross-section measurements at the SNS,
increasing the sensitivity of searches for new physics.
