  Neutron tagging in Gadolinium-doped water may play a significant role in
reducing backgrounds from atmospheric neutrinos in next generation proton-decay
searches using megaton-scale Water Cherenkov detectors. Similar techniques
might also be useful in the detection of supernova neutrinos. Accurate
determination of neutron tagging efficiencies will require a detailed
understanding of the number of neutrons produced by neutrino interactions in
water as a function of momentum transferred. We propose the Atmospheric
Neutrino Neutron Interaction Experiment (ANNIE), designed to measure the
neutron yield of atmospheric neutrino interactions in gadolinium-doped water.
An innovative aspect of the ANNIE design is the use of precision timing to
localize interaction vertices in the small fiducial volume of the detector. We
propose to achieve this by using early production of LAPPDs (Large Area
Picosecond Photodetectors). This experiment will be a first application of
these devices demonstrating their feasibility for Water Cherenkov neutrino
detectors.
