Accelerator Neutrino Neutron Interaction Experiment
[1] By implementing early prototypes of LAPPDs (Large Area Picosecond Photodetector), high precision timing is possible.
The neutrino beam originates in Fermilab where The Booster delivers 8 GeV protons to a beryllium target producing secondary pions and kaons.
The detector was upgraded for full science operation (so-called Phase II) in July 2019 and data collection began in 2020.
[2] The water target used by ANNIE is a cylindrical volume 3.8 m long and 2.3 m in diameter encased by a plastic liner and aluminium enclosure.
ANNIE examines the interactions between neutrinos and nuclei in water with the aim of producing measurements of final state neutron abundance as a function of total momentum transfer.
The ability to tag neutrons in the final state will also allow ANNIE to test specific nuclear models for validity in neutrino interactions.
ANNIE will characterize the neutron yield of events that generate signatures similar to those of proton decay in water Cherenkov detectors.
The subsequent decay of the charged pion produces a muon which is within the detectable threshold for water Cherenkov detectors.
Once these neutrons have been thermalized, they undergo radiative capture wherein they are incorporated into a nucleus to produce a more tightly bound state.
[11] In order to enhance visibility of neutron capture events, Gadolinium salts are dissolved into ANNIE's aqueous media.
Gadolinium has a larger capture cross-section, around 49,000 barns, and this occurs on the orders of microseconds after the free neutron is emitted.
ANNIE will attempt to characterize the exact confidence in rejection of background events based on neutron tagging experiments optimized to the application of atmospheric neutrino interactions.
Additionally, a movable, smaller volume of gadolinium doped water will be used to measure rates of neutron events as a function of position inside the tank.
This phase involves the use of a full gadolinium-doped water volume, 60 Type-S PMTs, a small but sufficient number of LAPPDs, and the refurbished MRD.
During this stage, detailed reconstruction of kinematics will be possible, and therefore, measurements of neutron yield for event classes determined by final state particles.
