MARS Detector -- Measuring the Neutron Background of the COHERENT Experiment

The Multiplicity and Recoil Spectrometer (MARS) is a portable fast neutron detector designed to measure low-intensity neutron flux. Built with gadolinium-doped plastic scintillators, MARS uses neutron capture signals to measure the weak fast-neutron flux, which is one of the major backgrounds of the CEvNS (Coherent Elastic Neutrino Nucleus Scattering) measurement of the COHERENT Experiment. We take advantage of the mobility of MARS and deploy it at various sites within Neutrino Alley, the COHERENT experimental hall at the Spallation Neutron Source at Oak Ridge National Laboratory, to characterize and monitor beam-related neutron backgrounds near our CEvNS detectors.

From 2018 to 2025, MARS has collected data at four distinct locations of interest to COHERENT subsystems. This presentation overviews the MARS analysis strategy, featuring the identification of neutron-capture events through muon-induced neutrons, and reporting on the status of beam-related neutron analyses for all past and planned deployment locations.