Characterization of the MARS Neutron Detector

Coherent Elastic Neutrino-Nucleus Scattering (CEvNS) was first measured by COHERENT in 2017 nearly 40 years after it was first proposed. The process involved measuring tiny nuclear recoils that result from a neutrino scattering off of atomic nuclei. COHERENT made the first two measurements of CEvNS at the Spallation Neutron Source (SNS) and is working toward additional measurements there with the goal of observing the dependence of the cross section on detector material. The SNS, as the name implies, is an intense neutron source. These neutrons must be fully accounted for as a background for CEvNS because they are coincident with the neutrinos and because they can leave a similar recoil signature in detectors.\\ \indent The Multiplicity And Recoil Spectrometer (MARS) was deployed at the SNS to measure neutrons. MARS takes advantage of capture-gating to identify neutrons separate from other environmental backgrounds. In order to measure neutrons at the SNS, MARS must be characterized there to assess detection efficiencies and performance. The detection efficiency for MARS was determined to be 3.9$\%$ for 14 MeV neutrons and is a function of cuts on the variables characteristic to the capture-gating method.\\ \indent In this work, other characterization measurements are detailed including trigger efficiency, light yield and resolution as a function of position, and neutron detection efficiency. A first measurement of the neutron fluence with MARS is described for its original location in Neutrino Alley. After determining cuts on the relevant variables, 179 $\pm$ 27 neutrons are used to measure a fluence of 415 neutrons/m$^2$/10$^{12}$J $\pm$15$\%_{stat}$ + $\pm$54$\%_{sys}$ of protons on target. This fluence is expectedly lower than previous measurements with other detectors as MARS was in a more neutron-quiet location. A first look at the deposited energy spectrum from these neutrons is also shown.