Status of the nEXO Outer Detector Design

The nEXO experiment is being designed to search for neutrino-less double beta decay (0νββ) in a time projection chamber (TPC) filled with 5000 kg of liquid xenon-136. Extremely low background conditions are necessary to achieve the > 1028-year sensitivity reach to the 0νββ half life. nEXO anticipates construction at a depth of approximately 2 km underground at SNOLAB where the flux from cosmic ray muons is greatly diminished. However, muon-induced neutrons are still of concern as they create background events through the production of Xe-137 in the TPC. The mitigation of this background is to be achieved by immersing the TPC in a cylindrical water tank instrumented with photomultiplier tubes (PMTs) in order to actively detect the Cherenkov light of passing muons (muon veto system). The water tank also passively shields the TPC against incident gamma-rays coming from the local U and Th decays. Together, the water tank and the muon veto system make up the nEXO Outer Detector (OD). In this poster, we will focus on the work that has been done to design the OD. Of particular relevance are the GEANT4 and GPU-based Chroma simulations and studies performed to determine the dimensions of the water tank, the arrangement of PMTs, the reflectivity of internal surfaces, the requirements for the water quality, the cover gas layer, the tagging efficiency, and the muon veto overall impact on the nEXO sensitivity. In addition we describe the plans and status of a dedicated facility to test, characterize, and select the Hamamatsu R-5912 PMTs to be repurposed from the Data Bay reactor neutrino experiment.