Estimating cosmogenic backgrounds in nEXO at SNOLAB using Geant4 and FLUKA
The nEXO experiment was designed to search for neutrinoless double beta decay in the isotope $^{136}$Xe, using a five-tonne, isotopically-enriched liquid xenon time projection chamber (TPC). To meet nEXO's sensitivity goal, significant effort is placed on background mitigation strategies. Here, we evaluate the expected cosmogenic background rate for nEXO if placed at its preferred underground host site, SNOLAB. A comparison of results from Geant4 and FLUKA Monte Carlo simulations are presented. We perform detailed Monte Carlo simulations of muons reaching the underground SNOLAB environment using both Geant4 and FLUKA to study the production of radioactive isotopes from both direct spallation reactions or subsequent neutron-induced reactions. We find that, of the isotopes produced, the only one of concern for nEXO is $^{137}$Xe, produced by neutron capture on the xenon target. We develop a veto strategy that achieves an anticipated 92\% \xe{137} rejection efficiency while keeping dead time to less than 1 day over a 10 year exposure.