  For next-generation neutrinoless double beta decay experiments, extremely low
backgrounds are necessary. An understanding of in-situ cosmogenic backgrounds
is critical to the design effort. In-situ cosmogenic backgrounds impose a depth
requirement and especially impact the choice of host laboratory. Often,
simulations are used to understand background effects, and these simulations
can have large uncertainties. One way to characterize the systematic
uncertainties is to compare unalike simulation programs. In this paper, a suite
of neutron simulations with identical geometries and starting parameters have
been performed with Geant4 and MCNP, using geometries relevant to the
LEGEND-1000 experiment. This study is an important step in gauging the
uncertainties of simulations-based estimates. To reduce project risks
associated with simulation uncertainties, a novel alternative shield of
methane-doped liquid argon is considered in this paper for LEGEND-1000, which
could achieve large background reduction without requiring significant
modification to the baseline design.
