Examining LEGEND-1000 cosmogenic neutron backgrounds in Geant4 and MCNP

TL;DR

This paper compares neutron background simulations for the LEGEND-1000 experiment using Geant4 and MCNP to assess uncertainties, and proposes a methane-doped liquid argon shield as a novel background reduction method.

Contribution

It provides a systematic comparison of two simulation tools for cosmogenic neutron backgrounds and introduces a new shielding approach to reduce backgrounds in neutrinoless double beta decay experiments.

Findings

Geant4 and MCNP simulations show consistent background estimates

The methane-doped liquid argon shield significantly reduces neutron backgrounds

Simulation uncertainties can be quantified through comparative analysis

Abstract

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.