Measuring Underground Neutron Fluxes for Neutrinoless Double Beta Decay

The goal of this project was to measure the muon-induced neutron flux in lead at sea level using the SciBath neutral particle detector. The muon-induced neutron rate is not well known, is challenging to measure and simulate, and potentially an important background for underground experiments such as the EXO neutrinoless double beta-decay experiment. A mass of 45 kg of lead was placed on top of the detector, and muon/neutron-capture correlated events were measured. Events with accidental neutron-capture signals were subtracted as were muon/neutron-capture correlated events from spallation in the detector liquid scintillator. This procedure resulted in a neutron yield from lead at sea level of \(1.4 \,(\pm1.1)\times10^{-5}\frac{\mathrm{n}}{\mu}{\,(\mathrm{g cm}^{-2})}^{-1}\) with the typical muon energy at sea level \(E_{\mu}\approx4 \mathrm{GeV}\). We compare this to parameterizations created for underground experimental sites extrapolated up to sea level.