Measuring muon-induced neutrons with liquid scintillation detector at Soudan mine

We report a direct detection of muon-induced high-energy neutrons with a 12-liter neutron detector fabricated with EJ-301 liquid scintillator operating at Soudan mine for about two years. The detector response to energy from a few MeV up to ~20 MeV has been calibrated using radioactive sources and cosmic-ray muons. Subsequently, we have calculated the scintillation efficiency for nuclear recoils, up to a few hundred MeV, using Birks' law in the Monte Carlo simulation. Data from an exposure of 655.1 days were analyzed and neutron-induced recoil events were observed in the energy region from 4 to 50 MeV, corresponding to fast neutrons with kinetic energy up to a few hundred MeV, depending on the scattering angle. Combining with the Monte Carlo simulation, the measured muon-induced fast neutron flux is determined to be $(2.23±{}0.52(sta)±{}0.99(sys))×{}{10}^{{-}9} {cm}^{{-}2} {s}^{{-}1}$ (${E}ₙ >20 MeV$), in a reasonable agreement with the model prediction. The muon flux is found to be ($1.65±{}0.02(sta)±{}0.1(sys))×{}{{0ex}{0ex}}{10}^{{-}7} {cm}^{{-}2} {s}^{{-}1}$ (${E}_{{μ}} >1 GeV$), consistent with other measurements. As a result, the muon-induced high-energy gamma-ray flux is simulated to be $7.08×{}{10}^{{-}7} {cm}^{{-}2} {s}^{{-}1}$ (${E}_{{γ}} > 1 MeV$) for the depth of Soudan.