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 (En >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))×10−7 cm−2 s−1 (Eμ >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 (Eγ > 1 MeV) for the depth of Soudan.