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 $\sim$ 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 MeV 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 muon-induced fast neutron flux is determined to be $(2.3 \pm 0.52 (sta.) \pm 0.99 (sys.) ) \times10^{-9}$ cm$^{-2}$s$^{-1}$ (E$_{n}$ $>$ 20 MeV), in a reasonable agreement with the model prediction. The muon flux is found to be ($1.65\pm 0.02 (sta.) \pm 0.1 (sys.) ) \times10^{-7}$ cm$^{-2}$s$^{-1}$ (E$_{\mu}$ $>$ 1 GeV), consistent with other measurements. As a result, the muon-induced high energy gamma-ray flux is simulated to be 7.08 $\times$10$^{-7}$cm$^{-2}$s$^{-1}$ (E$_{\gamma}$ $>$ 1 MeV) for the depth of Soudan.