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 $\sim 20\mathrm{MeV}$ 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 $(2.23\pm 0.52(\text{sta})\pm 0.99(\text{sys}))\times {10}^{-9}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ ($E_n>20\mathrm{MeV}$), in a reasonable agreement with the model prediction. The muon flux is found to be ($1.65\pm 0.02(\text{sta})\pm 0.1(\text{sys}))\times {10}^{-7}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ ($E_{\mu }>1\mathrm{GeV}$), consistent with other measurements. As a result, the muon-induced high-energy gamma-ray flux is simulated to be $7.08\times {10}^{-7}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ ($E_{\gamma }>1\mathrm{MeV}$) for the depth of Soudan.

• Received 11 July 2014

DOI:https://doi.org/10.1103/PhysRevD.90.122003