Depth-dependent background neutron distribution at the deep underground laboratory

The neutron energy spectrum measured at the Earth’s surface shows three distinct peaks: a thermal energy peak, an evaporation peak between 2-3 MeV, and a cascade peak near 100 MeV. However, the spectrum measured at a depth of 1604 metres water equivalent reveals the absence of cascade peak. This indicates an effective shielding of cosmogenic component of neutron background by the rock overburden. A comprehensive analysis of measured neutron fluence data from various facilities, combined with simulation data, indicates that the radiogenic component is the primary source of neutron background at the underground laboratories even at a depth of 1000 metres water equivalent. The concentrations of uranium and thorium in the surrounding rock are identified as the major factors influencing this background.