Measurements of the neutron background for the Colorado Underground Research Institute (CURIE)

We present the first direct measurements of the neutron background at the Colorado Underground Research Institute (CURIE), a shallow-underground facility with an effective overburden of approximately $400$~m.w.e.\ in the Edgar Experimental Mine in Idaho Springs, Colorado. The radiogenic and muon-induced components were characterized in the Subatomic Particle Hideout. The radiogenic component was measured with a custom Bonner Cylinder Spectrometer comprising a $^{3}$He proportional counter and six cylindrical high-density polyethylene moderators. Its response functions were simulated in geant4, empirically calibrated using a $^{239}$Pu-$^{9}$Be source and the shadow-cone method, and then corrected to account for the approximately isotropic underground field. Unfolding the measured count rates gives a total local scalar neutron flux, dominated by radiogenic production, for E_n\lesssim\SI{10}{\MeV} of $(2.51\pm0.58_{\rm stat}\pm0.39_{\rm sys})\times10^{-5}$~\si{\per\square\centi\meter\per\second}. The muon-induced component was measured with a BC-501A organic liquid scintillator operated for \SI{1431}{\hour}. Pulse shape discrimination identified 55~$\pm$~7 neutron candidates. Using the simulated detector response, these counts yield an integral muon-induced neutron flux at the rock–cavern boundary above \SI{10.5}{\MeV} of $(2.09\pm0.29_{\rm stat}\,{}^{+0.79}_{-0.53}{}_{\rm sys})\times10^{-7}$~\si{\per\square\centi\meter\per\second}. This flux uses the first-entry, boundary-area normalization of a previous CURIE simulation study. Its central value is approximately \SI{12}{\percent} higher than the site-specific prediction but is consistent within the reported uncertainties. Together, these measurements characterize the radiogenic and high-energy cosmogenic neutron environments at CURIE and provide quantitative input for planning future low-background experiments at the facility.