Simulation of Muon-induced Backgrounds for the Colorado Underground Research Institute (CURIE)

Abstract

We present a comprehensive Monte Carlo simulation of muon-induced backgrounds for the Colorado Underground Research Institute (CURIE), a shallow-underground facility with $\approx 415$~m.w.e. overburden. Using coupled \textsc{mute} and \textsc{geant4} frameworks, we characterize the production and transport of muon-induced secondaries through site-specific rock compositions and geometries, establishing a proof-of-concept for high-precision, end-to-end simulations. Our simulations employ angular-dependent muon energy distributions, which improve secondary flux accuracy. For the Subatomic Particle Hideout and Cryolab I research spaces, we predict total muon-induced neutron fluxes of $(8.52 \pm 1.30_{\text{sys}}) \times 10^{-3}$~m$^{-2}$s$^{-1}$ and $(8.86 \pm 1.62_{\text{sys}}) \times 10^{-3}$~m$^{-2}$s$^{-1}$, respectively. Additionally, we develop a Depth-Intensity Relation (DIR) to predict the muon-induced neutron flux as a function of facility depth, which is consistent with measurements across a broad range of underground depths. These results provide quantitative background predictions for experimental design and sensitivity projections at shallow- and deep-underground facilities. They further demonstrate that local geology and overburden geometry influence muon-induced secondary yields and energy spectra, emphasizing the need for site-specific simulations for accurate underground background characterization. Therefore, the simulation framework has been made publicly available at \href{https://doi.org/10.5281/zenodo.17196581}{https://doi.org/10.5281/zenodo.17196581}, for the broader low-background physics community to enable meaningful inter-facility comparisons.

Figures (14)

• Figure 1: Reconstruction of the geant4 geometry for the SPH. Red lines are primary muons and all others are muon-induced secondaries.
• Figure 2: Comparisons between ftfp_bert_hp and shielding_hp: (a) neutron creation processes, (b) neutron exit energies, and (c) neutron production distances.
• Figure 3: Comparison of the simulated liquid scintillator neutron yields, before and after applying the correction, against the experimental data from Refs. abe_mu_naberdeenhertenberger_mu_nboehm_mu_nEnikeev1987zhang_mu_n
• Figure 4: Comparison of the spectral ratios between $\langle E_{\mu}\rangle$ and $E_{\rm FULL}$.
• Figure 5: Average secondary production number per muon as a function of depth.