Cosmic-ray muon fluxes deep underground: Intensity vs depth, and the neutrino-induced component

The angular distribution of muons observed deep underground (10788 ft, or 8.89 ×{} 10⁵ g cm^-2 standard rock) has been measured with a 174-m² liquid scintillation detector in conjunction with 48 384 neon flash tubes. The data are fitted by a curve giving the vertical intensity of muons vs vertical depth h₀ as I_vμ(h₀)=A exp(-h₀λ)+I_vμ^(ν), where A=(2.26±0.16)×10^-6 cm^-2 sec^-1 sr^-1, and γ=(7.58±0.09)×10⁴ g cm^-2. The constant term, representing the measured depth-independent flux of muons produced in the surrounding rock by interactions of cosmic-ray neutrinos generated in the earth's atmosphere, has the value I_vμ^(ν)=(2.23±0.20)×10^-13 cm^-2 sec^-1 sr^-1. This observed flux is in fair agreement with that predicted assuming a cosmic-ray neutrino flux which is a composite of several theoretical estimates. Thus the flux of muons from extraterrestrial neutrinos is 10^-13 cm^-2 sec^-1 sr^-1.