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 × ${10}^5$ g ${\mathrm{cm}}^{-2\mathrm{}}$ standard rock) has been measured with a ${174-\mathrm{m}}^2$ 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 $h_0$ as $I_{v\mu }\left(h_0\right)=A \exp \left(\frac{-h_0}{\lambda }\right)+I_{v\mu }^{\left(\nu \right)}$, where $A=(2.26\mathrm{}\pm 0.16)\times {10}^{-6\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$, and $\gamma =(7.58\mathrm{}\pm 0.09)\times {10}^4$ g ${\mathrm{cm}}^{-2\mathrm{}}$. 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 $I_{v\mu }^{\left(\nu \right)}=(2.23\mathrm{}\pm 0.20)\times {10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$. 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 <${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$.