Measurement of cosmic-ray muon flux at CJPL-II

TL;DR

This study measures the cosmic-ray muon flux at China Jinping Underground Laboratory (CJPL-II), finding it to be the lowest worldwide due to extensive shielding, using a plastic scintillator muon telescope over several years.

Contribution

First measurement of muon flux at CJPL-II, providing a precise value and demonstrating the laboratory's exceptionally low background environment.

Findings

Muon flux at CJPL-II is (3.03 ± 0.24 (stat) ± 0.18 (sys)) × 10^{-10} cm^{-2}s^{-1}

Flux is the lowest among all underground laboratories worldwide

Measurement was conducted over 1098 days with a scintillator telescope.

Abstract

In China Jinping Underground Laboratory (CJPL), the deepest and largest underground laboratory globally, the cosmic-ray muon flux is significantly reduced due to the substantial shielding provided by the overlying mountain. From 2016 to 2020, we measured the muon flux in the second phase of CJPL (CJPL-II) with a plastic scintillator muon telescope system, detecting 161 muon events over an effective live time of 1098 days. The detection efficiency was obtained by simulating the underground muon energy and angular distributions and the telescope system's response to underground muons. The cosmic-ray muon flux is determined to be (3.03 $\pm$ 0.24 (stat) $\pm$ 0.18 (sys)) $\times$ 10$^{-10}$ cm$^{-2}$s$^{-1}$, which is the lowest among underground laboratories worldwide.