Measurement of cosmic-ray muon flux at CJPL-II

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.

Figures (9)

• Figure 1: Schematic of the muon telescope system
• Figure 2: Detector waveforms when a muon passed all three detectors of group A. The deposited energy in the A1, A2 and A3 detectors are 19.3, 9.7 and 12.3 MeV.
• Figure 3: Geometry model of the Jinping Mountain
• Figure 4: Visualization of the telescope system (zoomed in) at the hall C2 of CJPL-II. The green plane represents the sampling plane, which is in the same plane of the upper surface of the rock layer
• Figure 5: Energy spectra of triple coincidence signals