Muon Flux Measurement at China Jinping Underground Laboratory

TL;DR

This study measures the cosmic-ray muon flux at China Jinping Underground Laboratory using a 1-ton detector, providing essential data for neutrino research and comparing muon fluxes under different underground conditions.

Contribution

It presents the first precise muon flux measurement at CJPL and compares fluxes under mountain and mine shaft conditions, aiding future neutrino experiments.

Findings

Muon flux measured as (3.53±0.22±0.07)×10^{-10} cm^{-2}s^{-1}

Angular distribution matches terrain-based simulation

Muon flux under mountains is roughly four times higher than under mine shafts

Abstract

China Jinping Underground Laboratory (CJPL) is ideal for studying solar-, geo-, and supernova neutrinos. A precise measurement of the cosmic-ray background would play an essential role in proceeding with the R\&D research for these MeV-scale neutrino experiments. Using a 1-ton prototype detector for the Jinping Neutrino Experiment (JNE), we detected 264 high-energy muon events from a 645.2-day dataset at the first phase of CJPL (CJPL-I), reconstructed their directions, and measured the cosmic-ray muon flux to be $(3.53\pm0.22_{\text{stat.}}\pm0.07_{\text{sys.}})\times10^{-10}$ cm$^{-2}$s$^{-1}$. The observed angular distributions indicate the leakage of cosmic-ray muon background and agree with the simulation accounting for Jinping mountain's terrain. A survey of muon fluxes at different laboratory locations situated under mountains and below mine shaft indicated that the former is generally a factor of $(4\pm2)$ larger than the latter with the same vertical overburden. This study provides a convenient back-of-the-envelope estimation for muon flux of an underground experiment.