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In-situ environmental radiation background measurement in the second phase of CJPL

P. Zhang, Z. Zeng, J. Cheng, H. Ma

TL;DR

This paper documents in-situ measurements of environmental radiation in CJPL-II to establish a baseline background for future low-background experiments. The authors combine muon monitoring, radon assessment, gamma-ray spectroscopy, and neutron spectrometry, supported by the WRRS background-control system and deep rock overburden. Key findings include an ultra-low muon flux of $3.03 \pm 0.24$ (stat) $\pm$ $0.18$ (sys) $\times 10^{-10}$ cm$^{-2}$ s$^{-1}$, radon exhalation below $0.1$ mBq m$^{-2}$ s$^{-1}$, and dissolved gamma and neutron backgrounds that are orders of magnitude lower inside the polyethylene chamber. These results validate CJPL-II as an exceptional site for low-background physics and provide essential input for detector background modeling.

Abstract

China Jinping Underground Laboratory (CJPL), the deepest and largest underground laboratory worldwide, provides a low radiation background environment, which is necessary to frontier scientific research, such as the experimental studies of rare-event physics. Due to the almost filled space of CJPL-I and the requirement of future physics experiments, the construction of the second phase of CJPL (CJPL-II) was started in 2020 and all finished in 2024. In this work, we report the measured results of major environmental radiation in CJPL-II, including cosmic-ray muons, radon, gamma rays, and neutrons. Results indicate that the rock overburden and the radioactive background control effectively minimize the environmental radiation background. The scientific data presented also serve as an important basis to detector background modeling for the physics experiments in CJPL-II.

In-situ environmental radiation background measurement in the second phase of CJPL

TL;DR

This paper documents in-situ measurements of environmental radiation in CJPL-II to establish a baseline background for future low-background experiments. The authors combine muon monitoring, radon assessment, gamma-ray spectroscopy, and neutron spectrometry, supported by the WRRS background-control system and deep rock overburden. Key findings include an ultra-low muon flux of (stat) (sys) cm s, radon exhalation below mBq m s, and dissolved gamma and neutron backgrounds that are orders of magnitude lower inside the polyethylene chamber. These results validate CJPL-II as an exceptional site for low-background physics and provide essential input for detector background modeling.

Abstract

China Jinping Underground Laboratory (CJPL), the deepest and largest underground laboratory worldwide, provides a low radiation background environment, which is necessary to frontier scientific research, such as the experimental studies of rare-event physics. Due to the almost filled space of CJPL-I and the requirement of future physics experiments, the construction of the second phase of CJPL (CJPL-II) was started in 2020 and all finished in 2024. In this work, we report the measured results of major environmental radiation in CJPL-II, including cosmic-ray muons, radon, gamma rays, and neutrons. Results indicate that the rock overburden and the radioactive background control effectively minimize the environmental radiation background. The scientific data presented also serve as an important basis to detector background modeling for the physics experiments in CJPL-II.

Paper Structure

This paper contains 7 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Radiation measurements
  3. Cosmic-ray muons
  4. Radon and gamma rays
  5. Neutron
  6. Summary
  7. Acknowledgments

Figures (1)

  • Figure 1: Left: CJPL-II layout. Right: WRRS layer schematic diagram and photos taken during its construction. The WRRS layer was built with multiple stages applied from the rock to the surface: nano-modified steel fiber reinforced concrete (NMSFRC), HDPE dimpled drainage board (HDPE-DDB), rock bolts, rapid-Setting liquid rubber (RSLR), polymer cement concrete (PCC), polyurethane cement (PU cement), and polyurethane coat (PU coat).