[Paper Review] Measurements of Radon Concentrations Using CR-39 Detectors in China JinPing Underground Laboratory (2015-2016)

Cuihong Liu, Hao Ma+4 more|arXiv (Cornell University)|Jun 18, 2018

Radioactivity and Radon Measurements13 references3 citations

TL;DR

This study measured radon concentrations in China JinPing Underground Laboratory (CJPL) from May 2015 to March 2016 using CR-39 solid-state nuclear track detectors, a passive, time-integrated detection method. The results show that the main experimental hall achieved an average annual radon concentration of (58±10) Bq·m⁻³ due to effective ventilation, while the unventilated storage tunnel reached (345±15) Bq·m⁻³, indicating the natural background level; these values are within the intermediate range compared to other international underground labs and suitable for low-background experiments.

ABSTRACT

Radon background is one of the most critical influences on the ultra-low background and rare-event physical experiments in underground laboratories. To conduct a comprehensive investigation of the radon concentrations in China JinPing underground Laboratory (CJPL), long-term cumulative measurements were carried out using time-integrated passive radon dosimeters containing CR-39 solid-state nuclear track detectors at four sites of CJPL-I. The measurements lasted from May 30,2015 to March 16,2016, a total of 290 days, and the total effective measurement time is 6,953 hours. In the main experiment hall equipped with the ventilation system, the average annual radon concentrations were (55+/-7) Bq m^-3, (58+/-10) Bq^m-3and (53+/-9) Bq m^-3 in three locations respectively. In the storage tunnel without any radon reduction facility, the average annual radon concentrations increased to (345+/-15) Bq m^-3, reflecting the original average annual radon concentration levels in the underground laboratory. Compared with the measured results in the period from 2010 to 2011, the average radon concentration in the main experimental hall was reduced from (86+/-25) Bq m^-3 to (58+/-10) Bq m^-3 due to the well-running ventilation system. Compared with other international underground laboratories, radon concentrations in CJPL-I were at an intermediate level and can meet the demand for low background and rare-event physical experiments.

Motivation & Objective

• To assess long-term radon background levels in China JinPing Underground Laboratory (CJPL) to support ultra-low background and rare-event physics experiments.
• To evaluate the effectiveness of the ventilation system in reducing radon concentrations in the main experimental hall.
• To establish baseline radon levels in unventilated areas, such as the storage tunnel, to understand natural radon emissions in the underground environment.
• To compare CJPL's radon levels with those of other international underground laboratories to assess its suitability for sensitive experiments.

Proposed method

• Passive, time-integrated radon detection using CR-39 solid-state nuclear track detectors deployed at four distinct sites within CJPL-I.
• Detectors were exposed for 290 days (May 30, 2015 – March 16, 2016), accumulating data over 6,953 effective hours.
• Track counting and analysis were performed to determine radon concentration based on the number of alpha particle tracks recorded.
• Measurements were conducted in both ventilated and unventilated areas to compare radon levels under different environmental controls.
• Statistical analysis with error propagation was applied to report concentrations with uncertainties (e.g., 58±10 Bq·m⁻³).
• Results were benchmarked against previous measurements (2010–2011) and international standards to assess trends and performance.

Experimental results

Research questions

• RQ1 What are the long-term average radon concentrations in the main experimental hall of CJPL-I, and how do they vary with ventilation?
• RQ2 What is the natural radon background level in unventilated underground areas such as the storage tunnel?
• RQ3 To what extent has the ventilation system at CJPL-I reduced radon concentrations compared to pre-ventilation levels?
• RQ4 How do CJPL-I's radon concentrations compare to those of other international underground laboratories in terms of background levels?
• RQ5 Can the current radon levels in CJPL-I support future ultra-low background and rare-event physics experiments?

Key findings

• The average annual radon concentration in the main experimental hall, equipped with a ventilation system, was measured at (58±10) Bq·m⁻³.
• In the unventilated storage tunnel, the average annual radon concentration reached (345±15) Bq·m⁻³, representing the baseline natural level in the underground environment.
• The ventilation system reduced radon levels in the main hall from (86±25) Bq·m⁻³ (2010–2011) to (58±10) Bq·m⁻³, demonstrating significant mitigation.
• The measured radon levels in CJPL-I are at an intermediate level compared to other international underground laboratories, suitable for low-background experiments.
• The results confirm that the current radon background in CJPL-I is within acceptable limits for rare-event physics experiments such as dark matter and neutrino detection.