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Quantitative U/Th deposition and cleanliness control strategies in the JUNO site air

Jie Zhao, Chenyang Cui, Yongpeng Zhang, Gaosong Li, Nan Wang, Monica Sisti

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) employs a 20 kt liquid scintillator (LS) detector located 700 m underground. To meet its physics objectives, the LS must achieve an ultra-low $^{238}$U/$^{232}$Th content of 10$^{-17}$ g/g. Given that airborne dust exhibits radioactivity about 12 orders of magnitude higher, exceptional cleanliness is essential during on-site installation. The total permissible dust mass in the 20 kt LS is only about 8 mg. To attain this, the acrylic vessel interior must comply with class 1,000 cleanliness. Pre-filling water spray cleaning improves cleanliness by roughly two orders of magnitude, requiring the overall environment to be maintained between class 10,000 and 100,000. At JUNO, a cleanroom management system has been implemented across the 120,000 m$^3$ underground experimental hall. Since May 2022, continuous laser particle monitoring has consistently achieved an average cleanliness class of 74,000. Furthermore, we developed a method to directly measure $^{238}$U/$^{232}$Th deposition rates on detector surfaces. Using ICP-MS, sensitivity reaches sub-ppt levels ($<$10$^{-12}$ g/g), enabling effective cleanliness control and assessment of external contamination during detector construction.

Quantitative U/Th deposition and cleanliness control strategies in the JUNO site air

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) employs a 20 kt liquid scintillator (LS) detector located 700 m underground. To meet its physics objectives, the LS must achieve an ultra-low U/Th content of 10 g/g. Given that airborne dust exhibits radioactivity about 12 orders of magnitude higher, exceptional cleanliness is essential during on-site installation. The total permissible dust mass in the 20 kt LS is only about 8 mg. To attain this, the acrylic vessel interior must comply with class 1,000 cleanliness. Pre-filling water spray cleaning improves cleanliness by roughly two orders of magnitude, requiring the overall environment to be maintained between class 10,000 and 100,000. At JUNO, a cleanroom management system has been implemented across the 120,000 m underground experimental hall. Since May 2022, continuous laser particle monitoring has consistently achieved an average cleanliness class of 74,000. Furthermore, we developed a method to directly measure U/Th deposition rates on detector surfaces. Using ICP-MS, sensitivity reaches sub-ppt levels (10 g/g), enabling effective cleanliness control and assessment of external contamination during detector construction.

Paper Structure

This paper contains 17 sections, 5 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Overview of the JUNO detector
  3. Cleanliness requirement
  4. Cleanliness control strategies
  5. Overall environment
  6. Detector cleaning and protection
  7. U/Th settling measurement
  8. Plates and containers cleaning
  9. Sampling
  10. Sample treatment process
  11. Blanks and recovery efficiency
  12. Results
  13. Application to the JUNO detector systems
  14. Acrylic inner surface
  15. LS purification and filling systems
  16. ...and 2 more sections

Figures (5)

  • Figure 1: Conceptual diagram of the JUNO detector.
  • Figure 2: The long time monitoring of cleanliness inside the hall, as well as some key clean control measures are shown in the figure. Some of the larger peaks from March to July in 2023 mainly resulted from the dispersion of acrylic powder into the air during the polishing of the acrylic panel joints.
  • Figure 3: Sampling plate and vessels. The total sampling areas are 4 cm$^2$, 60 cm$^2$, and 390 cm$^2$ for the PTFE plate and acrylic plate, PFA bottle, and PFA flask, respectively.
  • Figure 4: Some sampling pictures in JUNO experimental hall. Fig. 1 and Fig. 2 depict the sampling photographs taken from the installation room and the hall floor, respectively. Fig. 3 shows the sampling image captured between the layers of acrylic sphere and the PMTs. Figure 4 illustrates the installation platform for the acrylic sphere, where we conducted our sampling.
  • Figure 5: The U/Th recovery efficiency for the entire pre-treatment. The average efficiencies of $^{233}$U and $^{229}$Th are shown in the black and red dash line.