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arXiv · 2507.05759

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
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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.

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Jie Zhao, Chenyang Cui, Yongpeng Zhang, Gaosong Li, Nan Wang, Monica Sisti. 2025-07-08. Quantitative U/Th deposition and cleanliness control strategies in the JUNO site air. https://arxiv.org/abs/2507.05759

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