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.
Keywords
Cite
@article{arxiv.2507.05759,
title = {Quantitative U/Th deposition and cleanliness control strategies in the JUNO site air},
author = {Jie Zhao and Chenyang Cui and Yongpeng Zhang and Gaosong Li and Nan Wang and Monica Sisti},
journal= {arXiv preprint arXiv:2507.05759},
year = {2026}
}