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

TL;DR

This paper details strategies for controlling U/Th deposition and maintaining ultra-clean conditions in the JUNO neutrino detector site, including environmental management, cleaning procedures, and direct contamination measurement techniques.

Contribution

It introduces a novel method for measuring U/Th deposition rates on detector surfaces with high sensitivity, enhancing cleanliness control during construction.

Findings

Achieved an average cleanliness class of 74,000 in the underground hall.

Pre-filling water spray cleaning improves cleanliness by about two orders of magnitude.

Developed ICP-MS based method for ultra-sensitive U/Th deposition measurement.

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.