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

Abstract: The Jiangmen underground neutrino observatory (JUNO) is made of a 20 kt liquid scintillator (LS) detector at a depth of 700 m underground. In order to meet all physics requirements, the $^{238}$U/$^{232}$Th content in the LS is required to reach a level of 10$^{-17}$ g/g. However, the radioactivity of dust in the air is about 12 orders of magnitude higher than that, so there is an extremely high requirement for the cleanliness of the installation environment on site. In this study, a clean room management mode was implemented in the 120,000 m$^3$ space of the JUNO underground experimental main hall, to control the environmental cleanliness at a level equivalent to a class 10,000-100,000 clean room. Additionally, we designed a method to directly measure the deposition rate of $^{238}$U/$^{232}$Th on the surface of the detector. Based on ICP-MS detection, the sensitivity to $^{238}$U/$^{232}$Th concentrations can reach the level of picograms (pg). This helps to implement the cleanliness control strategies and to assess the level of external contamination during the construction of the detector.