A practical approach of high precision U and Th concentration measurement in acrylic

The Jiangmen Underground Neutrino Observatory will build the world's largest liquid scintillator detector to study neutrinos from various sources. 20 kt be stored in a $\sim$600 t acrylic sphere with 35.4 m diameter due good light transparency, chemical compatibility and low radioactivity of acrylic. concentration U/Th is required less than 1 ppt (10$^{-12}$ g/g) achieve radioactive background fiducial volume JUNO detector. mass production has started, quality control requires fast reliable radioassay on We have developed practical method measuring sub-ppt level using Inductively Coupled Plasma Mass Spectrometer (ICP-MS). can concentrated by vaporizing class 100 environment, residue collected sent ICP-MS for U/Th. All other operation done clean room, measurement 1000 room. recovery efficiency studied adding natural nonexistent nuclei $^{229}$Th $^{233}$U as tracers. resulting detection limit (MDL) 99% confidence reach 0.02/0.06 pg $^{238}$U/$^{232}$Th /g $\sim$75% efficiency. This equipment not only used acrylic, but also further optimized materials extremely radioactivity, such ultra-pure water scintillator.