A practical approach of measuring ²³⁸U and ²³²Th in liquid scintillator to sub-ppq level using ICP-MS

Liquid scintillator (LS) is commonly utilized in experiments seeking rare events due to its high light yield, transparency, and radiopurity. The concentration of ²³⁸U and ²³²Th in LS consistently remains below 1 ppq (10⁻¹⁵ g/g), and the current screening result is based on a minimum 20-ton detector. Inductively coupled plasma mass (ICP-MS) spectroscopy is well-regarded for its high sensitivity to trace ²³⁸U and ²³²Th. This study outlines a method for detecting ²³⁸U and ²³²Th in LS at the sub-ppq level using ICP-MS, involving the enrichment of ²³⁸U/²³²Th from the LS through acid extraction. With meticulous cleanliness control, ²³⁸U/²³²Th in approximately 2 kg of LS is concentrated by acid extraction with 0.4 (0.3) pg ²³⁸U (²³²Th) contamination. Three standard adding methods are employed to assess recovery efficiency, including radon daughter, 2,5-diphenyloxazole (PPO), and natural non-existent ²³³U/²²⁹Th. The method detection limit at a 99% confidence level of this approach can reach approximately 0.2-0.3 ppq for ²³⁸U/²³²Th with nearly 100% recovery efficiency.