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

TL;DR

This paper presents a highly sensitive ICP-MS method for measuring ultra-trace levels of uranium and thorium in liquid scintillator, achieving detection limits below 1 part per quadrillion through acid extraction and meticulous contamination control.

Contribution

It introduces a novel acid extraction technique combined with ICP-MS to detect $^{238}$U and $^{232}$Th at sub-ppq levels in liquid scintillator, surpassing previous sensitivity limits.

Findings

Detection limits of 0.2-0.3 ppq for $^{238}$U and $^{232}$Th.

Nearly 100% recovery efficiency achieved.

Method applicable for ultra-trace radiopurity screening.

Abstract

Liquid scintillator (LS) is commonly utilized in experiments seeking rare events due to its high light yield, transparency, and radiopurity. The concentration of $^{238}$U and $^{232}$Th 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 $^{238}$U and $^{232}$Th. This study outlines a method for detecting $^{238}$U and $^{232}$Th in LS at the sub-ppq level using ICP-MS, involving the enrichment of $^{238}$U/$^{232}$Th from the LS through acid extraction. With meticulous cleanliness control, $^{238}$U/$^{232}$Th in approximately 2 kg of LS is concentrated by acid extraction with 0.4 (0.3) pg $^{238}$U ($^{232}$Th) contamination. Three standard adding methods are employed to assess recovery efficiency, including radon daughter, 2,5-diphenyloxazole (PPO), and natural non-existent $^{233}$U/$^{229}$Th. The method detection limit at a 99% confidence level of this approach can reach approximately 0.2-0.3 ppq for $^{238}$U/$^{232}$Th with nearly 100% recovery efficiency.