The Jiangmen Underground Neutrino Observatory (JUNO) will build the world’s largest liquid scintillator detector to study neutrinos from various sources. The 20 kilo tonne (kton) liquid scintillator will be stored in a 0.6 kton acrylic sphere with 35.4 m diameter due to the good light transparency, chemical compatibility and low radioactivity of acrylic. The concentration of U/Th in acrylic is required to be less than 1 ppt (10⁻¹² g/g) to achieve a low radioactive background in the fiducial volume of the JUNO detector. The mass production of acrylic has started, and the quality control requires a fast and reliable radioassay on U/Th in acrylic. We have developed a practical method of measuring U/Th in acrylic to sub-ppt level using the Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The U/Th in acrylic can be concentrated by vaporizing acrylic in a class 100 environment, and the residue will be collected and sent to ICP-MS for measuring U/Th. All the other chemical operation is done in a class 100 clean room, and the ICP-MS measurement is done in a class 1000 clean room. The recovery efficiency is studied by adding the natural nonexistent nuclei ²²⁹Th and ²³³U as the tracers. The resulting method detection limit with 99% confidence can reach 0.04/0.09 ppt ²³⁸U/²³²Th in acrylic with (77.5 ± 3.4)% and (73.6 ± 4.1)% recovery efficiency. This equipment and method can not only be used for the quality control of JUNO acrylic, but also be further optimized for the radioassay on other materials with extremely low radioactivity, such as ultra-pure water and liquid scintillator.

江门地下中微子观测站（JUNO）将建造世界上最大的液体闪烁体探测器，以研究各种来源的中微子。由于丙烯酸具有良好的透光性，化学相容性和较低的放射性，因此，该20吨（kton）液体闪烁器将存储在直径为35.4 m的0.6吨丙烯酸丙烯酸球中。丙烯酸中U / Th的浓度必须小于1 ppt（10 ^-12g / g）以在JUNO检测器的基准体积中实现低放射性本底。丙烯酸的大规模生产已经开始，质量控制要求对丙烯酸中的U / Th进行快速可靠的放射分析。我们已经开发了一种使用电感耦合等离子体质谱仪（ICP-MS）来测量丙烯酸中U / Th到亚ppt级的实用方法。可以通过在100级环境中蒸发丙烯酸来浓缩丙烯酸中的U / Th，然后将残留物收集并送至ICP-MS进行U / Th的测量。所有其他化学操作均在100级无尘室中进行，而ICP-MS测量则在1000级无尘室中进行。通过添加天然不存在的核²²⁹ Th和²³³研究回收效率U作为示踪剂。所获得的方法检测限（置信度为99％）在丙烯酸​​树脂中可以达到0.04 / 0.09 ppt ²³⁸ U / ²³² Th，回收率为（77.5±3.4）％和（73.6±4.1）％。该设备和方法不仅可以用于JUNO丙烯酸的质量控制，而且可以进一步优化用于放射性极低的其他材料（如超纯水和液体闪烁体）的放射分析。