Developing a μBq/m³ level ²²⁶Ra concentration in water measurement system for the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory (JUNO), a 20~kton multi-purpose low background Liquid Scintillator (LS) detector, was proposed primarily to determine the neutrino mass ordering. To suppress the radioactivity from the surrounding rocks and tag cosmic muons, the JUNO central detector is submerged in a Water Cherenkov Detector (WCD). In addition to being used in the WCD, ultrapure water is used in LS filling, for which the ²²⁶Ra concentration in water needs to be less than 50~μBq/m³. To precisely measure the ²²⁶Ra concentration in water, a 6.0~μBq/m³ ²²⁶Ra concentration in water measurement system has been developed. In this paper, the detail of the measurement system as well as the ²²⁶Ra concentration measurement result in regular EWII ultrapure water will be presented.