Developing the radium measurement system for the water Cherenkov detector of Jiangmen Underground Neutrino Observatory.

The Jiangmen Underground Neutrino Observatory is proposed to determine neutrino mass hierarchy using a 20~kton liquid scintillator detector. Strict radio-purity requirements have been put forward for all the components of the detector. One of the main background source is the $^{222}$Rn dissolved in the water Cherenkov detector, which should be below 200~mBq/m$^3$. $^{226}$Ra, the progenitor of $^{222}$Rn, should also be taken seriously into account. In order to measure the radium concentration in water, a radium measurement system, which consists of the radium extracting system, the radon emanation chamber and the radon concentration measurement system, has been developed. In this paper, the updated radon concentration measurement system with a sensitivity of $\sim$5~mBq/m$^3$ for one day measurement, the production of Mn-fibers, which is the key part of radium extracting system, the calibration process as well as the measurement results of Daya Bay water samples will be presented.