Developing the radium measurement system for JUNO's water Cherenkov detector

The Jiangmen Underground Neutrino Observatory(JUNO) 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 is the $^{222}$Rn dissolved in the water Cherenkov detector supposed to be less than 200~mBq/m$^3$ in JUNO. $^{226}$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 and the radium measurement system, has been developed. In this paper, the manufacture of Mn-fiber, which is the key part of radium extracting system, the calibration result of extracting efficiency as well as the measurement results of Daya Bay water samples will be presented.