[en] As photodisintegration of deuterons mimics the disintegration of deuterons by neutrinos, the accurate measurement of the radioactivity from thorium and uranium decay chains in the heavy water in the Sudbury Neutrino Observatory (SNO) is essential for the determination of the total solar neutrino flux. A radium assay technique of the required sensitivity is described that uses hydrous titanium oxide adsorbent on a filtration membrane together with a β-α delayed coincidence counting system. For a 200 tonne assay the detection limit for ²³²Th is a concentration of ∼3x10^-16 g Th/g water and for ²³⁸U of ∼3x10^-16 g U/g water. Results of assays of both the heavy and light water carried out during the first 2 years of data collection of SNO are presented