We describe a method to measure the concentration of 224 Ra and 226 Ra in the heavy water target used to detect solar neutrinos at the Sudbury Neutrino Observatory () and in the surrounding light water shielding. A water volume of (50-400) m 3 from the detector is passed through columns which contain beads coated with a compound of manganese oxide onto which the Ra dissolved in the water is adsorbed. The columns are removed, dried, and mounted below an electrostatic chamber into which the Rn from the decay of trapped Ra is continuously flowed by a stream of N 2 gas. The subsequent decay of Rn gives charged Po ions which are swept by the electric field onto a solid-state α counter. The content of Ra in the water is inferred from the measured decay rates of 212 Po, 214 Po, 216 Po, and 218 Po. The Ra extraction efficiency is >95%, the counting efficiency is 24% for 214 Po and 6% for 216 Po, and the method can detect a few atoms of 224 Ra per m 3 and a few tens of thousands of atoms of 226 Ra per m 3 . Converted to equivalent equilibrium values of the topmost elements of the natural radioactive chains, the detection limit in a single assay is a few times 10 −16 g Th or U/cm 3 . The results of some typical assays are presented and the contributions to the systematic error are discussed.

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