Measurement of 222Rn dissolved in water at the Sudbury Neutrino Observatory

Abstract The technique used at the Sudbury Neutrino Observatory (SNO) to measure the concentration of 222 Rn in water is described. Water from the SNO detector is passed through a vacuum degasser (in the light water system) or a membrane contact degasser (in the heavy water system) where dissolved gases, including radon, are liberated. The degasser is connected to a vacuum system which collects the radon on a cold trap and removes most other gases, such as water vapor and N 2 . After roughly 0.5 tonnes of H 2 O or 6 tonnes of D 2 O have been sampled, the accumulated radon is transferred to a Lucas cell. The cell is mounted on a photomultiplier tube which detects the α-particles from the decay of 222 Rn and its progeny. The overall degassing and concentration efficiency is about 38% and the single-α counting efficiency is approximately 75%. The sensitivity of the radon assay system for D 2 O is equivalent to ∼3×10 −15 g U/g water. The radon concentration in both the H 2 O and D 2 O is sufficiently low that the rate of background events from U-chain elements is a small fraction of the interaction rate of solar neutrinos by the neutral current reaction.