Measurement of <f>222Rn</f> dissolved in water at the Sudbury Neutrino Observatory

The technique used at the Sudbury Neutrino Observatory (SNO) to measure the concentration of <f>222Rn</f> 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 <f>N2</f>. After roughly <f>0.5 tonnes</f> of <f>H2O</f> or <f>6 tonnes</f> of <f>D2O</f> have been sampled, the accumulated radon is transferred to a Lucas cell. The cell is mounted on a photomultiplier tube which detects the <f>α</f>-particles from the decay of <f>222Rn</f> and its progeny. The overall degassing and concentration efficiency is about 38% and the single-<f>α</f> counting efficiency is approximately 75%. The sensitivity of the radon assay system for <f>D2O</f> is equivalent to <f>∼3×10-15 g U/g</f> water. The radon concentration in both the <f>H2O</f> and <f>D2O</f> 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. [Copyright &y& Elsevier]