The SNO Water System

The SNO experiment poses a number of extremely difficult technical challenges. Neutrino reactions are very rare so that large detectors are essential and control of background sources is critical. Radioactivity in any of the detector components can mimic the neutrino signal and it is particularly important to control and measure these backgrounds. As an example of the levels of purity that are required, the presence of some atomic species must be at levels less than one atom per tonne of water.

To exploit the physics potential of the detector it is essential to make an accurate measurement of the neutral-current rate which gives the total neutrino flux independent of the neutrino type. The Carleton group have developed a technique which greatly enhances the sensitivity of the SNO detector to this signal. It involves the addition of about 2 tonnes of sodium chloride to the heavy water. This salt must be extremely pure in order that the enhancement is not masked by increased background rates.