The Water Systems

The water systems in SNO must be able to maintain very large volumes of water at very high levels of purity. Elements from the Uranium  and Thorium radioactive chains must be reduced to concentrations a million times lower than is found in natural water. If radioactive elements decay in or near the detector the signal could be large enough to be indistinguishable from a neutrino event.

The purpose of the outer light water is to provide a medium for absorption of gamma rays and neutrons that result from activity within the rock.

For the 1000 tonnes of heavy water, impurities must be less than 10^-14g/g of water, while for the 7000 tonnes of light water the levels must be maintained at less than 10^-13g/g of water.

All detector materials that come into contact with the water have been carefully analyzed and selected for low radio-impurity levels and low rate of impurity "out-gassing". (For example, it is well known that many plastics "out-gas", that is why they often smell). Even then, the water systems will need to purify water at a rate of hundreds of liters per minute.

The purification systems must also prevent any biological activity in the water, as this could severely compromise the UV transmittance of the water.

 The main components are:

• Large high pressure reverse osmosis units                                                   
• Seeded ultra filtration                                                                          
• Ion exchangers                                                                                        
• Ultra violet sterilization and vacuum degassing.             
• Specialized filters and complexing agents have been developed to remove particular elements and to provide for impurity monitoring.
• Lead and MnO₂ bead filters have been developed for extraction of Uranium and Thorium derivatives, and for water quality tests.