Controlling surface contamination at SNO

The ability of the Sudbury Neutrino Observatory (SNO) detector to measure the energy spectrum of the 8B solar neutrinos will depend on the background radiation arising from uranium and thorium contamination in the bulk material and on the surfaces of the detector. A principle surface contaminant is the ubiquitous dust found in the working nickel mine where the detector will be assembled. The thorium content of mine dust is about 6 ppm, which is a factor of 6 x 106 greater than is present in the acrylic material that holds the heavy water. The result of this is that the detector cavity, 6800 feet underground and having a volume of about 9000 cubic meters, must become a dust-free cleanroom. (It will be one of the larger cleanrooms in the world, and certainly the lowest lying.) After an 18 month construction period, the amount of dust present on the surfaces of the detector must be less than 0.4 micrograms/cm2. A variety of techniques has been developed to measure these small amounts of surface contamination. These will be described along with the measures planned to achieve the surface cleanliness requirements of the SNO detector