Low-background He-3 proportional counters for use in the Sudbury Neutrino Observatory

Current solar neutrino detectors measure a considerably lower flux of elect ron-flavor neutrinos than predicted by solar models. This could be an indic ation of neutrino oscillations, which would provide direct evidence that ne utrinos have mass. The Sudbury Neutrino Observatory (SNO) was designed to d etect all flavors of neutrinos, and will provide a rigorous test of this th eory. The SNO detector's heavy water target gives it the unique ability to detect all non-sterile neutrino flavors via the neutral-current (NC) break- up of the deuteron. This NC interaction liberates a neutron which may be de tected with an array of discrete He-3 proportional counters. The strict rad ioactivity requirements imposed by the need for low backgrounds dictate the use of ultra-pure materials and processes in building these counters. Addi tionally, they must survive in the heavy water environment for several year s. The design, construction, and testing of these unique counters are descr ibed.