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Current solar neutrino detectors measure a considerably lower flux of electron-flavor neutrinos than predicted by solar models. This could be an indication of neutrino oscillations, which would provide direct evidence that neutrinos have mass. The Sudbury Neutrino Observatory (SNO) was designed to detect all flavors of neutrinos, and will provide a rigorous test of this theory. 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 detected with an array of discrete 3He proportional counters. The strict radioactivity requirements imposed by the need for low backgrounds dictate the use of ultra-pure materials and processes in building these counters. Additionally, they must survive in the heavy water environment for several years. The design, construction, and testing of these unique counters are described.

This paper discusses the phenomenon of surface microdischarge induced by high voltage, and techniques for the reduction and discrimination of such breakdowns in ultra-low background proportional counters. An array of ultra-low background [sup.3]He-filled proportional counters will measure the neutral-current interaction rate of all active neutrino species in the Sudbury Neutrino Observatory. The sensitivity of these Neutral Current Detectors and their stringent background criteria make it essential to minimize all spurious pulses including signals induced by high voltage. Such pulses can originate from microscopic surface discharges in the various proportional counter components. Studies have shown that this discharge effect occurs mainly at interfaces and in microscopic voids between dielectric surfaces, on contaminated surfaces, and on surfaces with imperfections. Because of its occurrence in various materials and environments, microdischarge is a concern for all low-background detectors that operate under high voltage.