High-voltage microdischarge in ultra-low background He-3 proportional counters

This paper discusses the phenomenon of surface microdischarge induced by hi gh voltage, and techniques for the reduction and discrimination of such bre akdowns in ultra-low background proportional counters. An array of ultra-to w background He-3-filled proportional counters will measure the neutral-cur rent interaction rate of all active neutrino species in the Sudbury Neutrin o Observatory. The sensitivity of these Neutral Current Detectors and their stringent background criteria make it essential to minimize all spurious p ulses including signals induced by high voltage. Such pulses can originate from microscopic surface discharges in the various proportional counter com ponents. Studies have shown that this discharge effect occurs mainly at int erfaces and in microscopic voids between dielectric surfaces, on contaminat ed 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.