Sporadic electron jets from cathodes - the main breakdown-triggering mechanism in gaseous detectors

We demonstrated experimentally that the main breakdown-triggering mechanism in most gaseous detectors, including micropattern gaseous detectors, is sporadic electron jets from the cathode surfaces. Each jet may, depending on conditions, contain randomly between a few up to 10⁵ primary electrons being emitted in a time interval of 0.1 μs up to the range of ms. After the emission these primary electrons experience a full gas multiplication in the detector and create spurious pulses. The rate of these jets increases with applied voltage and very sharply at voltages close to the breakdown limit. We found that these jets are responsible for the breakdown-triggering at any counting rates used in our measurements, from 10^-2 Hz/mm² up to 10⁸ Hz/mm². We demonstrated on a few detectors that an optimized cathode-geometry, a high electrode surface quality and a proper choice of the gas mixture, allows one to considerably improve the performance characteristics and reach the highest possible gains.