engleski

Electrical discharge propagation in GEM detector

The stable operation of a gaseous detector is, besides its’ performance, one of the most important design features. It is well known that gaseous detectors are prone to the occurrence of electrical discharges. In this dissertation, the delayed discharge propagation (DP) to the readout board in the GEM (Gas electron multiplier) detector has been studied. Sometimes, after a primary discharge, within the GEM foil hole, a propagating discharge (PD) to the readout electrode, adjacent GEM foil or the drift electrode can occur. Primary discharges are usually not destructive, but a PD to the readout board can seriously compromise the stable operation of the detector and cause permanent damage. The delayed DP does not happen instantaneously after the initial primary discharge, but with a delay in the range of 1-100 μs. The physical mechanism behind the microsecond time delay was not fully understood. Measurements conducted on a newly developed experimental setup, for the first time, enabled simultaneous electrical and optical measurements of the delayed DP in GEM detectors, both with a still and a high-speed camera. A custom made single hole THGEM foil enabled a large number of measurements and allowed a simple induction of a primary discharge with an over-voltage. Measurements of the delayed DP were made for different gas mixtures and powering configurations. A current in the induction region at induction field values below the onset field for the delayed DP was observed at the standard induction field orientation after the primary discharge. The measurements showed that the charge transfer through the induction region significantly differs with an inverted induction field. The induction region current was an indication of a physical process inside the GEM detector that precedes the delayed DP. Three different current regimes in the induction region were identified in the time in between the primary and the delayed DP: initial current decay, constant current regime and pre-delayed current rise. Important information about the source of the induction current has been revealed with a high-speed camera, where the current regimes were correlated to the observed glow at the GEM bottom electrode. The physical mechanism that includes the heat generation and the thermionic emission was proposed as a possible explanation of the delayed DP occurrence.

GEM detector ; GEM foil discharge ; microsecond time delay ; discharge propagation ; glow discharge ; thermionic emission

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