Investigating discharge formation in Micro-Pattern Gas Detectors
Towards a universal charge limit for MPGDs
Micro-pattern gas detectors are widely used in high-energy physics experiments thanks to their high resolution, low cost when utilized over large areas, and high radiation hardness. Nonetheless, one of the key limitations to the long-term sustainability of these devices is electrical discharges, which might occur during high-gain operation. Discharge events lead to dead-times and can even result in irreparable damage to the detector components; therefore, there has been extensive research aiming to develop methods to mitigate or subdue such events. This effort has yielded great success and enabled the development of very reliable and stable instruments. However, there are still major unanswered questions remaining concerning the fundamental mechanisms leading to the formation of electrical discharges in GEMs. In our studies, we investigated discharge formation in GEMs, Thick-GEMs (THGEM), and Micromegas detectors.
We observed that the discharge probability is the highest when the charge deposit occurs in the closest vicinity of the TH/GEM holes or Micromegas mesh, and that the breakdown limit is lower for argon mixtures than for neon mixtures. These findings are in line with the well-grounded hypothesis of the charge density being the limiting factor of MPGD stability against discharges.
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Investigating discharge formation in Micro-Pattern Gas Detectors
A very peculiar effect observed with GEM and THGEM detectors operated with strong induction fields (electric field between the bottom-most GEM and the readout anode) is the onset of delayed secondary discharges. As opposed to the standard primary GEM discharge (which occurs inside the GEM amplification hole), secondary discharges are breakdown events occurring in the gap between the GEM and the anode. Due to the large gap, they are also much more energetic, and with the direct involvement of the electrodes connected to the readout electrodes, much more dangerous for the detector. Secondary discharges are always triggered by primary discharges inside a GEM hole, but the time delay between the two events can be significantly longer than what can be expected in any streamer-based discharge formation process. As such, the initial formation and evolution of these events remain a puzzle. We have conducted many studies aiming to find ways not only to mitigate but also to understand these discharges. Especially, in our studies using THGEMs with electrode layers of a variety of different metals, we observed that some electrode materials (especially Molybdenum) are much more effective in mitigating the onset of secondary discharges compared to others. Our current understanding points towards the surface smoothness properties as the leading source of the observed material dependency, which in turn supports the thermal emission of secondary ions as the delayed trigger of secondary discharges, which was proposed by other groups.
Influence of humidity on MPGD stability
MPGDs (Micro-Pattern Gaseous Detectors) are gaseous detectors used in high-energy physics experiments like ALICE or ATLAS at the LHC. Despite 30 years long experience in the production and successful operation of this type of detector, the effect of water contamination on the gas composition on their performance is still a subject of debate. We contribute to this topic with systematic studies using several MPGDs (GEMs, THGEMs, and Micromegas) operated with an Ar-CO2 mixture and introducing water content in a range of 0-5000 ppmV. Detector performance is evaluated while varying the humidity level for each type of MPGD under study. The water is introduced to the detector vessel by incorporating a water-filled bubbler into the gas system, through which gas can be flushed at different rates. It is observed that the presence of increased humidity does not degrade any of the studied performance criteria. On the contrary, our measurements suggest an improvement in discharge stability with increasing humidity levels, at the highest gains and fields. We conclude that adding a small amount of water to the gas mixture may be beneficial for the stable operation of an MPGD.
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