# Charge density as a driving factor of discharge formation in GEM-based detectors

## TL;DR

This study investigates how charge density influences discharge formation in GEM detectors, revealing that discharge probability depends on proximity of charge deposit and gas mixture, with critical charge density identified for spark formation.

## Contribution

It provides experimental evidence linking charge density to GEM stability and quantifies the critical charge density for spark formation across different gas mixtures.

## Key findings

- Discharge probability peaks when charge deposit is near GEM holes.
- Breakdown limit is lower in argon mixtures compared to neon.
- Critical charge density for spark formation is approximately (5-9)×10^6 electrons.

## Abstract

We report on discharge probability studies with a single Gas Electron Multiplier (GEM) under irradiation with alpha particles in Ar- and Ne-based gas mixtures. The discharge probability as a function of the GEM absolute gain is measured for various distances between an alpha source and the GEM. We observe that the discharge probability is the highest when the charge deposit occurs in the closest vicinity of the GEM holes, and that the breakdown limit is lower for argon mixtures than for neon mixtures. Our experimental findings are in line with the well-grounded hypothesis of the charge density being the limiting factor of GEM stability against discharges. A detailed comparison of the measurements with GEANT4 simulations allowed us to extract the critical charge density leading to the formation of a spark in a GEM hole. This number is found to be within the range of $(5-9)\times10^6$ electrons after amplification, and it depends on the gas mixture.