Systematic investigation of critical charge limits in Thick GEMs

TL;DR

This study investigates discharge probabilities in Thick GEMs, revealing how gas composition affects stability, and identifies a critical charge threshold for sparking, with findings compared to traditional GEMs.

Contribution

It provides the first detailed measurement of critical charge limits in Thick GEMs and compares their discharge behavior to GEMs, highlighting geometrical influences.

Findings

Discharge probability varies with gas mixture composition.

Critical charge for sparking is 3-7×10^6 electrons.

THGEMs have higher discharge probability than GEMs due to geometrical factors.

Abstract

We present discharge probability studies performed with a single Thick Gas Electron Multiplier (THGEM) irradiated with alpha particles in Ar-CO$_2$ and Ne-CO$_2$ mixtures. We observe a clear dependency of the discharge stability on the noble gas and quencher content pointing to lighter gases being more stable against the development of streamer discharges. A detailed comparison of the measurements with Geant4 simulations allowed us to extract the critical charge value leading to the formation of a spark in a THGEM hole, which is found to be within the range of 3-7$\times10^6$ electrons, depending on the gas mixture. Our experimental findings are compared to previous GEM results. We show that the discharge probability of THGEMs exceeds the one measured with GEMs by orders of magnitude. This can be explained with simple geometrical considerations, where primary ionization is collected by a lower number of holes available in a THGEM structure, reaching higher primary charge densities and thus increasing the probability of a spark occurrence. However, we show that the critical charge limits are similar for both amplification structures.