Ion space-charge effects in multi-GEM detectors: challenges and possible solutions for future applications

Gaseous Electron Multiplier (GEM) detectors are well known both for stable operation under irradiation with high particle fluxes and high achievable effective gains. The aim of this thesis is two-fold: to investigate the limits of GEM detector operation due to space-charge effects, and to develop a means to reduce the magnitude of the observed effects and thus extend those limitations. The first part of the thesis presents a comprehensive study of the intrinsic limits of triple-GEM detectors under exposure to very high fluxes of soft X-rays or operation at very large effective gains. The behaviour of the effective gain, ion back-flow and the pulse-height spectra is explained in terms of the movement and accumulation of positive ions throughout the detector volume, resulting in distortions of the transfer and amplification fields. Numerical computations, and measurements on double-stage and single-stage detectors confirm the model describing the observed effects. Discussions on ways to extend the limits of gaseous detectors conclude the first part. In the second part of the thesis the use of graphene to eliminate the ion back-flow into the conversion volumes of gaseous detectors is investigated. Graphene is a single layer of carbon atoms arranged in a honeycomb lattice, with a reported strong asymmetry in transmission of low energetic electrons and ions. The techniques to achieve graphene layers, freely suspended in holes of several tens of micrometres diameter, are described, as well as the methods to measure the charge transfer properties through the graphene samples. Results on the electron and ion transparency of graphene in gaseous detectors are presented, with special attention to the challenges arising from detects in the graphene layers. Discussions on ways to improve the layer quality and an outlook on further studies are found at the end of this work