Characterization of a hybrid GEM-Micromegas detector with respect to its application in a continuously read out TPC

In the context of the upgrade of the LHC during the second long shutdown the interaction rate of the ALICE experiment will be increased up to 50 kHz for Pb-Pb collisions. As a consequence, a continuous read-out of the Time Projection Chamber (TPC) will be required. To keep the space-charge distortions at a manageable size, the ion backflow of the charge amplification system has to be significantly reduced. At the same time an excellent detector performance and stability of the system has to be maintained. A solution with four Gaseous Electron Multipliers (GEMs) has been adopted as baseline solution for the upgraded chambers. As an alternative approach a hybrid GEM-Micromegas detector consisting of one Micromegas (MM) and two GEMs has been investigated. The recent results of the study of the hybrid GEM-Micromegas detector will be presented and compared to measurements with four GEM foils.

Multi-GEM Detectors in High Particle Fluxes

Gaseous Electron Multipliers (GEM) are well known for stable operation at high particle fluxes. We present a study of the intrinsic limits of GEMdetectors when exposed to very high particle fluxes of the order of MHz/mm2. We give an interpretation to the variations of the effective gain, which, as a function of the particle flux, first increases and then decreases. We also discuss the reduction of the ion back-flow with increasing flux. We present measurements and simulations of a triple GEM detector, describing its behaviour in terms of accumulation of positive ions that results in changes of the transfer fields and the amplification fields. The behaviour is expected to be common to all multi-stage amplification devices where the efficiency of transferring the electrons from one stage to the next one is not 100%.

Hole-Type Gaseous Electron Multipliers (GEM)

This chapter describes another popular micropattern detector, the Gas Electron Multiplier (GEM). The GEM belongs to the family of hole-type detectors made of a dielectric sheet metalized on both sides with a matrix of holes through it. When a voltage is applied between the metalized electrodes, a strong electric field is created inside the holes. The electric field is sufficiently strong for avalanche multiplication of primary electrons produced by radiation in a drift region adjacent to the plate. In contrast to other hole-type detectors, GEM is manufactured by a photolithographic technology from thin metalized Kapton sheets. This detector has several unique features (e.g. the possibility to operate in cascade mode to increase the maximum achievable gain or to combine a GEM with other gaseous detectors, MSGC, MICROMEGAS, etc.). Cascaded GEMs are used today in several experiments at CERN and elsewhere. A modified robust version of the GEM, called a “thick GEM,” can operate at gas gains higher than ordinary GEMs and is used in various designs of photodetectors.