Ratza, Viktor: Multi-stage Micropattern Gaseous Detectors for the ALICE TPC Upgrade - Studying and Modelling Charge Transfer and Energy Resolution. - Bonn, 2020. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-58779
@phdthesis{handle:20.500.11811/8421,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-58779,
author = {{Viktor Ratza}},
title = {Multi-stage Micropattern Gaseous Detectors for the ALICE TPC Upgrade - Studying and Modelling Charge Transfer and Energy Resolution},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2020,
month = jun,

note = {With the upgrade of the LHC (Large Hadron Collider) at CERN, the interaction rate of ALICE (A Large Ion Collider Experiment) will be increased up to 50 kHz for Pb-Pb collisions. Thus the gated and rate-limited readout technology of the TPC (Time Projection Chamber) requires a complete redesign to allow for a continuous operation. Micropattern Gaseous Detectors (MPGD) are considered a promising solution to overcome the gating required for the existing Multiwire-Proportional Chambers (MWPC) technology. Several solutions like a multi-GEM (Gaseous Electron Multipliers) stack and a hybrid detector consisting of two GEM stages and a single Micromegas were under investigation. A solution with four GEMs has been adopted as baseline solution for the upgraded chambers since the operation of multi-GEM stages was more understood and studied at this time.
Within the scope of this work an alternative approach consisting of two GEM foils and a single Micromegas (MM) has been investigated in terms of the energy resolution, the ion backflow and the gain. The hybrid 2GEM-MM detector as well as the newly developed Slow Control to operate the setup are presented in detail. A systematic study of the recorded 55Fe energy spectra is a central part which finally leads to a dedicated fit model to obtain the energy resolution. A comparison yields that fitting a single Gaussian distribution to the photo peak overestimates the energy resolution 1% up to 2% (difference of absolute values). The measurements are compared to the baseline solution of the ALICE TPC upgrade program as well as to a hybrid 2GEM-MM setup which has been studied at the Yale University. The hybrid 2GEM-MM detector clearly competes with the baseline solution of the ALICE TPC upgrade and the Yale measurements can be reproduced.
A major part of this work is the investigation of the charge transfer processes in GEM stacks, as these transfer efficiencies highly determine the energy resolution, the ion backflow and the gain. Within two-dimensional electrostatic calculations of electric fluxes, analytic expressions of the electron as well as of the ion transfer efficiencies can be derived as functions of the hole size, the pitch and the thickness of a GEM. The equations are compared to simulations, allowing to immediately calculate transfer efficiencies for arbitrary electrostatic configurations and GEM geometries. A big advantage is the short calculation time compared to the time-consuming simulations. The calculations lead to a profound and detailed understanding of the formation of the characteristic transfer efficiency curves.
The transfer efficiencies are used in order to derive models to calculate the energy resolution, the ion backflow and the gain of stacks with multiple GEM stages and for arbitrary electric field configurations. The model calculations allow for in-depth studies of the processes within multiple amplification stages and to understand the contributions of the individual stages to the measured quantities of the detector, i.e. energy resolution, ion backflow and gain. The models are compared to the measurements of the Bonn and the Yale hybrid 2GEM-MM detector as well as to the quadruple GEM stack of the ALICE TPC upgrade. Finally, the developed charge transfer models as well as the energy resolution model are implemented in the new ALICE O2 framework which will be used with the ongoing upgrade of ALICE for the online as well as for the offline data acquisition.},

url = {http://hdl.handle.net/20.500.11811/8421}
}

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