Charging-up Behaviour of MicroPattern Gaseous Detectors

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Title: Charging-up Behaviour of MicroPattern Gaseous Detectors
Author: Monira, Shirajum
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
Thesis level: master's thesis
Discipline: Fysiikka
Abstract: Particle and nuclear physics experiments require state-of-the-art detector technologies in a pursuit to achieve high data collection efficiency, and thus ensuring reliable data recording from the particle collisions at the Large Hadron Collider (LHC) experiments of CERN. High demand for data to be used for precision analysis has led to the development of MicroPattern Gaseous Detector (MPGD) based structures: Gas Electron Multiplier (GEM) and MICROMEGAS (MM). A systematic study is conducted on charging-up behaviour in a two-stage amplification structure, consisting of a single GEM foil above a MM detector with 2D readout chamber. Charging-up effect arises in the detector system due to combined effects from polarization of dielectric surfaces and accumulation of charges on the dielectric surfaces of MM resistive strips under high external electric field. The internal fields created from charging-up of dielectric surfaces can lead to a change in the applied electric field and gain of the detector suffers. In this thesis, the instability of gain due to characteristic charging-up process in GEM and MM is observed for different event rates and humidity level in the detector fill gas (P-10 gas mixture). MM gain decreased with time due to charging-up of dielectric surfaces and an exponential drop of gain by ≈30% is detected in case of dry gas i.e. fill gas without moisture. By adding a small amount of water content into fill gas, the MM gain is observed to drop around 22-30%. Addition of 1320±280 ppmV water content into MM gas volume yielded a higher gain of about 10% compared to dry gas. In case of higher rate measurement, achieved by using GEM foil as a pre-amplification stage between drift and readout electrode, the gain saturates at 70%. For low rate measurement, the gain saturates at 68%. GEM gain is observed to increase slowly by 17% as the dielectric surfaces inside its holes charged-up gradually over time.
Subject: ALICE TPC Upgrade
MicroPattern gaseous detectors
charging-up effect

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