Optimization of the Gas Flow and Quality Check of a GEM Tracker

In this paper we report results of a fluid-dynamics performance study of Gas Electron Multiplier (GEM) detector. The GEM technology has been proven to tolerate a rate larger than 50 MHz/cm2 without noticeable aging and to provide the sub-millimeter resolution on working chambers up to 45x45 cm2 [1]. A new GEM based tracker is under development for the upgrade of the Hall A equipment at Jefferson Lab. The chambers of the tracker have been designed in a modular way: each chamber consists of 3 adjacent GEM modules, with an active area of 40x50 cm2 each [2]. We optimized the gas flow inside the GEM module volume, a mixture of Ar/CO2 (70/30), using a COMSOL code. Our simulation includes design of the inlet-outlet pipes, maximization of the uniformity of the gas flux and minimization of the zones where such flux is too low. Quality checks of GEM foil consist of: Optical and High voltage test. The first is an inspection of the anomalous sector(s) by using a microscope and the second is a high voltage test, in order to check leakage current in the GEM foil.