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The Gas Electron Multiplier (GEM) foil is an amplification stage that hasbeen introduced to overcome the problem of discharges observed in gaseousdetectors. There are two major production techniques of GEM foils: double-maskand single-mask etching. Despite being an effective method, an asymmetry isobserved between the top and bottom diameters of GEM holes in single maskcompared to double mask. In this paper we describe extensive simulations andexperimental measurements to study this hole asymmetry and its effect on theperformance of GEM based detectors. The experimental data is collected usingGEM foils of various hole geometries and orientations. In simulations, the samedimensions are used to study the properties of the detector. Simulations areperformed with the Garfield$^{++}$ simulation package along with ANSYS forcreating the geometry of the GEM foils as well as the triple-GEM detector andthe meshing needed for the field calculations. The simulation results match theobservations from experimental measurements. The gains measured with single andtriple-GEM detectors are lower if asymmetric foils are oriented with thesmaller diameters towards the readout plane. Detailed simulation of theamplification and collection steps indicates that the lower gain is attributedto a loss of electrons at the GEM3 foil for the first time.

Impact of the hole orientation of asymmetric GEM foils on the performance of single and triple GEM detectors