Impact of Single-Mask Hole Asymmetry on the properties of GEM Detectors

A single-mask Gas Electron Multiplier (GEM) technique overcomes the cumbersome practice of alignment of two masks and allows the production of foils with very large area as needed for the CMS muon forward upgrade. However, the holes obtained with refinements in the single-mask technique are conical in shape and hence asymmetric compared to the symmetric holes of double-mask technology where the holes are bi-conical. The hole geometry and their uniformity define the performance of the detectors which are constructed with such GEM foils.To evaluate the effect of this asymmetry on triple-GEM detectors, such foils have been characterized experimentally. A series of tests have been conducted on a special prototype with three single-mask GEM foils, studying effective gain and its uniformity, rate capability, charging-up behaviour, energy resolution and their variations with time when continuously irradiated with a particle source. The results have also been compared for two different hole orientations.