A new technique to establish the uniformity of the induction gap in GEM based detectors

This work explores the use of multichannel readout electronics, already in use for quality assurance in gain uniformity studies, to measure the uniformity of the induction gap in Gas Electron Multipliers (GEM) based detectors. The devised procedure also provides a qualification of the readout electrodes in terms of disconnected or shorted channels. The measurement is based on inducing a signal on the readout strips by pulsing the bottom layer of the GEM foil, and measuring the amplitude of the induced signal. In this work, signals are readout using the analog APV25 front-end chip and the Scalable Readout System (SRS) developed by the RD51 collaboration at CERN. Studies on small and large area GEM detectors, effects of mechanical stress, and of electric fields are presented. Sensitivity to defects in the readout plane is also verified.