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

This work is proposing and exploring the use of multichannel readout electronics, already used in quality assurance for gain uniformity studies, to measure the uniformity of the induction gap in GEM based detectors. The measurement will furthermore provide a qualification of the readout electrodes in terms of disconnected or shorted channels. The proposed method relies on the dependence on the induction gap capacitance between the readout strips and the bottom of the GEM. The measurement is obtained inducing capacitively a signal on the readout strips pulsing the bottom layer of the GEM foil. In this work, the signals are read with the analog APV25 front-end chip and the RD51 Scalable Readout System (SRS). Studies on small and large area GEM detectors, relative variations under mechanical stress, and in presence of electrical fields will be shown. Sensitivity to defects in the readout plane will be proven.