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T: Fachverband Teilchenphysik

T 107: Combined instrumentation session I: Gaseous detectors (joint session HK/T)

T 107.4: Vortrag

Freitag, 3. April 2020, 11:45–12:00, J-HS C

The Charge-Up Effect in GEM Detectors – Simulations and Measurements — •Philip Hauer, Karl Flöthner, Dimitri Schaab, Markus Ball, and Bernhard Ketzer — Univ. Bonn, Helmholtz-Institut für Strahlen- und Kernphysik, Bonn, Germany

Gas Electron Multipliers (GEM) are widely used as an amplification stage in gaseous detectors exposed to high rates, e.g. in the Time Projection Chamber of the ALICE (A Large Ion Collider Experiment) experiment after its upgrade. Typically, the GEM consists of a polyimide foil which is coated from both sides with thin layers of copper. Holes are etched into this structure in which electrons can get multiplied.

During the multiplication process, some electrons and ions diffuse to the polyimide part of the GEM and are adsorbed there, which change the electric field inside the holes. This is known as the charge-up effect. Many publications suggest that it is causing a change of the effective gain with time but a quantitative description is often missing.

In this work, the charge-up effect was investigated quantitatively in simulations and measurements. The simulations are based on an iterative approach, where new field maps are calculated with a finite element method and the deposition of charges is simulated with Garfield++. For the measurements, a dedicated test detector was set up with a single (standard) GEM foil as amplification stage. In this talk, the results from both approaches will be shown and compared to each other. A special focus lies on the influence of initial rate, applied voltage and different hole shapes on the charge-up effect.

Supported by BMBF.