Cascaded GEM and MHSP detectors for TPC readout

Abstract

Cascaded gas electron multipliers (GEMs) are used for the fast detection and imaging of single and multiple charges; they operate in a variety of gases, with very high gains attained due to an efficient suppression of avalanche-induced photon feedback. However, since they are highly transparent to the flow of charges it is not straightforward to restrict the ion backflow without losing charge gain. When the cascaded-GEM detector is coupled to a TPC it may benefit from favorable electric field configuration, resulting in an efficient trapping of a large fraction of the ions on the GEM electrodes and a reduced ion backflow to the TPC volume. We have recently shown that the ion backflow is further reduced by replacing the last GEM in the cascade with the newly proposed Micro-Hole-and-Strip Plate (MHSP). A new operation-mode arises, reducing the ion backflow by about an order of magnitude without losing speed or gain of the detector.