Investigation of the performance of an optimised MicroCAT, a GEM and their combination by simulations and current measurements

Abstract

A MicroCAT (Micro Compteurà Trous) structure which is used for avalanche charge multiplication in gas filled radiation detectors has been optimised with respect to maximum electron transparency and minimum ion feedback. We report on the charge transfer behaviour and the achievable gas gain of this device. A threedimensional electron and ion transfer simulation is compared to results derived from electric current measurements. Similarly, we present studies of the charge transfer behaviour of a GEM (Gas Electron Multiplier) by current measurements and simulations. Finally, we investigate the combination of the MicroCAT and the GEM by measurements with respect to the performance at different voltage settings, gas mixtures and gas pressures.

Gas gain and signal length measurements with a triple-GEM at different pressures of Ar-, Kr- and Xe-based gas mixtures1

“…These local vari- ations are mainly due to two aspects: First, the GEMs, themselves, produce intrinsic gain inhomogeneities because of geometry variations. Furthermore, the distances between the individual GEMs and the distance between undermost GEM and anode are not constant over all the detector area, which leads to transfer and induction field inhomogeneities und finally to variations in the charge transfer behaviour and in the effective gas gain of the GEMs [13].…”

“…Three-dimensional field calculations with Maxwell [29] using a geometry of 1 mm transfer gap, the 60 µm thick GEM foil and 1 mm induction gap (the geometry of the GEM and more simulation details are described in Ref. [13]) show that the electric field in the induction region along the symmetry axis of a GEM hole is larger than the quotient E = U GEM-bottom /d of the applied voltage on the bottom side of the GEM U GEM-bottom and the distance d between GEM bottom and anode. This effect is some relict due to the very high electric field in the holes.…”

“…Besides the operation in detectors at high energy projects like HERA-B [2], COMPASS [3] [4] the GEM is also used in gaseous photomultiplier development [5,6] or Xray imaging detectors [7,8]. Thereby, the GEM is usually combined with other micro pattern devices like MSGCs [9,10], MGCs [11], micromegas [12] or MicroCAT [13]. Also the cascade of multiple GEMs has been tested extensively [14].…”

R&D of gas filled detectors for high energy physics experiments

“…It is again to be mentioned here that a gain variation up to 25% is possible because of the intrinsic gain inhomogeneities for GEM geometry variations and also for the inhomogeneity in the gap between the individual GEM foils. This effects are extremely well described in the References [71,72].…”

“…With the new cylinder also there is no decrease in the normalized gain other than a fluctuation. It is to be mentioned here that because of intrinsic gain inhomogeneities for GEM geometry variations and also for the inhomogeneity in the gap between the individual GEM foils a gain variation up to 25% is possible which is extremely well described in References [71,72]. (However, in the experimental method described here, the source irradiates a particular region of the detector and the total summed anode current is measured to calculate the gain as stated in Section 2.1.…”