MSGC, GEM, GEM-MSGC

The idea behind a Micro Strip Gas Counter (MSGC) [#!oed!#] is the same as that of a Multi-Wire Proportional Chamber (MWPC). Instead of wires in the MWPC an electrode structure in the MSGC is used, it is produced on a solid substrate, in case of HERA-B specific glass is used. The solid support prevents different types of instability and allows to select a small readout pitch. As a result the intended granularity and spatial resolution could be achieved.

The electrode structure consists of anodes (typical width $\approx10\mu m$) and cathodes (typical width $\approx170\mu m$). The ground potential is applied to the anodes and negative voltage to the cathodes ( $\approx500$ V), they play the role of field wires in case of the MWPC. A drift field of several kV/cm is applied between the drift electrode and the MSGC structure which forces electrons deposited by an ionizing particle to drift toward the MSGC plane. The simulated distribution of the electrical field is shown in Fig. 3.1. The amplification occurs near the anodes, where the electrical field strength is maximum. The amplification factor can reach the order of several thousands. The advantage of the MSGC construction is that field cathodes are placed close to the anodes, the ions produced in the avalanche near the anodes are quickly removed, avoiding the build-up of space charges. A high field strength allows to speed up drift velocity and as a result provide fast chamber signals.

Figure 3.1: Simulated electrical field of the MSGC is shown. Gas amplification possible near the anodes which are located in the region with a maximum field strength. The typical sizes are indicated.

The robustness of the MSGC was tested with the help of an electron beam and photon sources (Fe 55) [#!bspaper98!#]. The device demonstrated stable behaviour and promised to survive during five years of operation in such environment. In the further tests, it turned out that the electrode structure of the detector in flux of pions and protons is destroyed nearly immediately. The reason for such a behaviour are discharges between anodes and cathodes, caused by heavy ionizing particles passing close to the surface of the MSGC and depositing a large amount of charge between cathode and anode. This phenomenon was observed for the first time during beam tests in PSI and later reproduced in the laboratory with the help of $\alpha$-particle sources. These discharges ruled out the use of the pure MSGC solution in case of HERA-B.

The Gas Electron Multiplier (GEM) [#!sauli!#] is a thin insulating foil, copper-clad on both sides. The foil is perforated with a regular pattern of holes (for the perforating a chemical etching procedure is used). The GEM is placed in the homogeneous drift field between drift electrode and MSGC structure. The difference of potential between the two metal sides is $\approx$ 400 V. The field lines are forced through the GEM holes and inside the holes gas amplification can occur. The amplification depends on the geometry of the holes, thickness of the metal-clad and field configuration above and below the GEM. With the voltages chosen for the Inner Tracker chambers the amplification at the GEM is in the range of 20-100.

Figure 3.2: Left: a GEM foil used for the ITR system is shown. Right: The simulated electrical field inside the GEM hole is shown.

After a variety of tests in order to overcome the MSGC discharge problems, the solution was found by using MSGC with GEM as pre-amplification structure for the ITR system, this solution has proven to be resistant enough to be used. The main advantage of this setup is that the gas amplification occurs at two well separated stages and the gas gain at the MSGC structure can be reduced. With the help of the amplification separation, the discharge probability could be reduced by several orders of magnitude and a stable and efficient operation of the GEM-MSGC can be achieved also in hadronic beams.