GEM-MSGC at HERA-B

The GEM-MSGC chamber used in the Inner Tracker system of HERA-B is schematically shown in Fig. 3.3.

In case of HERA-B the GEM foil is a 50 $\mu$m thick Kapton foil claded with 7 $\mu$m copper layers on both sides. The GEM holes have a double conical shape with a diameter of about 55 $\mu$m at the narrowest point. The smallest distance between two holes is about 140 $\mu$m. All foils used for the construction of the ITR chambers were produced at the CERN workshop with the help of a photolithographic method. The GEM foil is stretched and mounted on a hollow frame.

The MSGC wafer is made out of 400 $\mu$m thin DESAG AF45 glass [#!desag!#], coated with a 80 nm layer of amorphous carbon (``diamond like coating''). This coating has been produced by a CVD (chemical vapor deposition) process [#!fhg!#]. The coating is used to get a well-defined surface resistivity. The electrode structure is produced on top of this coating with a photo-lithographic process. The strips are made out of 500 nm gold, the anode strips have a width of 10 $\mu$m the cathode strips of 170 $\mu$m, between anode and cathode is a 60 $\mu$m gap resulting in a pitch size of 300 $\mu$m. The number of strips per chamber is up to 756, depending on the type of the chamber.

The drift cathode is made of a compoud of 125 $\mu$m Kapton and 300 $\mu$m G10 copper cladded with 18 $\mu$m copper.

Three types of Inner Tracker chambers are used, they differ in the radius of the beam-pipe recess and outer dimensions. The chambers situated behind the RICH have a larger dimensions. In order to keep the number of readout channels per chamber constant the pitch size has been increased to 350 $\mu$m. The geometrical parameters of the chambers used in the different superlayers are summarized in Table 3.1.

As counting gas for the GEM-MSGC chambers a mixture of $Ar/CO_2$ has been chosen with a composition of 70 % $Argon$ and 30% $CO_2$.

Figure 3.3: Sketch of the GEM-MSGC chamber used in the Inner Tracker system. The typical dimensions are indicated.

The use of the GEM in additional to the MSGC improved stability and solved the MSGC problems. However due to the introduction of the GEM foil the operation of the coupled GEM-MSGC device with 5 different electric potentials became more complicated [#!itrpaper!#].

Table 3.1: The sizes of the Inner Tracker chambers corresponding to the different parts of the HERA-B detector.

Type	Size	Superlayer	Pitch	Beam-pipe	Number of
			size	recess	anode strips
I	25 cm x 23 cm	MS01	300 $\mu$m	5.5 cm	752
II	25 cm x 23 cm	MS10 - MS13	300 $\mu$m	8 cm	752
III	27.5 cm x 27 cm	MS14,MS15	350 $\mu$m	12.5 cm	756