The Inner Tracker system

One GEM-MSGC chamber allows to measure one coordinate of a track impact point. In order to perform track reconstruction several chambers have to be combined, to provide measurements of the track impact points in different stereo projections.

With four L-shaped detectors one plane (stereo layer) around the beam-pipe can be covered. With the aim to avoid dead regions in the acceptance the chambers are arranged in such a way that their active volumes overlap. The detectors are mounted on support plates made of carbon fibre and Nomex honeycomb (see Fig. 3.6). Several layers with different stereo angles are combined into one tracking station (superlayer).

Each superlayer consists of two separate half-stations, they cover the area to the left and to the right of the beam-pipe ($-x$,$+x$). This allows to get access to the stations for maintenance and installation. The half-stations are mounted on the frames of the corresponding Outer Tracker stations (see Fig. 3.5).

The total number of Inner Tracker chambers is 184. They are grouped according to the tasks which they have to carry out (see Fig. 3.4): magnet tracking (MC, in the magnet spectrometer), pattern recognition (PC, between magnet and RICH) and triggering (TC, chambers between RICH and ECAL). The chambers placed in the TC area are mainly needed for trigger purposes, with the aim to prolongate seeds found by the calorimeter and muon pretrigger system into the main tracking system (PC, MC).

During the comissioning run in 1999/2000, 136 chambers were installed and routinely operated in the Inner Tracker system, missing were the TC chambers. During the shutdown in 2000-2001, the decision was made to remove the chambers inside the magnet region in order to improve the resolution of the ECAL. Therefore, in the running period 2002, 149 chambers were installed and operated, in the TC area only half (24 chambers) of the installed chambers were read-out.

In order to identify each chamber, the following naming convention was used. The first two numbers identify the superlayer (01, ... 15). The third and fourth character are a +- (x) and (y) indicating the position in the $x$ and $y$ direction respectively. The last digit indicates the position inside the superlayer. For example MS10+-3 is the third layer in station MS10, in the quadrant covering positive $x$ and negative $y$.

The information summarizing the setup 2002 can be found in Tab. 3.2.

Figure 3.4: Location of the Inner Tracker superlayers along the beam axis (configuration of 2002). The $z$ positions are indicated.

Figure 3.5: Mounting of the two half stations on the frame of the Outer Tracker station. The dimensions given for the chambers situated in the PC area.

Table 3.2: The configuration of the ITR tracking stations in 2002.

ITR	number of	Type	stereo	contribution to
station	chambers		views	the FLT
MS01	16	I	$0^0,-5^0,+5^0,0^0$	no
MS10	32	II	$0^0,0^0,-5^0,-5^0,0^0,+5^0,+5^0,0^0$	yes
MS11	16	II	$0^0,-5^0,+5^0,0^0$	no
MS12	16	II	$0^0,-5^0,+5^0,0^0$	no
MS13	32	II	$0^0,0^0,-5^0,-5^0,0^0,+5^0,+5^0,0^0$	yes
MS14	24	III	$0^0,0^0,-5^0,-5^0,+5^0,+5^0$	yes
MS15	24	III	$0^0,0^0,-5^0,-5^0,+5^0,+5^0$	yes

Figure 3.6: The drawing shows the construction of the support plates with mounted GEM-MSGC chambers. In order to cover all four quadrants, two layers of the supports are needed.