2 Central Muon Detector

The concept of the construction of a redundant detector was propagated through all the design phases. In fact the basic barrel chamber module is composed by three superlayers ( two measuring the coordinate in the bending plane and one in the longitudinal plane) each one split in four layers of staggered drift tubes as shown in Figure 2.

Figure 2: Cross section of a barrel muon chamber.

The inner superlayer in the bending plane is separated from the other ones by a 20cm thick aluminium honeycomb plate that supplies the module with the required stiffness, permits the opening of an electromagnetic shower eventually generated inside the iron allowing a better measurement in the superlayers away from the iron and provides a lever arm in the bending plane useful for triggering purposes.

The barrel chambers were supposed to be drift chambers and were required to provide a final resolution of 100µm per station and to be linear in the time to space conversion, i.e. to have a drift velocity constant along the whole drift path.

Most of the past studies were therefore devoted to careful evaluation of the drift cell layout in order to meet these requirements. The final result was a multi-electrode cell built glueing insulated aluminium I-beams on grounded aluminium plates carrying one shaping electrode facing the anode wire. The cell layout is shown in Figure 3, where the electron drift lines as computed from the simulation program GARFIELD [2] are also shown. The shaping effect of the electrode facing the anode and of the C-shape of the cathode is evident, since all the drift field lines are swept to the cathode.

Figure 3: Drift cell layout showing drift lines.

Several tests were done on a muon beam using various chamber prototype and different gas mixtures [3]. The results of the tests fully matched the requirements. Figure 4 shows that the deviation from linearity of the space-time relationship was below 100 µm all along the drift cell while Figures 5a and 5b show that also the single hit resolution is reasonably flat, even for large angles. The resolution averaged on the drift cell is reported in Figure 5c, where we see that the target resolution of 100 µm per eight planes is achieved for the whole expected angular range, since we do not expect angles of incidence larger than 45.

Figure 4: Deviation from linearity along drift cell for normal incident muons.

Figure 5: Performance of the drift cell: resolution along drift cell (a) at normal incidence; (b) at large angle ; (c) resolution versus incident angle.