This configuration is that of a time projection chamber ( TPC ); it allows three-dimensional reconstruction of the interaction site, since the depth coordinate ( z ) can be measured very accurately from the time separation between the two light pulses.
22.
The detector is a time-projection chamber ( TPC ), using the time between S1 and S2 signals to find the interaction depth since electrons move at constant velocity in the liquid ( around 1 2 km / s, depending on the electric field ).
23.
The Inner Tracking System ( ITS ) ( consisting of three layers of detectors : Silicon Pixel Detector ( SPD ), Silicon Drift Detector ( SDD ), Silicon Strip Detector ( SSD ) ), the Time Projection Chamber ( TPC ) and the Transition Radiation Detector ( TRD ) measure at many points the passage of each particle carrying an electric charge and give precise information about the particle's trajectory.
24.
The advantages of GEMs, compared to multiwire proportional chambers, include : ease of manufacturing, since large-area GEMs can in principle be mass-produced, while wire chambers require labor-intensive and error-prone assembly; flexible geometry, both for the GEM and the readout pads; and suppression of positive ions, which was a source of field distortions in time-projection chambers operated at high rates.
