Calibration of the ALICE Transition Radiation Detector

The Transition Radiation Detector (TRD) belongs to the central barrel detectors of the ALICE experiment at the LHC at CERN. It is composed of 540 large area drift chambers, with a total of 1181952 readout pads (1728 or 2304 per chamber). To ensure the optimal performance of the detector, designed for electron/pion identification and charged particle tracking, including triggering for high-pt electrons, a calibration procedure has been devised and tested with simulated events. Operating with a Xe-CO2 [85-15] gas mixture at atmospheric pressure, the TRD chambers will exhibit a short-term variability in gas gain (g) and drift velocity (vd). For these two parameters, the calibration factors will be updated per chamber at intervals of several hours. The long-term variability arises from the chamber mechanical nonuniformities and will influence the values of the gas gain and the width of the pad response function (wPRF ), while the time reference (t0) is influenced by the details of signal transmission via the traces from pads to the front-end electronics. Calibration factors are foreseen in this case for each pad and the update will be done yearly. The values of g, vd, t0 and wPRF are expected to vary, respectively, within ± 20%, 10%, 5% and 5% around their nominal values over all the detectors.