Transition Radiation Detector
The Transition Radiation Detector (TRD) is located in the ALICE central barrel, which is used for the precise reconstruction of tracks of charged particles. The special feature of the TRD is that it allows distinguishing electrons from other charged particles. This is based on the emission of transition radiation (TR): A few real photons in the X-ray regime, every time an electron traverses the detector. The TRD has the capability to mark events with identified electrons and this can lead to a trigger decision within about 8 microseconds after a collision. This feature was used in LHC Run2 to select collisions with charmonia, jets or atomic nuclei. Requiring that electrons are measured with an efficiency of 90 %, pions are suppressed by more than a factor 100 using the TR signature and likelihood methods enhanced by machine learning.
The TRD is composed of 522 individual detectors filled with a Xenon-CO2 gas mixture. A charged par- ticle traversing a detector leaves an ionization trail in a 30 mm deep drift region where also the TR photons are converted into electrons. All electrons drift to thin anode wires (25 micrometer diameter) and lead to a concurrent induced charge signal on cathode pads of typical size 0.7 cm x 8.7 cm. The charge signal is sampled in 20 time bins, 100 ns apart, leading to a track segment (tracklet) in each individual detector. The detectors are of a typical size of 110 cm x 140 cm and cover a total area of 675 m2 with 1.15 million front-end electronics (FEE) channels using custom designed ASICs directly on the detectors. There, the charge signals are shaped, digitized, and filtered; the tracklet parameters are computed by massive parallel processing in about 250,000 CPUs in the FEE and readout via 1044 optical fibres.
The individual detectors are arranged in 18 sectors in azimuth, each of 5 stacks in longitudinal direction, with 6 layers per stack, weighing about 1.65 tons each. The 18 sectors of the TRD form a cylindrical layer of about 70 cm depth starting at a distance of 2.9 m from the beams and with an overall length of 7 m. The TRD is located outside the TPC and is directly followed in radial direction by the TOF. The TRD depth of 70 cm represents 25 % of a radiation length.