ATLAS Experiment CERN

Transition Radiation Tracker (TRT)

The Transition radiation Tracker (TRT) is based on the use of straw detectors, which can operate at the expected high rates due to their small diameter and the isolation of the sense wires whitin individual gas volumes. Electron identification capability is added by employing Xenon gas to detect transition radiation photons created in a radiator between the straws. Each straw is 4 mm in diameter and equipped with a 30 μm diameter gold-plated W-Re wire. The maximum straw length is is 144 cm in the barrel, which contains about 50 000 straws, each divided in two at the center and read out at both end, to reduce the occupancy. The end-caps contain 320 000 radial straws, with the readout at the outer radius. The total number of channels that are read out is 420 000. Each channel provides a drift time measurement, giving a spatial resolution of 170 μm per straw, and two independent thresholds. These allow the detector to discriminate between tracking hits, which pass the lower threshold, and transition radiation hits, which pass the higher one. The nonflammable gas mixture is now Xe(70%)CO2(27%)O2(3%) (compared to 70% Xe, 20% CO2, and 10% CF4 originally) with a total volume of 3 m2. The barrel section is built of individual modules between 329 and 793 straws each, covering the radial range from 56 to 107 cm. The first six layers are inactive over the central 80 cm of their length to reduce their occupancy. Each end-cap consists of 18 wheels. The innermost 14 cover the radial range from 64 to 103 cm, while the last four extend to an inner radius of 48 cm. Wheels 7 to 14 have half as many straws per cm in z as the others, to avoid an unnecessary increase of crossed straws and material at medium rapidity.

   TRT barrel, just before SCT barrel insertion.


Other Inner Detector Sections

General | Pixel Detector | SCT | TRT

Return to Detector Description...