by Peter Glassel. Published: 27 May 2014

The ALICE Transition Radiation Detector (TRD) is an ambitious project that provides ALICE with electron and positron identification. The physics goals are the investigation of the quarkonia (J/psi and Upsilon and their excited states) via their leptonic decays, semi-leptonic decays of D and B mesons and the study of the e+e- continuum down to below 1 GeV. Also, the TRD provides a level-1 trigger on high momentum electrons, electron pairs and jets.

A TRD module

The TRD exploits transition radiation, an effect related to the well-known Cherenkov effect. However, unlike Cherenkov light, the transition radiation is not associated with the volume of material traversed by charged particles above the speed-of-light of the medium but with the number of interfaces of such media. Particles with Lorentz factor larger than about 500 give transition radiation photons, which are in the X-ray range (order 10-30 keV). In accelerator based experiments for all practical purposes only electrons emit transition radiation due to their low mass.

Insertion of one of the TRD modules.

The TRD is an ALICE central barrel detector with (eventually) full azimuthal coverage and polar angle coverage from about 45 to 135 deg with respect to the beam. Like the other central barrel detectors it is segmented azimuthally into 18 sectors. There are 6 layers of detectors resulting in the desired 99% rejection of pions, each with a radiator made of polypropylene fibers, and a 3 cm drift chamber. Longitudinally the detector is divided into 5 stacks. This makes all in all 540 chambers. The 30 chambers of one of the 18 sectors are housed in a so-called TRD supermodule.

During the running period before LS1, 13 of the 18 supermodules were already installed in ALICE. The full implementation of the TRD had been delayed for several reasons: funding was initially available only for 60%, i.e. about 12 supermodules. After full funding was granted, problems appeared at the companies which did the mass production of the sophisticated read-out electronics. It took more that a year to trace the origin of the problems and fix them.

We are now in the process of installing the last 6 TRD modules (1 was removed for repair during LS1) into the central barrel of ALICE. In May, three sectors below the beam line were installed successfully. This was a very difficult location as it required to dismantle mayor parts of the service structure of ALICE. In the fall the last three supermodules above the beam line will be installed, in principle much easier, but we will have to work at real height (about 10 m).