page_title: ALICE experiment
section_title: Transition radiation detector
hierarchical_section_title: ALICE experiment / The ALICE detectors / Barrel tracking / Transition radiation detector
caption_attribution_description: Deutsch: Bild des Teilchendetektors des ALICE Experiments im CERN LHC.English: Picture of the particle detector from the ALICE Experiment at the CERN LHC.
context_page_description: ALICE is one of eight detector experiments at the Large Hadron Collider at CERN. The other seven are: ATLAS, CMS, TOTEM, LHCb, LHCf, MoEDAL and FASER.
context_section_description: Electrons and positrons can be discriminated from other charged particles using the emission of transition radiation, X-rays emitted when the particles cross many layers of thin materials.
  The identification of electrons and positrons is achieved using a transition radiation detector (TRD). In a similar manner to the muon spectrometer, this system enables detailed studies of the production of vector-meson resonances, but with extended coverage down to the light vector-meson ρ and in a different rapidity region. Below 1 GeV/c, electrons can be identified via a combination of particle identification detector (PID) measurements in the TPC and time of flight (TOF). In the momentum range 1–10 GeV/c, the fact that electrons may create TR when travelling through a dedicated "radiator" can be exploited. Inside such a radiator, fast charged particles cross the boundaries between materials with different dielectric constants, which can lead to the emission of TR photons with energies in the X-ray range. The effect is tiny and the radiator has to provide many hundreds of material boundaries to achieve a high enough probability to produce at least one photon. In the ALICE TRD, the TR photons are detected just behind the radiator using MWPCs filled with a xenon-based gas mixture, where they deposit their energy on top of the ionization signals from the particle's track.
  The ALICE TRD was designed to derive a fast trigger for charged particles with high momentum and can significantly enhance the recorded yields of vector mesons. For this purpose, 250,000 CPUs are installed right on the detector to identify candidates for high-momentum tracks and analyse the energy deposition associated with them as quickly as possible (while the signals are still being created in the detector). This information is sent to a global tracking unit, which combines all of the information to search for electron–positron track pairs within only 6 μs.
  To develop such a Transition Radiation Detector (TRD) for ALICE many detector prototypes were tested in mixed beams of pions and electrons.
caption: The completed ALICE detector showing the eighteen TRD modules (trapezoidal prisms in a radial arrangement).
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