The Transition Radiation Detector (TRD)
The Transition Radiation Detector (TRD) is the main electron detector in ALICE. In conjunction with the TPC and the ITS, it provides the necessary electron identification capability to study:
- Production of light and heavy vector mesons as well as the continuum in the di-electron channel
- Semi-leptonic decays of hadrons with open charm and open beauty via the single-electron channel using the displaced vertex information provided by the ITS
- Correlated DD and BB pairs via coincidences of electrons in the central barrel and muons in the forward muon arm
- Jets with high Et by requiring several high pt tracks in one single TRD module.
Credits: Antonio Saba
Principle of Operation
An individual detector module consists of a radiator and a drift chamber
operated with Xe/CO2 mixture (85%/15%). The drift electrode is glued directly to
the radiator. The particles pass through the radiator, the generator of the
Transition Radiation (TR) and then enter the conversion and drift region of the
The conversion and drift region is 30 mm deep. The drift field of 700 V/cm corresponds to a drift velocity of 1.5 cm/μs. The secondary electrons are amplified in the multi-wire proportional counter with a gas gain of around 5000. The wires run in φ-direction where the best position resolution has to be achieved for an accurate momentum determination. The readout pads are rectangular with an average area of about 6 cm2. The induced signal on the pads is recorded in 15 time samples. The ratios of charges recorded on adjacent pads for each time sample allow position determination along the track segments in one chamber, called tracklet. From the inclination of the tracklet one can infer the momentum
The radiator is optimized to provide the best compromise between transition radiation yield, radiation thickness and mechanical stability. The final radiator consists of polypropylene fibre mats of 3.2 cm total thickness, sandwiched between two Rohacell foam sheets of 0.8 cm thickness each. The foam is reinforced by carbon fibre sheets with a thickness of 0.1 mm laminated onto the outer surface. The measured radiator performance, with a pion rejection factor of 100 at an electron efficiency of 90%, is as required.
TRD radiator design
Scanning electron microscope images of the used radiator materials.
The front readout electronics consists of: . an analog ASIC containing 18 channels of the Pre-Amplifier Shaper (PASA) and output buffers; . a mixed analog/digital ASIC containing 18 channels of ADCs, Tracklet PreProcessor (TPP) and Event Buffers and a single Tracklet Processor (TP) for all channels; . a Global Tracking Unit (GTU). Both ASICs, assembled in a Multi-Chip Module (MCM), are mounted directly on the readout chambers. The GTU serves the TRD stack with 6 readout chambers in order to provide trigger information on high pt tracks.
During the drift time the data processing is performed in the Tracklet Preprocessor (TPP). At the end of the drift time the Tracklet Processor (TP) processes the data of all time bins in order to determine potential tracklets. Selected tracklets are shipped to the GTU. The GTU combines and processes the trigger information from individual readout chambers and ships selected tracks to the Central Trigger Processor (CTP).
The Trigger Concept
Local Tracking Unit
Tasks of digital chip on detector:
. select good clusters
. assemble parameters for linear regression
. perform straight-line fit
. select minimum inclination
Global Tracking Unit
Tasks of FPGAs near detector:
. match tracklets and compute momentum using vertex
. cut on momentum, curvature check
. identify electrons by likelihood on energy loss (TR)
. compute pair invariant mass
Tasks of physics trigger at Level 1:
. select high pt electron pairs with J/Ψ and Υ mass
. select high pt electron and high pt muon
. select several high pt particles in jet cone
. select high pt electrons, or several high pt particles in jet cone with respect to HMPID, PHOS
Transition Radiation Signature The figure shows the drift time distribution of the average pulse height summed over adjacent pads for pions and electrons. For electrons there is a significant increase in the average pulse height at later drift times, due to the preferential absorption of the transition radiation near the entrance of the drift chamber.
At 90% electron efficiency the pion efficiency as a function of the momentum is shown in the right figure for 6 TRD layers in case of a pure fibre radiator. The steep decrease at momenta around 1 GeV/c is due to the onset of the transition radiation production. Near the highest measured momentum of 2 GeV/c we observe saturation in pion efficiency due to TR saturation and the pion relativistic rise.
Going from well isolated tracks (data) to full multiplicity (simulation) we observe a deterioration up to a factor of 6-7. However, the rejection factor at 90% electron efficiency is still close to 100 as required.
Pion efficiency as a function of particle momentum.
Pion efficiency as a function of electron efficiency and the total charged particle multiplicity.
In the TRD readout chamber the position resolution along the wires is 400
(600) micron for low (high) multiplicity.
This results in the following momentum resolution:
. low multiplicity: Δp/p = 2.5% ⊕ 0.5% p in (GeV/c);
. high multiplicity: Δp/p = 2.5% ⊕ 0.8% p in (GeV/c).
Momentum resolution as a function of Pt for low multiplicity events.
Y Invariant Mass Resolution
Electrons traversing the beam pipe, the ITS, the TPC and the TRD itself can lose large amounts of energy due to Bremsstrahlung. This leads to tails in the reconstructed invariant mass distribution of ϒ from their decay into e+ e- as shown in left-hand figure for tracks reconstructed in the TRD alone. The intrinsic invariant mass resolution for tracks reconstructed in the combined ITS, TPC, and TRD improves to ~100 MeV/c2 at the magnetic field of B = 0.4 T as shown in the right-hand figure as a function of charged-particle multiplicity.