TOF
Time of Flight for ALICE Experiment
 


Overview
More info from
TOF TDR Add Chap.1 Chairman of the TOF Project and Project Leader: Antonino Zichichi



The TOF detector in ALICE is dedicated to charged particle identification over a very large part of the phase space.
The basic physics goals of the ALICE experiment demand a Time of Flight detector with outstanding characteristics:

  • the TOF rapidity acceptance has to be large enough to cover the full central acceptance of ALICE, in order to allow a significant study of the observables of interest on a Event-by-Event basis. This implies that a large number of hadrons of average momenta ~1 GeV should be detected.
    More precisely, the TOF detector should cover the hadron momentum range from about 0.5 GeV/c (upper limit for dE/dx measurements in both the ITS and TPC detectors for kaon/pion separation) to about 2.5 GeV/c (statistics limit in single events).
  • the TOF intrinsic time resolution must be well below 100 ps; an 'overall' time resolution of 120 ps, including all other sources of timing errors, would guarantee a 3 sigma separation up to 1.9 GeV/c for kaon/pion and up to 3.2 GeV/c for proton/kaon.

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R&D
More
info from TOF TDR Add Chap.2

Since a large area (~150 square meters) has to be covered, a gaseous detector is the only choice.
In the framework of the LAA project at CERN an intensive R&D programme has proved that the best solution for the Time of Flight detector is the Multigap Resistive Plate Chamber (see the following figure).
The key aspect of this technology is that the electric field is high and uniform over the whole sensitive gaseous volume of the detector. Any ionisation produced by a through-going charged particle will immediately start a gas avalanche process which will eventually generate the observed signals on the pick-up electrodes. There is no time jitter associated with the drift of the electrons to a region of high electric field.

MRPC

The main advantages of the MRPC technology are:
  • it operates at atmospheric pressure;
  • the construction requires commercially available glass;
  • the signal is the analogue sum of signals from many gaps, so there is no late tail and the charge spectrum is not of an exponential shape, it has a peak well separated from zero;
  • the resistive plates quench the streamers so there are no sparks, thus high gain operation becomes possible;
  • both an array of single cells and a multicell strip design produce good uniformity: for the second option, the geometric arrangement is far simpler and it explains the choice of this design.
The results obtained with a 10-gap double-stack strip design (see previous and following figures)

Strip

show that this new detector has an intrinsic time resolution smaller than 50 ps and an efficiency of 99.9% (see following figure).

Experimental results

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Detector Description
More
info from TOF TDR Add Chap.3

The TOF detector covers a cylindrical surface (at 3.7 m from the beam line) of polar acceptance [45,135] degrees and a full coverage in phi angle using MRPC strips. The area of the detector is ~150 square meters.
The system has a modular structure corresponding to 18 sectors in phi and five modules along the beam direction (with a total of 90 modules and 1638 MRPC strips).
The active area of each strip is 121.5*7.4 square centimeters.
Taking into account that each strip contains 96 pickup pads, it results a total number of read-out channels (pads) equal to 157248.
The general design has been conceived taking into account the results of the simulations, the feasibility of the proposed solution, the performances of the detector and the need to keep the dead area inside the module to a minimum.

  • In particular, it is important to minimise the transversal path of the particles through the strip that can create a sharing effect of the signal among adjacent pickup pads, thereby increasing the occupancy and the time jitter of the detected signals. To overcome this effect, a special positioning of the strips has been envisaged: their angle with respect to the axis of the cylinder is progressively increased from 0 degrees in the central part (theta=90 degrees) of the detector to 45 degrees in the extreme part of the external module (theta=45 degrees).
  • To avoid dead areas, adjacent strips inside one module have been overlapped by 2 mm and adjacent modules have specially shaped boundaries (see the following figure for the intermediate module).
Intermediate Module



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Simulation
More
info from TOF TDR Add Chap.5

The Monte Carlo simulation studies are performed using AliROOT which is the ALICE off-line code implemented in the ROOT framework for event simulation, reconstruction and analysis.

  • AliROOT icludes the well-known GEANT3.21 detector description and tracking package in a user-friendly environment handled by C++.
  • The AliROOT framework is interfaced with several external event generators (JETSET, PYTHIA, HIJING, SHAKER out of the internal AliROOT generators) thus providing a complete set of instruments to simulate ion-ion collisions.
The following figure is an example ('overall' time resolution : 120 ps) of the TOF particle identification capability (PID). It shows the reconstructed mass in the momentum range [0.5, 2.5] GeV/c for 50 HIJING Pb-Pb events at B = 0.4 T. The coloured-line histograms show the individual mass distributions of the true pions, kaons, protons.

PID

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