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The ALICE Time of Flight Detector

Charged particles in the intermediate momentum range are identified in ALICE by the Time Of Flight (TOF) detector. The time measurement with the TOF, in conjunction with the momentum and track length measured by the tracking detectors is used to calculate the particle mass. A time resolution of 100 ps will provide 3σ π/K separation up to 2.2 GeV/c and K/p separation up to 4 GeV/c.

The goal is to study:

  • The QCD thermodynamics via the measurement of π, K and p transverse momentum distributions and particle ratios on an Event-by- Event basis.
  • Signatures of QGP formation via open charm and φ meson production.

One TOF super module (Credits: Antonio Saba)

System Description

The TOF detector has a cylindrical shape, covering polar angles between 45 degrees and 135 degrees over the full azimuth. It has a modular structure with 18 sectors in φ; each of these sectors is divided into 5 modules along the beam direction. The modules contain a total of 1638 detector elements (MRPC strips), covering an area of 160 m2 with 157248 readout channels (pads).TOF Cylinder

Principle of Operation

The MRPC is a stack of resistive glass plates. A high voltage is applied to the external surfaces of the stack. Further out there are pickup electrodes. A charged particle ionises the gas and the high electric field amplifies this ionization by an electron avalanche. The resistive plates stop the avalanche development in each gap; they are however transparent to the fast signal induced on the pickup electrodes by the movement of the electrons. So the total signal is the sum of the signals from all gaps (the reason for many gaps is to achieve high efficiency), whereas the time jitter of the signal depends on the individual gap width (the reason for narrow gaps is to achieve good time resolution).

Shema of the working principle of the TOF Multigap Resistive Plate Chamber (MRPC)

Detector Description

The detector chosen for the ALICE TOF is the Multigap Resistive Plate Chamber (MRPC), developed within the CERN LAA project. The detector element is a long MRPC strip with an active area of 7.4 × 120 cm2. It has 96 readout pads of 2.5 × 3.5 cm2 arranged in two rows. As shown in the figure of the cross section, it consists of 2 stacks of glass, each with 5 gas gaps of 250 μm; spacers made of nylon fishing line keep the distance between the glass plates fixed.

MRPC strips construction

Washing glasses in the ultrasonic tank. (Credits: Antonio Saba)

MRPC Strips storage in Bologna (Credits: Antonio Saba)

Detector assembly in Bologna (Credits: Antonio Saba)


The signal from the MRPC must be amplified and discriminated and the time measured with an accuracy matching the intrinsic time resolution of the detector. Ideally the charge needs to be measured in addition, to correct for slewing; however, the measurement of the time width of the pulse (leading edge - trailing edge) is equivalent.

Based on these requirements, the electronics for the ALICE TOF will consist of the following basic components.

For the readout the HPTDC (High Performance Time to Digital Converter), a chip developed by the CERN EP/MIC group, with bin width of 25 ps. Prototype readout card with 4 HPTDC chips on board

The photograph shows a prototype readout card with 4 HPTDC chips on board (8 channels per chip).

For the Front End the NINO ASIC, developed recently by the CERN LAA project, which combines a fast amplifier, discriminator and stretcher.

The photograph shows the NINO ASIC (8 channels) directly bonded on the PC board (no packaging).

Detector Performance

The time resolution of the TOF MRPC is in the 50 ps range, as can be seen in the figure showing the time distribution after slewing correction. A typical efficiency and time resolution plateau as a function of the high voltage is more than 2 kV long before the onset of streamers, with efficiency reaching 99.9 %.

Peaks in the momentum range up to 3GeV/c in a 250 Pb-Pb events

π, K and p mass peaks clearly show up in the momentum range up to 3 GeV/c, in a simulation of 250 Pb-Pb events generated with HIJING for magnetic field B = 0.4 T.


The dE/dX measurement by the TPC is correlated to the mass measurement by the TOF array.

More information about the Time of Flight (TOF)