Time of Flight
The Time Of Flight (TOF) detector of ALICE measures, with a precision better than a tenth of a billionth of a second, the time that each particle takes to travel from the vertex to reach the detector so that one can measure its speed. Charged particles in the intermediate momentum range are identified in 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 provides 3σ π/K separation up to 2.2 GeV/c and K/p separation up to 4 GeV/c. The ALICE Time-Of-Flight (TOF) detector at LHC is based on the Multigap Resistive Plate Chambers (MRPCs). It is located at about 3.7 m from the interaction point and includes a cylindrical array (∼ 141 m2 of active area) the central region (−0.9 < η < 0.9). The TOF provides charged-particle PID in the intermediate momentum range and also trigger for cosmic ray events and ultraperipheral collisions.
The Multigap Resistive Plate Chamber (MRPC) strip detector is the basic unit of the ALICE TOF detector, with a 120 × 7.4 cm2 active area. The ALICE TOF array consists of 1593 MRPC strips, subdivided into 18 azimuthal sectors. To guarantee low detector occupancy even in the highest charged-particle-density scenario each MRPC strip is segmented into two rows of 48 pickup pads of 3.5 × 2.5 cm2 , for a total of 96 pads for each strip and 152928 total readout channels. The TOF MRPC is based on a double-stack design: it is made of two stacks of five gas gaps. The resistive plates are made with commercially available soda-lime glass sheets. The gap (250 µm) is realised by commercial fishing line stretched across the glass sheets. In a beam test setup the average MRPC time resolution, including the contributions of the complete front-end and readout electronics, was measured to be better than 50 ps.