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ALICE Upgrade: rejuvenating the wire chambers

The ALICE muon spectrometer [1] consists of 5 tracking stations and 2 triggering stations which detect charged particles and determines their trajectories to allow energy/momentum calculations. Amongst the charged particles, muons are of particular interest since they may originate from charm and beauty quarks decay. 

About 5 meters away from the collision point, protected by a 40 tons absorber made of carbon, concrete, lead, polyethylene with boron and tungsten, is located the first tracking station of the muon spectrometer. This station is composed of 8 identical wire chamber detectors, or quadrants, with cathode pads readout filled with an argon and carbon dioxide gas mixture (Fig.1).

Figure 1. The two cathode planes of a wire chamber of the ALICE muon spectrometer first tracking station.The total active area is close to one square-meter.

To sustain the high collision rate expected when LHC will restart in 2021, the wire chambers need a rejuvenation treatment. In February 2019, the 8 quadrants were dismounted from the experimental area and brought back at IPNO. Built in the years 2003-2004, the chambers are operating since 2009 and have nicely fulfilled their mission [2].

Figure 2. Zoom on the wires that create the electric field inside the chamber (left). Space located in the center of the chamber to ensure a constant gap between both cathode planes (right).

voltage of about 1650 volts with respect to the cathode planes located 2.1 mm away on both sides of the wire plane (figure 2). The cathode planes are divided into about 15000 rectangles (or pads) which are all readout allowing to measure precisely a track position (better than 100 micrometers). Pad sizes vary depending on the position on the chamber (8, 16 ou 32 mm2).

High precision maintenance work is now ongoing : opening of the chambers, replacement of wires, cleaning of glue traces, etc… in order to insure the chambers will operate properly for the next decade for LHC runs 3 and 4 until year 2030. During these runs the interaction rate will reach 50 kHz and 1 MHz for the lead-lead and proton-proton collisions respectively. 

This rejuvenation treatment is being performed by the Detector Team of IPNO and require the skills, the meticulousness and the technical expertise of many people. This work is completed by validation tests using a radioactive source and X-ray tube to assess the performance of the chambers. In 2020, these chambers will go back to the ALICE experiment, equipped with their new readout electronic [3], and collect ultra-relativistic heavy-ions collisions data to study the Quark Gluon Plasma.







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