The ALICE TPC is upgraded
"One more centimeter," said the chief technician, while operating the hydraulic jack system on 14 August. The 5-m-diameter, 5-m-long cylindrical detector gently slid into the parking position, 56 meters below the ground in the ALICE cavern at LHC Point 2, where it will stand for some time. This operation culminates the many-years-long upgrade of ALICE's Time Projection Chamber (TPC), the large tracking device of the LHC's heavy-ion specialist.
The ALICE TPC is a big, gas-filled cylinder with a hole in the center – to accommodate the silicon tracker as well as the beam pipe – where the charge produced by ionizing radiation is projected onto detectors arranged in the two endplates. These detectors used to be multi-wire proportional chambers, 72 in total, which have now been replaced by detectors based on Gas Electron Multipliers (GEM), a micro-pattern structure developed at CERN 20 years ago. These new devices, together with new readout electronics that feature a continuous readout mode, will allow ALICE to record the information of all tracks produced in lead–lead collisions at rates of 50 kHz, producing data at a staggering rate of 3.5 TB/s. The average load on the chambers under these conditions is expected to be as high as 10 nA/cm², and the GEM detectors are able to cope with this. But will these new devices perform as nicely as their predecessors?
In order to answer this question, several years of intensive R&D were necessary, since the large number of positive ions produced at the detectors would lead to excessive track distortions. This, combined with the necessity of keeping excellent energy-loss (dE/dx) resolution for particle identification, and the imperative robustness against discharges, posed an exciting challenge that led to a novel configuration of GEM-based detectors.
As the fabrication of over 800 GEM foils was taking place at the CERN PCB workshop, the new chambers and electronics were being constructed and thoroughly tested around the world – quite a logistic exercise. The ALICE team proceeded with the final steps of the upgrade process during the ongoing second long shutdown of CERN's accelerator complex (LS2). First, the TPC was extracted from the underground cavern and brought, inside its blue frame, to a large clean room at the surface. Cranes, jacks and a huge truck were used for careful transportation. The chamber replacement, electronics installation and tests with a laser system, cosmic rays and X-rays took over a year. In July 2020, the TPC was declared ready for being re-installed in the cavern. Cranes, truck and jacks once again.
ALICE achieved a major milestone with the completion of the TPC upgrade, after many years of intense R&D, construction and assembly. At the end of 2020, all the services will be connected and the full, upgraded TPC will be operated and commissioned together with all other detectors in the experiment. The real excitement will be when the first post-LS2 collisions from the LHC are delivered.