Image credits: ATLAS Collaboration.
On 7 August, the technical teams in charge of closing activities in the ATLAS collaboration started to move the first pieces back into position around the LHC beam pipe. The subdetectors had been moved out in February 2013, at the beginning of the first LHC Long Shutdown (LS1) – a manoeuvre that was needed to allow access and work on the planned upgrades.
LS1 has seen a great deal of work on the ATLAS detector. In addition to the upgrades carried out on all of the subdetectors, when the next LHC run starts in 2015 the experiment will have a new beam pipe and a new inner barrel layer (IBL) for the pixel detector. For the work to be carried out in the cavern, one of the small wheels of the muon system had to be moved to the surface.
The various pieces are moved using an air-pad system on rails, with the exception of the 25-m-diameter big wheel (in the muon system), which moves on bogies. One of the most difficult objects to move is the endcap calorimeter: it weighs about 1000 tonnes and comes with many “satellites”, i.e. electric cables, cryogenic lines and optical fibres for the read-out. Thanks to the air pads, the 1000 tonnes of the calorimeter can be moved by applying a force of only 23 tonnes. During the movement, the calorimeter, with its cryostat filled with liquid argon, remains connected to the flexible lines whose motion is controlled by the motion of the calorimeter.
The inflation of the air pads must be controlled perfectly to avoid any damage to the delicate equipment. This is achieved using two automated control units –one built during LS1 – which perform hydraulic and pneumatic compensation. This year, the ATLAS positioning system has been improved thanks to the installation of a new sensor system on the various subdetectors. This will allow the experts to achieve an accuracy of 300 μm in placing the components in their final position. The position sensors were originally developed by Brandeis University within the ATLAS collaboration, but the positioning system itself was developed with the help of surveyors from CERN, who are now using this precision system in other experiments.
All of the equipment movements in the cavern happen under the strict control of the technical teams and the scientists in charge of the various subdetectors. It takes several hours to move each piece, not only owing to the weight involved, but also because several stops are necessary to perform tests and checks.
The closing activities are scheduled to run until the end of September. By then, the team will have moved a total of 12 pieces, that is, 3300 tonnes of material.
