An iron hand in a velvet glove
The various parts of the ATLAS detector were designed to be moveable, just like in a toy construction set, allowing access to the sub-detectors inside. However, unlike toy bricks, the components of this giant of physics weigh up to 1000 tonnes and need to be repositioned with a precision of as high as 300 microns.
The ATLAS endcap calorimeter. This element, weighing 1000 tonnes and measuring 9m in diameter, is the most difficult to move.
It’s certainly not child’s play: opening and closing the ATLAS detector takes several weeks and requires the expert eye of the team who developed the system. It involves moving no fewer than 12 elements that weigh a combined total of 3300 tonnes. “Each element is several metres in diameter, and that increases to 25 metres for the biggest ones – the large muon wheels,” explains Michel Raymond, the engineer leading the team responsible for the closing and opening of the ATLAS experiment. “The sub-detectors sit on air cushions that slide along rails nine metres above the ground (see picture 2). The whole thing is controlled by two automated units, which we’ve fine-tuned over the years that the machine’s been running."
The post-LS1 closing operation began on 7 August and will continue until the end of September. This year, for the first time, the sub-detectors will be repositioned with a precision of as high as 300 microns. “In order to achieve that, we installed sensors during LS1 that give us information on the relative positions of the different elements," explains Frédéric Rosset, a member of the technical team. “The detectors are moved with all their ‘accessories’ – things like electrical cables, flexible chains containing cryogenic lines and fibre optics. We have a tolerance of barely three millimetres during these manoeuvres. The equipment that we’re moving is very delicate, so we have to handle it as if we’re using an iron hand inside a velvet glove!”
It takes about half a day to move one element and put it back in its original position with the required precision. The operation is always carried out by the technical team (comprising around 15 people) under the watchful eye of the physicists responsible for the detector. “We have eyes everywhere while this is going on," stresses Cédric Sordé, another member of the team responsible for the opening and closing of ATLAS. “Our machines are very reliable, but nothing beats an expert eye, which is why we monitor and check everything step by step."
As the sub-detectors are gradually put back in their original positions, boxed one inside the other, the technical teams gain more and more space to manoeuvre, while the physicists gradually lose access to the detectors. “All the upgrade work on the sub-detectors planned for LS1 has to be done before the elements in question are put back in place,” Raymond explains. “Only the muon chambers, on the outside of the detector, will remain accessible during and after the closing operations."
Once the enormous detector has been fully reassembled, the physicists will test the functioning of all systems to ensure that the giant is ready for operation as soon as the LHC collisions begin in 2015.
