CMS tracker slides into centre stage
As preparations for the magnet test and cosmic challenge get underway, a prototype tracker has been carefully inserted into the centre of CMS.
The CMS prototype tracker to be used for the magnet test and cosmic challenge coming up this summer has the same dimensions -2.5 m in diameter and 6 m in length- as the real one and tooling exactly like it. However, the support tube is only about 1% equipped, with 2 m2 of silicon detectors installed out of the total 200 m2. This is already more than any LEP experiment ever used and indicates the great care needed to be taken by engineers and technicians as these fragile detectors were installed and transported to Point 5.
Sixteen thousand silicon detectors with a total of about 10 million strips will make up the full tracker. So far, 140 modules with about 100 000 strips have been implanted into the prototype tracker. These silicon strips will provide precision tracking for cosmic muons during the summer.
In the dead of night on 11-12 May, a special truck with atmospheric controls to avoid humidity travelled from the Meyrin site to Cessy at Point 5 at a maximum speed of 10 kilometres per hour to avoid shocks to the equipment. It took two and a half hours to make the trip, but as Hans Postema, CMS tracker project engineer, explained, it is better not to rush, as once the detector is finally closed underground there will be no way to open it again before the start-up of the LHC machine. This trip was an exercise for the transport of the final tracker early next year.
The next morning workers began lifting the 2-tonne tracker up 8.5 metres to the future beam height and to meet the entrance to the giant solenoid magnet (The real tracker is expected to weigh as much as 3 tonnes). Special platforms mounted on concrete blocks were designed just for this purpose. On the other side of the solenoid an 8-metre long support mechanism, known as the Eiffel Tower, takes half of the load of the tracker as it is moved into place in the centre of the magnet.
Surveyors spent hours painstakingly aligning the pieces using two already installed supermodules of the electronic calorimeter (ECAL) to check the nominal distance. This was all extremely crucial considering that the real tracker costs about 65 million Swiss francs, and the ECAL costs around 100 million Swiss francs. Once the total ECAL is installed, there will only be about a centimetre clearance between it and the tracker.
'This was the only time we could test the fitting before putting in the real tracker. Surveyors measured permanently the positions of the tracker during its insertion using optical measurement equipment, which allows them to know within a fraction of a millimetre deviations from the correct place,'explained Horst Breuker, CMS tracker mechanics and engineering section leader.
