The ATLAS electromagnetic calorimeter moves home

The first cylinder of the ATLAS electromagnetic calorimeter barrel and the pre-sampler have just been inserted in the cryostat. Wiring operations can now begin.

A portion of the big ATLAS accordion has now been moved into position. One of the two half-barrels of ATLAS's cylindrical electromagnetic calorimeter has just been inserted in its cryostat. This is its last move before being lowered into the experimental cavern, once the other half-barrel has also been assembled, inserted and tested. That is fortunate, since the operation carried out on 26 February was not a simple one. The detector had to be ferried from the clean room, where it was assembled, to the cryostat entry point. "One of the many difficulties comes from the heaviness of the calorimeter. It actually weighs 90 tonnes, much greater than the maximum permissible load for the overhead crane", commented Michel Chalifour, in charge of operations involving the electromagnetic calorimeter in Building 180. So two cranes were coupled together to move the load to the front of the cryostat. To ensure an ultra-clean environment, a tent was erected round the calorimeter and entry point to the cryostat. The detector and pre-sampler, fitted together, could then be slid gradually into the cryostat like a drawer. To do so, the team had to fine-tune the alignment of the rails in the cryostat and the guides on the calorimeter, with a high degree of precision. A system of hydraulic glides at the rail ends allowed the detector to be slowly moved into place.

The ATLAS electromagnetic calorimeter is intended to detect electrons, positrons and photons by measuring the energy they deposit on being absorbed. The cylinder of the calorimeter is in two halves, that will be sunk in a liquid argon bath cooled to 90 Kelvins (-180°C). Both of the two hollow cylinders are 3.2 metres long, 53 cm thick and are formed by assembling 16 modules. Each module is made up of alternate lead absorbers and electrodes pressed into 64 layers folded accordion-fashion (c.f. Bulletin 18/2001)These modules were built at LAPP in Annecy, CEA at Saclay, and at CERN. The pre-sampler, set up inside the cylinder, is an integral part of the calorimeter system. It measures the energy lost by a particle before it reaches the calorimeter. The pre-sampler was built at ISN Grenoble and at KTH Stockholm. The barrel cryostat, which is 7 metres high, 5.8 metres wide and 7.2 metres long, weighing 100 tonnes, was manufactured in Japan by the Kawasaki Heavy Industries Ltd company (see Bulletin 29/2001).
The module has undergone high-voltage, electrical and low-temperature tests. "2000 volts will be applied between each absorber and electrode layer. The modules must therefore be kept free of any metal dust that might cause a short circuit," stresses Martin Aleksa, a physicist in the EP-ATA-LA group. So the assembly work, as well as the insertion in the cryostat, was done in a clean room. Once the modules were assembled, the ceiling of the clean room was removed so that the detector could be pivoted into its final alignment before transportation began. To check that no damage was done, a series of tests was carried out after each move.
Now that the operation is over, it remains to install the wiring system for the electronics and to run the requisite tests. The team has to assemble the other 16 modules for the second half-barrel. The assembly work must be completed by the end of June and insertion in the cryostat is planned for the late summer of 2003.