ATLAS: last few metresfor the Calorimeter
On Friday 4th November, the ATLAS Barrel Calorimeter was moved from its assembly point at the side of the ATLAS cavern to the centre of the toroidal magnet system. The detector was finally aligned, to the precision of within a millimetre, on Wednesday 9th November.
The Barrel Calorimeter which will absorb and measure the energy of photons, electrons and hadrons at the core of the ATLAS detector is 8.6 meters in diameter, 6.8 meters long, and weighs over 1600 Tonnes. It consists of two concentric cylindrical detector elements. The innermost comprises aluminium pressure vessels containing the liquid argon electromagnetic calorimeter and the solenoid magnet. The outermost is an assembly of 64 hadron tile calorimeter sectors.
Assembled 18 meters away from its final position, the Barrel Calorimeter was relocated with the help of a railway, which allows the movement of these heavy detector elements at the centre of the ATLAS toroidal magnet.
The movement gear consists of a hydraulic power pack, valve table and associated pipe-work, pressure and displacement sensors, and a computer control system.
During movements, the detectors are supported vertically by air-pads. Effectively on air, the friction is decreased so the detector slides on the railway with minimal force. Traction cylinders enable the movement of the detector horizontally along the rail. Blocking jacks are used for the final support and adjustments in the vertical direction.
In fact, the movement was performed downhill, since the LHC plane is not horizontal, such that there is a difference of 40 cm in height from one side of ATLAS to another. As a result, the traction cylinders actually worked as breaks, so that the Barrel Calorimeter, once set in motion, was sliding by itself thanks to gravity!
The operation was computer-controlled by a PLC (Programmable Logic Controller) system, which reads in sensor data and displays this information for the operator, who can then steer the movement of the detector. The PLC also monitors the safety systems and has a self-levelling feature to maintain constant height during the movements.
The entire procedure of sliding the Barrel Calorimeter along the 18 meters of railway was performed over 12 hours at a constant speed of 4 mm/s. The most time-consuming operation was the recurrent procedure of reconnecting the traction cylinders to new railway anchoring positions. Accelerometers were installed to register all possible shocks above 0.02g, and to monitor that no such events occurred.
Aligning the Barrel Calorimeter into its final position is a major milestone for ATLAS, as this is the first set of detector elements to be placed in their final running position. The Barrel Calorimeter will support the Inner Detector, underlining the importance of high precision in its positioning. A large effort by the CERN survey team in an iterative alignment process made it possible to confirm the final position on 8 targets to a precision below 1mm.
However, it is already known from measurements that the ground of the ATLAS cavern is rising. Re-adjustments are already foreseen, in case the natural forces push the Barrel Calorimeter away from the interaction point.
With this final act, the Barrel Calorimeter has landed in its final position, after years of construction at various home institutes, long journeys, as far as from Japan, assembly work at CERN and many laboratory and beam tests.
