ALICE installs its TPC
The ALICE time projection chamber has been transported to the experimental cavern. The handling of this extremely fragile detector was a long and delicate process.
For ALICE the year started with a flurry of activity...but at a snail's pace. On 8 January, the day CERN reopened after the end-of-year break, teams from ALICE and the TS Department began the transportation of the experiment's time projection chamber (TPC), the largest ever built. This 5-metre long and 5-m diameter cylinder was transported from the clean room where it had been assembled to the experimental cavern. The 300-metre journey took no less than four days! Since the TPC is an extremely fragile object, the utmost precautions were exercised in its transportation.
The TPC, which is designed to reconstruct the trajectories of charged particles, comprises a very light, and therefore highly fragile, cylindrical carbon-fibre structure. The surface of this structure, known as the field cage, is covered with 30,000 mylar strips secured with the utmost precision. The cylinder's two end-caps carry the electronic read-out channels and account for nearly all the TPC's 8-tonne weight. The end-caps are connected by several thousand flat cables to two service support wheels, each weighing 2.8 tonnes. These wheels support the electrical, electronic and gas supply services.
It was vital for the transport teams to minimise the movements of the whole complex. 'We specified that the movement of the service wheels relative to the TPC must not exceed 3 mm,' explains TPC Project Leader Peter Braun-Munzinger. During transportation, all movements were controlled in real time by means of electronic sensors connected to a monitor.
The TPC had been assembled inside a steel transport frame, formerly used by the DELPHI experiment at LEP. Before transportation got under way, the TPC was sealed in insulating sheeting. The roof and walls of the clean room had to be dismantled. Once these preliminaries had been completed, the main operations to move the 30-tonne TPC and its protective frame could begin.
First, an overhead travelling crane lifted the TPC and its transport frame from the lower level of the clean room and positioned them above four hydraulic jacks. These jacks were then carefully raised to gradually take the structure's weight. The next day, the structure was slowly raised to a height of 80 cm. The aim of this six-hour procedure was to enable a flat-bed trailer to be slid underneath the structure. It was imperative that the trailer should not be inclined by more than 2 degrees at any time during transportation. It took one hour to travel the 200 metres from the assembly hall to the access hall leading to the experiment's shaft. The structure was then lowered the 50 metres into the pit by another travelling crane. This spectacular operation was made all the more difficult by the fact that the gap between the structure and the shaft wall was never more than 10 centimetres! The detector's position during the descent therefore had to be closely monitored to avoid any impact with the shaft wall. One handling operator standing in the cavern held a rope connected to one corner of the structure, and any movement of this rope indicated a rotation of the TPC on itself, which the operator could rectify by pulling on the rope. This may seem like a rather low-tech system, but it is nonetheless extemely effective! After an hour and a half, the TPC finally reached the bottom, where four jacks were ready and waiting to cushion its arrival. Finally, the TPC was transported by gantry along the last 30 metres before reaching what is practically its final position within the magnet. It is now ready to receive the internal tracking system. In May, the TPC will be tested again with cosmic rays.
