ALICE Holds Up to Challenge



ALICE's main austenitic stainless steel support structure (the Space Frame) has recently gone through many tests that proved quite challenging: insuring the structure is sound and lowering it horizontally into the ALICE cavern. This structure is constructed to hold the large volume detectors, such as the Time Projection Chamber, Transition Radiation Detector and Time of Flight inside the ALICE solenoid magnet.

After the final assembly at CERN, two large mobile cranes were needed for the job of lifting and turning the 14 tonne frame onto its side. Once shifted, it was placed in Building SX2, one of the surface assembly areas designated for ALICE. The structure, which is 8 m in diameter and 7 m long, underwent many tests in its new position. Geometric control tests were performed by measuring each of the 18 cells and placing wooden or metal samples constructed to the same dimensions as the real thing inside the structure. The most important check was the movement of the real Time Projection Chamber from its support module to the working position inside the Space Frame and the test assembly of the central detectors and related services.

A full load test was conducted by using common PVC pipes along the perimeter of the supporting structure and filling them with water; this method proved to be relatively simple and economic. In addition, the central area was filled with iron bars; the combined weight simulating the 77 tonnes of detectors needed to be supported inside.

These tests are crucial for the Space Frame, as it is impossible to check each of the 600 welds done to hold it all together. Some of them are in very inconspicuous places and the only way to really know if the frame will hold up to the stresses is to literally load it up.

After successfully completing these tests, the austenitic stainless steel structure was carefully lowered into the ALICE pit using a 60 m long crane. This was a delicate operation, as the pit opening is 7.2 m, only 20 cm larger than the frame. Diego Perini, Engineer for ALICE, explained that the driver of the crane had to carry the structure through the building roof and lower it into the tunnel shaft by communicating via radio to a co-worker.