Two new wheels for ATLAS

Juergen Zimmer (Max Planck Institute), Roy Langstaff (TRIUMF/Victoria) and Sergej Kakurin (JINR), in front of one of the completed wheels of the ATLAS Hadronic End Cap Calorimeter. A decade of careful preparation and construction by groups in three continents is nearing completion with the assembly of two of the four 4 m diameter wheels required for the ATLAS Hadronic End Cap Calorimeter. The first two wheels have successfully passed all their mechanical and electrical tests, and have been rotated on schedule into the vertical position required in the experiment. 'This is an important milestone in the completion of the ATLAS End Cap Calorimetry' explains Chris Oram, who heads the Hadronic End Cap Calorimeter group. Like most experiments at particle colliders, ATLAS consists of several layers of detectors in the form of a 'barrel' and two 'end caps'. The Hadronic Calorimeter layer, which measures the energies of particles such as protons and pions, uses two techniques. The barrel part (Tile Calorimeter) consists of sheets of steel to create showers of lower energy particles. These sheets are interleaved with scintillator to sample the energy. However, because of high radiation levels, the end caps use a different technique with copper to create the showers and liquid argon to measure the energy. Both Hadronic End Cap Calorimeters consist of two wheels placed one behind the other. Each wheel is made from 32 wedge-shaped modules consisting of 25-mm thick copper plates for the front wheel and 50-mm thick plates for the rear wheel. The modules have been built by groups in Western Canada, Munich (Germany), Dubna and Protvino (Russia), with contributions to the electronics equipment by Kosice (Slovakia), while a group from Nanjing (China) has procured part of the copper and contributed to the equipment for assembling the wheels. The 8-mm gaps between the copper plates house three large wedge-shaped HV electrodes, produced by a collaboration from Mainz, TRIUMF, and the Lebedev Institute. The electrodes are separated by honeycomb mat spacers to ensure a well defined geometry and mechanical stability. Once immersed in liquid argon at -185 C, the inner electrode will collect the electric charge left by the charged particles that ionise the liquid. Fast radiation-hard GaAs-based amplifiers, which have been specially designed at the Max Planck Institute of Physics in Munich, are installed at the outer radius of the wheel. They have been pre-tested to be fully operational at both room temperature and liquid argon temperature. The 70-tonne front and 90-tonne rear wheels were initially assembled from pre-tested modules on a table 3 m above ground and later rotated to the vertical position by a special device designed and built in Canada. The assembly of the first complete wheel took 45 days - two days less than scheduled. Construction of the second wheel was completed in November and all four wheels should be finished by summer 2003. Of the 128 modules required to make the four wheels only two remain to be manufactured and all but 12 have been pre-tested at liquid argon temperatures in the test beam area at CERN. The Electromagnetic Calorimeter, which measures the energies of electrons and photons, and the Forward Calorimeter close to the beam pipe, will also use liquid argon, so a common cryostat structure will keep the liquid argon at -185 C. The first such cryostat, provided by a French-German consortium, will be filled with an Electromagnetic End Cap Calorimeter, a Forward Calorimeter and two wheels of the Hadronic End Cap Calorimeter and cooled down in February 2004. The second cryostat is expected to follow one year later.

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