Commissioning and calibration of the Zero Degree Calorimeters for the ALICE experiment

The Zero Degree Calorimeters (ZDCs) for the ALICE experiment will measure the energy of the spectator nucleons in heavy ion collisions at the CERN LHC. Since all the spectator nucleons have the same energy, the calorimeter response is proportional to their number, providing a direct information on the centrality of the collision. Two identical ZDC systems are located at opposite sides with respect to the interaction point (IP), 116 m away from it. Each ZDC system consists of a neutron (ZN) calorimeter, placed between the two beam pipes, and a proton (ZP) calorimeter, positioned externally to the outgoing beam pipe. The ZDCs are spaghetti calorimeters, which detect the Cherenkov light produced by the shower particles in silica optical fibers embedded in a dense absorber. In summer 2007 the ZN and ZP calorimeters have been assembled on the surface on a movable platform and then installed in the LHC tunnel. The results of the commissioning on the surface, in particular the solutions adopted to control the stability of the PMTs response will be shown: light injection with a laser diode and cosmic rays. The forseen physical calibration in Pb-Pb collisions will be also discussed. Finally the first measurements of the commissioning in the LHC tunnel will be presented.