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

The ALICE experiment at the CERN LHC will study the properties of matter at the extreme temperature and energy density conditions produced in heavy ion collisions. The impact parameter of the collision will be estimated by means of the Zero Degree Calorimeters (ZDC), that measure the energy carried away by the non-interacting (spectator) nucleons. All the spectator nucleons have the same energy, therefore the calorimeter response is proportional to their number, providing a direct information on the centrality of the collision. Two identical sets of hadronic calorimeters are located at opposite sides with respect to the interaction point, 116 m away from it, where the two LHC beams circulate in two different pipes. Each set of detectors 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 ZDC are spaghetti calorimeters, which detect the Cherenkov light produced by the charged particles of the shower in the quartz fibers, acting as the active material embedded in a dense absorber matrix. In summer 2007 the ZN and ZP calorimeters have been placed on a movable platform and then installed in the LHC tunnel. The results of the commissioning studies are shown. The monitoring systems adopted to control the stability of the PMT responses, i.e. light injection with a laser diode and cosmic rays, are described in detail. The foreseen calibration with e.m. dissociation events in Pb-Pb collisions will also be discussed. Finally the first measurements carried out during the commissioning in the LHC tunnel will be presented.