A POSITION-SENSITIVE HIGHLY RADIATION-HARD AND FAST HADRON CALORIMETER FOR A LEAD-ION EXPERIMENT AT CERN SPS

We present the performance of the Zero Degree Calorimeter (ZDC) built for the NA50 experiment at the CERN SPS. This detector measures the Ch erenkov light produced in silica optical fibres embedded in tantalum a nd offers the double advantage of being highly radiation resistant (up to several Grad) and very fast (signal width of the order of 10 ns). It has an active volume of 5 x 5 x 65 cm(3) with a fibre to tantalum v olume ratio of 1/17; the fibres are positioned at an angle of 0 degree s with respect to the beam direction and have a diameter of 365 mu m. The measured energy resolution (sigma/E) is 30% for protons at 205 GeV and 5% for lead ions at 160 GeV/nucleon. The detector exhibits also v ery good localising properties since it can detect the impact point of the lead beam on its front face with a precision better than 0.4 mm M ms.