A Silicon pad counter for first level triggering in ultrarelativistic proton - nucleus and nucleus-nucleus collisions

Abstract The CERES/NA45 experiment at CERN uses a silicon pad detector with 64 ring shaped pads for first level triggering on minimum bias proton-nucleus as well as on high multiplicity ion-ion collisions. In addition, the counter serves as a multiplicity detector to estimate the energy density obtained in the collision. The design and performance of the pad counter in the laboratory and under real beam conditions together with the first level trigger electronics are reported.