Array of Cherenkov radiators for PHOBOS at RHIC

An array of Cherenkov radiators (“the Cherenkov counters”) for the measurement of the vertex position of heavy-ion gold–gold collisions for the PHOBOS experiment at relativistic heavy ion collider is described. These simple, versatile, and highly efficient detectors provide a low bias and easily understood hardware (on-line) vertex trigger. This trigger is ready for the data-acquisition system in about 650 ns. The position resolution of the vertex distribution found by the Cherenkov counters is approx. 4 cm and is very stable as function of the centrality of the collisions. The general characteristics of the Cherenkov counters, their design parameters, and performance are presented, along with the implementation of the hardware vertex trigger used for PHOBOS during the 2001 run.