Front-end electronics for the silicon partition of the Phobos detector at RHIC

The Phobos experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory uses silicon pad detectors to measure charged particles from Au-Au collisions. We describe here the front-end electronics which instruments the 135,000-channel silicon partition of the detector. The system records pad "hits" and 12-bit pulse height data for later reconstruction of charged-particle trajectories, and for identification of particle species using the dE/dx method. The compactness of the detector puts challenging spatial and power dissipation constraints on the design of the electronics; the proximity of the silicon sensors and front-end chips to the heavy ion collisions presents a challenge in terms of dealing with latchup. We describe the design features of the principal elements of the front-end including the silicon detector modules, the front-end controller, and the data concentrator. The performance of the system and our experience with the latchup protection during the first two physics runs are presented.