Calibration system of the Phobos silicon detectors

The Phobos experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory uses silicon pad detectors to measure charged particles in Au-Au collisions. Precise knowledge of the ionization signal amplitude in the silicon pad detector is vital for the determination of particle multiplicity and for particle identification. We describe here a system that precisely calibrates the gain curve for each channel in the 135 000-channel silicon readout system. The calibration covers the range up to 100 minimum ionizing particles, and the calibration circuit has a linearity better than /spl plusmn/1% over a dynamic range of 1:250.