Calibration system of the phobos silicon detectors

The Phobos experiment at the Relativistic Heavy Ion Collider (RHIC) at Broo khaven National Laboratory uses silicon pad detectors to measure charged pa rticles in Au-Au collisions. Precise knowledge of the ionization signal amp litude in the silicon pad detector is vital for the determination of partic le multiplicity and for particle identification. We describe here a system that precisely calibrates the gain curve for each channel in the 135 000-ch annel silicon readout system. The calibration covers the range up to 100 mi nimum ionizing particles, and the calibration circuit has a linearity bette r than +/-1% over a dynamic range of 1:250.