The pulsed light calibration system of the ZEUS calorimeter

Abstract

The ZEUS calorimeter is a compensating calorimeter consisting of uranium and scintillator plates. Light is transported via light guides to photomultiplier tubes (PMTs). The design goal is an energy calibration good to within 1%. Stability is measured and PMT gains are set using the signal from uranium radioactivity. Another important component of the calibration is the pulsed light system, which distributes light from a central laser or from distributed LEDs to the photomultiplier tubes via optical fibers. The light pulse gives a similar PMT response as a signal generated by a particle in the calorimeter. This has allowed the monitoring of the following properties of the readout chain: • • Number of photoelectrons/GeV/PMT. This enables us to differentiate between changes in the photomultiplier tubes and changes in the calorimeter (such as radiation damage). • • Linearity of the PMT readout chain between 0 and 400 GeV. • • Time delays from the PMTs and from the electronics are known to within 1 nsec. It is necessary to know these delays in order to precisely calculate the reconstructed charge and to eliminate background to e-p interactions from beam gas events. • • Short-term monitoring of PMT gain. This has been used to measure PMT gain changes under varying magnetic fields to better than 1% and will be used to measure the PMT gain under varying HERA background conditions.