A high stability light emitting diode system for monitoring lead glass electromagnetic calorimeters

We have designed, built and tested a high stability system based on blue light emitting diodes (LED) and current pulse generators for calibration and monitoring of lead-glass calorimeters. This apparatus is presently being used for the electromagnetic calorimeter of the WA96 (NOMAD) experiment at CERN. The system was developed to minimize the sensitivity to temperature variations, different loadings, and ageing effects. Tests performed both in the laboratory and in the experiment showed that the response of the lead-glass calorimeter modules to LED light pulses could be kept within the precision of ±1% for periods of several months.