LED Based Light Monitoring System for the Primex Experiment at Jefferson Lab

A multichannel Light Monitoring System (LMS) based on super-bright blue LED, has been developed for the high resolution electromagnetic hybrid calorimeter (HYCAL) at Jefferson Lab. The central part of the calorimeter consists of 1200 PbWO₄ crystal modules. The prototype LMS has been constructed and tested, and has demonstrated a capability of providing an adequate light pulse per each channel, equivalent to the signal of 5 GeV electrons in the detector. The final Light Monitoring System with 1800 output channels and fiber optics light distribution system is presently under construction. The light source of the LMS represents an assembly of 31 super bright blue LEDs connected in series. The uniformity of the output light is achieved by mixing light from the LED assembly in an integrating sphere. The light intensity is being monitored using PIN photo diode and reference photomultipliers with Am²⁴¹ radioactive source. In order to achieve (0.1 - 0.3)% long term stability, the system including the light source, the photo diode with the preamplifier and the reference PMT, is thermally stabilized.