Monitoring system

A monitoring system for ACC has been designed and constructed [51]. It consists of blue LEDs, a diffuser box, and optical distributors. The monitoring system is required to supply roughly the same light intensity as that of Cerenkov lights generated by light velocity particles in each ACC module, i.e. approximately 18 photoelectrons. The monitoring light intensity should be stable ($\sim$95 %) in terms of aging and temperature variation to measure possible drifts of PMT gains and light yields of aerogels in a long period and those due to temperature changes.
The employed LED light source has been observed to have the long term stability of 98.0 $\pm$ 0.4 %. The measured temperature coefficient of the LED is (- 0.003 $\pm$0.035)%/$^oC$ over a temperature range of 5 - 70$^oC$. The uniformity of light intensity of the monitoring system has been measured to be 94.20 $\pm$ 0.09 %. The overall performance on uniformity and intensity of the light output has been found to satisfy all the requirements for the ACC monitoring system.