Monitoring for telescope array fluorescence detector PMT camera by YAP and Xe flasher

Abstract

Gain stability of the photomultiplier (PMT) cameras for The Fluorescence Detectors (FD) in the Telescope Array experiment has been monitored by using Am-241 loaded scintillator pulsers (YAP) and diffused xenon flashers (TXF). Each FD camera consists of 16 × 16 PMT(Hamamatsu-R9508) array, and contains two or three absolutely calibrated PMTs carrying a YAP. YAP, which is attached on an PMT cathode, is a small light source which emits UV photons at 50Hz. A TXF which is a relative calibration installed on each camera shoots photons simultaneously on all the 256 PMTs uniformly from about 3 meter away. From the monitoring of YAP pulsers over 4 years of FD operation we found an annual average of slow monotonic PMT gains drift is 0.01 ± 0.31(stat.) ± 0.21(syst.)% and it’s sigma is 1.7%. In conclusion, no systematic PMT gain decrease by the night sky background was observed. A gains monitoring by the TXF also shows the same PMT gain drifts within the annual magnitude of 0.86 ± 0.13(stat.)%/yearl.