Gain monitoring of telescope array photomultiplier cameras for the first 4 years of operation

Abstract

The stability of the gain of the photomultiplier (PMT) camera for the Fluorescence Defector (FD) of the Telescope Array experiment was monitored using an Am-241 loaded scintillator pulsers (YAP) and a diffused xenon flasher (TXF) for a selected set of 35 PMT-readout channels. From the monitoring of YAP pulses over four years of FD operation, we found slow monotonic drifts of PMT gains at a rate of -1.7 similar to +1.7%/year. An average of the PMT gains over the 35 channels stayed nearly constant with a rate of change measured at -0.01 +/- 0.31(stat) +/- 0.21(sys)%/year. No systematic decrease of the PMT gain caused by the night sky background was observed. Monitoring by the TXF also tracked the PMT gain drift of the YAP at 0.88 +/- 0.14(stat)%/year.