PMT regime and time resolution

High energy electrons passing perpendicular to the S2 detector plane yield about 400-500 photons at the photo cathode of each PMT. In a fresh PMT this leads to 80-100 photo electrons. On HRS the discriminators have a threshold of 45 mV and a typical PMT has gain $~ 3*10^6$. The HV for a fresh PMT should be in the range 1800 to 2000 V. Based on PMT pulse rise time ( 2.8 ns ) and photo electron statistics the time resolution is about 0.2 ns. The propagation time of the light inside the detector is about 10 ns, which needs to be corrected by using track position information. LeCroy HV 1460 modules are used to supply HV power for the trigger counters. The HV can be controlled from a VT100 terminal connected through a terminal server or through the EPICS system based on the HAC computer. Current HV settings for the trigger counters should be found from a printout of the EPICS control in the last experimental logbook. Figures  5.6 and  5.7 give examples which are included for guidance only. The settings used in the plots may be not correct.

Figure 5.6: EPICS HV summary screen.

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Figure 5.7: HV screen for a single card.

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