Detector Rates and Lifetime

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As the rates close to the photon beam line can be substantial it is necessary to investigate the consequences on the performance and lifetime of the scintillator itself and its readout devices.

Rate Estimates

The estimated rates in a given paddle depend on the upstream detector geometry and target material. Some estimates can be found in GlueX-DB 1471. In figure 7 on page 9 of this document you find the estimated rates for all paddles with the half sized paddles on the side (41 - 44). The rates for the central paddles are expected to be rather large up to 12 MHz!

PMT specifications

According to the specifications for the Hamamatsu PMT H9779 (8-stage) the maximum operating current is 100 nA. Elton estimated the signal amplitude based on this maximum current assuming a gain of 5*10^5 and 50ns signal base line. This results in an amplitude of 50mV. From bench tests with an LED on an H10534 PMT assembly with a 10-stage H9779 PMT and operating voltage of 1900V resulting in a gain of 4.1*10^6 and a measured 270 photo electrons per LED pulse a signal amplitude of 1.45V was recorded. The signal base line was about 20ns. Assuming a triangular signal shape the total charge of the signal is 0.5 * 1.45V * 20*10^-9s / 50Ohm = 2.9*10^-10 C. At a rate of 3MHz this would correspond to a current of 0.9 mA!