The PMT selected for the HEXTE is the Hamamatsu R877-04 ruggedized 5" PMT¹⁴. This phototube exhibits excellent pulse height response in combination with the phoswich detector, and has extremely low noise. The tube is guaranteed to meet our environmental specifications, and is screened for acceleration and random vibration. The bleeder string is of "standard" design, with 1.2M ohm divider resistances and bypass capacitors over the last four dynodes. A signal is obtained from the 8th dynode (out of 10) for determining when charged particles have penetrated the detector, and optionally provide a veto window of 50 - 500 µs after the occurrence of such an event. The entire PMT/bleeder assembly is potted with an opaque silicone. The extemal surface of the potting material includes ribbing to accommodate thermal expansion and provide a light seal for the PMT.

The charged particle background at the HEXTE orbit results in a maximum variation in count rate in the phoswich detector of 40 to 400 cps. The change in average tube current between these extremes (using the bleeder assembly described above) results in a gain change of 2%, within the gain control system requirement. The bypass capacitor values were chosen to minimize the instantaneous gain change due to the charge depletion of the capacitors immediately following a large energy-loss event. The design should result in a negligible degradation of the overall energy resolution, especially when the large energy-loss veto window is used. This feature of the PMT assembly was recently tested at the Indiana University Cyclotron Facility, and preliminary analysis validates the design.