GAMMASPHERE-timing and signal processing aspects of the BGO Compton shield

We describe the design of the signal processing system and considerations involved in the timing performance of the BGO scintillator Compton shield that surrounds each of the 110 large germanium detectors used in a geodesic array covering the complete sphere surrounding the target in GAMMASPHERE. It is shown that the main timing limitation results from the statistics of photoelectron emission from the photocathode of the photomultiplier tubes (PMTs) that observe the light from the scintillators and that achieving the required timing demands triggering on the first photoelectron. The circuits to do this for a single detector assembly must be able to deal with signals from 14 PMTs associated with the 7 elements of the scintillator shield surrounding each germanium detector