Trigger processor for the APEX positron-electron spectrometer

The APEX experiment is designed to study anomalous positron-electron pair production in heavy-ion collisions. Its trigger processor is designed to give a fast, first-level trigger which initiates data readout for interesting events. This is achieved by identifying the back-to-back 511 keV annihilation radiation from positron decay. The radiation is detected in a segmented NaI(Tl) calorimeter and the trigger processor identifies those patterns of hit detectors which are consistent with back-to-back decay. A fast pattern recognition algorithm gives triggering pulses within the 82.5 ns accelerator beam pulse period. Valid hit patterns are software-coded into memory and can be easily changed. A novel two-level discriminator, constructed from standard IC devices, provides self-vetoing to reduce background associated with cosmic ray muons.