TRIGGER PROCESSOR FOR THE APEX POSITRON-ELECTRON SPECTROMETER

The APEX experiment is designed to study anomalous positron-electron p air production in heavy-ion collisions. Its trigger processor is desig ned to give a fast, first-level trigger which initiates data readout f or interesting events. This is achieved by identifying the back-to-bac k 511 keV annihilation radiation from positron decay. The radiation is detected in a segmented NaI(TI) 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 trigger ing 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, p rovides self-vetoing to reduce background associated with cosmic ray m uons.