ARCHITECTURE OF THE MEGA DETECTOR TRIGGER

The trigger for the MEGA detector system is based on signals from sing le, high-energy photons interacting in one of the three MEGA pair spec trometers. The trigger is divided into a fast and a slow stage. The fi rst stage produces a fast output if a specific pattern of detector hit s is observed in the scintillators and high-speed wire chambers of a p air spectrometer. The second, slow-stage interrogates drift chamber hi t patterns and provides a veto when the pattern fails a minimal requir ement for reconstruction of the hits into a pair of circular orbits. T he trigger interacts with the photon detector electronics by gating li mited sections of the detector during the read-out of an event. This p aper describes the two stage trigger system, the photon detector elect ronics, and the implementation of the trigger outputs to strobe the da ta acquisition system. The performance of the trigger is compared to M onte Carlo simulations of the photon detector response.