Positron identification in APEX

The trigger which signals than an event of interest has occurred in APEX is based on the identification of back-to-back 511 keV positron annihilation radiation. The gamma rays are detected in 24- element position sensitive NaI(Tl) barrels which surround each silicon array. The gamma-ray information gives positron identification and allows the reconstruction of the location of the decaying positron on the silicon array. The principle of operation, design and performance will he presented including the energy, time and position resolutions. Examples of in-beam data show the successful suppression of false triggers from room background and beam-induced radiation, resuIting in the unambiguous identification and location of reaction positrons and the clean separation of positrons from the copious background of delta electrons.