PERFORMANCE OF THE N - ANTI-N SCINTILLATION COUNTERS TRIGGER SYSTEM.

Seventy-four plastic scintillation counters, 2.1 and 3.0 m long, 21 cm wide, 2 cm thick, were built, tested and used in the trigger and TOF systems of the Grenoble free neutron-antineutron oscillation experiment. They showed efficiency and stability at a very high level. The time resolution of the counters was between 0.2 and 0.4 ns, allowing to measure the time of flight of particles crossing two planes of such counters 54 cm apart, with a resolution of the order of 0.5 ns. A spatial resolution of few centimeters was also obtained. An optical system, based on a pulsed dye laser and optical fibers, was used during data taking to monitor the performance of the counters and to keep the timing stable at the 150 ps level.