ONLINE DETECTION OF NEUTRONS WITH A LEAD SCINTILLATING-FIBER CALORIMETER AND A SCINTILLATING TILE HODOSCOPE

We present the set-up and performance of a high energy neutron trigger based on a detector system consisting of a scintillating tile hodosco pe and a compensating lead/scintillating fibre calorimeter, A first le vel trigger signal is generated by a minimum hadronic energy detected in the fiducial volume of the calorimeter, This signature is complemen ted at a second trigger stage by the isolation of a shower whose origi n cannot be attributed to a charged particle. We detect such a topolog y by comparing the hit pattern of the hodoscope with online available calorimeter information, This complex task can be fulfilled in an effi cient and flexible way by Digital Signal Processors (DSP),The presente d method of online neutron detection was developed in the framework of the hyperon beam experiment WA89 at CERN and yields an average trigge r decision time of approximate to 11 mu s. From an analysis based on S igma(-) decays (Sigma(-) -->n pi(-)), we find a second level trigger e fficiency for neutrons of 70% at a rejection level of 95% for hadronic showers originating from charged particles.