A data acquisition system and a trigger of the SCAN setup

The trigger, data acquisition, and monitoring systems of the double-arm SCAN spectrometer are described. The SCAN spectrometer has been designed for seeking eta-mesic nuclei in reaction pA -> (A-1)(eta) + np at the internal target of the nuclotron at the Joint Institute for Nuclear Research. A brief description of the spectrometer is presented. A system of triggers has been developed for selection of events with eta-mesic nuclei, which are identified by a two-particle decay of the S-11(1535) resonance produced in a target nucleus (S-11 -> p pi(-)). A two-level trigger is used. At the first level, fast selection of events with two charged particles from a stream of 10(4) triggers/s is performed. The second level includes extraction of events with a pair of particles (p, pi(-)) flying apart at angle of similar to 180 and subsequent identification of pairs corresponding to the kinematics of the S-11 resonance decay, which is effected by a special trigger module.