The many faces of FOPI from fragment to strangeness detector

The FOPI detector system has been built to study the properties of dense an d hot nuclear matter with the beam (0.1-2 GeV/nucleon) of the heavy ion syn chrotron SIS at GSI Darmstadt. A modular system covering nearly the full 4 pi solid angle has been assembled in stages between 1990 and 1995, having a lready been used since 1991 in various experiments employing its expanding capabilities. Since the first phase of experiments, when nucleons and inter mediate mass fragments could be detected at projectile- and midrapidity, FO PI developed into a specialized system detecting also pions, kaons, antikao ns and identifying signals from decays of neutral particles with a strange quark content like Lambda, Phi and K-S(0). Studies of strangeness productio n and propagation became an important new line of the FOPI research program me. In order to extend the strangeness research and include kaon studies in heaviest systems an upgrade program for the FOPI detector system has been started. It aims at broadening the phase space of K+ and K- identification and at extending the efficiency as well as selectivity of data collecting. The upgrade of the subdetectors mainly involves the modification of TOF det ectors by replacing a part of the scintillator layer with Pestov spark coun ters. New experiments are expected to begin in 2001. The capabilities of th e FOPI system are illustrated by examples of recent and earlier results.