The Gsi-Lbl Plastic Ball/Wall Spectrometer*

Since the Plastic Wall can detect beam velocity particles the time calibration is taken from each individual experiment. The inner part of the Wall, covering the angular region from 0 deg. to 2.5 deg., and the outer part, from 2.5 deg. to approximately 10 deg. The Plastic Wall serves in addition as a trigger counter for the Ball. The inner Wall extends the angular measurement to 0 deg. In addition, the center region of the inner Wall makes up the event trigger in the first round of experiments. The inner Wall covered the region within 2 deg. of the beam and was finely divided because of the high multiplicity of fragments near the beam. Its main purpose was to form the fast trigger with the upstream beam counter and collimator to decide which events were to be recorded.