The plastic ball - a multi-detector, large solid angle spectrometer with charged particle identification for the bevalac

For the study of central relativistic nuclear heavy ion collisions, which are characterized by the emission of a large number of particles, one needs a detector which covers a large solid angle - 4π if possible - and which is capable of identifying charged particles. The high multiplicity requires a large number of detectors, and the need for charged-particle identification requires a measurement of the energy loss, and the total energy for each particle detected. The spectrometer consists of 815 detector modules, which cover 94% of 4π. The geometry of these modules has been taken from the Stanford crystal ball detector for γ-rays - with minor modifications. This geometry is suited for the high multiplicities of particles emitted in relativistic heavy ion collisions. The dimension of the individual elements have been chosen to stop 240 MeV protons. Above this energy reaction losses start to dominate, so that the light output of a scintillator would no longer be a true indication of the energy. Out of 100 charged particles, 94 will hit the Plastic Ball, 87 will fire a detector element, and 80 will be identified uniquely. For the individual detector modules we have used the "Phoswich" idea, by gluing a 4 mm thick CaF₂ scintillator to a 35 cm thick plastic scintillator (NE114) with the shape of a truncated pyramid, which is viewed by one photomultiplier tube (PM2202B).The extremely different decay times of the CaF₂ scintillator and the plastic allow us to separate their light output by integrating the signal from the phototube for the first 20 nsec (the "E signal") and then integrating the "delayed" signal for 2 μsec (the "ΔE signal"). This constitutes a simple low cost detector telescope, which can identify the hydrogen and helium isotopes. We describe the electronic setup to separate the "ΔE" and "E" signals, and give some test results, which show the separation and identification, of protons- - , deuterons, tritons, ³He, and alpha particles.