A Plastic-BaF2 Phoswich Telescope for ChargedNeutral Particle and Photon Detection'

BaFz has become a very versatile scintillator for particle and photon detection commonly applied in medium and high energy physics experiments. The intrinsic properties allow particle identification via time-of-flight, AE-E and pulse-shape techniques. A new NE102A-BaFz phoswich detector using the standard TAPS crystals has been tested at relativistic energies. The drastically improved particle identification and first results obtained with a large annular forward wall consisting of 120 phoswich modules with 15" NE102A attached to the BaF2-crystal will be discussed in detail. I. INTRODUCTION During the past years BaF2 has become one of the most commonly used inorganic scintillators for the detection of neutral or charged particles as well as photons in medium and high energy physics experiments. The extreme fast response [ 1,2,3], strong luminescence and the different intrinsic sensitivity of the two major scintillation components to the nature of the impinging probe [4,5] combined with high density and short radiation length fulfill the main requirements for a versatile detector material. The production of large, homogeneous and high quality crystals has encouraged the construction of complex spectrometers such as the calorimeter TAPS [6]. The Two/Three Arm Photon Spectrometer has been designed to investigate primarily neutral mesons via an invariant mass analysis of the two high energy decay photons in relativistic and ultra-relativistic heavy ion collisions or photonuclear reactions, respectively [7,8]. The point of impact and the total energy of the electromagnetic shower contained in several calorimeter modules have to be determined precisely. In case of heavy ion reactions, the much higher multiplicity of hadronic reaction products requires the efficient discrimination against chargedheutral particles. In contrast, the exclusive study of photonuclear processes aims for the simultaneous spectroscopy of the coincident hadrons. The short decay time and the high light output of BaF2 allow time resolutions better than 0=85ps even for large crystals such as the TAPS modules [9]. Therefore, particle identification and even energy determination can be performed based on the time-of-flight technique (TOF) at a typical distance of 1-2m from the vertex using a start-counter system as time reference. In particular, low and medium energy neutrons, which react via (n,y)-processes with BaFz and induce a signal-shape identical to that of photons, can only be identified via TOF. Restricted to the typical geometrical arrangement of TAPS in blocks consisting of 64 individual modules, a plastic scintillator array of identical granularity and shape can be mounted in front. The individually read-out components (0.5cm NE102A) serve as an on-line charged particle vetodetector (CPV) and measure the specific energy loss required for particle identification by means of AE-E-correlations.