A Preshower Photon Multiplicity Detector for the ALICE Experiment

A preshower Photon Multiplicity Detector (PMD) is proposed to be implemented in the ALICE experiment to study event shapes and isospin fluctuations. The PMD, to be mounted on the magnet door at 6m from the vertex, has fine granularity and full azimuthal coverage in the pseudo-rapidity region 1.8<eta<2.8. Charged hadrons are rejected using a charged particle veto (CPV) in front of the converter. The detector is based on a cellular honeycomb proportional chamber design for both the PMD and the CPV, and has a total of about 2 x 105 cels of 1 cm2 area. The honeycomb walls form a common cathode, operated at a high negative voltage. The signal is read out from the anode wires at ground potential using gassiplex electronics. The detector employs an inert gas mixture of Ar (70%) and CO2 (30%). Beam test results for a small prototype indicate that about 80% of the central volume of the detector has almost uniform efficiency (about 95%) for MIP detection. The average number of cells fired by a MIP is close to unity. Using VENUS events and GEANT simulation along with a neural network algorithm for photon-hadron discrimination, it is found that in the case of the PMD alone in ALICE, one can achieve photon counting efficiency of better than 65% for a photon sample having 80% purity. For the actual ALICE environment,the efficiency and purity reduce to about 70% each. By combining the PMD with the FMD, which measures the multiplicity of charged particles, one can study DCC and other effects leading to event-by-event fluctuation of the measured Ngamma/Nch ratio (