A preshower photon multiplicity detector for the WA93 experiment

A fine granularity preshower detector has been designed, fabricated and installed in the WA93 experiment for the first ever measurement of photon multiplicity in ultra-relativistic heavy ion collisions. Photons are counted by detecting the preshower signal in plastic scintillator pads placed behind a lead converter and applying a threshold on the scintillator signal to reject the minimum ionizing particles. The photon detection efficiency estimated using Monte-Carlo and beam test results is found to be ∼65-75% depending on centrality. Simulation and performance results of the detector for counting of photons and rejection of hadrons in the large multiplicity environment of ultra-relativistic heavy ion collisions are described.