A high granularity preshower detector has been fabricated and installed in
the WA98 Experiment at the CERN SPS for measuring the spatial distribution
of photons produced in the forward region in lead-ion-induced interactions.
Photons are counted by detecting the preshower signal in plastic scintilla
tor pads placed behind a 3 radiation length thick lead converter and applyi
ng a threshold on the scintillator signal to reject the minimum ionizing pa
rticles, Techniques to improve the imaging of the fibre and performance of
the detector in the high multiplicity environment of lead-lead collisions a
re described. Using Monte-Carlo simulation methods and test beam data of pi
(-) and e(-) at various energies the photon counting efficiency is estimate
d to be 68% for central and 73% for peripheral Pb+Pb collisions. (C) 1999 E
lsevier Science B.V. All rights reserved.