  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 scintillator
pads placed behind a 3 radiation length thick lead converter and applying a
threshold on the scintillator signal to reject the minimum ionizing particles.
  Techniques to improve the imaging of the fibre and performance of the
detector in the high multiplicity environment of lead-lead collisions are
described.
  Using Monte-Carlo simulation methods and test beam data of pi- and e- at
various energies the photon counting efficiency is estimated to be 68% for
central and 73% for peripheral Pb+Pb collisions.
