About the PMD:
The Photon Multiplicity Detector (PMD) is a Particle shower detector which measures the multiplicity and spatial distribution of photons produced in the collisions. It utilizes as a first layer a veto detector to reject charged particles. Photons on the other hand pass through a converter, initiating an electromagnetic shower in a second detector layer where they produce large signals on several cells of its sensitive volume. Hadrons on the other hand normally affect only one cell and produce a signal representing minimum-ionizing particles.