Photon Multiplicity Detector
The Photon Multiplicity Detector (PMD) measures the multiplicity and spatial distribution of photons produced in collisions. It utilizes a veto detector as a first layer, to reject charged particles. Photons 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.