CERN Accelerating science

The PHOS (PHOton Spectrometer) is a high resolution electromagnetic calorimeter consisting of 12544 detection channels based on lead-tungstate crystals, PbWO₄ (or PWO). It is positioned on the bottom of ALICE set-up at a distance 460 cm from the interaction point. The PHOS covers the pseudorapidity range –0.12 < eta < 0.12 and 70^o in azimuthal angle. The PHOS is dedicated to the search for electromagnetic radiation from the hot strongly interacting matter in nucleus-nucleus interactions at high energies, as well as for measurements of hadron specra via their radiative decays. The dynamic range for photon and neutral meson detection in PHOS spans from 0.1 to 100 GeV. The PHOS detector is equipped by the Charged Particle Veto detector (CPV) for photon identification via suppression of charged-particle contamination. CPV covers the most of PHOS acceptance of  60^o in azimuth. CPV is a set of proportional chambers with cathode pad readout installed in front of PHOS modules. PHOS is also a trigger detector delivering the hardware triggers of the L0 and L1 levels to the ALICE central trigger processor to select events with high-energy photons.