PHOS is a high-resolution electromagnetic calorimeter which measures the photons coming out of the extremely hot plasma created in the lead-lead collisions at the LHC. PHOS covers a limited acceptance domain at central rapidity. It is made of lead tungstate crystals read out by avalance photodiodes.
When high energy photons strike lead tungstate, they make it glow or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron) and stops most photons that reach it. The crystals are kept at a temperature of 248 K, which helps minimize the deterioration of the energy resolution due to noise and optimize the response for low energies.