Heavy metals see the light

17 July 2002

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Containing  lead and tungsten, and more than eight times denser than water, this crystal of lead tungstate (PbWO4) is  destined for the ALICE experiment at CERN, the European particle physics centre  near Geneva. ALICE will study high-energy collisions between nuclei ("heavy ions") at the  Large Hadron Collider (LHC) which is currently under construction at CERN. The crystals will form the Photon Spectrometer, which is designed, in effect, to measure the temperature of the collisions by detecting the high-energy photons emerging from them. Similar crystals are also being used in the Electromagnetic Calorimeter of the CMS experiment at the LHC. When electrons and  photons strike the lead tungstate, they create showers of further photons and electrons in the dense material and cause it to glow, or scintillate. The total amount of light detected in this way gives a measure of the energy of the original electron or photon.

Credit: CERN Photo
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