OPAL's lead-glass barrel

25 November 1998

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This week's picture is of OPAL (Omni Purpose Apparatus for LEP), one of the four large underground detectors positioned around the Large Electron Positron Collider (LEP) at CERN. Like the other detectors, OPAL is constructed basically in cylindrical layers around the beam pipe where the electrons and positrons collide. Each layer is designed with a specific role in detecting and identifying the sprays of new particles produced when the electrons and positrons annihilate. This photograph - taken in 1993, during maintenance when the detector was opened up - shows the inner curved surface of half of OPAL's electromagnetic calorimeter. This is the layer that stops and identifies electrons and photons, and catches all their energy. As many as 4720 blocks of lead-glass (wrapped in black to keep out unwanted light) together make up this half of the cylindrical portion of the electromagnetic calorimeter. The calorimeter is completed by the lead-glass sections (not shown) of two "end caps" which ensure that the electron-positron collision point is completely surrounded and that all electrons and photons produced are stopped and detected.

Credit: CERN Photo

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