ALEPH's iron barrel

2 December 1998

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This week's picture is of ALEPH (Apparatus for LEP pHysics), one of the four large underground detectors positioned around the Large Electron Positron Collider (LEP) at CERN. Like the other detectors, ALEPH 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 during construction in 1987, two years before LEP started up - shows the segmented structure of ALEPH's iron hadron calorimeter or "barrel". This is the layer that stops and identifies hadrons (protons and other particles made from quarks, such as pions) and catches all their energy. The barrel is made up of 24 segments (half-modules), each of which is a multi-layer sandwich of iron and detectors (streamer chambers) weighing 70 tonnes.The iron also forms the "yoke" of the electromagnet (a superconducting solenoid) which bends the tracks of particles to provide vital information on momentum and hence energy. In the final apparatus, the calorimeter is completed by two "end caps" of similar construction which ensure that the electron-positron collision point is completely surrounded and that all hadrons produced are stopped and detected.

Credit: CERN Photo

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