A technician works on one of two end caps of the OPAL detector at CERN, the European centre for particle physics near Geneva. OPAL is one of 4 giant particle detectors at the LEP collider. LEP collides electrons & positrons accelerated to an energy of 50 GeV in a circular tunnel 100m underground & 27km in circumference. OPAL is a cylindrical assembly of many types of apparatus which fit together like layers of an onion around the point where the particles collide. The OPAL end caps consist of pre-shower counters (the spokes" of the wheel) & electro-magnetic & hadron calorimeters. The technician is Martin Sproston of the Rutherford Appleton Laboratory in Britain. Photo taken April 1989."