ALEPH first measured events in LEP in July 1989. LEP operated at around 91 GeV – the predicted optimum energy for the formation of the Z particle. From 1995 the accelerator operated at energies up to 200 GeV, above the threshold for producing pairs of W particles.
The ALEPH detector was built in cylindrical layers around a beam pipe made of beryllium, with the electron-positron collision point in the middle. Working outwards from the beam pipe, ALEPH held a vertex detector composed of two layers of double-sided silicon microstrips; an inner drift-chamber that provided 8 tracking coordinates and a trigger signal for charged particles from the interaction point; a time projection chamber – 4.4 metres long and 3.6 metres in diameter – to detect charged particles; an electromagnetic calorimeter to identify electrons and photons; a hadron calorimeter to detect hadrons; and a superconducting coil, 6.3 metres long and 5.3 metres in diameter, to provide the 1.5 tesla magnetic field necessary to work out a particle's charge and allow measurements of momentum. The whole system was housed inside a 12-sided cylinder and surrounded by a muon-detection system.