The Proton Synchrotron (PS) is the oldest major particle accelerator at CERN, built as a 28 GeV proton accelerator in the late 1950s and put into operation in 1959. It takes the protons from the Proton Synchrotron Booster at a kinetic energy of 1.4 GeV and lead ions from the Low Energy Ion Ring (LEIR) at 72 MeV per nucleon. It has been operated as an injector for the Intersecting Storage Rings (ISR), the Super Proton Synchrotron (SPS) and the Large Electron-Positron Collider (LEP). Starting in November 2009, the PS machine delivers protons and will provide lead ion beams for the Large Hadron Collider (LHC).
It has also been used as a particle source for other experiments, such as the Gargamelle bubble chamber for which it supplied a neutrino beam. This led to the discovery of the weak neutral current in 1974.
The PS machine is a circular accelerator with a circumference of 628.3 m. It is a versatile machine having accelerated protons, antiprotons, electrons, positrons and species of ions. Major upgrades have improved its performance by more than a factor of 1000 since 1959. The only significant components remaining from its original installation some 50 years ago are the bending magnets and the buildings.