On 27 January 1971, two beams of protons collided in the Intersecting Storage Rings (ISR) for the first time. Physicists had planned this machine in the 1960s, based on the idea that smashing two particle beams head on would give much higher energies than colliding a single beam of particles with a fixed target. It was the world’s first hadron collider, and proved pivotal in changing the views of members of the physics community who doubted the usefulness of hadron colliders. The ISR began operation in 1971 and ran until 1984, holding the luminosity record for hadron colliders until 2004.
The ISR was composed of two interlaced rings each with a diameter of 150 meters. Each ring contained a beam pipe surrounded by magnets to direct the circulating particles. Protons circulated in opposite directions and collided with a maximum centre-of-mass energy of 62 GeV. This is the equivalent of a 2000 GeV beam hitting a stationary target. The Proton Synchrotron, which is still in operation, fed proton beams into the ISR.
The ISR performed the first-ever proton-proton and proton-antiproton collisions. It was also where stochastic cooling was first developed. This technique reduces both the transverse dimension of the beam and the spread in the energy of the particles. The technique was adopted by the Super Proton Synchrotron for proton-antiproton collisions and is still used in the Antiproton Decelerator. The ISR paved the way for later accelerators as it hinted at the smaller elements of protons which are now known to be quarks and gluons.
About 15% of the research done at the ISR was directed at improving the machine itself, which grew increasingly sophisticated during its 13 years of operation. The detector expertise gained through operating the ISR made later projects such as the Large Hadron Collider possible.
After the ISR shut down in 1984, CERN’s focus shifted to the planned Large Electron-Positron Collider but the ISR tunnel is still used for storage and magnet work.