Particle Accelerators

The synchrotron is the most recent and most powerful member of the accelerator family. A synchrotron consists of a tube in the shape of a large ring through which the particles travel; the tube is surrounded by magnets that keep the particles moving along the centre of the tube. The particles enter the tube after already having been accelerated to several million electronvolts. They are accelerated at one or more points on the ring each time they make a complete circle around the accelerator. To keep the particles in a rigid orbit, the strengths of the magnets in the ring are increased as the particles gain energy. In a few seconds, the particles reach energies greater than 1 GeV and are ejected, either directly into experiments or towards targets that produce a variety of elementary particles when struck by the accelerated particles. The synchrotron principle can be applied to either protons or electrons, although most of the large machines are proton-synchrotrons.

The first accelerator to exceed the 1 GeV mark was the cosmotron, a proton-synchrotron at Brookhaven National Laboratory, in New York State. The cosmotron was operated at 2.3 GeV in 1952 and later increased to 3 GeV. In the mid-1960s, two operating synchrotrons were regularly accelerating protons to energies of about 30 GeV. These were the Alternating Gradient Synchrotron at Brookhaven National Laboratory, and a similar machine near Geneva, Switzerland, operated by CERN, the European Organization for Nuclear Research. By the early 1980s, the two largest proton-synchrotrons were a 500-GeV device at CERN and a similar one at the Fermi National Accelerator Laboratory (Fermilab) near Batavia, Illinois. The capacity of the latter, called the Tevatron, was increased to a potential 1 TeV (1 tera-electronvolt, or 1 trillion eV) in 1983 by installing superconducting magnets, making it the most powerful accelerator in the world. In 1989, CERN began operating the Large Electron-Positron Collider (LEP), a 27-km (16.7-mi) ring that can accelerate electrons and positrons to an energy of 50 GeV.

A collider is a combination of an accelerator and a storage ring, or rings, that produces more energetic collisions between particles than a conventional accelerator. The latter slams accelerated particles into a stationary target, whereas a collider accelerates two sets of particles that are injected into the storage rings and are then made to collide head-on. All major accelerators have now been converted to storage-ring colliders. Since electrons and positrons have opposite electric charges, they can be stored in the same ring, circulating in opposite directions. When particles of the same kind of charge are to be collided, they have to be stored in separate rings. In 1987, Fermilab converted the Tevatron into a storage ring collider and installed a detector three storeys high that observed and measured the products of the head-on particle collisions.

As powerful as today’s colliders are, physicists need even more powerful devices to test today’s theories. Unfortunately, building larger rings is extremely expensive. CERN’s Large Hadron Collider (LHC) is designed to share the 27-km (16.7-mi) tunnel that houses LEP. In 1988, the United States began planning for the construction of the Superconducting Super Collider (SSC) near Waxahatchie, Texas. The SSC was to be an enormous storage ring collider 87 km (54 mi) long. However, in October 1993, when about one fifth of the tunnel had been completed, the US Congress voted to cancel the project as a result of its estimated total cost of over US$10 billion.

Relatively small-scale particle accelerators are used in industry and medicine. For example, beams of high-energy particles can be used to kill tumours. They can also create radioisotopes (radioactive forms of elements), which in turn can be used in radiotherapy, or as biological or industrial tracers.

Accelerators are used to explore atomic nuclei, thereby allowing nuclear scientists to explain the structure and behaviour of atoms. The most powerful machines, exceeding 1 GeV, are used to study the fundamental particles that appear in particle interactions. Several hundred of these particles have been identified. Colliders permit scientists to bring about violent particle collisions that mimic the state of the universe in the Big Bang (see Cosmology), when it was just microseconds old. Continued study of their findings should increase scientific understanding of the make-up of the universe.

See also Particle Detectors.