Particle accelerator

Tevatron, particle accelerator that was located at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. Fermilab is and the Tevatron was operated for the U.S. Department of Energy by the Universities Research Association, a consortium of 85 research universities in the United States and four universities representing Canada, Italy, and Japan. The Tevatron was the world’s highest-energy particle accelerator until 2009, when it was supplanted by the Large Hadron Collider of the European Organization for Nuclear Research (CERN). The Tevatron closed on September 30, 2011.

The Tevatron was constructed in the 1980s below Fermilab’s first particle accelerator, a proton synchrotron in a circular tunnel with a circumference of 6.3 km (3.9 miles). The Tevatron was a superconducting synchrotron that took advantage of the higher magnetic-field strengths produced by 1,000 superconducting magnets to accelerate protons to significantly higher energy levels. The whole ring was kept at 4.5 kelvins (−268.7 °C, or −451.6 °F) by liquid helium. The original synchrotron became part of the preaccelerator injection system for the Tevatron, accelerating particles to 150 GeV (1 GeV = 1 giga electron volt = 1 billion electron volts) and then transferring them to the new superconducting ring for acceleration to 900 GeV. In 1987 the Tevatron began operation as a proton-antiproton collider—with 900-GeV protons striking 900-GeV antiprotons to provide total collision energies of 1.8 teraelectron volts (TeV; 1.8 trillion electron volts). The original main ring was replaced in 1999 by a new preaccelerator, the Main Injector, which had a 3.3-km (2.1-mile) magnet ring. The Main Injector delivered more intense beams to the Tevatron and thus increased the number of particle collisions by a factor of 10.

The Tevatron’s premier discovery was that of the top quark, the sixth and most-massive quark, in 1995. Scientists inferred the existence of the top quark, produced as a result of 1.8-TeV proton-antiproton collisions, on the basis of its decay characteristics. In 2010 scientists used the Tevatron to detect a slight preference for B-mesons (particles that contain a bottom quark) to decay into muons rather than antimuons. This violation of charge symmetry could lead to an explanation for why there is more matter than antimatter in the universe.

At Fermilab the proton beam, initially in the guise of negative hydrogen ions (each a single proton with two electrons), originated in a 750-kV Cockcroft-Walton generator and was accelerated to 400 MeV in a linear accelerator. A carbon foil then stripped the electrons from the ions, and the protons were injected into the Booster, a small synchrotron 150 metres (500 feet) in diameter, which accelerated the particles to 8 GeV. From the Booster the protons were transferred to the Main Injector, where they were further accelerated to 150 GeV before being fed to the final stage of acceleration in the Tevatron.

The antiprotons were produced by directing protons accelerated to 120 GeV from the Main Injector at Fermilab onto a nickel target. The antiprotons were separated from other particles produced in the collisions at the target and were focused by a lithium lens before being fed into a ring called the debuncher, where they underwent stochastic cooling. They were passed on first to an accumulator ring and then to the Recycler ring, where they were stored until there were a sufficient number for injection into the Main Injector. This provided acceleration to 150 GeV before transfer to the Tevatron.

Protons and antiprotons were accelerated simultaneously in the Tevatron to about 1 TeV, in counterrotating beams. Having reached their maximum energy, the two beams were stored and then allowed to collide at points around the ring where detectors were situated to capture particles produced in the collisions.

During storage in the Tevatron, the beams gradually spread out so that collisions became less frequent. The beams were “dumped” in a graphite target at this stage, and fresh beams were made. This process wasted up to 80 percent of the antiprotons, which were difficult to make, so, when the Main Injector was built, a machine to retrieve and store the old antiprotons was also built. The Recycler, located in the same tunnel as the Main Injector, was a storage ring built from 344 permanent magnets. Because there was no need to vary the energy of the antiprotons at this stage, the magnetic field did not need to change. The use of permanent magnets saved energy costs. The Recycler “cooled” the old antiprotons from the Tevatron and also reintegrated them with a new antiproton beam from the accumulator. The more-intense antiproton beams produced by the Recycler doubled the number of collisions in the Tevatron.

Until 2000, protons at 800 GeV were extracted from the Tevatron and directed onto targets to yield a variety of particle beams for different experiments. The Main Injector then became the principal machine for providing extracted beams, at the lower energy of 120 GeV but at much higher intensities than the Tevatron provided.

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