Introduction to Particle Accelerator
Particle Accelerator, a device that accelerates electrons, protons, or other charged particles to very high speeds. These particles are either electrically charged atoms or subatomic particles—objects smaller than an atom. The particles travel through an accelerator in a narrow beam. In accelerating the beam, the machine increases the particles' energy of motion. In storage rings, accelerated particles are kept moving with a constant energy for hours or even days. Some particle accelerators direct particles against a fixed target. In particle accelerators, known as colliders, two beams of particles are directed against each other. Particle accelerators are popularly known as "atom smashers," since the particles they accelerate acquire enough energy to break apart the nucleus of an atom.
Uses of Particle Accelerator
Accelerators have found some important applications in industrial and medical settings. Physicists use accelerators to determine properties of subatomic particles and their interaction. Accelerators give high energy to subatomic particles, such as positively charged protons or negatively charged electrons, which then collide with a stationary target or with another beam that is moving in the opposite direction. Due to this interaction particles break away from the target or cause other subatomic particles to form. Devices called particle detectors located near the points of impact provide information about particles that fly away from these points.
Experiments conducted with particle accelerators have contributed greatly to the scientific knowledge of the nucleus of the atom, nuclear forces, and elementary particles. This knowledge was essential in building the first atom bomb and in the design of nuclear reactors. In industry and other areas, particle accelerators are used to produce beams of various kinds of radiation or subatomic particles. For example, electrons in storage rings can be used to produce beams of synchrotron radiation—mainly x-rays and ultraviolet radiation. A beam of electrons sends out x-rays when it passes through a magnetic field, a region of space in which a moving electric charge is acted on by magnetic force. One use of these rays is to produce transistors on experimental computer chips. Various kinds of particle beams are used for medical diagnostics and treatment, for example, to destroy tissue in cancerous tumors. Accelerated particles are also used for producing elements heavier than uranium and for making radioactive isotopes. In this case, nuclei (singular, nucleus) in beams of ions combine with target nuclei to form heavy elements. The nucleus (central core of an atom) consists of charged particles called protons and electrically neutral neutrons. Particle accelerators are very complex devices and their construction and operation require extreme precision. The first particle accelerators were small laboratory instruments. The world's most powerful accelerator, the Tevatron, occupies an underground tunnel that forms a ring 3.9 miles (6.3 km) in circumference, and it is operated by a staff of more than 2,000 persons. It is located at Fermilab (Fermi National Accelerator Laboratory) near Chicago.
How Particle Accelerators Work
Most particle accelerators use electric fields to accelerate charged particles and magnetic fields to guide them. In general, the electric field can be set up to accelerate either negatively or positively charged particles, including particles of antimatter. A magnetic field exerts a force on moving charged particles in such a way that it acts at right angles to the direction in which the particles are moving. The strength of the magnetic field controls the degree to which the charged particles will curve as they move through the field. The magnetic field can thus be used to make charged particles move in a circular path while the electrical fields are used to boost the energy of the particle to higher and higher levels.
Electrons are very light particles and are easier to accelerate to high speed than protons, which are 1,836 times as heavy. Some accelerators are used to accelerate atomic nuclei that contain a large number of protons and neutrons. They are given an electrical charge by stripping them of all or most of their electrons. Such nuclei are called ions.
In modern particle accelerators, charged particles are accelerated to relativistic speeds —that is, to speeds at which the effects predicted by Einstein's theory of relativity become important. As the speed of a particle approaches the speed of light, increasingly greater energy is needed to accelerate the particle. It is not possible to accelerate any particle to the speed of light, but electrons in accelerators have reached more than 99.9999 per cent the speed of light.
It is customary in nuclear physics to measure energy in terms of the electron volt (eV), the amount of energy an electron gains when passing through an electrical potential of one volt. Compared to the energies involved in many nuclear phenomena, the electron volt is a small unit. For example, protons require energies of several hundred keV (kilo, or thousand, electron volts) or more to penetrate the nuclei of the various elements, and the production of antiprotons requires 6 GeV (giga, or billion, electron volts) or more. When an accelerated particle strikes another particle, energy is released that can result in the creation of new particles. The higher the energy of the particle accelerator, the more massive the particles it can create. The particles are studied by various means. Electronic particle detectors are used to track the particles and to measure their energy. Using this information, physicists can identify the particles and determine their properties. In the most powerful accelerators, many thousands of particles are produced almost simultaneously, and computers are used to help select the data obtained for the particles under study. With earlier particle detectors, photographs were taken of the tracks formed by the particles produced after each collision, and hundreds of thousands of these photographs had to be studied to obtain information about rare subatomic particles.
Kinds of Particle Accelerators
Among the several kinds of particle accelerators are the following:
BetatronThe betatron uses a varying magnetic field to accelerate electrons. The electrons are fed into a doughnut-shaped ring placed between the poles of an alternating-current electromagnet. As the magnetic field is varied, it increases the energy of the electrons as they whirl around the ring. The betatron is now largely obsolete.
Cockcroft-Walton AcceleratorCockcroft-Walton accelerator uses a system of capacitors and rectifiers to produce a high voltage. Cockcroft-Walton accelerators, a type of linear machine, were once commonly used to give the initial acceleration to particles that would then be fed to larger particle accelerators. The machine was invented in 1929 by physicists John D. Cockcroft of Britain and Ernest T. S. Walton of Ireland. In 1932, Cockcrof and Walton used it to disintegrate nuclei and accelerate protons to 500 keV. Presently, Cockcroft-Walton accelerators are used to produce high-speed beams of particles such as protons, electrons, and ions. Most of them serve as beam sources for more powerful accelerators. A Cockcroft-Walton accelerator can boost particles to about 750 keV.
CyclotronThe simplest circular accelerator, cyclotron, was invented by United State physicist Ernest O. Lawrence in 1930. It consists of a cylindrical chamber containing two hollow D-shaped electrodes placed between the poles of a powerful electromagnet. A constant, high-frequency alternating current is applied to the electrodes. Charged particles at the center of the chamber are attracted to one electrode and then the other. As the charged particles are accelerated, they move outward along ever-larger circular paths. This process continues until the particles hit a target or spiral out of the cyclotron.
The diameter of the magnet and the strength of the magnetic field in a cyclotron is responsible for the energy produced by particles. The largest cyclotron in the world can accelerate protons to 720 MeV.
Linear AcceleratorsIn a linear accelerator, the charged particles move through a series of hollow tubes placed end to end. The tubes are separated by a small gap. An alternating electrical current is fed to adjacent tubes in such a way that the charged particles are accelerated as they pass between the tubes.
Linear accelerators fall into two categories: standing-wave linear accelerators and traveling-wave linear accelerators. Standing-wave linear accelerators can boost protons to energies up to 200 MeV by generating an alternating electric field, which is part of an electromagnetic wave produced by vacuum tubes called klystrons. This wave is called standing wave. The electric field alternates between positive and negative and accordingly accelerates the protons in the desired or opposite direction. The traveling-wave linear accelerator is used to accelerate electrons. In a traveling-wave linear accelerator, a wave is generated by klystrons which travels the length of a horizontal pipe. The electrons travel with the wave's negative field, gaining energy as they go. The electrons move so quickly that the positive field cannot catch up with them, so the accelerator needs no drift tubes. Stanford Linear Accelerator Center (SLAC) in Palo Alto, California, has developed the longest linear accelerator in the world. The 2-mile (3.2 km) long machine began operating in 1966. In the late 1980s, SLAC added a circular structure to one end of the machine, enabling it to operate as a two-beam collider. It accelerates electrons and positrons to energies of 50 GeV. The positron has a positive electrical charge, in contrast to the electron's negative charge. In a head-on collision of a matter particle and its antimatter counterpart, all the mass in the two particles turns into energy. An instant later, most of the energy turns into new particles.
SynchrocyclotronThis accelerator is a type of cyclotron. The frequency of the alternating current fed to the electrodes is varied with time to account for the slowing rate at which the charged particles are accelerated as the particles increase in mass.
SynchrotronIn a synchrotron, the accelerated particles are confined to a closed path by a ring of electromagnets. As the particles are accelerated, the strength of the magnetic field is increased to keep them confined to the same path. Other accelerators are used with large synchrotrons to accelerate the charged particles in several steps before they are injected into the synchrotron. Storage rings are synchrotrons in which high-energy particles are kept circulating through the machine while more charged particles are added to the beam. Usually two beams of oppositely charged particles are produced, the particles in each beam moving in opposite directions. Once the beams contain a large number of particles, they are made to collide head-on at a designated point. The head-on collisions release much more energy than would be released if the particle struck a fixed target. The Tevatron is a storage ring in which protons collide with antiprotons.
Van De Graaff GeneratorIn 1931, U.S. physicist Robert J. Van de Graaff pioneered Van de Graaff generator, which accelerates particles by means of a very powerful electrostatic discharge.
The Van de Graaff generator can accelerate particles such as protons, electrons, and ions to energies as high as 15 MeV. Van de Graaff generator has a hollow metal sphere at one end of a pipe. At the lower end of the machine the generator uses a pulley. A belt passes over the pulley located beside a source of electric charge. The source puts a charge on the belt. A metallic brush transfers the charge to the sphere. When the charge on the sphere builds to a certain strength, particles that have the same kind of charge—positive or negative—are released near the sphere's inner surface. These particles move to the other end of the machine, pass through a hole, and hit a target.
The maximum energy obtainable from a Van de Graaff can be greatly increased by the application of the "tandem" principle. A tandem machine has the capacity to produce ions with twice the energy of a single Van de Graaff generator. The tandem Van de Graaff generator has a sphere at the center of a long pipe, which receives a positive charge. This charge attracts negative ions that are inserted at one end of the pipe. When the ions reach the center, they pass through a thin target that removes two electrons from some of them. The electrons are thus rendered positive, and the sphere repels them toward the other end of the pipe. They gain additional energy as they pass to the end of the pipe.
History
The cyclotron was invented by Ernest O. Lawrence in 1930-31. During the same period, Robert Jemison Van de Graaff invented the accelerator named after him. In 1932, Sir John D. Cockroft and Ernest T. S. Walton produced the first successful artificial transmutation of an element with the type of particle accelerator that was later named after them.
Donald Kerst built the first betatron in 1940. The synchrotron was developed in the late 1940s and early 1950s. In 1955, the antiproton was discovered with a synchrotron called the Bevatron, one of the first machines to accelerate particles to an energy of more than one billion electron volts.
Since the late 1970s, superconducting electromagnets have been used in many particle accelerators. These electromagnets are made with wire that loses all resistance to an electric current at a low temperature. Additional power is needed to cool the magnets, but they can produce much stronger magnetic fields than conventional magnets of comparable size. Using superconducting magnets, the Tevatron in the late 1980s achieved an energy of more than 1 TeV (tera, or thousand billion, electron volts). In 2000, the Relativistic Heavy Ion Collider, built with 1,740 superconducting magnets, achieved even greater energies.
The most powerful existing accelerator is a synchrotron with a circumference of 9/10 miles (6.3 km). The Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, developed this synchrotron. On the first year of operation, in 1972, the machine accelerated protons to 400 GeV. Since 1987, it has collided beams of protons with beams of antiprotons, giving a total collision energy of 980 GeV.
The world's largest accelerator tunnel, called LEP2, is assembled at the CERN research center near Geneva, Switzerland.It has a circumference of 17 miles (27 km). The Large Electron-Positron (LEP), built inside the tunnel, was switched on in 1989. In 1996, LEPs name changed to LEP2 to reflect a major increase in beam energy. LEP2 collided electrons and positrons at energies up to 104.5 GeV per beam.
LEP2s era came to an end in 2000 to give way to the new machine, the Large Hadron Collider (LHC). It has a capacity to collide two beams of protons at energies up to 7 TeV per beam.
The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory on Long Island, New York, began its operation in 2000. RHIC accelerates gold ions around two 2.4 miles (3.9 km) circular tubes. The machine can operate at energies as high as 100 GeV per proton or neutron.