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Radiology => Particle Accelerators
Particle Accelerators
INTRODUCTION
Particle Accelerators, in physics, devices used to accelerate charged elementary particles or ions to high energies. Particle accelerators today are some of the largest and most expensive instruments used by physicists. They all have the same three basic parts: a source of elementary particles or ions, a tube pumped to a partial vacuum in which the particles can travel freely, and some means of speeding up the particles.
Charged particles can be accelerated by an electrostatic field. For example, by placing electrodes with a large potential difference at each end of an evacuated tube, British scientists John D. Cockcroft and Ernest Thomas Sinton Walton were able to accelerate protons to 250,000 eV (see Electron Volt). Another electrostatic accelerator is the Van de Graaff accelerator, which was developed in the early 1930s by the American physicist Robert Jemison Van de Graaff. This accelerator uses the same principles as the Van de Graaff Generator. The Van de Graaff accelerator builds up a potential between two electrodes by transporting charges on a moving belt. Modern Van de Graaff accelerators can accelerate particles to energies as high as 15 MeV (15 million electron volts).
LINAC
Another machine, first conceived in the late 1920s, is the linear accelerator, or linac, which uses alternating voltages of high magnitude to push particles along in a straight line. Particles pass through a line of hollow metal tubes enclosed in an evacuated cylinder. An alternating voltage is timed so that a particle is pushed forward each time it goes through a gap between two of the metal tubes. Theoretically, a linac of any energy can be built. The largest linac in the world, at Stanford University, is 3.2 km (2 mi) long. It is capable of accelerating electrons to an energy of 50 GeV (50 billion, or giga, electron volts). Stanford's linac is designed to collide two beams of particles accelerated on different tracks of the accelerator.
CYCLOTRON
The American physicist Ernest O. Lawrence won the 1939 Nobel Prize in physics for a breakthrough in accelerator design in the early 1930s. He developed the cyclotron, the first circular accelerator. A cyclotron is somewhat like a linac wrapped into a tight spiral. Instead of many tubes, the machine has only two hollow vacuum chambers, called dees, that are shaped like capital letter Ds back to back (thus: ? D). A magnetic field, produced by a powerful electromagnet, keeps the particles moving in a circle. Each time the charged particles pass through the gap between the dees, they are accelerated. As the particles gain energy, they spiral out toward the edge of the accelerator until they gain enough energy to exit the accelerator. The world's most powerful cyclotron, the K1200, began operating in 1988 at the National Superconducting Cyclotron Laboratory at Michigan State University. The machine is capable of accelerating nuclei to an energy approaching 8 GeV.
When nuclear particles in a cyclotron gain an energy of 20 MeV or more, they become appreciably more massive, as predicted by the theory of relativity. This tends to slow them down and throws the acceleration pulses at the gaps between the dees out of phase. A solution to this problem was suggested in 1945 by the Soviet physicist Vladimir I. Veksler and the American physicist Edwin M. McMillan. The solution, the synchrocyclotron, is sometimes called the frequency modulated cyclotron. In this instrument, the oscillator (radio-frequency generator) that accelerates the particles around the dees is automatically adjusted to stay in step with the accelerated particles; as the particles gain mass, the frequency of accelerations is lowered slightly to keep in step with them. As the maximum energy of a synchrocyclotron increases, so must its size, for the particles must have more space in which to spiral. The largest synchrocyclotron is the 600-cm (236-in) phasotron at the Dubna Joint Institute for Nuclear Research in Russia; it accelerates protons to more than 700 MeV and has magnets weighing 6984 metric tons (7200 tons).
BETATRON
When electrons are accelerated, they undergo a large increase in mass at a relatively low energy. At 1 MeV energy, an electron weighs two and one-half times as much as an electron at rest. Synchrocyclotrons cannot be adapted to make allowance for such large increases in mass. Therefore, another type of cyclic accelerator, the betatron, is employed to accelerate electrons. The betatron consists of a doughnut-shaped evacuated chamber placed between the poles of an electromagnet. The electrons are kept in a circular path by a magnetic field called a guide field. By applying an alternating current to the electromagnet, the electromotive force induced by the changing magnetic flux through the circular orbit accelerates the electrons. During operation, both the guide field and the magnetic flux are varied to keep the radius of the orbit of the electrons constant.
SYNCHROTRON
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 through the center of the tube. The particles enter the tube after already having been accelerated to several million electron volts. Particles are accelerated at one or more points on the ring each time the particles 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 toward 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 Brookhaven, New York. 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 (also known as the European Laboratory for Particle Physics). 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 Tevatron, was increased to a potential 1 TeV (trillion, or tera, 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.
STORAGE RING COLLIDER ACCELERATORS
A storage ring collider accelerator is a synchrotron that produces more energetic collisions between particles than a conventional synchrotron, which slams accelerated particles into a stationary target. A storage ring collider accelerates two sets of particles that rotate in opposite directions in the ring, then collides the two sets of particles. CERN's Large Electron-Positron Collider is a storage ring collider. In 1987, Fermilab converted the Tevatron into a storage ring collider and installed a three-story-high detector that observed and measured the products of the head-on particle collisions.
As powerful as today's storage ring colliders are, physicists need even more powerful devices to test today's theories. Unfortunately, building larger rings is extremely expensive. CERN is considering building the Large Hadron Collider (LHC) in the existing 27-km (16.7-mi) tunnel that currently houses the Large Electron-Positron Collider. In 1988, the United States began planning for the construction of the Superconducting Super Collider (SSC) near Waxahachie, Texas. The SSC was to be an enormous storage ring collider accelerator 87 km (54 mi) long. However, after about one-fifth of the tunnel had been completed, the Congress of the United States voted to cancel the project in October 1993, as a result of the accelerator's projected cost of over $10 billion.
APPLICATIONS
Accelerators are used to explore atomic nuclei, thereby allowing nuclear scientists to identify new elements and to explain phenomena that affect the entire nucleus. Machines exceeding 1 GeV are used to study the fundamental particles that compose the nucleus. Several hundred of these particles have been identified. High-energy physicists hope to discover rules or principles that will permit an orderly arrangement of the proportion of subnuclear particles. Such an arrangement would be as useful to nuclear science as the periodic table of the chemical elements is to chemistry. Fermilab's accelerator and collider detector permit scientists to study violent particle collisions that mimic the state of the universe when it was just microseconds old. Continued study of their findings should increase scientific understanding of the makeup of the universe.
See also Particle Detectors.
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