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Cyclotron

Cyclotron

Background

The modern cyclotron uses two hollow D-shaped electrodes held in a vacuum between poles of an electromagnet. A high frequency AC voltage is then applied to each electrode. In the space between the electrodes an ion source produces either positive or negative ions depending on the configuration. These ions are accelerated into one of the electrodes by an electrostatic attraction, and when the alternating current shifts from positive to negative, the ions accelerate into the other electrode. Because of the strong electromagnetic field, the ions travel in a circular path. Each time the ions move from one electrode to another they gain energy, their rotational radius increases, and they produce a spiral orbit. This acceleration continues until they escape from the electrode. The accelerated particles are extracted from the cyclotron when they reach the end of the spiral acceleration path. This beam of accelerated subatomic particles can be used to bombard a variety of target materials to produce radioactive isotopes.

Various isotopes are used in medicine as tracers that are injected into the body and in radiation treatments for certain types of cancers. Cyclotrons are also used for research purposes in academic and industrial settings, and for positron emission tomography (PET). Positron emission tomography (PET) is a technique for measuring the concentrations of positron-emitting radioisotopes within the tissue of living subjects. The usefulness of PET is that, within limits, it has the ability to assess biochemical changes in the body. Any region of the body that is experiencing abnormal biochemical changes can be seen through PET. PET has had a huge impact on the clinical applications of neurological diseases, including cerebral vascular disease, epilepsy, and cerebral tumors.

History

E. O. Lawrence and his graduate students at the University of California, Berkley tried many different configurations of the cyclotron before they met with success in 1929. The earliest cyclotron was very small, using electrodes, a radio frequency oscillator producing 10 watts, a vacuum, hydrogen ions, and a 4 in (10 cm) electromagnet. The accelerating chamber of the first cyclotron measured 5 in (12.7 cm) in diameter and boosted hydrogen ions to energy of 5-45 MeV depending on the settings. One mega electron volt (MeV) is 1.602 × 1013 J. (J stands for Joule, the standard unit for energy.) The design, construction, and operation of increasingly larger cyclotrons involved a growing number of physicists, engineers, and chemists. Lawrence was never certain as to whether his research should be classified as nuclear physics or nuclear chemistry.

Raw Materials

The magnets in the cyclotron are made from 25 tons of low carbon steel with two nickel plated poles. Physically, the cyclotron weighs 55 tons, and is located inside an inner vault with concrete walls and doors about 6.6 ft (2 m) thick to shield the surroundings from the nuclear radiation present when the machine runs. Fortunately, most of this radiation has a half-life of only seconds to minutes, so there are no long-term waste disposal problems. Actual dimensions are approximately 100 × 100.5 × 39 ft (30.5 × 30.6 × 11.9 m). The coils are manufactured from annealed copper, insulated with fiber-glass and covered with an epoxy resin. The aluminum vacuum tank is sealed by polyurethane o-rings. The ion source uses a tungsten filament to energize the hydrogen gas and borated polyethylene packing is used to reduce the build up of thermal neutrons around components of the cyclotron. The target changer allows the cyclotron operator to select different targets on each of the beamlines to be irradiated and are made primarily from aluminum, with a minimum of stainless-steel to minimize neutron activation.

Design

The design of the cyclotron varies according to the specifications of the purchaser. Ebco Technologies Inc. builds two different types of negative ion cyclotrons, one capable of accelerating protons to a maximum energy level of 19 MeV (TR19) and the other capable of accelerating protons to 32 MeV (TR32). The standard configuration of the TR19 cyclotron is with two external beamlines but there is a scaled down version with an option of one beamline. The TR19 standard target configuration is with two external beamlines and eight targets. There is a design option of two to four targets on one beamline, with the upgrade to up to eight targets at a later date. The TR19 is also available in a self-shielded or unshielded configuration. The self-shielded feature eliminates the need for a cyclotron vault or major upgrades to existing facilities. Additionally, the magnet gap in the TR19 is vertical to minimize space.

The radio frequency (RF) system consists of a RF amplifier, a coaxial transmission line from the RF amplifier to the cyclotron, a power supply, and instrumentation and read-back devices, an oscilloscope, current/voltage, power gauges, and interfaces with the computerized control system. A mass flow controller, needle valve, and pneumatic valve regulate the gas pressure and flow.

A tungsten filament is placed inside the ion source and when heated will ionize the hydrogen gas. A plasma filter is placed on the ion source aperture to enhance conditions for negative ion production.

The negative ions generated will be injected into the cyclotron at its X-axis. The injection system is manufactured from a set of steering magnets to focus the negative ions onto the plane of acceleration by the tilted spiral inflector.

Ernest Orlando Lawrence was born in South Dakota on August 8, 1901. He received his bachelor's degree in physics in 1922 from the University of South Dakota. Lawrence entered the University of Minnesota graduate school, completing his master's degree in one year. He received his Ph.D. at Yale in 1925, remaining there for three years as a fellow of the National Research Council, then as assistant professor. In 1928 he became associate professor at the University of California at Berkeley. Two years later Lawrence became the youngest full professor at Berkeley.

Lawrence conceived his most famous invention, the cyclotron, in 1929. He realized that to achieve particle energies of a few MeV (million electron volts) required for nuclear experiments, he could convert the particle's linear trajectory into a circular one by superimposing a magnetic field at right angles to the particle's path. Lawrence immediately proved that a particle's frequency of revolution depends only upon the strength of the magnetic field and the charge-mass ratio of the particle, not upon the radius of its orbit. This was the basic principle of the cyclotron, which Lawrence first re-ported in the fall of 1930.

In 1932, Lawrence married and had six children. He was elected to the National Academy of Sciences in 1934, awarded the Nobel Prize in physics in 1939, and received the Medal of Merit in 1946 and the Fermi Award in 1957. Lawrence remained at Berkeley until his death August 27, 1958 from an intestinal ulcer.

The Manufacturing
Process

  1. Project teams coordinate conduit, cable tray, floor duct, and related equipment prior to the shipping, rigging, and installation of the cyclotron and its sub-systems.
  2. The manufacturing process begins with the 25-ton steel magnet. It is machined from 10-in (25.4-cm) slabs and placed in-between the poles of a powerful electromagnet until the magnetic field area is precisely measured.
  3. Two nickel plated magnetic poles are forged from low-carbon steel.
  4. Two magnet coil assemblies are manufactured from annealed hollow copper and harden after being bent into shape. They are mounted in the yoke of the magnet, connected to water cooling headers, insulated with fiberglass, and coated in an epoxy resin.
  5. The aluminum vacuum tank is placed between the nickel plated poles and bolted into place. The vacuum tank has cryopumps that are bolted externally to cool the tank close to −459°F (−273°C) in order to freeze out any gases that may be present.
  6. The electrodes are machined from a single 0.06-in (1.6-mm) low resistively copper sheet (to optimize the energy transfer from the RF system to the accelerating hydrogen ions), cut out, and etched using boaring tools and drill bits.
  7. Next, the tank is sealed with polyurethane 0-rings after the copper electrodes are mounted inside. The electrodes are set, using nylon screws and spacers, into a round piece of industrial lisex nylon. A few holes are drilled in the nylon. Two are for the oscillator wiring. The third is meant for the vacuum pump; there is also a vacuum gauge attached to this port.
  8. On top of the nylon and surrounding the electrodes is a ring of poly vinyl chloride (PVC) pipe. This has several holes drilled into it, the largest of which is the detector storage tube. Also located in this material are smaller holes sufficient for supplying a voltage source to the deflector plate, for the set screws required to control its position, and attachment holes for the solid brass hook that will be used to hang the complete apparatus on a set of Helmholtz coils.
  9. Atop the PVC pipe is a piece of industrial strength clear plastic. This is both to allow people to see the inside workings of the mechanism, should anything go wrong, as well as increase the strength of the casing.
  10. On either side of the PVC is silicon gel, in order to maintain a sufficient seal around the main chamber. This is so that the vacuum will be as efficient as possible. The vacuum is needed because the alpha particles are heavily influenced by particles of any kind, especially air. That is why alpha particles are considered so safe; by the time they contact a person through any medium, their energy has been so severely affected, they are not able to do damage.
  11. The walls are guided in place by a thin I cut in the face of both the top and bottom sheet and both electrodes are held together with the use of 2 in (5.1 cm) nylon screws. No solder was used in these pieces so as to keep the inner chamber as clean and constant as possible. In one wall is cut a window, roughly 0.79 in (2 cm) long.
  12. Pivoted on a nylon screw is a slightly smaller copper plate (the deflector) separated electrically from the rest of the component. Outlying set screws can control the deflector position and both it and each electrode have an electrical connection. This is to allow the oscillator to be supplied to the electrodes and a large negative charge to be put on the deflector plate.
  13. The RF system is assembled inside a 19-in (48-cm) square, 6-ft(1.8-m) high metal chassis. Here, the resistors, transmitters, switches, tuning circuits, inductors, and capacitors are assembled by hand.
  14. Power supply cabinets are purchased and assembled for the water-cooled targets and magnets, ion sources, cryopump, and the water circuitry.
  15. The ion source will be injected after assembly of the cyclotron. A magnetic cylinder, 4 in (10 cm) in diameter and 4.7 in (12 cm) long comprises the ion source. Hydrogen gas will be injected through a capillary tube.
  16. The tilted spiral inflector is enclosed J b y a grounded helical shaped electrode. The electrode is machined on a fixed axis milling machine.
  17. Next, the target bodies are made of high purity silver, aluminum, and titanium and designed with helium-cooled thin foil windows. The two foil windows separate the target material from the high vacuum within the cyclotron.
  18. A recirculating closed loop cooling system is placed in the target services metal cabinet to cool the foil windows with high speed streams of helium gas.
  19. The tubing connections, solenoid valves, water-cooled beam stops, and electrically isolated collimators are assembled and attached to the target assembly.
  20. The target assembly has a solid aluminum plug that is pierced by a 4 in (10 cm) hole that will act as the target collimator.
  21. Grooves are machined onto the outside of the plug, and the o-ring is mounted to create the vacuum seal between the target body and the four position target changer.
  22. A collimating disc is placed between the plug and the target body with a window on both sides.
  23. Finally, the entire system is integrated with supervisory software to control and monitor the PLC hardware.

Quality Control

Each step of the manufacturing process must be monitored to ensure that the parts are of standard quality. If any of the components have a crack or leak, radiation may get into the environment. The steel used in the magnets of the cyclotron is carefully monitored to ensure it has the desired properties. Magnetic fields are constantly checked by Nuclear Magnetic Resonance (NMR).

Byproducts/Waste

The manufacturing process yields 2-3 tons of metal waste during production. This is recycled for future manufacturing processes. Due to the number of parts, the excess material from the manufacturing of the cyclotron is large. If any defective parts are found they are salvaged to the best of their ability, but the majority are scrapped.

The Future

The improvements in sealing the cyclotron unit are requiring that less concrete shielding be provided at the installation site and provide a safer and more compact cyclotron unit. More powerful cyclotron units are being designed for commercial isotope production. The latest series of cyclotrons are state of the art, compact, strong focusing, four sector negative ion cyclotrons, with external ions sources, cryopumps, high precision power and control systems, and superb manufactured quality. They are now modular in design and share a common technology irrespective of the size and type of cyclotron.

Where to Learn More

Books

Lawrence, Ernest 0., and Irving Langmuir. Molecular Films: The Cyclotron & TheNew Biology. New Brunswick: Rutgers University Press, 1942.

Periodicals

Burgerjon, J. J., and A. Strathdee, eds. Cyclotrons1972. New York: American Institute of Physics, 1972.

BonnyP.McClain

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Cyclotron

CYCLOTRON

CYCLOTRON, a machine for accelerating charged nuclear particles, commonly protons, so that they may be used to probe the nuclei of target atoms. Such "atom smashers" are considered the microscopes of nuclear physics.

In the nineteenth century, some physicists still labored under the theory—really, the dream of alchemists for centuries—that elements could be made to transmute into other elements through chemical processes. In 1902, Ernest Rutherford and Frederick Soddy explained the new phenomenon of radioactivity as a "transformation" of one element into another, occurring spontaneously in nature; and in 1919, Rutherford succeeded in deliberately causing transmutations by bombarding light elements with the alpha particles emitted from naturally decaying radio-elements. Since very few of the projectile alpha particles collided with nuclei of the target atoms, the number of


transmutations was relatively small. Therefore, scientists sought new ways to increase the number of projectile particles and to accelerate them to higher energies. The copious production of charged particles was the easier task; the high-voltage engineering required for acceleration proved far more difficult.

Scientists tried a number of different approaches to the acceleration problem, including a voltage multiplier circuit (Sir John Douglas Cockcroft and Ernest Walton) and an electrostatic generator (Robert J. Van de Graaff), both linear accelerators. In 1930, University of California at Berkeley physicist Ernest O. Lawrence, with the help of one of his students, M. Stanley Livingston, designed and constructed the first of many magnetic resonance accelerators. Lawrence's accelerator operated at voltages much lower than other machines, yet imparted as much or more energy to its projectiles. Lawrence won the 1939 Nobel Prize for Physics for his work on the cyclotron. During World War II he headed a unit of the Manhattan Project that worked to perfect the process of separating uranium-235 for the atomic bomb.

These cyclotrons, destined to be the chief tool of nuclear physics, worked on the principle that charged particles, accelerated across a voltage gap, travel in a circular path under the influence of a magnetic field. If confined to a hollow disk-shaped chamber built in two D-shaped halves (called "D's") and if subjected to a radio-frequency voltage alternation as the particle passes from one half to the other, the particle receives two accelerations per cycle and travels at higher velocities in ever-larger circles. The beam of rapidly moving particles may then be deflected onto a target, producing observable nuclear reactions.

The D's of Lawrence's first cyclotron were only about 4 inches in diameter. Subsequent models of 9, 11, 27, 37, and 60 inches followed, with a new model built almost every other year. These larger machines surpassed an early goal of one million electron volts projectile energy; many different types of atoms were split; and scores of new radioisotopes were identified, including the first trans-uranium elements.

Higher energies, suitable for the production of mesons, were impossible with the fixed-frequency cyclotrons, because the projectiles would experience a relativistic mass increase at the required velocities, destroying the resonant operating condition. After World War II scientists overcame this handicap with a new generation of accelerators that use a variable-frequency voltage alternation that exactly balances the mass-velocity change. The synchrocyclotron was the largest machine to use a single magnet.

This postwar synchrocyclotron became the foundation for a government-funded national accelerator. Work on a four-mile-long circular machine in Weston, Illinois, thirty miles west of Chicago, was completed in 1971. Project leader Robert O. Wilson envisioned a series of magnets to boost particle speeds, and he insisted on allowing for space in the tunnel of the main ring for the addition of a second magnet system. When the main ring was about to operate in 1971 he described his idea of a "doubler" that would take the protons from the magnetic ring and inject them into a new ring of super-conducting magnets and double their energy. Physicists working at the laboratory, which in 1974 was named the Fermi National Laboratory for physicist Enrico Fermi, solved the technical problems of building the doubler. The principal Fermilab accelerator subsequently became known as the Tevatron (one TeV is a trillion electron volts). In 1994 the Tevatron revealed the existence of the so-called top quark, the last of twelve subatomic building blocks of all matter.

BIBLIOGRAPHY

Livingston, Milton Stanley. Particle Accelerators: A Brief History. Cambridge, Mass.: Harvard University Press, 1969.

Mladenovic, Milorad. The Defining Years in Nuclear Physics, 1932– 1960s. Bristol, Pa.: Institute of Physics, 1998.

Riordan, Michael. The Hunting of the Quark: A True Story of Modern Physics. New York: Simon and Schuster, 1987.

Wilson, Robert R., and Raphael Littauer. Accelerators: Machines of Nuclear Physics. Garden City, N.Y.: Anchor Books, 1960.

LawrenceBadash/a. r.

See alsoPhysics: Overview ; Physics: High-Energy Physics ; Physics: Nuclear Physics ; Superconducting Super Collider .

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cyclotron

cyclotron See accelerator, particle It has two semicircular or D-shaped electrodes across which is applied an alternating voltage. The charged particles are attracted by electrostatic forces to the electrode of opposite charge, but since the polarity alternates, the particles are swung outwards at increasing speed.

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cyclotron

cy·clo·tron / ˈsīkləˌträn/ • n. Physics an apparatus in which charged atomic and subatomic particles are accelerated by an alternating electric field while following an outward spiral or circular path in a magnetic field.

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cyclotron

cyclotron: see particle accelerator.

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cyclotron

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Cyclotron

Cyclotron

Resources

A cyclotron is a type of particle accelerator designed to accelerate charged particles, such as protons and ions, to high velocities and, then, release them so as to strike a target. Observations of such collisions yield information about the nature of atomic particles. In contrast to the enormous particle accelerators used in particle physics today, the first cyclotron, built in 1930 by American nuclear physicist Ernest O. Lawrence (19011958) measured just 4.5in(12 cm)indiameter.

A charged particle moving at right angles to a magnetic field is subject to a force that is at right angles both to the field and to the charged particles direction of motion. This force makes the particle follow a circular path. If the particle loses energy, then it begins to spiral inward, but if more energy is applied, then the particle spirals outward. In a cyclotron, a pair of hollow, D-shaped pieces of metal are mounted above a powerful electromagnet, with their flat sides facing one another. One of the D-shaped metal pieces (called simply D) is given a negative charge and the other is given a positive charge.

A charged particle, say a proton, is injected into this environment. With its positive charge, the proton is attracted by the negative D and repelled by the positive D; these forces start it into motion toward the negatively charged D. Once the particle is moving, the magnetic field deflects it into a curved path, back toward the positive D. Before the positive D can repel the proton, it is switched to a negative charge, thus attracting the proton rather than repelling it. Thus, the magnetic field keeps the particle on a circular path, while the alternating positive and negative charges on the D-shaped pieces of metal keep the proton chasing a negatively charged target indefinitely. As the proton circles inside the cyclotron it gains speed and thus energy; for a fixed magnetic-field strength, the size of the circle it travels increases correspondingly. Ultimately, before it can strike either of the metal Ds, it is propelled out of the cyclotron by a bending magnet and directed toward a target.

The cyclotron was a revolutionary device for its time, but has since been outmoded for particle-physics research purposes as cyclotrons are not capable of accelerating particles to the high speeds required for todays experiments in subatomic physics. High speeds are required for such research because, as GermanAmerican physicist Albert Einstein (1879 1955) proved, mass is proportional to energy. When a particle moves at high speed, it has considerable energy of motion and its mass is therefore greatly increased. One way to boost the speed of the particle in a cyclotron further is to switch the electrical polarities of the Ds at a gradually lower frequency. A more sophisticated version of the cyclotron, the synchrocyclotron, includes the complicated electronics necessary to do this. However, the most efficient method of compensating for the increased mass of high-energy particles is to increase the applied magnetic field as the particle speed increases. The class of device that does this is called a synchrotron, and includes the most

powerful particle accelerators in existence today. These installations have rings more than 1.2 mi (2 km) in diameter, a far cry from Lawrences first cyclotron.

Cyclotron-type warping of charged-particle paths occurs in nature as well as in cyclotrons: wherever charged particles move through a magnetic field (for instance, when charged particles from the sun encounter the magnetic field of a planet), they are forced to follow spiraling paths. Since acceleration of a charged particleany change in the particles direction or velocitycauses it to emit electromagnetic radiation, charged particles encountering magnetic fields in space emit radiation. This radiation, termed cyclotron radiation, can reveal the interactions of particles and magnetic fields across the cosmos, and is of importance in astronomy.

Human-built cyclotrons of the fixed-field type are not used in physics research any more, but are increasingly important in medicine. Proton-beam therapy is a recent innovation in radiosurgery (surgery using radiation) in which protons accelerated by a cyclotron are beamed at a target in the human body, such as a tumor at the back of the eye. The energy of these protons can be carefully controlled, and their stopping distance inside living tissue (that is, the depth at which they deposit their energy) precisely predicted. These features mean that tumors inside the body can be targeted while minimizing damage to healthy tissues.

Resources

BOOKS

Lee, Shyh-Yuan. Accelerator Physics. Hackensack, NJ: World Scientific, 2004.

Strehl, Peter. Beam Instrumentation and Diagnostics. Berlin, Germany, and New York: Springer, 2006.

OTHER

De Martinis, C., et al.Beam Tests on a Proton Linac Booster for Hadronotherapy.Proceedings of European Particle Accelerator Conference, Paris, France, 2002. <http:accelconf.web.cern.ch/AccelConf/e02/PAPERS/MOPRI095.pdf> (Feb. 6, 2003).

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Cyclotron

CYCLOTRON

High-energy charged particles have many applications in fundamental and applied research in the physical and biological sciences. They are produced by starting with a source of low-energy charged particles, such as a plasma in a gaseous discharge, and then accelerating the particles to high energy. A cyclotron is one of the devices that can be used as a charged-particle accelerator.

To accelerate a particle, it is necessary to exert a force on it. Forces on charged particles can be exerted by electric fields and by magnetic fields. The force exerted by a magnetic field is always exactly perpendicular to the velocity of the particle. Such a force can change the direction in which a particle is moving, but it cannot change the energy of the particle. However, the force exerted by an electric field acts in the direction of the electric field. If this direction has a non zero component in the direction of the velocity of the particle, then the effect of the electric field will be to increase the speed, and hence the energy, of the particle. Thus any device that increases the energy of a charged particle must use electric fields (not magnetic fields) to produce the increase. In a cyclotron, the sideways forces produced by magnetic fields are used to keep the particles moving in approximately circular orbits, while electric fields at certain places around these orbits provide the increases in particle energy.

Figure 1 shows a simplified drawing of a cyclotron. The two shaded regions in Figure 1 are semicircular metal chambers, called "dees," which are shown in

FIGURE 1

more detail in the cross-section drawing in Figure 2. An alternating potential difference between the dees is produced by an oscillator. Because of this potential difference, there is an electric field between the dees which accelerates a particle as it moves from one dee to the other. Within a dee, there is only a uniform magnetic field perpendicular to the plane of the dee. The associated magnetic force provides the centripetal acceleration needed to keep the particle in a circular orbit as it moves around the dee. The dee is enclosed in an evacuated chamber, and the particles can pass unimpeded from one dee to another.

The source of low-energy particles is near the center of the cyclotron, in the space between the dees. The particles are accelerated across this space, into a dee, then move halfway around the cyclotron and reenter the electric field between the dees traveling in the opposite direction. In the time it takes the particle to move halfway around the dee, the sign of the potential difference between the dees must change so that the electric field is still in the direction needed to accelerate the particles. For example, if the particle is a proton, it must always move from the dee of high potential to the dee of low potential, and so the oscillator must ensure that this is always the situation when a proton is in the space between the dees. If the time required for a half-orbit were the same at all radii, acceleration would occur every time the particle crossed from one dee to another as it spiraled outward. This is true as long as the particle moves slowly compared to the speed of light, in a uniform magnetic field. Then the frequency of the oscillator must be given by where q is the charge of the particle in coulombs, m is its mass in kilograms, and B is the magnetic field in tesla. If the particle spirals out to a radius R , in meters, it will reach a kinetic energy of

For example, a proton cyclotron using a magnetic field of 0.1 tesla (1 kilogauss) requires an oscillator of frequency of 1.5 MHz (1.5 × 106 Hz). If the maximum orbit radius is 1 meter, then the proton will reach a kinetic energy of 7.7 × 10-14 joules = 0.48 MeV (0.48 × 106 eV).

The description above referred to a "standard" cyclotron, with fixed magnetic field and fixed frequency

FIGURE 2

This device was developed by E. O. Lawrence and M. S. Livingston in the early 1930s. Its great merit was that particles could be accelerated to high speeds in repeated small increments, so that high voltages were not required. It produces a continuous beam of particles, whose energy can be varied by adjustment of the magnetic field and oscillator frequency. Much of the research in nuclear reactions done before about 1960 depended upon beams produced by standard, fixed-frequency cyclotrons.

If the speed of the particle being accelerated is not small compared to the speed of light, then the time taken for a half-orbit around the cyclotron ceases to be independent of the radius of the orbit. This prevents the use of a fixed-frequency cyclotron to produce very high-energy beams. One solution to this difficulty is provided by the synchrocyclotron. In this device, low-energy particles are injected into the space between the dees in short bursts (bunches). The magnetic field is still uniform, but the frequency with which the potential on the dees is alternated varies as the bunch spirals outward, in such a way that the electric field in the space between the dees always has the direction needed to accelerate the pulse passing through it. In a typical synchrocyclotron, each bunch lasts about 10 about a 10-4 seconds, with about a 10-2-second interval between bunches. Thus a target put in a synchrocyclotron beam would be bombarded for only about 1 percent of the total time it was exposed to the beam, whereas a standard cyclotron would produce continuous bombardment. For some purposes, such as the measurement of the lifetime of a state produced by bombardment, the bunching of the beam is an advantage.

In both the standard cyclotron and a synchrocyclotron, the radius of the orbit of the particle starts out small, and increases with each half-cycle, all the while immersed in a uniform magnetic field. To reach very high energy it would be necessary to reach very large radii, which would then require very large (and very expensive) magnets. A more economical solution would be to have each bunch move in an orbit of constant radius, while the strength of the magnetic field and the oscillator frequency are increased as the energy of the particles in the bunch increases. The beam is confined to an evacuated "beam pipe," containing one or more accelerating sections, with the magnets arranged around the pipe. This device, in which both the frequency and magnetic field strength are varied in order to produce acceleration at constant radius, is called a synchrotron. The Tevatron is a proton synchrotron at Fermilab, with a diameter of 2 kilometers. Superconducting magnets produce a maximum field strength of 4.2 tesla, about 15,000 times stronger than the Earth's magnetic field. It takes about 20 seconds for the magnetic field to rise from 0.66 to 3.54 tesla, while the proton energy increases from 150 to 850 GeV (1 GeV = 109 eV). The final beam energy is approximately 980 GeV. The world's most powerful accelerator, the Large Hadron Collider (LHC), is under construction at the European Laboratory for Particle Physics (CERN) in Geneva. It is designed to accelerate protons to an energy of 7 TeV (7 × 1012 eV). It is planned that it will begin operation in 2007.

There are many technical problems that must be overcome in the design and operation of a working particle accelerator. An especially important issue is the stability of the beam with respect to spatial and temporal fluctuations. This requires careful shaping of the magnetic field and electric fields, and very high vacuum in the beam pipe. Accelerator technology is a very important component of modern physical science.

See also:Accelerators, Early; Lawrence, Ernest Orlando

Bibliography

European Laboratory for Particle Physics (CERN). <http://www.cern.ch/>.

Fermi National Accelerator Laboratory (FNAL). <http://www.fnal.gov/>.

Fishbane, M. F.; Gasiorowicz, S.; and Thornton, S. T. Physics for Scientists and Engineers (Prentice-Hall, Upper Saddle River, NJ, 1993).

Krane, K. S. Introductory Nuclear Physics (Wiley, New York, 1988).

Lawrence, E. O., and Livingston, M. S. "The Production of High Speed Light Ions Without the Use of High Voltages." Physical Review40 , 19 (1932).

Benjamin Bayman

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Cyclotron

Cyclotron

A cyclotron is a type of particle accelerator designed to accelerate protons and ions to high velocities and then release them so as to strike a target. Observations of such collisions yield information about the nature of atomic particles. In contrast to the enormous particle accelerators used in particle physics today, the first cyclotron, built in 1930 by U.S. physicist E. O. Lawrence (1901–1958), measured just 4.5 in (12 cm) in diameter.

A charged particle moving at right angles to a magnetic field is subject to a force that is at right angles both to the field and to the charged particle's direction of motion at instant; this force follows the particle to follow a spiraling path. In a cyclotron, a pair of hollow, D-shaped pieces of metal are mounted above a powerful electromagnet, with their flat sides facing one another. One of the Ds is given a negative charge and the other is given a positive charge.

A charged particle, say a proton , is injected into this environment. With its positive charge, the proton is attracted by the negative D and repelled by the positive D; these forces start it into motion toward the negatively charged D. Once the particle is moving, the magnetic field deflects it into a curved path, back toward the positive D. Before the positive D can repel the proton, it is switched to a negative charge, thus attracting the proton rather than repelling it. Thus, the magnetic field keeps the particle on a circular path, while the alternating positive and negative charges on the D-shaped pieces of metal keep the proton chasing a negatively charged target indefinitely. As the proton circles inside the cyclotron it gains speed and thus energy ; for a fixed magnetic-field strength, the size of the circle it travels increases correspondingly. Ultimately, before it can strike either of the metal Ds, it is propelled out of the cyclotron by a bending magnet and directed toward a target.

The cyclotron was a revolutionary device for its time, but has since been outmoded for particle-physics research purposes as cyclotrons are not capable of accelerating particles to the high speeds required for today's experiments in subatomic physics. High speeds are required for such research because, as Einstein proved, mass is proportional to energy. When an particle moves at high speed, say in a cyclotron, it has considerable energy of motion and its mass is therefore greatly increased. One way to boost the speed of the particle further is to switch the electrical polarities of the Ds at a gradually lower frequency . A more sophisticated version of the cyclotron, the synchrocyclotron, includes the complicated electronics necessary to do this. However, the most efficient method of compensating for the increased mass of high-energy particles is to increase the applied magnetic field as the particle speed increases. The class of device that does this is called a synchrotron, and includes the most powerful particle accelerators in existence today. These installations have rings more than 1.2 mi (2 km) in diameter, a far cry from Lawrence's first cyclotron.

Cyclotron-type warping of charged-particle paths occurs in nature as well as in cyclotrons: wherever charged particles move through a magnetic field (e.g., when charged particles from the Sun encounter the magnetic field of a planet ), they are forced to follow spiraling paths. Since acceleration of a charged particle—any change in the particle's direction or velocity—causes it to emit electromagnetic radiation , charged particles encountering magnetic fields in space emit radiation. This radiation, termed cyclotron radiation, can reveal the interactions of particles and magnetic fields across the cosmos, and is of importance in astronomy .

Human-built cyclotrons of the fixed-field type are not used in physics research any more, but are increasingly important in medicine. Proton-beam therapy is a recent innovation in radiosurgery (surgery using radiation) in which protons accelerated by a cyclotron are beamed at a target in the human body, such as a tumor at the back of the eye . The energy of these protons can be carefully controlled, and their stopping distance inside living tissue (i.e., the depth at which they deposit their energy) precisely predicted. These features mean that tumors inside the body can be targeted while minimizing damage to healthy tissues.


Resources

other

De Martinis, C., et al. "Beam Tests on a Proton Linac Booster for Hadronotherapy." Proceedings of European Particle Accelerator Conference, Paris, France. 2002 (cited Feb. 6, 2003) <http:accelconf.web.cern.ch/AccelConf/e02/PAPERS/MOPRI095.pdf>.

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