The cyclotron is a type of particle accelerator invented by physicist Ernest O. Lawrence (1901-1958) in 1930. The first working cyclotron was built by Lawrence and an assistant, M. Stanley Livingston, at the University of California, Berkeley, in 1931. Lawrence was awarded the 1939 Nobel Prize in physics for this pioneering achievement, which gave scientists direct experimental access to the atomic and subatomic world.
Particle accelerators produce a beam of fast-moving, electrically charged atomic or subatomic particles for use in fundamental research into the structure of nuclei, the nature of the nuclear forces, the interactions of elementary subatomic particles, and the creation and study of nuclei not found in nature. Of special interest are the nucleon rich nuclei (i.e., atomic nuclei with large numbers of protons and neutrons) found in the transuranic elements, so called because they are heavier (i.e. have a greater atomic mass) than uranium, the heaviest naturally occurring element. The transuranic element lawrencium, was discovered in 1961 and named in honor of the inventor of the cyclotron. The most famous, or infamous, transuranic element is plutonium, the deadliest substance known in terms of leathality per dose. A fissionable isotope of plutonium, plutonium-239, is used in nuclear weapons and reactors.
Following the 1919 discovery by physicist Ernest Rutherford of a reaction between a nitrogen nucleus and an alpha particle, scientists had searched for a way to artificially accelerate ions to higher energies than those found in nature. In 1928, a German physicist, Rolf Wideröe, devised a linear accelerator which, while demonstrating the possibility of artificial acceleration, did not provide sufficient velocity to the accelerated particles to initiate a nuclear reaction in the target nucleus.
Lawrence chanced to read an account of the linear accelerator and realized immediately that the solution to the velocity problem lay in accelerating the particles along a spiral path shaped by application of a magnetic field. Thus was born the magnetic resonance accelerator, or cyclotron.
Cyclotrons modeled on Lawrence's invention are called classical cyclotrons. They consist of two hollow semicircular electrodes (called "dees" because of their shape) mounted back to back and separated by a narrow gap in a vacuum chamber between the poles of a magnet. An electric field, alternating in polarity, is created in the gap by a radio-frequency oscillator. The particles to be accelerated are introduced into the gap and propelled by the electric field into one of the dees. There the magnetic field takes over and guides them in a semicircular path back toward the gap. By the time the particles reach the gap again, however, the electric field has oscillated, or reversed, accelerating them into the other dee. The speed of each particle and the radius of its orbit increase each time it crosses the gap, until finally it smashes into the target nucleus (hence the name atom smashers) with sufficient velocity to break the nucleus apart.
The velocity of accelerated particles in a classical cyclotron is limited by the laws of relativistic physics. The faster an object goes, the more its mass increases. As accelerated particles approach the speed of light, their increasing mass causes their orbital frequency to decrease until they cross the gap between the dees out of phase with the oscillations of the electric field. Instead of kicking them up to a faster speed, the field begins to decelerate them.
Physicist Edwin Mattison McMillan (1907-1991) solved this problem in 1945, when he found that by altering the frequency of the accelerating voltage, he could keep the particles in phase and thus avoid the limitations of relativistic mass increase. McMillan called his improved cyclotron a synchrocyclotron. Subsequent improvements in the design and functionality of particle accelerators, along with technological advances, have yielded still more powerful accelerators such as betatrons, synchrotrons, and storage rings. Lawrence's first cyclotron, whose diameter was a mere 4.5 inches and was capable of accelerating particles to only 80,000 electron volts (80 keV) of energy, is dwarfed in size and power by particle accelerators such as the Cosmotron at Brookhaven National Laboratory in New York, the Tevatron at Fermilab in Illinois, and the Large Electron-Positron Collider (LEP) at the European Organization for Nuclear Research (CERN) outside Geneva, Switzerland.
Cyclotrons and their technological offspring have important applications not only in nuclear physics but in archeology (radiocarbon dating), medicine (cancer diagnosis and treatment), and industry (polymerization of plastics).
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