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.
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.
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.
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.
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).
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 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.
Lawrence, Ernest 0., and Irving Langmuir. Molecular Films: The Cyclotron & The New Biology. New Brunswick: Rutgers University Press, 1942.
Burgerjon, J. J., and A. Strathdee, eds. Cyclotrons — 1972. New York: American Institute of Physics, 1972.
Bonny P. McClain