A cyclotron is a type of particle accelerator invented by Ernest Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. Method and apparatus for the acceleration of ions, filed: January 26, 1932, granted: February 20, 1934 A cyclotron accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying electric field. Lawrence was awarded the 1939 Nobel Prize in Physics for this invention.
The cyclotron was the first "cyclical" accelerator. The primary accelerators before the development of the cyclotron were electrostatic accelerators, such as the Cockcroft–Walton generator and the Van de Graaff generator. In these accelerators, particles would cross an accelerating electric field only once. Thus, the energy gained by the particles was limited by the maximum electrical potential that could be achieved across the accelerating region. This potential was in turn limited by electrostatic breakdown to a few million volts. In a cyclotron, by contrast, the particles encounter the accelerating region many times by following a spiral path, so the output energy can be many times the energy gained in a single accelerating step.
Cyclotrons were the most powerful particle accelerator technology until the 1950s, when they were surpassed by the synchrotron. Nonetheless, they are still widely used to produce particle beams for nuclear medicine and basic research. As of 2020, close to 1,500 cyclotrons were in use worldwide for the production of radionuclides for nuclear medicine and ultimately, for the production of radiopharmaceuticals. In addition, cyclotrons can be used for particle therapy, where particle beams are directly applied to patients.
History
Origins A key limitation of the earliest charged particle accelerators was that increasing the particle energy required extending the length of the acceleration path, which was only feasible and practical up to a certain point. In 1927, while a student at Kiel, German physicist Max Steenbeck was the first to formulate the concept of the cyclotron, but he was discouraged from pursuing the idea further.Lawrence and His Laboratory - II — A Million Volts or Bust 81–82 in Heilbron, J. L., and Robert W. Seidel Lawrence and His Laboratory: A History of the Lawrence Berkeley Laboratory', Volume I. (Berkeley: University of California Press, 2000) In late 1928 and early 1929, Hungarian physicist Leo Szilárd filed patent applications in Germany for the linear accelerator, cyclotron, and betatron. In these applications, Szilárd became the first person to discuss the resonance condition for a circular accelerating apparatus. However, neither Steenbeck's ideas nor Szilard's patent applications were ever published and therefore did not contribute to the development of the cyclotron.
Several months later, in the early summer of 1929, Ernest Lawrence independently conceived the cyclotron concept after reading a paper by Rolf Widerøe describing a drift tube accelerator. He published a paper in Science in 1930 (the first published description of the cyclotron concept), after a student of his built a crude model in April of that year. He patented the device in 1932.
To construct the first such device, Lawrence used large electromagnets recycled from obsolete arc converters provided by the Federal Telegraph Company. He was assisted by a graduate student, M. Stanley Livingston. Their first working cyclotron became operational on January 2, 1931. This machine had a diameter of , and accelerated protons to an energy up to 80 keV.
At the Radiation Laboratory on the campus of the University of California, Berkeley (now the Lawrence Berkeley National Laboratory), Lawrence and his collaborators went on to construct a series of cyclotrons which were the most powerful accelerators in the world at the time; a 4.8 MeV machine (1932), a 8 MeV machine (1937), and a 16 MeV machine (1939). Lawrence received the 1939 Nobel Prize in Physics for the invention and development of the cyclotron and for results obtained with it.
The first European cyclotron was constructed in 1934 in the Soviet Union by Mikhail Alekseevich Eremeev, at the Leningrad Physico-Technical Institute. It was a small design based on a prototype by Lawrence, with a 28 cm diameter capable of achieving 530 keV proton energies. Research quickly refocused around the construction of a larger MeV-level cyclotron, in the physics department of the V.G. Khlopin Radium Institute in Leningrad, headed by . This instrument was first proposed in 1932 by George Gamow and and was installed and became operative in March 1937 at 100 cm (39 in) diameter and 3.2 MeV proton energies.V. G. Khlopin Radium Institute. Chronology . Retrieved 25 February 2012.
The first Asian cyclotron was constructed at the Riken laboratory in Tokyo, by a team including Yoshio Nishina, Sukeo Watanabe, Tameichi Yasaki, and Ryokichi Sagane. Yasaki and Sagane had been sent to Berkeley Radiation Laboratory to work with Lawrence. The device had a 26 in diameter and the first beam was produced on April 2, 1937, at 2.9 MeV deuteron energies.
During World War II Cyclotrons played a key role in the Manhattan Project. The published 1940 discovery of neptunium and the withheld 1941 discovery of plutonium both used bombardment in the Berkeley Radiation Laboratory's 60-inch cyclotron. Furthermore Lawrence invented the calutron (California University cyclotron), which was industrially developed at the Y-12 National Security Complex from 1942. This provided the bulk of the uranium enrichment process, taking low-enriched uranium (<5% uranium-235) from the S-50 and K-25 plants and electromagnetically separating isotopes up to 84.5% highly enriched uranium (HEU). This was the first production of HEU in history, and was shipped to Los Alamos and used in the Little Boy bomb dropped on Hiroshima, and its precursor Water Boiler and Dragon test reactors.
In France, Frédéric Joliot-Curie constructed a large 7 MeV cyclotron at the Collège de France in Paris, achieving the first beam in March 1939. With the Nazi occupation of Paris in June 1940 and an incoming contingent of German scientists, Joliot ceased research into uranium fission, and obtained an understanding with his German former colleague Wolfgang Gentner that no research of military use would be carried out. In 1943 Gentner was recalled for weakness, and a new German contingent attempted to operate the cyclotron. However, it is likely that Joliot, a member of French Communist Party and in fact president of the National Front resistance movement, sabotaged the cyclotron to prevent its use to the Nazi German nuclear program.
In Nazi Germany, one cyclotron was built in Heidelberg, under the supervision of Walther Bothe and Wolfgang Gentner, with support from the Heereswaffenamt. At the end of 1938, Gentner was sent to Berkeley Radiation Laboratory and worked most closely with Emilio Segrè and Donald Cooksey, returning before the start of the war. Construction was slowed by the war and completed in January 1944, but difficulties in testing made it unusable until the war's end.
In Japan, the large Riken cyclotron was used to bombard uranium processed in their Clusius tube gaseous diffusion device. The experiment indicated that no enrichment of the uranium-235 content had occurred. Following the occupation of Japan, American forces, fearing continuation of the Japanese nuclear weapons program, disassembled the Riken laboratory's cyclotron and dumped it in Tokyo Bay. During the disassembly, Yoshio Nishina begged otherwise, saying "This is ten years of my life ... It has nothing to do with bombs." Secretary of War Robert P. Patterson later admitted the decision was a mistake.
Post-war By the late 1930s it had become clear that there was a practical limit on the beam energy that could be achieved with the traditional cyclotron design, due to the effects of special relativity. As particles reach relativistic speeds, their effective mass increases, which causes the resonant frequency for a given magnetic field to change. To address this issue and reach higher beam energies using cyclotrons, two primary approaches were taken, synchrocyclotrons (which hold the magnetic field constant, but decrease the accelerating frequency) and isochronous cyclotrons (which hold the accelerating frequency constant, but alter the magnetic field).
Lawrence's team built one of the first synchrocyclotrons in 1946. This machine eventually achieved a maximum beam energy of 350 MeV for protons. However, synchrocyclotrons suffer from low beam intensities (< 1 μA), and must be operated in a "pulsed" mode, further decreasing the available total beam. As such, they were quickly overtaken in popularity by isochronous cyclotrons.
The first isochronous cyclotron (other than classified prototypes) was built by F. Heyn and K.T. Khoe in Delft, the Netherlands, in 1956. Early isochronous cyclotrons were limited to energies of ~50 MeV per nucleon, but as manufacturing and design techniques gradually improved, the construction of "spiral-sector" cyclotrons allowed the acceleration and control of more powerful beams. Later developments included the use of more compact and power-efficient superconducting magnets and the separation of the magnets into discrete sectors, as opposed to a single large magnet.
Principle of operation A cyclotron is essentially a linear particle accelerator wrapped in a circle. A uniform magnetic field perpendicular to the plane of particle motion causes the particles to orbit. During each orbit the particles are accelerated by electric fields.
Cyclotron principle
In a particle accelerator, charged particles are accelerated by applying an electric field across an . The force on a particle crossing this gap is given by the Lorentz force law:
where is the charge on the particle, is the electric field, is the particle velocity, and is the magnetic flux density. It is not possible to accelerate particles using only a static magnetic field, as the magnetic force always acts perpendicularly to the direction of motion, and therefore can only change the direction of the particle, not the speed.
In practice, the magnitude of an unchanging electric field which can be applied across a gap is limited by the need to avoid electrostatic breakdown. As such, modern particle accelerators use alternating (radio frequency) electric fields for acceleration. Since an alternating field across a gap only provides an acceleration in the forward direction for a portion of its cycle, particles in RF accelerators travel in bunches, rather than a continuous stream. In a linear particle accelerator, in order for a bunch to "see" a forward voltage every time it crosses a gap, the gaps must be placed further and further apart, in order to compensate for the increasing speed of the particle.
A cyclotron, by contrast, uses a magnetic field to bend the particle trajectories into a spiral, thus allowing the same gap to be used many times to accelerate a single bunch. As the bunch spirals outward, the increasing distance between transits of the gap is exactly balanced by the increase in speed, so a bunch will reach the gap at the same point in the RF cycle every time.
The frequency at which a particle will orbit in a perpendicular magnetic field is known as the cyclotron frequency, and depends, in the non-relativistic case, solely on the charge and mass of the particle, and the magnetic flux density: