Theoretical advances by Edwin Mattison McMillan and Vladimir Iosifovich Veksler led to the practical development of the first synchrocyclotron, a powerful particle accelerator that overcame problems of its predecessor, the cyclotron.
The synchrocyclotron is a large electromagnetic apparatus designed to accelerate atomic and subatomic particles at high energies. Therefore, it falls under the broad class of scientific apparatuses known as particle accelerators. Accelerated subatomic and atomic particles occur naturally in such sources as cosmic rays and the radioactive decay of elements. Abundant as these sources may be, they allow the scientist no means of controlling the properties of the particles. By the early 1920’s, the experimental work of physicists such as Ernest Rutherford
Edwin Mattison McMillan at the controls of the synchrocyclotron in 1948.
In September, 1930, Lawrence, together with a group of his graduate students at the University of California Radiation Laboratory
Particles are given their initial energy by the rf source, which sends them across the gap, where the magnetic field forces them into circular paths, or orbits, bringing them into the gap once again. This time, when the particles enter the gap, the rf source has been reversed. Because it is an oscillating source, it sends them into the opposite dee, increasing their energy and orbital radii. This process continues until the particles reach the desired energy and velocity and are extracted from the outer rim of the dees for use in experiments ranging from particle-to-particle collisions to the synthesis of radioactive elements.
Particle energy is measured in units called electron volts, which are defined as the amount of energy a particle of unit charge, such as an electron, receives when it is passed through an electrical field with a strength of 1 volt. Between 1931 and 1932, the Lawrence cyclotron generated protons, which are subatomic particles, with energies in excess of 1.2 million electronvolts. By mid-1934, the Lawrence cyclotron was producing deuterons at 5 million electronvolts. Deuterons are the positive ions of deuterium, a radioactive form of hydrogen.
Although Lawrence was interested in the practical applications of his invention in medicine and biology, the cyclotron also was applied to a variety of experiments in a subfield of physics called “high-energy physics.” Among the earliest applications were studies of the subatomic, or nuclear, structure of matter. The energetic particles generated by the cyclotron made possible the very type of experiment that Rutherford and Gamow had attempted earlier. These experiments, which bombarded lithium targets with streams of highly energetic accelerated protons, attempted to probe the inner structure of matter.
Among other experiments was the confirmation of Sir James Chadwick’s 1932 discovery of the neutron, an electrically neutral subatomic particle that, together with the proton, constitutes the atomic nucleus. These 1933 experiments were made possible by the acceleration of deuterons.
Although funding for scientific research on a large scale was scarce before World War II, Lawrence nevertheless conceived of a 467-centimeter cyclotron that would generate particles with energies approaching 100 million electronvolts. By the end of the war, increases in the public and private funding of scientific research and a demand for even higher energy particles created a situation in which this plan looked as if it would become reality, were it not for an inherent limit in the physics of cyclotron operation.
In 1937, Hans Albrecht Bethe discovered a severe theoretical limitation to the energies that could be produced in a cyclotron. Albert Einstein’s theory of special relativity had postulated that as any particle gains velocity relative to the speed of light, its mass increases. Bethe showed that this increase in mass would eventually slow the rotation of the particle. Therefore, as the rotation of each particle slows and the rf frequency of the cyclotron remains constant, particle velocity will decrease eventually each time particles cross the gap between the cyclotron dees. This effect of “relativistic mass” set an upper limit on the energies that any cyclotron could produce.
Edwin Mattison McMillan, a colleague of Lawrence at Berkeley, proposed a solution to Bethe’s problem in 1945. Simultaneously and independently, Vladimir Iosifovich Veksler of the Soviet Union proposed the same solution. They suggested that the frequency of the rf source be slowed to meet the decreasing rotational frequencies of the accelerating particles, in essence, synchronizing the rf frequency to match the particle frequency. In principle, the frequency of the rf source is to be matched to the frequency of a reference particle. This reference particle sets the rf source so that particles at either higher or lower frequencies, within a preset margin of error, are restored to the reference frequency. This process is called phase focusing. The synchrocyclotron was only one of a family of so-called synchronous accelerators developed as a result of McMillan’s insight into phase focusing.
Prior to World War II, Lawrence and his colleagues had obtained the massive electromagnet for the new 100-million-electronvolt cyclotron. This 467-centimeter magnet became the heart of the new Berkeley synchrocyclotron. McMillan’s 1938 theory was first put to experimental test in 1945 in the older 94-centimeter cyclotron. The new synchronous rf source overcame the relativistic mass effect. With this test deemed a success, the Berkeley team decided that it would be reasonable to convert the cyclotron magnet to one in a new synchrocyclotron. The apparatus was operational in November of 1946 and produced deuterons at 190 million electronvolts and helium ions, or alpha particles, at 380 million electronvolts.
These high energies combined with economic factors to make the synchrocyclotron a major achievement for the Berkeley Radiation Laboratory. The synchrocyclotron required less voltage to produce higher energies than the cyclotron because the relativistic mass effects were virtually nonexistent. In essence, the energies produced by synchrocyclotrons are limited only by the economics of building them. These factors led to the planning and construction of other synchrocyclotrons in the United States and Europe. In 1957, the Berkeley apparatus was redesigned in order to achieve energies of 720 million electronvolts, at that time the record for cyclotrons of any kind.
The economic and scientific benefits of the synchrocyclotron were not without problems. When the change was made from cyclotrons to synchrocyclotrons, an important property of the generated particle beams was lost: intensity. Beam intensity is directly related to the number of particles leaving the accelerator. Particles leave the synchrocyclotron at a much lower rate than the cyclotron because particles are essentially “held up” until the rf source comes into agreement with the reference particle, resulting in fewer particles per unit of time.
Although physicists had higher energies with which to experiment, beam intensity had dropped by a factor of one hundred. This greatly limited the number of nuclear and subatomic collisions, or “events,” which could be observed in one experimental run. The synchrocyclotron was, however, still a more powerful instrument than the cyclotron in terms of the energies it could produce, and the intensity problem was eventually solved in the 1950’s, with the advent of the isochronous cyclotron.
Previously, scientists had to rely on natural sources for highly energetic subatomic and atomic particles with which to experiment. In the mid-1920’s, Robert Andrews Millikan began his experimental work in cosmic rays, one natural source of energetic particles called mesons. Mesons are charged particles that have a mass in excess of two hundred times that of the electron and are therefore of great benefit in high-energy physics experiments. In February of 1949, McMillan announced the first synthetically produced mesons.
The mesons were produced by exploiting the high energies generated by the Berkeley synchrocyclotron. Electrons are accelerated to an energy of 300 million electronvolts. The synchrocyclotron’s ability to avoid the pitfalls of its predecessor were evident in this experiment. At such a high energy, electrons weigh six hundred times more than they do before acceleration. The synchronized rf source allowed the electrons to continue to gain energy while traveling around the chamber 480,000 times, with each electron completing an average of six orbits per second.
Upon reaching this energy, the electrons are extracted and allowed to collide with a heavy metal target, which liberates a strong X-ray beam, which is responsible, in turn, for the creation of muons. The production of such particles has a wide variety of experimental applications in high-energy physics. Muons exist in both positively and negatively charged varieties. The negatively charged meson, known as the mu meson, or muon, has been linked since the late 1950’s by physicists such as Luis Walter Alvarez with the possibility of attaining controlled, low-temperature nuclear fusion.
Finally, McMillan’s theoretical development led not only to the development of the synchrocyclotron but also to the development of the electron synchrotron, the proton synchrotron, the microtron, and the linear accelerator. Both the proton and electron synchrotrons have been used successfully to produce precise beams of muons and another species of meson, the pi-meson, or pion.
The increased use of accelerator apparatus ushered in a new era of physics research, which became dominated by the technical and economic magnitude of increasingly large accelerators and, subsequently, larger teams of scientists and engineers required to run individual experiments. As a rule, particle accelerators were run on the joint funding of major research universities and national governments. This joint venture led to the generation of energies in excess of 2 trillion electronvolts at the United States’ Fermi National Accelerator Laboratory, or Fermilab, in Illinois. Part of the huge Tevatron apparatus at Fermilab, which generates these particles, was a proton synchrotron, a direct descendant of McMillan and Lawrence’s early efforts.
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Lee, S. Y. Accelerator Physics. 2d ed. Hackensack, N.J.: World Scientific, 2004. General survey of the use of particle accelerators in theoretical and practical physics. Bibliographic references and index. -
Livingston, M. Stanley. Particle Accelerators. Cambridge, Mass.: Harvard University Press, 1969. This small, technically concise volume presents the history of particle accelerators from their earliest conception in the early twentieth century to the 1967 Fermilab achievement of 200 to 400 billion electronvolts. It traces many of the technical and economic difficulties faced in the construction and use of particle accelerators. Livingston was involved in the early Lawrence groups at Berkeley and uses this experience to tell a well-written story that has sufficient technical detail to remain useful. -
Segrè, Emilio. From X-rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. Segrè was actively involved in nuclear physics during the 1930’s and 1940’s and presents an excellent history of the rise of high-energy physics, which was the impetus for particle accelerator development. -
_______. “Synchrotron Makes Mesons.” Science Newsletter 55 (February, 1949): 99. This is a very brief discussion of McMillan’s announcement of the production of mesons in the Berkeley synchrocyclotron. Although it is written in a journalistic style, it includes enough technical detail to make it an informative article for the layperson.
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