The invention: A powerful particle accelerator that performed better than its predecessor, the cyclotron.
The people behind the invention:
Edwin Mattison McMillan (1907-1991), an American physicist
who won the Nobel Prize in Chemistry in 1951 Vladimir Iosifovich Veksler (1907-1966), a Soviet physicist Ernest Orlando Lawrence (1901-1958), an American physicist Hans Albrecht Bethe (1906- ), a German American physicist
The First 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 devices known as “particle accelerators.” By the early 1920′s, the experimental work of physicists such as Ernest Rutherford and George Gamow demanded that an artificial means be developed to generate streams of atomic and subatomic particles at energies much greater than those occurring naturally. This requirement led Ernest Orlando Lawrence to develop the cyclotron, the prototype for most modern accelerators. The synchrocyclotron was developed in response to the limitations of the early cyclotron.
In September, 1930, Lawrence announced the basic principles behind the cyclotron. Ionized—that is, electrically charged—particles are admitted into the central section of a circular metal drum. Once inside the drum, the particles are exposed to an electric field alternating within a constant magnetic field. The combined action of the electric and magnetic fields accelerates the particles into a circular path, or orbit. This increases the particles’ energy and orbital radii. This process continues until the particles reach the desired energy and velocity and are extracted from the machine for use in experiments ranging from particle-to-particle collisions to the synthesis of radioactive elements.
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.
Although funding for scientific research on a large scale was scarce before World War II (1939-1945), Lawrence nevertheless conceived of a 467-centimeter cyclotron that would generate particles with energies approaching 100 million electron volts. By the end of the war, increases in the public and private funding of scientific research and a demand for 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.
Overcoming the Problem of Mass
In 1937, Hans Albrecht Bethe discovered a severe theoretical limitation to the energies that could be produced in a cyclotron. Physicist Albert Einstein’s special theory of relativity had demonstrated that as any mass 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 each particle. Therefore, as the rotation of each particle slows and the frequency of the alternating electric field remains constant, particle velocity will decrease eventually. This factor 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 alternating electric field be slowed to meet the decreasing rotational frequencies of the accelerating particles—in essence, “synchronizing” the electric field with the moving particles. The result was the synchrocyclotron.
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 would become the heart of the new Berkeley synchrocyclotron. After initial tests proved successful, the Berkeley team decided that it would be reasonable to convert the cyclotron magnet for use in a new synchrocyclotron. The apparatus was operational in November of 1946.
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 obstacles cited by Bethe 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 electron volts, at that time the record for cyclotrons of any kind.
Impact
Previously, scientists had had to rely on natural sources for highly energetic subatomic and atomic particles with which to experiment. In the mid-1920′s, the American physicist Robert Andrews Millikan began his experimental work in cosmic rays, which are one natural source of energetic particles called “mesons.” Mesons are charged particles that have a mass more than 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 using the synchrocyclotron.
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 proton and electron synchrotrons have been used successfully to produce precise beams of muons and pi-mesons, or pions (a type of meson).
The increased use of accelerator apparatus ushered in a new era of physics research, which has become dominated increasingly by large accelerators and, subsequently, larger teams of scientists and engineers required to run individual experiments. More sophisticated machines have generated 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, is a proton synchrotron, a direct descendant of McMillan and Lawrence’s early efforts.
See also Atomic bomb; Cyclotron; Electron microscope; Field ion microscope; Geiger counter; Hydrogen bomb; Mass spectrograph; Neutrino detector; Scanning tunneling microscope; Tevatron accelerator.
