Accelerators, Early

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ACCELERATORS, EARLY

The study of particle physics has involved artificial means of accelerating particles since the discovery of the electron, which required the manipulation of cathode rays by an electromagnetic field in an evacuated tube. The need for more powerful sources of accelerated particles was articulated by Ernest Rutherford, the discoverer of the atomic nucleus, in1927. The development of electron acceleration for X-ray tubes in order to provide high-voltage X rays provided a more practical rationale for the development of early accelerators. Discovered in 1895, X rays were in common medical use within a very short time, although it was not until the First World War that tubes with reliable output were manufactured by W. D. Coolidge at General Electric.

At the California Institute of Technology (Caltech), Charles Lauritsen and his colleagues capitalized on a million-volt testing laboratory built by Southern California Edison Company to devise a million-volt accelerating tube for a high-voltage X-ray machine that was designed to provide deep therapeutic X rays to treat cancer.

It is not surprising that physicists turned to electrical engineers for high voltages in order to conduct nuclear investigations in the 1920s. In addition to the Caltech high-voltage tube, they turned to sources such as Tesla coils, electrostatic generators, and even lightning as power sources for their accelerating tubes. Like Benjamin Franklin's early experiments, these had fatal consequences. More generally, the problems attending the insulation and regulation of high voltage made their straightforward application problematic.

Robert J. Van de Graaff developed the electrostatic accelerator. While working on his Ph.D. at Oxford University in 1926, he conceived of a vacuuminsulated high-voltage generator composed of concentric Faraday cages. He subsequently became a National Research Fellow at Princeton University working with Karl T. Compton, who encouraged him to pursue the idea. When Compton became President of the Massachusetts Institute of Technology (MIT) in 1929, he invited Van de Graaff to be a Research Associate, and it was agreed that MIT would have a half-interest in any patents acquired for a source of extremely penetrating X rays as well as for an electrostatic motor and the artificial transmutation of elements. Compton acquired a dirigible hanger at the Round Hill Estate of railroad magnate Edward Howland Robinson Green in South Dartmouth, Massachusetts, to perfect the tube.

At Round Hill, Van de Graaff built a pair of accelerators that could be used to double the potential along an accelerating tube. Although this made more than a million volts available, it did not enable him to achieve the "transmutation" of the atom that he sought. This was done by Rutherford's students, John Douglas Cockcroft and Thomas Sinton Walton, in the Cavendish Laboratory in Cambridge, UK. Cockcroft and Walton shared a background in electrical engineering and an interest in nuclear physics and used a tube developed by T. E. Allibone to achieve the transmutation of the atom. They benefited from the theoretical calculations of George Gamow, then at the Niels Bohr Institute in Copenhagen, that showed that the nucleus could be penetrated by particles with energies below the Coulomb potential through a quantum mechanical tunneling effect. Gamow came to the Cavendish in early 1929, where he discussed his theory with Cockcroft and Walton. Their accelerator was at first a transformer coupled to a vacuum-tube rectifier built by Allibone that produced 300 kilovolts (kV). When the transformer failed, Cockcroft conceived a voltagemultiplying circuit to produce a high-voltage direct current from the transformer's alternating current. This voltage was applied along an accelerating tube made of two glass cylinders and evacuated by oil pumps producing over 700,000-volt protons. Using one of Rutherford's scintillation detectors, they observed the disintegration of the lithium nucleus into two alpha particles on April 14, 1932. The reaction of lithium and hydrogen nuclei produced two alpha particles and energy corresponding to the difference in masses between the reactants and products according to Einstein's famous equation E = mc². The accelerator served to disintegrate many other elements and has become a staple in more complex accelerators where it serves as the first stage. Cockcroft and Walton received the Nobel Prize in Physics in 1951 for transmutation of atomic nuclei by artificially accelerated atomic particles.

Cockcroft and Walton also invented this accelerator before competing American physicist Ernest Orlando Lawrence had developed a means to avoid the use of high voltages in accelerating particles to high energies by reusing the same potential in a series of accelerating gaps through which the particles passed in circular orbits in resonance with the radio frequency of the voltage. Resonance acceleration had been proposed by Swedish physicist Gustaf Ising and experimentally demonstrated by Rolf Wideröe in a thesis written at the Aachen Technische Hochscule in 1927. Although linear accelerators for mercury and other heavy ions could be developed using these frequencies, the acceleration of light particles required acceleration in a radial direction by a magnetic field so that the particles traveled in a spiral as they were accelerated. Lawrence learned of the latter's work in 1929 and pursued both techniques in 1930 and 1931 with the assistance of N. E. Edlefsen, who built a 10-centimeter prototype, and M. Stanley Livingston, who demonstrated resonance acceleration in his Ph.D. thesis at the University of California, Berkeley in 1931 with a 4-inch-diameter chamber. During the years of 1931 and 1932, Livingston and Lawrence built a magnet with 10-inch- diameter pole faces and a brass chamber that achieved 1.2-million-electron-volt (MeV) protons in early 1932.

By increasing the diameter of the magnetic field, one could accelerate particles to higher energies, and Lawrence was already working on a machine with magnetic poles of 27 inches in diameter when he learned of Cockcroft and Walton's success. Ironically, he might have anticipated them with a machine 10 inches in diameter that had been built by his graduate student, M. Stanley Livingston, had he recognized the quantum mechanical implications of George Gamow's work, which had been done independently by Ronald W. Gurney and Edward U. Condon in the United States. Moreover, he had not constructed the detectors required; they were not installed until the summer of 1932 when Yale physicists Donald Cooksey and Franz Kurie introduced them to the Radiation Laboratory that Lawrence had built to house his larger cyclotron.

Although the Van de Graaff, Cockcroft-Walton, and cyclotron accelerators dominated the field of particle acceleration in the 1930s, their contributions to the development of nuclear and particle physics were eclipsed by more conventional techniques. The positron, the first antiparticle predicted by Paul Dirac, was detected in cosmic rays by Carl Anderson at Caltech, and the neutron was established as a nuclear constituent by Rutherford's associate, James Chadwick, both of whom used natural sources of particles. The meson, predicted by the Japanese physicist Hidekei Yukawa, was also discovered in cosmic rays by Anderson.

The new field of artificial radioactivity was opened by others using traditional techniques, especially Frédéric and Irène Joliot-Curie in 1934. Like nuclear disintegration, this discovery rested upon the availability of suitable detectors, and the recognition that radioactivity persisted after the original source was removed. In 1934 and 1935 Enrico Fermi and his associates in Rome used neutrons slowed by light elements to induce radioactivity in a variety of elements. The nuclear reactions that produced these unstable isotopes had been unsuspected by the accelerator builders, who were able to duplicate them almost immediately.

Ernest Lawrence pursued the art with his cyclotrons that were capable of much higher energies and produced isotopes that he and his sponsors hoped to market for medical purposes, as they had marketed high-voltage X-ray tubes developed in his laboratory. Because of the higher energies available to them, Lawrence and his Radiation Laboratory at the University of California were responsible for the discovery of the majority of the reactions producing artificially radioactive substances (radioisotopes) in the 1930s. In 1936, he won funding for a Medical Cyclotron as well as for the Crocker Radiation Laboratory, which would house a 60-inch cyclotron to be used for medical research and for experimental therapy with neutrons produced by the cyclotron. Samuel Ruben and Martin Kamen, in a search for biologically useful radioisotopes, found carbon-14 in 1940.

Emilio Segrè used parts of the 27-inch cyclotron supplied to him in Sicily to discover a new element, technetium, in 1937. After he left Italy, Segrè joined the Radiation Laboratory in Berkeley. In 1940, Edwin M. McMillan, investigating the newly discovered fission of uranium, found that it could be transmuted into another new element, heavier than uranium, which he called neptunium. Emilio Segrè and Glenn Seaborg picked up this work and discovered the next of the "transuranic" elements, plutonium, in 1940. Seaborg and McMillan received the Nobel Prize in Chemistry in 1951 for their discoveries of the transuranium elements. Seaborg and Albert Ghiorso continued this program at Berkeley after World War II, discovering the elements 95 (Americium) and 96 (Curium) in 1946, elements 97 (Berkelium) and 98 (Californium) in 1950, element 102 (Nobelium) in 1958, and element 103 (Lawrencium) in 1961.

The development of early particle accelerators depended upon a variety of historical factors, some of which were remote from the interest in subatomic particles, although it was regarded as "pure science" by physicists at the time. While participants have focused upon the experimental utility of particle accelerators in describing the atomic nucleus, it is clear that entrepreneurs like Lawrence were successful in presenting these machines as a variety of X-ray equipment as well as production machines for radioisotopes at a time when nuclear reactors were not yet available. The perceived increase in the incidence of cancer in the interwar period enhanced the market for technological cures promised by radiologists and physicists who built their giant machines. The public demonstration of the use of radioisotopes using Geiger counters to detect the circulation of radioactive sodium in the blood was a standard marketing tool used by Lawrence, while universities saw medical applications as an easily understood rationale for the support of physics research with accelerators. The National Cancer Institute encouraged experiments with accelerators as did a number of medical philanthropies. Lawrence even went so far as to postulate the production of nuclear energy using the cyclotron, although Rutherford felt compelled to brand this as "moonshine." Nevertheless, the Cavendish Laboratory acquired a cyclotron.

In addition to Great Britain and the United States, cyclotrons found homes in Denmark, France, Sweden, the Soviet Union, and Japan in the 1930s. The construction of these cyclotrons was based upon Lawrence's designs, which he shared freely with physicists elsewhere. In most cases, physicists who had worked on the original machines at the Radiation Laboratory in Berkeley were among those who built the accelerators, since the problems related to creating and maintaining a suitable vacuum in the larger cyclotrons as well as the focusing of magnetic fields in all such machines had not been reduced to engineering practice but was, at the University of California and elsewhere, still tacit knowledge.

Engineering and theoretical understanding of cyclotron behavior was developed only in the late 1930s by William Brobeck and Robert R. Wilson of the Radiation Laboratory. Before World War II, however, empirical techniques of shimming magnets and leak prevention remained an important part of cyclotron construction and operation.

Van de Graaff electrostatic generators were also widespread in Europe and the United States. Here the interest extended to the generation of electrical power as well as cancer therapy and nuclear physics, as might be expected given Van de Graaff's early experience in electrical power generation. The steady currents available from the accelerators made them more reliable for nuclear experiments in the 1 to 10 MeV range, and Merle Tuve at the Carnegie Institution of Washington, William Fowler's group at Caltech, Ray Herb's group at the University of Wisconsin, and John Williams's group at the University of Minnesota made significant contributions to nuclear physics and Van de Graaff machine design in the 1930s.

At the University of Illinois, Donald M. Kerst, aided by Radiation Laboratory veteran Robert Serber, built a magnetic induction electron accelerator, the betatron, just before World War II. The scheme, which had been proposed by Wideröe and Walton, was made to work by shimming of the magnetic field in much the same way that the cyclotron had been.

World War II brought an end to particle accelerator development as physicists turned to work on radar and the atomic bomb. Lawrence's cyclotrons were rebuilt as calutrons to separate the isotopes of uranium for the bomb, and a large Oak Ridge facility was created to house these machines, which processed the uranium-235 that was used in the Hiroshima bomb. The war did not prevent physicists from thinking about particle accelerators, however, and the thoughts of Luis Alvarez and Edwin M. McMillan bore postwar fruit in a new generation of accelerators. McMillan, who had been assigned to the new Los Alamos Laboratory of the University of California where the first nuclear weapons were constructed, conceived of a means of escaping the energy limits on conventional cyclotrons, which was caused by the increase in mass of accelerated particles as they approached the speed of light with their increasing mass. By changing the frequency as the particles were accelerated, it was possible to keep them in synchrony. This principle, called phase stability, is the basis of modern proton synchrotrons. It was first demonstrated in the 184-inch cyclotron in 1946 and was applied to an electron synchrotron at Berkeley subsequently. William Brobeck designed a very large proton synchrotron to provide protons with energies of 10 billion electron volts, which became the basis of the design of the first two American machines, the Cosmotron at the new Brookhaven National Laboratory and the Bevatron (named for its billion electron volt energies) at Berkeley. These machines, completed in 1951 and 1954, respectively, were the first to produce particles with energies approaching those in cosmic rays. The 184-inch cyclotron produced the first human-made mesons in 1948. The Cosmotron produced a series of strange particles, such as the K -meson, so named because of their unexpectedly long lifetimes, and demonstrated the principle of associated production. The Bevatron produced an entirely new particle, the antiproton, in 1955, an accomplishment for which Emilio Segrè and Owen Chamberlain won the Nobel Prize in Physics. Marcus Oliphant built a smaller proton synchrotron.

Luis Alvarez pursued the development of linear proton accelerators using microwave-frequency generators similar to those used in wartime radar and built a 40-foot accelerator at Berkeley after World War II. The linear accelerator (LINAC), as it was called, produced 40-MeV protons, and a larger accelerator, the materials testing accelerator, was built at Livermore, California, to produce fissile and other neutron-enriched elements to supply America's need for nuclear explosives. It was abandoned after two years of development when the discovery of natural sources of uranium made it economically unfeasible. The Alvarez linear accelerator, like the Cockcroft-Walton machine, is often used to preaccelerate particles fed into high-energy synchrotrons. One of Alvarez's associates, Wolfgang Panofsky, applied the principle to the acceleration of electrons in the Stanford Linear Accelerator.

With the liquid hydrogen bubble chamber, a 6-foot-long detector built in the 1950s, Alvarez discovered many new subatomic particles, providing the empirical basis for the Standard Model. He was awarded the Nobel Prize in Physics in 1967 for this work.

The Bevatron and subsequent proton synchrotrons were the principal instruments of particle physics, replacing cosmic rays as sources of particles and, after the discovery of strong focusing at Brookhaven in 1953, produced increasingly higher energies to probe the nucleus.