John Douglas Cockcroft and Ernest Thomas Sinton Walton bombarded a lithium atom with protons, producing the first artificial nuclear disintegration with accelerated particles.
Sir John Douglas Cockcroft and Ernest Thomas Sinton Walton opened a new era in physics in 1932 when they successfully split the lithium nucleus using 500-kilovolt protons accelerated in a voltage multiplier. Modern nuclear and particle physics depends on subatomic particle accelerators for sources of probes to investigate the behavior of nuclei of atoms, their constituent particles, and the forces that influence them. Cockcroft and Walton’s achievement made the heart of the atom accessible for nuclear physicists.
Ernest Thomas Sinton Walton.
Following Ernest Rutherford’s discovery of the atomic nucleus in 1910, his transformation of nitrogen into radioactive oxygen using natural radioactive substances in 1917, and his assumption of the directorship of the Cavendish Laboratory
Cockcroft was educated in physics at the University of Manchester and Cambridge University, and in electrical engineering at the Manchester College of Technology. At Metropolitan Vickers Company, one of the engineers, T. E. Allibone, had made the first attempts to produce nuclear disintegration with accelerated electrons produced by a modified tesla coil in 1927, thus inspiring Rutherford’s call “for a copious supply of atoms and electrons which have an individual energy far transcending that of the alpha- and beta-particles from radioactive bodies.”
Walton had earned his doctorate in physics at Cambridge University by investigating a variety of means of accelerating subatomic particles: electrons in a circular magnetic field similar to the one later successfully used by Ernest Orlando Lawrence
Late in 1928, Cockcroft became aware of a theory propounded by Soviet theoretical physicist George Gamow.
Cockcroft realized that several-million-volt subatomic particles were not required to penetrate the nucleus. Given enough subatomic particles, however, about six in one thousand particles with energies of about 300,000 volts should penetrate a boron nucleus. Cockcroft informed Rutherford of this prospect and immediately began to build an accelerator to test this hypothesis. Rutherford assigned Walton to assist Cockcroft in building a source of protons (positive hydrogen ions) and a vacuum tube to withstand several hundred kilovolts to accelerate them. Rutherford then arranged for the team to receive a grant to purchase a transformer and rectifiers to produce steady direct-current voltage for the experiments; these were provided by the Metropolitan Vickers Company. The 350-kilovolt transformer was custom-built to fit the room in which it was housed by B. L. Goodlet and was installed in December, 1928. The rectifiers were designed by Allibone to produce steady direct currents to accelerate ions from the transformer’s alternating current. When the transformer failed in August, Cockcroft determined a new means of producing higher voltages than the 280 kilovolts they had achieved.
Cockcroft reinvented the “voltage multiplying” circuit in which condensers, which store electric charges like a battery, were linked alternately in parallel with rectifying diodes.
Cockcroft and Walton also built an accelerating tube strong enough to bear this high voltage, basing their design on that of a high-voltage X-ray tube invented by W. D. Coolidge of General Electric in the United States. The particles to be accelerated were generated in a small glass chamber at its top, to which 60 kilovolts were applied. Then they entered two evacuated glass tubes placed end to end in which the electrodes, supported by a steel plate to withstand the stresses induced by the high voltages applied to them, supplied the energy to accelerate the tubes to 710 kilovolts.
At this point, Cockcroft and Walton’s experiments were interrupted by the demands of the laboratory in which they had built their apparatus. They moved the apparatus to a larger laboratory in May of 1931 and resumed their experiments. By early 1932, 710-kilovolt protons had been produced by the tube. The two researchers, however, interrupted these studies to look for gamma rays that they expected to be produced when alpha particles struck beryllium, as Irène Joliot-Curie and Frédéric Joliot had observed in Paris. Although it had yet to be revealed by their colleague James Chadwick,
After a fruitless search, Cockcroft and Walton returned to accelerating protons and measured their magnetic deflection in order to determine their energies. At this point, Rutherford intervened and reminded them of the fundamental technique for detecting alpha particles that had been developed by the Cavendish Laboratory: the use of a fluorescent screen, a paper or a card coated with zinc sulfide. He was convinced that when protons bombarded lithium, alpha particles must be produced. Rutherford believed they should give up their search for gamma rays to hunt them. On April 14, 1932, they inserted a lithium target in the tube, and Walton climbed into a darkened cabin built at its base to look for fluorescence. He saw them immediately and summoned Rutherford, who confirmed that the fluorescence was produced by alpha particles. This was the first human-made artificial disintegration of any atom: The proton had united with lithium and broken it up into two atoms of helium, releasing 17 million electronvolts of energy. This energy conformed to the difference between the masses of the lithium and hydrogen before the disintegration and the helium afterward. Mass had been converted to energy according to the formula E = mc2
, exactly as predicted by Albert Einstein’s theory of special relativity.
The disintegration of the lithium atom by Cockcroft and Walton unleashed the power of particle accelerators on the nucleus and led to the rapid development of the field of nuclear physics in the succeeding decades. Rutherford and Marcus Laurence Oliphant, who came to Cavendish in 1927 from Australia, developed a low-energy accelerator of protons to follow up the experiments of Cockcroft and Walton with 200-kilovolt protons in order to examine the thresholds of proton-induced nuclear reactions. They succeeded in producing the disintegration of lithium with only 100-kilovolt protons by February, 1933, and subsequently found that 20-kilovolt protons would suffice; boron required only 60-kilovolt protons.
Other accelerator developers, such as Lawrence at the University of California’s Radiation Laboratory in Berkeley, Robert J. Van de Graaff at Princeton University, and Charles C. Lauritsen at the Kellogg High-Voltage Laboratory at the California Institute of Technology, quickly entered the field with more powerful particle accelerators. They had not paused to look for disintegration of atoms at the energies Cockcroft and Walton used because they were not as familiar with nuclear theory. Once they had developed the appropriate detectors, they were able to surpass the Cambridge experimenters, whose machine was not capable of much higher energies. Indeed, Cockcroft built a cyclotron at the Cavendish in 1937, after trying and failing to develop a larger machine of their design in collaboration with a physicist in the Netherlands. Lawrence’s cyclotron easily outpaced direct-current accelerators in energy, although very significant scientific work was done with the accelerators developed at Cambridge in the mid-1930’s.
The voltage multiplier remained, however, a very successful source of potentials around 1 million volts and has been used extensively as the first stage of many larger accelerators. Cockcroft and Walton received the Nobel Prize in Physics in 1951 for their pioneering work with this accelerator.
Their achievement also reflected the new constellation of interests that was to give rise to modern science. The involvement of industrial firms such as Metropolitan Vickers and General Electric with the Cavendish Laboratory in the investigation of nuclear physics presaged the industrial scale of the particle accelerators that were to be developed in the twentieth century. Allibone and Cockcroft needed engineering skills along with training in physics to accomplish the goal of artificially accelerating protons to energies sufficient to split the atom. The state of the art in high-voltage engineering had to be advanced for them to do this; therefore, industry benefited from the quest to understand the nucleus just as did physics. These early collaborations engendered gigantic efforts such as the construction of the Superconducting Super Collider, on which the U.S. government spent $2 billion before the partially completed project was ultimately canceled in 1993.
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Cathcart, Brian. The Fly in the Cathedral: How a Group of Cambridge Scientists Won the International Race to Split the Atom. New York: Farrar, Straus and Giroux, 2005. Relates the story of Cockcroft and Walton’s work in an exciting way, focusing on the Cambridge physicists’ struggle to split the atom ahead of competing scientists in the United States and Germany. -
Cockburn, Stewart, and David Ellyard. Oliphant. Adelaide, S.Aust.: Axiom Books, 1981. Biography of Rutherford’s associate chronicles aspects of nuclear physics at Cambridge during Oliphant’s tenure there from 1927 to 1936 and his work with Rutherford in following up the Cockcroft and Walton experiment. Includes illustrations, bibliography, and index. -
Crowther, J. G. The Cavendish Laboratory, 1874-1974. New York: Science History Publications, 1974. Anecdotal history of the Cavendish Laboratory is a well-illustrated introduction to its work. Includes references and index. -
Hartcup, Guy, and T. E. Allibone. Cockcroft and the Atom. Bristol, England: Adam Hilger, 1984. Biography of Cockcroft, written in part by Allibone, who was a participant in the work, is an authoritative source of information on Cockcroft’s life and work. Includes illustrations, appendixes, bibliography, and index. -
Hendry, John, ed. Cambridge Physics in the Thirties. Bristol, England: Adam Hilger, 1984. Collection of retrospective accounts interspersed with historical commentary places the achievements of the 1930’s in a broad context and illuminates many of the more obscure technical developments that ensured the Cambridge physicists’ leadership in the field. Includes illustrations, selected bibliography, and name index. -
Oliphant, Mark. Rutherford: Recollections of Cambridge Days. New York: Elsevier, 1972. Anecdotal account intended for a wide audience includes a chapter on the work of Cockcroft and Walton by a participant who knew them. Illustrated with contemporary photographs. -
Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. An overview of the scientific achievements of the twentieth century. Chapter 3 discusses Cockcroft and Walton’s work. Includes many illustrations and index. -
Rutherford, Lord Ernest. The Newer Alchemy. 1937. Reprint. Whitefish, Mont.: Kessinger, 2003. A brief discussion of nuclear physics by its founder. Written for a general audience. -
Wilson, David. Rutherford: Simple Genius. Cambridge, Mass.: MIT Press, 1983. Massive biography of the founder of nuclear physics summarizes what has been said in many other volumes on the “force of nature” who led the Cavendish Laboratory at the time of Cockcroft and Walton’s work. Includes illustrations, bibliography, and index.
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