Cockcroft, John Douglas

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Cockcroft, John Douglas

(b. Todmorden, Yorkshire, England, 27 May 1897; d. Cambridge, England, 18 Septmber 1967),

physics.

Cockcrofts have lived in the Calder Valley of the West Riding of Yorkshire for generations. During the nineteenth century they owned a cotton mill in Todmorden, but the business declined and in 1899 was transferred to Birks Mill, situated a few miles away at Walsden. Cockcroft’s father, J. A. Cockcroft devoted himself to rebuilding the prosperity of the firm, with the wholehearted support of his wife, daughter of a milowner. John, eldest of their five sons, was sent first to the Walsden Church of England School and then, after attending the Todmorden Elementary School, went to the Todmorden Secondary School in 1909. Perhaps because physics and mathematics were well taught there, he read more widely in the physical sciences, and accounts of the work of J. J. Thomson, Rutherford, and others gave him an ambition to do research. Cockcroft entered the University of Manchester in the autumn of 1914 with a county major scholarship, chiefly to study mathematics under Horace Lamb, and was fortunate enough to be able to attend some first-year lectures given by Rutherford.

In the summer of 1915, after only one year at the university, Cockcroft volunteered for war service with the Y. M. C. A. Later in the year he was called up for military service and became a signaler in the Royal Field Artillery. Cockcroft took part in several of the major battles on the western front, was commissioned in the spring of 1918, and was released from the army when peace came in the autumn. He did not return to the science faculty of the university but entered the college of technology to study electrical enginerring, possibly because as a signaler he had developed a professional interest in this subject. In 1920 he became a college apprentice at the Metropolitan Vickers Electrical Company, where he carried out some work in the research department under the direction of Miles Walker. He was awarded the degree of M. Sc. Tech. in 1922. On Walker’s advice he went on to Cambridge to study mathematics at St. John’s College and distinguished himself in part two of mathematical tripos, obtaining a b.A. degree in 1924. Cockcroft thus had an exceptionally long period of training—seven years excluding the war years—before joining Rutherford’s team at the Cavendish Laboratory.

He married Eunice Elizabeth Crabtree at Todmorden on 26 August 1925. She was the daughter of Herbert Crabtree of Stansfield Hall, a cotton manufacturer, and John had known her since childhoo. They had four daughters and one son. Their happy, close-knit family life was a source of great strength to Cockcroft.

His work with Miles Walker on the harmonic analysis of voltage and current wave forms at commercial power frequencies was published in 1925 and was followed by two further papers describing some detailed studies of boundary effects in electrical conductors. By this time Cockcroft had acquired a deep insight into certain aspects of electrical engineering and commanded some powerful theoretical techniques. He helped the Russian physicist Peter Kapitza, who was in Cambridge, with his work on very high magnetic fields by designing extremely efficient magnet coils in which the stresses were minimized. Later, he designed an electromagnet for α-ray spectroscopy for Rutherford and a permanent magnet for β-ray spectroscopy. Cockcroft also carried out an elegant investigation into the properties of molecular beams, following the classical work of Otto Stern and Immanuel Estermann. It was shown that the Frenkel theory of surface condensation applied, and attention was drawn to the role of adsorbed gaseous impurities. This work, published in 1928, afforded valuable experience in vacuum technology that was later to be curcial.

In the meantime, Thomas Allibone and E. T. S. Walton had been experimenting with different methods of accelerating electrons, and after Cockcroft had calculated from George Gamow’s theory of a emission that protons of a few hundred kilovolts’ energy should have an appreciable probability of penetrating the energy barriers of light nucile, he became interested in the possibility of designing an accelerator for protons. Walton decided to join him, and with Rutherford’s support they eventually constructed a voltage multiplier of a type originally proposed by Greinacher in 1920 and connected this to an accelerating tube provided with a proton source designed by M. L. E. Oliphant. Four stages of voltage multiplication yielded a steady potential of 710,000 volts at the accelerating tube. The maintenance of the necessary degree of vacuum in the system was difficult, and the success achieved was, in part, attributable to current improvements in vacuum techniques. When a lithium target was exposed to the proton beam and the voltage was raised progressively, scintillations due to a particles appeared on the zinc sulfide detector screen at 100,000 volts, increasing rapidly in number at higher volatges. The identification of a particles was confirmed by several methods, and it was also shown that two particles are ejected simultaneously in opposite directions, each with 8.6 MEV energy, in accordance with the equation

This was the first unclear transformation to be brought about by artificial means. Similar effects were observed when other elements, such as boron and fluorine, were bombarded.

At the same time E. O. Lawrence and M. S. Livingston were constructing another device for accelerating protons, later known as the cyclotron, at the University of California, and they were soon able to confirm Cockcroft and Walton’s results. By adding further stages to the voltage multiplier, the beam energy of the Cockcroft-Walton machine could be increased to about 3 MEV, whereas the cyclotron was capable of reaching much higher energies.

The year 1932 became an outstanding one for the Cavendish Laboratory with the discovery of the neutron by James Chadwick. In 1933 Rutherford received, through Cockcroft, some of the first heavy water from G. N. Lewis at Berkeley, and Cockcroft and Walton bombarded lithium, boron, and carbon with deuterons from their accelerating apparatus. After the announcement of the artificial production of radioactivity by Irene Joliot-Curie and Frédéric Joliot in 1934, Cockcroft and Walton showed that radioactive nuclei were produced from boron and carbon exposed to proton and deuteron beams from the machine. Finally, a more detailed study of the disintegration of boron by protons and deuterons was carried out in collaboration with W. B. Lewis, using highly purified deuterium and with magnetic separation of unwanted ions from the beam.

From 1935 until the outbreak of war, Cockcroft was made personally responsible by Rutherford for a major reconstruction and reequipping of the Cavendish Laboratory, including the building of a cyclotron. Although habitually economical in the use of words, he was always genial and approachable yet made firm, impartial, and prompt decisions that were accepted almost without question. These qualities proved to be extremely valuable when Cockcroft became involved in the development of radar during the war. After building radar stations for detection of submarines and low-flying aircraft at various remote sites, he went to the United States in August 1940 as a member of the famous Tizard mission to negotiate scientific and technical exchanges of military importance. On his return he was appointed chief superintendent of the Air Defence Research Development Establishment at Christchurch, Hampshire, and during the same year he became a member of the committee which had been formed to investigate the possible applications of nuclear fission. After three more grueling years on radar development, Cockcroft was sent as director to the Anglo-Canadian atomic energy research laboratory in Montreal in April 1944. There he was faced not only with technical and scientific problems but also with a delicate diplomatic situation involving Canadian. British, American, and French interests. The NRX reactor, built under his direction at Chalk River, Ontario, was for a long time a unique instrument for nuclear research as well as for technology.

Cockcroft returned to Britain in 1946 to become director of the new Atomic Energy Research Establishment at Harwell, where he remained until 1959. During this period, in the latter part of which he was member for research of the Atomic Energy Authority, he guided and stimulated nuclear developments of all kinds, from basic research to power stations. He was especially concerned with the succession of particle accelerators constructed at Harwell and was largely responsible for obtaining approval to build the 7 GEV proton synchrotron and the Rutherford Laboratory at nearby Chilton. Cockcroft also did a great deal to promote science through international organizations such as CERN, the European laboratory for highenergy physics, and in many other ways. Nor did these activities cease when he became master of Churchill College, Cambridge, in 1959. Indeed, he was elected president of the well-known Pugwash Conferences on Science and World Affairs just before his death on 18 September 1967.

Cockcroft shared the 1951 Nobel Prize for physics with E. T. S. Walton and received many other awards, including a knighthood and the O. M., in recognition of his work on radar and more especially for his outstanding role in promoting the peaceful; uses of atomic energy.

BIBLIOGRAPHY

Among Cockcroft’s many papers are “An Electric Harmonic Analyser,” in Journal of the Institution of Electrical Engineers, 63 (1925), 69–113, written with R. T. Coe, J. A. Lyacke, and Miles Walker; “The Design of Coils for the Production of Strong Magnetic Fields,” in Philopsophical Transactions of the Royal Society, A227 (1928), 317–343; “The Effect of Curved Boundaries on the Distribution of Electrical Stress Round Conductors,” in Journal of the Institution of Electrical Engineers, 66 (1928), 385–409; “On Phenomena Occurring in the Condensation of Molecular Streams on Surfaces,” in Proceedings of the Royal Society, A119 (1928), 293–312: “Skin Effect in Rectangular Conductors at High Frequencies,” ibid., A122 (1929), 533–542; “Experiments With Velocity Positive lons,” ibid., A129 (1930), 477–489, written with E. T. S. walton; “Experiments With High Velocity Positive Ions. I. Further Developments in the Method of obtaining High Velocity Positive lons,” ibid., A136 (1932), 619–630, written with Walton; “Experiments With High Velocity Positive lons.” ibid., A136 (1932), 619–630, written with Walton; “Experiments With High Velocity Positive lons.” II. The Disintegration of Elements by High Velocity Protons,” ibid., A137 (1932), 229–242, written with with Walton; “A Permanent Magnet for β-Ray Spectroscopy,” ibid., A135 (1932), 628–636, written with C. D. Ellis and H. Kershaw; “Disintegration of Light Elements by Fast Neutrons,” in Nature, A131 (1933), 23, written with Waltons; “A Magnet for α-Ray Spectropscopy,” in Journal of Scientific Instruments, 10 (1933), 71–75; “Experiments With High Velocity Positive lons. III. The Disintegration of Lithium, Boron and Carbon by Heavy Hydrogen lons.” in Proceedings of the Royal society, A144 (1934). 704–720, written with Walton; “Experiments With High Velocity Positive lons. IV. The Production of Induced Radioactivity by High Velocity Protons and Diplons,” ibid., A148 (1935), 225–240, written with C. W. Gilbert and Walton; “Experiments With High Velocity Positive lons. V. Further Experiments on the Disntegration of Boron,” ibid., A154 (1936), 246–261, written woth W. B. Lewis; “Experiments With High Velocity Positive lons. VI. The Disintegration of Carbon, Nitrogen, and Oxygen by Deuterons,” ibid., 261–279, written with Lewis; “High Velocity Positive lons. Their Application to the Transmutation of Atomic Nuclei and the Production of Artificial Radioactivity.” in British Journal of Radiology;10 (1937), 159–170: “The Cyclotron and Betatron,” in Journal of Scientific Instruments, 21 (1944), 189–193: “Rutherford: Life and Work after year 1919, With Personal Reminiscences of the Second Cambridge Period,” in Proceedings of the Physical Society 58(1946), 625–633, the second Rutherfird memorial mecture; “The Development of Linear Accelerators and Synchrotrons for Radiotherapy, and for Research in Physics,” in Proceedings of the Institution of Electrical Engineers 96, pt. 1 (1949), 296–303; “Modern Concepts of the Structure of Matter,” ibid., 98, pt. 1 (1951), 301–308, the forty-second Kelvin lecture; “Experiments on the Interaction of High Speed Nucleons With Atomic Nuclei,” in Les prix Nobel en 1951 (Stockholm, 1952), pp. 101–118; “The Scientific Work of the Atomic Energy Research Establishment, “in Proceedings of the Royal Society A211 (1952), 155–168; and “High energy Particle Accelerators,” in Endeavour 24 (1955), 61–70, written with T. G. Pick avance.