Chadwick, Sir James
- Isobel Falconer
Chadwick, Sir James (1891–1974), nuclear physicist, was born on 20 October 1891 at Bollington, near Macclesfield, the eldest of the three sons and one daughter (who died young) of John Joseph Chadwick, cotton spinner, and his wife, Annie Mary Knowles. Chadwick's father moved to Manchester to establish a laundry and, when that failed, became a railway storekeeper. James meanwhile was left in the care of his grandmother and attended Bollington Cross School. About 1902 he rejoined his family in Manchester, finding the change in lifestyle difficult to adjust to. Unable to afford to take up a scholarship to Manchester grammar school, he attended the Central Grammar School for Boys instead, being greatly encouraged in mathematics by Mr Wolfenden.
At sixteen Chadwick won a scholarship to Manchester University where he enrolled as a physicist by mistake, but found himself fascinated by the professor, Ernest Rutherford, and his approach. In Chadwick's third year Rutherford asked him to devise a method of making accurate inter-comparisons of different radium sources, important for setting international standards. Chadwick was successful and graduated with first-class honours in 1911, publishing his work jointly with Rutherford in 1912 ('A balance method for comparison of quantities of radium and some of its applications', Proceedings of the Physical Society, 24, 141–51). Chadwick remained at Manchester as demonstrator, receiving his MSc in 1912 and being elected Beyer fellow of the university. He made the first direct measurements of absorption of gamma rays by gases, pointing out incidentally that the concentration of ions in the upper atmosphere could not be 'wholly due to the radiation from the radioactive material in the earth' (The absorption of gamma rays by gases and light substances, Proceedings of the Physical Society, 24, 152–6), and then, together with A. S. Russell, undertook a difficult series of measurements of the excitation of gamma rays by alpha and beta rays. The precision of his results, coupled with the elegance of his published accounts, established Chadwick as a first-class experimentalist. In 1913, on Rutherford's strong recommendation, he was awarded an 1851 scholarship to study for two years with Hans Geiger in Berlin. Here he learned German and met a number of eminent physicists, including Albert Einstein, Otto Hahn, and Lise Meitner. Using a new prototype electric counter he rapidly found that beta radiation gave a continuous, rather than a line, spectrum, contradicting the then generally accepted view and posing a theoretical problem that was not resolved until Fermi's postulation of the neutrino twenty years later ('Intensitätsverteilung im magnetischen Spektrum der Beta-Strahlen von Radium B + C', Deutsches Physical Gesellschaft. Verh., 16, 383–91).
Chadwick's work ceased abruptly in August 1914 on the outbreak of the First World War, when he was interned as an enemy alien at Ruhleben racecourse near Spandau. Conditions were hard and food was appalling, but a number of societies were established, including a science circle of which Chadwick was the secretary. They set up a rudimentary laboratory, and here Chadwick continued some simple research using radioactive toothpaste. A fellow internee, Charles Ellis, was attracted to physics by Chadwick and became a long-term colleague. Chadwick returned home in 1918 having, as he considered, grown up, but with his digestive system shattered beyond recovery.
Rutherford immediately offered Chadwick a part-time teaching job at Manchester (for he was too debilitated to work more). Here he used an alpha-particle scattering technique to make the first direct measurements of the charge on atomic nuclei, demonstrating unambiguously that the nuclear charge was indeed equivalent to the chemical atomic number. He also showed that the electrostatic inverse square force law holds to a high degree of accuracy in the neighbourhood of the nucleus and concluded that there are no electrons in the region between the nucleus and the innermost electron ring—a hitherto untested assumption (The charge on the atomic nucleus and the law of force, Philosophical Magazine, 40, 1920, 734–46). This work, begun in Manchester, was finished at the Cavendish Laboratory in Cambridge, to which Rutherford was appointed professor in 1919. Gonville and Caius College offered Chadwick a Wollaston research studentship and then, in 1920, the university awarded him the Clark Maxwell studentship. He was elected a fellow of Gonville and Caius in 1921 and in the same year gained one of the first of Cambridge's new PhD degrees; half of his thesis comprised an account of his work on alpha-particle scattering and half described his work with Etienne Bieler on the size and shape of the helium nucleus, work which has been cited as 'marking the birth of strong [nuclear] reactions' (A. Pais, Inward Bound, 1986, 240, cited in Brown, 51), for they provided direct evidence that electrostatic force alone could not explain the extremely strong forces present in and just around the nucleus.
Chadwick's de facto position as Rutherford's right-hand man and overseer of research at the Cavendish was officially recognized in 1923 when the Department of Scientific and Industrial Research appointed him assistant director of research at the Cavendish. Rutherford in 1919 had succeeded in the first artificial disintegration of an atomic nucleus, by bombarding nitrogen with alpha particles, and now the main thrust of research at the Cavendish was understanding the structure of the nucleus. Chadwick and Rutherford undertook a systematic study which established incontrovertibly the transmutation of light elements by incident alpha particles and provided further evidence for a, hitherto unknown, strong force within the nucleus. Their results were contested in 1924 by Hans Pettersson and Gerhard Kirsch, working in Vienna. The ensuing ‘Vienna controversy’ raged for four years and was resolved, largely in Rutherford's and Chadwick's favour, only when Chadwick travelled to Vienna and the two groups compared their particle counting procedures. It brought about a realization that visual scintillation counting was highly susceptible to observer bias, and prompted the Cavendish move to develop automatic counting techniques.
In 1925 Chadwick married Aileen Maud Stewart-Brown, daughter of a stockbroker and member of one of Liverpool's most prominent families. Vivacious and self-assured, she had a devastating effect on the shy, reserved Chadwick. Peter Kapitza, his best man, wrote to his mother in Russia, 'Chadwick is up to his ears in love and the crocodile [Rutherford] growls that he is not working enough' (J. W. Boag and others, eds., Kapitza in Cambridge and Moscow, 187–8). Twin daughters, Joanna and Judy, were born in 1927, the year in which Chadwick was elected a fellow of the Royal Society.
Through the 1920s Rutherford and Chadwick had speculated occasionally about the possibility of a neutral particle, which they initially thought of as a tightly bonded proton–electron pair, playing a fundamental role in nuclear structure. Chadwick sporadically tried two experimental approaches to finding these ‘neutrons’, either by synthesizing them from protons and electrons, or by knocking them out of an atomic nucleus. These experiments were minor adjuncts to his main experimental programme, and were hampered by his weak polonium source of alpha particles. By 1930, however, he had obtained a stronger polonium source, and had set a research student, Webster, to work on investigating the puzzling radiation from beryllium which was bombarded by alpha particles. When in 1932 the Joliot-Curies in Paris reported that gamma radiation from beryllium disintegration was sufficiently energetic to knock protons out of paraffin wax, Chadwick was in an excellent position to recognize what the radiation was: not gamma radiation but neutrons which, because of their lack of charge, were very penetrating and very difficult to detect by conventional methods. Working night and day for about three weeks Chadwick showed that the radiation from beryllium ejects particles from most of the light elements, and studied the tracks of target atoms as they recoiled from the collision. He published his results in a letter to Nature, 'Possible existence of a neutron', in February 1932, concluding that the difficulties of interpreting the radiation as gamma rays disappear 'if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons' (p. 312). He followed this up with a full paper, 'The existence of a neutron', in the Proceedings of the Royal Society (136, 1932, 692–708). Chadwick was awarded the Nobel prize in 1935 for his discovery. He was soon immersed in further investigations of protons, electrons, neutrons, and the newly discovered positrons, showing in 1935 (with Goldhaber) that the neutron was indeed an elementary particle ('The nuclear photoelectric effect', Proceedings of the Royal Society, 151, 479–93).
The Liverpool laboratory
By then it was clear to Chadwick that further investigation of nuclear physics necessitated large-scale accelerators, particularly E. O. Lawrence's new cyclotron, and he was frustrated by Rutherford's inability to agree that the Cavendish should develop in this direction. When in 1935 the University of Liverpool offered Chadwick the Lyon Jones chair of physics he accepted with pleasure, for he was offered money and facilities to develop the moribund physics department in the direction he wanted. Aileen was glad to return to her native city as a professor's wife. They already had many friends there and Chadwick found the atmosphere more relaxing than at Cambridge. Chadwick galvanized the department and obtained an unprecedentedly large grant from the Royal Society to build a cyclotron, proposing a multidisciplinary research programme which looked not only at the structure of the nucleus, but also at the biological effects of neutrons and the use of radioactive isotopes in biochemistry. He also made his mark in the general affairs of the university. James Mountford, later the vice-chancellor, recalled that his
disinterested devotion to excellence in any academic sphere inevitably demanded and gained attention, respect and eventual gratitude … he stripped sloppy thinking of its pretensions and though invariably kind to individuals and sensitive to their difficulties, he was never deflected from his own high standards of judgement.Brown, 138
In the city, also, he became a member of a commission reporting on cancer research and treatment in Liverpool.
Military work: the nuclear bomb
The Liverpool cyclotron was finally completed in 1939 a few months after the discovery of nuclear fission: uranium bombarded with neutrons split into two smaller atoms, and released sufficient neutrons to continue the process; a chain reaction could ensue. With the outbreak of the Second World War an atomic bomb was seen as a possible weapon. Chadwick, as a member of the Maud committee, was soon overseeing the collaborative efforts of a number of universities and Imperial Chemical Industries to investigate uranium fission and production. The Liverpool cyclotron did more war work than any outside Los Alamos. By the spring of 1941 Chadwick had realized that 'a nuclear bomb was not only possible—it was inevitable. Sooner or later these ideas could not be peculiar to us. Everybody would think about them before long, and some country would put them into action' (C. Weiner, Sir James Chadwick, oral history, 1969, American Institute of Physics, College Park, Maryland; cited in Brown, 205). So worried that he could not sleep, he resorted to sleeping pills, which he continued to take for most of his remaining years. He realized that full-scale production of a bomb would need North American help, but American nuclear research was still proceeding at a leisurely pace, aimed more at power plants than at bombs. The Maud report of July 1941, written largely by Chadwick, convinced both the British and, crucially, the USA, that a nuclear bomb was achievable in the current war. When, two years later, collaboration was finally agreed between the British efforts, now codenamed Tube Alloys, and the vast American Manhattan project, Chadwick became the scientific adviser to the British members of the Combined Policy Committee. He rapidly gained the confidence of General Groves, who was in charge of the joint effort, and became the only man apart from Groves and his second in command to have access to all the American research and production facilities. His support for Groves and his diplomacy in reconciling American and British interests during this period ensured a continuing stake for Britain in the project and her future as a nuclear power after the war. In 1946 Chadwick persuaded a reluctant Groves to share future supplies of uranium with Britain, essential to her further nuclear development. Chadwick was a member of the government's advisory committee on atomic energy, and chairman of the nuclear physics subcommittee and his views both that production of independent nuclear weapons was essential to Britain's defence strategy, and on the related question of the types of nuclear power to develop, were influential in post-war policy. He was knighted on 1 January 1945.
On his return to Liverpool in the summer of 1946, Mountford recorded that he had never seen a man 'so physically, mentally and spiritually tired' as Chadwick, for he 'had plumbed such depths of moral decision as more fortunate men are never called upon even to peer into … [and suffered] … almost insupportable agonies of responsibility arising from his scientific work' (Brown, 323). Turning down the offer of the Jacksonian professorship at Cambridge, Chadwick plunged into rebuilding the Liverpool department, obtaining money from the Royal Society to build the new, high-energy synchrocyclotron now deemed necessary for investigating the mesons which had been found to mediate proton–neutron interactions. However, although he realized that it was inevitable in nuclear physics, the ‘Big Science’ and teamwork consequent on the Manhattan project held little appeal for Chadwick, and he found Liverpool, now launched on a downward economic spiral, depressing. In 1948 when his old Cambridge college, Gonville and Caius, invited him to be master, having failed to agree on an internal candidate, Chadwick felt duty-bound to accept, especially since his Liverpool department was now prospering. His main regret was having to resign as vice-president of the Royal Society because college meetings clashed with council dates; he thus forfeited the chance of becoming president.
Chadwick's chief aim as master of Caius was to raise the academic standards of the college. His tenure saw a gradually increasing proportion of students accepted from state schools, a broadening of the fellowship to canvas the best available from other colleges and universities, and a great increase in the number of research fellows, all made possible by the improvement in the college finances that Chadwick engineered. However, the fellowship of Caius at the time were a contentious body in whom ultimate authority in the college rested and Chadwick was deeply upset by the antagonism engendered by the college's internal politics. He resigned the mastership in 1958.
The Chadwicks bought a house in north Wales, Wynne's Parc on the outskirts of Denbigh, where they spent ten happy years. Chadwick spent much of his time editing Rutherford's collected papers for publication. In 1968 they moved back to Cambridge to be near their daughters. Chadwick had received many honours: the Copley and Hughes medals of the Royal Society, the Faraday medal, and the Franklin medal. He held the American medal for merit and Germany's order of merit, the highest honours these countries can bestow on foreign citizens, and honours from universities and scientific societies all over the world. In 1970 the queen made Chadwick a Companion of Honour. Tall and dark, with a dry sense of humour, Chadwick was a very humble man who found public speaking so daunting that ill health often precluded him from delivering the lecture. Yet behind the reserve his students and colleagues detected a great kindness and an overriding concern for his duty. Chadwick died in his sleep on 24 July 1974.
- A. Brown, The neutron and the bomb: a biography of Sir James Chadwick (1997)
- H. S. W. Massey and N. Feather, Memoirs FRS, 22 (1976), 11–20 [incl. list of publications]
- M. Goldhaber, ‘With Chadwick in the Cavendish’, Bulletin of Atomic Science, 38/10 (1982), 12–13
- M. Oliphant, ‘The beginning: Chadwick and the neutron’, Bulletin of Atomic Science, 38/10 (1982), 14–18
- L. Arnold, ‘A modest maker of modern physics’, Science, 282 (1998), 422
- The Times (25 July 1974), 20g
- Atomic Energy Research Establishment, Harwell, corresp. and papers
- CAC Cam., papers
- U. Lpool L., corresp.
- Bodl. Oxf., corresp. relating to Society for Protection of Science and Learning
- CAC Cam., corresp. with Sir Edward Bullard
- CUL, Rutherford collection, corresp. with Rutherford, MS 7653
- Nuffield Oxf., corresp. with Viscount Cherwell
- Trinity Cam., corresp. with Egon Bretscher
- University of Copenhagen, Niels Bohr Institute for Astronomy, Physics, and Geophysics, corresp. with Bohr
- A. Barrington-Brown, photograph, 1953, Daily Herald [see illus.]
- J. Gunn, oils, 1953, Gon. & Caius Cam.
- photograph, repro. in Massie and Feather, Memoirs FRS
- photograph, NPG
- photograph, Hult. Arch.
- photographs, repro. in Brown, Neutron and the bomb, pl. 1–18
- photographs, U. Cam., Cavendish Laboratory
Wealth at Death
£33,146: probate, 4 Oct 1974, CGPLA Eng. & Wales