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Chadwick, James

CHADWICK, JAMES

(b. Bollington, near Macclesfield, England, 20 October 1891; d. Cambridge, England, 24 July 1974)

physics.

Chadwick was the son of J. J. Chadwick. who had a laundry business in Manchester, and of Ann Mary Knowles. After attending the Manchester Municipal Secondary School he won a scholarship to Manchester University, where he studied physics under Ernest 0. Rutherford. He was awarded a first-class degree in 1911, then was accepted by Rutherford as a research student for the M.Sc. At this time the department of physics at Manchesterwas at its height, for besides Rutherford its staff included Hans Geiger. Ernest Marsden, Charles Galton Darwin, György Hevesy, and Henry G. J. Moseley. as well as, for a while, Niels Bohr. The Rutherford and Bohr atoms both date from this period. In 1913 Chadwick went to work with Geiger in Berlin and was still there when war broke out the following year. He was interned until the end of the war in 1918.

Internment did not prevent Chadwick from pursuing scientific interests—he was even allowed to visit German scientific colleagues—but the materials available were basic and the literature nonexistent: the science was more of an aid to survival than anything else. In 1918 he returned to Manchester and a job with Rutherford, moving to Cambridge with him when he was appointed Cavendish Professor in 1919. In 1921 Chadwick was elected to a research fellowship at Gonville and Caius College, and the following year he was appointed assistant director of research under Rutherford at the Cavendish Laboratory, a post funded by the department of scientific and industrial research to take some of the load off Rutherford, For the next thirteen years Chadwick took day-to-day charge of all the research at what was then the leading laboratory in experimental atomic and nuclear physics. He also contributed significantly to this research, often in collaboration with others. Because of his administrative duties he had no teaching load. In 1925 he married Eileen Stewart-Brown; they had twin daughters.

Chadwick’s relationship with Rutherford seems to have been generally very good, but in the early 1930’s the development of nuclear physics brought with it the prospect of a quarrel. Chadwick believed that the cyclotron particle accelerator invented by Ernest Lawrence would rapidly become an essential tool for nuclear physics research, and he wanted one at Cambridge, Rutherford refused to have one. In 1935, deciding it was time to move. Chadwick accepted the Lyon Jones chair of physics at Liverpool University. Over the next few years he built up the physics department, which had virtually ceased to exist as a research center, with a cyclotron as its centerpiece.

When World War II broke out in 1939, Chadwick again found himself in Europe, but this time he was able to return to England, For the next four years he divided his attention between the university and government service, with the latter increasingly predominant. Late in 1943 he moved to the United States to take charge of the British part of the atomic bomb project. Chadwick returned to Liverpool in 1946 and resumed the work of building up the physics department, In 1948 he was offered the mastership of Gonville and Caius College, Cambridge, which he decided to accept. He seems to have felt that his debt to the college, which had been very kind to him when he first arrived at Cambridge, outweighed his preference for remaining active in physics. The decision may not have been a wise one, however, for college politics led to his resignation in 1958. He retired to North Wales but returned to Cambridge in 1969 to be near his daughters.

Chadwick’s early research, assigned to him by Rutherford, was concerned with gamma-ray absorption; first with its use as a precision test of radium standards and then with applications of the method devised for standardization. He investigated the excitation of gamma rays by beta rays (electrons) and then by alpha rays (helium nuclei), the latter in collaboration with the radiochemist A. S. Russell. In both cases the excitation was confirmed. In Berlin with Geiger, Chadwick set out to determine by direct observation, using a primitive Geiger point counter, the relative intensities of the discrete lines observed by Rutherford and Robinson in radioactive betaray spectra. Although he was able to identify a few of the most intense of the observed lines, he also found a continuous spectrum alongside the discrete one. He tried changing the detection apparatus, but this merely confirmed the conclusion. Theresult came as a complete surprise and could not readily be explained theoretically, but it was a clear indication of Chadwick’s experimental skill. Both spectra, and the relation between them, became an important problem in atomic and nuclear physics.

After moving with Rutherford to Cambridge, Chadwick resumed research begun before the war. He worked, as before, under Rutherford’s direction, effectively supplying his own solutions to the master’s problems. One of his first assignments was to use the determination of alpha-ray scattering probabilities to confirm van den Broek’s hypothesis that the nuclear charge of an atom on the Rutherford-Bohr model was the same as the chemical atomic number. Employing an axially symmetric scattering arrangement and a much improved optical arrangement for the counting of deflected alpha-particle scintillations, Chadwick confirmed the hypothesis for platinum with an accuracy within 1 percent and for silver and copper with slightly less accuracy. In 1921, working with E. S. Bieler, he applied the same experimental arrangement to the study of the scattering of alpha particles by hydrogen in sheets of paraffin wax. Using hydrogen gas, Rutherford had already noted discrepancies between theory and experiment; the more sophisticated analysis of Chadwick and Bieler confirmed this, leading them to propose an asymmetric model of the alpha particle. The same experimental setup was used by Chadwick and P. H. Mercier for an analysis of beta-ray scattering.

Chadwick also collaborated during this period with C. D. Ellis, whom he had met in the internment camp in Germany, on a continuation of the analysis of radioactive beta spectra and with K. G. Emeléus on the cloud chamber analysis of alpha-particle collisions. His main research throughout the 1920’s, however, was in direct collaboration with Rutherford. Following up Rutherford’s discovery of the artificial transmutation of nuclei under alpha-ray bombardment (they called it artificial disintegration, thinking wrongly that the alpha particles were not absorbed), they demonstrated transmutations in a range of elements besides the nitrogen of the original experiment. Chadwick and Ellis investigated the properties of the disintegration particles, confirming that they were protons. After demonstrating the existence of disintegration particles moving in different directions, they used this to eliminate the effects of hydrogen contamination (which gave spurious protons) and thereby demonstrated transmutations in still more elements. When workers in Vienna claimed to have found transmutations of elements for which Rutherford and Chadwick had found no effect, including carbon and oxygen, Chadwick’s experimental skill was called on. quite successfully, to uphold the Cambridge view. Other work with Rutherford in this period, on radioactively emitted alpha particles of unusually long range, was also done with a view to the Vienna group, who had reported that the particles didn’t exist.

In the second half of the 1920’s Rutherford and Chadwick turned to the problem of nuclear structure raised by the earlier experiments on alpha-particle scattering by hydrogen. In 1925 they first looked at scattering by a range of other elements; magnesium, aluminum, gold, and uranium. They then turned to helium scattering, in which scattered and scattering particles were identical (alpha particles are helium nuclei), so that there was only one nuclear structure to contend with. Once again they concluded that some asymmetry in the structure would be required. They were assuming, however, that thescattering predicted by quantum mechanics in this case was the same as that predicted by classical mechanics, in 1928 Nevill Mott showed that this was not true for identical particles, and in 1930 Chadwick showed that the results of helium scattering could in fact be interpreted by quantum mechanics without any need for asymmetries.

Apart from his work on beta spectra using Geiger point counters and his one foray into cloud chamber techniques, all of Chadwick’s published research in the 1920’s was based on scintillation counting (the optical observation of the scintillations produced when a proton or alpha particle hits a screen of zinc sulfide). This technique had its limitations, however, and by the end of the decade electrical techniques able to surpass it were coming into use. In 1928 Geiger and Walther Müller improved Geiger’s earlier point counter to make what is generally known as a Geiger counter, a very sensitive detector of beta and gamma rays. The counter was rather unreliable, in that it was subject to spurious counts in the wake of genuine ones; but it could be used reliably for coincidence counting. Chadwick responded to the new invention by quickly building some for use in the Cavendish Laboratory. Meanwhile, HL Greinacher in Hern had succeeded in detecting individual alpha particles and protons by linearly amplifying the ionization currents produced by the particles in a small ionization chamber.

By 1928 Walther Bothe and Johannes Fränz in Berlin had applied the new technique to the study of the transmutation of boron, using a polonium source of alpha rays in place of the traditional radium active deposit. Rutherford’s experience had always been that polonium alpha rays did not produce transmutations; but since the new counting technique was sensitive to background gamma radiation, the use of radium active deposits, with very high gammaray outputs, was ruled out. Bothe and Fräz’s work showed that polonium alpha rays did indeed produce transmutations despite their very low energies, a phenomenon that was soon explained by the new quantum mechanics.

Under Chadwick’s direction the new counting technique was quickly taken up and developed by C. E. Wynn-Williams and others at the Cavendish. In 1930 Chadwick, J. L. R. Constable, and E. C. Pollard used the electrical linear amplification of ionization currents and. for the first time, a polonium source to study the relationship between the energies of the incident alpha rays and emitted protons in nuclear transmutations. A year later Chadwick and Constable, with an improved polonium source and an improved ionization chamber, were able to give a detailed quantitative analysis of atomic transmutations.

Meanwhile, interest had been mounting in the production of gamma radiation under alpha-particle bombardment. Gamma rays were known to be emitted along with the radioactive alpha rays. Looking at the energy spectra of the alpha rays, George Gamow suggested in 1931) that when an alpha particle was emitted from a radioactive source with less than the maximum possible energy, a gamma-ray quantum would subsequently be emitted to restore the energy balance. It had become increasingly evident in the Cavendish experiments and elsewhere that the protons emitted from nuclear transmutations were not all of I he same energy, and it was therefore natural to look for gamma rays in that context as well.

In 1930 Bothe and H. Becker detected penetrating radiation, assumed to be gamma rays, emitted when light elements were bombarded with polonium alpha rays. They also noted a surprising effect for beryllium: the intensity of the penetrating radiation from this element was nearly ten times that for any other element, and the radiation was exceptionally penetrating, Soon after, H.C. Webster, working under Chad wick’s direction on the same subject, observed a similar phenomenon. In June 1931 Chadwick and Webster considered the possibility that the extreme penetrating radiation from beryllium might be not gamma rays, as was generally assumed, but neutrons.

The possible existence of a neutron, envisaged as a bound state of proton and electron, had been suggested by Rutherford in 1920, and over the intervening years there had been a number of attempts made in the Cavendish to detect such particles. Chadwick himself had looked for evidence of neutrons in hydrogen in 1923 and again, with the new Geiger counters, in 1928, and throughout all the work on nuclear transmutations the possibility of neutron emissions had been kept in mind. Beryllium in particular was seen as a promising source of neutrons, because it did not emit protons under alpha-ray bombardment and, through a false argument, because naturally occurring beryl was known to contain a great deal of helium: this suggested that under cosmic radiation the beryllium nucleus might split into two helium nuclei and one neutron. Chadwick had looked for neutrons from beryllium on and off for a number of years, and his interpretation of Webster’s observation was a natural one. The energy of the extremely penetrating particles was related to their direction in a way that suggested they might be material particles rather than gamma rays, and their penetrating power suggested that if this was the case, they must be uncharged. Attempts to observe their passage through an ionization chamber failed, however, and the problem was put aside.

Early in 1932 Irène Joliot-Curie and Frédéric Joliot in Paris reported that the radiation from beryllium was even more penetrating than had been thought. They still assumed it to be gamma radiation; but when Chadwick read the report, he saw, as did Rutherford, that the energy arithmetic of the collisions producing it did not add up. By now Chadwick was convinced that the radiation must be something new and might well be neutrons, Using the ionization chamber and linear amplifier of his recent investigations, together with a new and improved polonium source, he investigated the effects of collisions between the penetrating rays and a range of various substances, measuring the energies of the recoil atoms in each case. He quickly showed that the results accorded completely with the theory that the penetrating radiation was composed of neutral particles of roughly the mass of the proton, and required implausible assumptions if they were supposed to be gamma rays. A short paper announcing the discovery of the neutron was submitted in February 1932. Detailed papers by Chadwick, by Norman Feather, and by Philip Dee, who used cloud chamber techniques to further analyze the neutrons properties, followed in May.

In 1933 Chadwick did some work with Patrick Blackett and Giuseppe Occhialini, who had just demonstrated the existence of the positron. The idea was that positrons might be produced in neutron interactions, but it transpired that the observed effects were in this case due to gamma rays. The team then concentrated on the quantitative analysis of the gamma-ray production of positrons. With D. Lea. Chadwick also conducted a search of the neutrino postulated by Wolfgang Pauli to account for the continuous spectra of beta rays first demonstrated by Chadwick. Unable to detect any particles, they showed, using a very-high-pressure ionization chamber, that if the neutrino did exist, it could not produce more than one ionization in 150 kilometers of air at normal pressure.

Chadwick’s last major work before leaving Cambridge for Liverpool was with Maurice Goldhaber, who joined him as a personal assistant in 1934. Following up a suggestion of Goldhaber’s, they demonstrated the nuclear photoelectric effect in the form of the disintegration of deuterium under gammaray illumination. This work also led to the first accurate figure of the mass of the neutron, and to speculation as to the significance of slow neutrons. It was not published, however, and a few months later Enrico Fermi observed and realized the significance of the same phenomenon. Following Fermi’s work, Chadwick and Goldhaber investigated slow-neutron-induced transmutations of lithium, boron, and nitrogen. After moving to Liverpool in 1935, Chadwick did some further work on the photodisintegration of deuterium with N. heather and F. Bretscher, although he concentrated his attention on the construction of a cyclotron and the building up of the physics department there. So far as scientific publications were concerned, his career was effectively over. He still had one major contribution to make as a scientist, however, and that was to the wartime atomic energy program.

Chadwick’s first response to the discovery of fission was to reproach himself for not having done it himself earlier; he had studied uranium under slow neutron bombardment with Goldhaber. but in filtering out alpha-particle releases they had also filtered out any fission products that might have been present. Chad wick did not at first respond to fission with any experimental work of his own; but once G. P. Thomson, who did respond in this way, had alerted the authorities to the possibilities of a fission bomb, Chad wick was consulted. Like Thomson, he at first saw no real prospect of a bomb— the critical mass would be enormous, and the reaction would be too slow to go far before the uranium expanded to stop it. His having read Bohr and J. A. Wheeler’s analysis, in which fission was attributed to the relatively rare isotope uranium 235, led him to decide, at the end of 1939, that the possibilities could not be completely dismissed and that more information was needed. Using the Liverpool cyclotron, he set out to obtain this information.

Following the memorandum of Otto Frisch and Ronald Peierls (April 1940), in which it was estimated that a bomb might be made with just a few pounds of pure uranium 235, Chadwick was made a member of the M.A.U.D. Committee on military use of uranium and took on the coordination of relevant scientific work at British universities. By the end of the year he was thoroughly involved in this work and convinced that the development of a bomb was inevitable. As work continued through the early years of the war, Chadwick played an increasingly major role in discussions. When the British finally decided to abandon efforts at a bomb project of their own and transfer their scientists to the American project, Chadwick was appointed technical adviser to the British representatives on the Combined Policy Committee—the only scientist in the British group to have full access to all project information. The British had wanted Wallace Akers, who had been in charge of their project, to hold this post, but the Americans were suspicious of his commercial connections (he was seconded from Imperial Chemical Industries). Chadwick commanded the highest respect as a scientist and, a naturally discreet man, was completely trusted. He also had exceptional diplomatic skills.

Chadwick’s abilities as a scientist and diplomat ensured that the Anglo-American collaboration proceeded well And although he could not always stop British politicians and civil servants from upsetting the Americans, his sober advice prevailed sufficiently for the latter not to give up the joint effort. Even after the war in Europe ended. Chadwick insisted that the British put all their effort into the American project. Although the British came out of the war with less information than they would have liked, what they had, they owed substantially to Chadwick.

At the end of the war an exhausted Chadwick let it be known that he was not interested in the post of director of the planned Atomic Energy Research Establishment at Harwell, preferring to return to university life. He continued, however, to play a major consultative role in the British atomic energy program. Following up an earlier concern with medical physics, he was instrumental in the establishment of the Radiochemical Centre at Amersham for the production of radioisotopes.

Chadwick won the Nobel Prize for physics in 1935. He was knighted in 1945 and made a companion of honor in 1970. Chadwick was elected a fellow of the Royal Society in 1927, he was awarded its Hughes (1932) and Copley (1950) medals, and served as a vice president in the Near 1948–1949 He also received a wide range of other scientific honors and awards.

BIBLIOGRAPHY

I. Original Works. A bibliography of Chadwick’s writings is in the obituary by Massey and Feather (see below), His discovery of the neutron was reported in “Possible Existence of a Neutron,” in Nature, 129 (1932). 312, and “The Existence of a Neutron,” in Proceedings of the Royal Society. A136 (1932), 692–708.

A substantial collection of Chadwick’s papers and correspondence is in the archives of Churchill College, Cambridge, which also has a transcript of an interview with Chadwick conducted by C. Weiner in 1969.

II. Secondary Literature. The principal published source of information on Chadwick’s life is Sir Harrie Massey and Norman Feather, “James Chadwick,” in Biographical Memoirs of Fellows of the Royal Society. 22 (1976), 11–70. Articles dealing with the discovery of the neutron and other aspects of Chadwick’s work are collected in John Hendry, ed., Cambridge Physics in the Thirties (Bristol, 1984) which also contains an extensive bibliography of related secondary literature: see also Norman Feather. “Chadwick’s Neutron,” in Contemporary Physics. 15 (1974), 565–572, Chadwick’s wartime career is documented in Margaret M. Cowing, Britainand Atomic Energy1939–1945 (London and New York. 1964), and Independence and Deterrence (London and New York, 1974).

John Hendry

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Chadwick, James

Chadwick, James


ENGLISH PHYSICIST
18911974

Sir James Chadwick was born of humble origins on October 20, 1891, to John Joseph and Anne Mary Chadwick in Clarke Lane just outside of Bollington, England. Primarily raised by his grandparents, he won a scholarship to nearby Victoria University in Manchester, where he entered the physics program by mistake. He had intended to enroll in the mathematics program, but waited in the wrong registration area so ended up being admitted to the physics department instead. Chadwick graduated in 1911 and then went to work in Ernest Rutherford's laboratory, beginning a long, productive relationship between the two men. Chadwick earned his M.S. in 1913; by the age of twenty-one he had already published five scientific papers. At that point, he won an 1851 Exhibition scholarship to study abroad for two years, whereupon he traveled to Germany to work with Hans Geiger.

During his period of study in Germany, Chadwick discovered that β -rays (electrons) are emitted in a continuous spectrum, at odds with other groups' results, and a finding that eventually led to the theory and discovery of the neutrino. While he was in Germany, World War I broke out, and Chadwick was rounded up with other English in the country and interned at Ruhleben.

After his release from Ruhleben in 1919, Chadwick followed Rutherford to the Cavendish Laboratory at Cambridge University, where he was named assistant director of research in 1923. Rutherford had been working on the disintegration of nitrogen nuclei under bombardment by α -particles , and Chadwick's first research upon his return to England involved the disintegration of different nuclei.

It was in the investigation of beryllium disintegration in 1932 that the neutron was discovered. Since the neutron has no charge, the typical electromagnetic methods of detection were useless. Chadwick bounced the mystery particle off atomic nuclei that were detectable, and, by the conservation of momentum and energy, he was able to determine that the neutron had a mass slightly greater than that of a proton.

With the discovery of the neutron as a fundamental particle, many paradoxes of physics and chemistry were finally resolved, and new areas of research evolved. Prior to the discovery of the neutron as a fundamental particle, scientists generally believed that the nucleus was comprised of protons and "nuclear electrons." However, one could not explain, for example, the spin of nuclei with that model. Now, at last, theory could predict the properties of the nucleus quite well. Also, since neutrons are not repelled by the charge on the atomic nucleus, they interact easily with nuclei. Neutron scattering enables the determination of crystal structures by probing the positions of nuclei in a sample. Neutrons can also catalyze fission reactions, for example, the fission of uranium nuclei that led to the creation of nuclear power plants and the atomic bomb.

Only three years after the discovery of the neutron, Chadwick was awarded the Nobel Prize in physics in 1935. He was lured away from Cambridge to accept the chair in physics at Liverpool University, where he oversaw the construction of the first cyclotron in England. As World War II broke out, Chadwick played a prominent role in the effort to create the atomic bomb, both in England, and, beginning in 1943, as the leader of the British effort on the Manhattan Project . Chadwick returned to his chair in Liverpool in 1946, but soon thereafter he accepted an offer from his alma mater, the College of Gonville and Caius at Cambridge, to serve as its master, a post he held until his retirement in 1958. He died in Cambridge on July 24, 1974.

see also Beryllium; Nuclear Chemistry; Radiation; Rutherford, Ernest.

Michael J. Fosmire

Bibliography

Brown, Andrew (1997). The Neutron and the Bomb. New York: Oxford University Press.

Internet Resources

Nobel e-Museum. "James ChadwickBiography." Available from <http://www.nobel.se/physics/laureates/1935/chadwick-bio.html>.

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Sir James Chadwick

Sir James Chadwick

The English physicist Sir James Chadwick (1891-1974) made his most outstanding contribution to modern physics by demonstrating the existence of the neutron.

James Chadwick was born in Manchester on Oct. 20, 1891, the eldest son of John Joseph and Anne Mary Knowles Chadwick. In 1908 he enrolled at Victoria University in Manchester. Although his intention was to study mathematics, Chadwick was admitted to the physics programs and was too shy to correct the error. He graduated from the Honours School of Physics in 1911. During the next 2 years his education was continued in Ernest Rutherford's laboratory at the same university. It was there that Rutherford outlined his planetary theory of the atom. Chadwick's acquaintances in the physics department included Hans Geiger and Niels Bohr… After Chadwick received his master's degree in 1913, he was awarded the 1851 Exhibition Scholarship, which he used to finance his studies under Geiger in the foremost German research institute, the Physikalisch-Technische Reichsanstalt in Charlottenburg near Berlin. An early result of his work there was the establishment of the first energy spectrum of beta particles. Years later, subsequent developments along these lines prompted Wolfgang Pauli to postulate the existence of the neutrino.

Discovered the Neutron

After spending the years of World War I in a civilian internment camp in Ruhleben, Chadwick returned to England and used his fellowship at Gonville and Caius College to work with Rutherford at Cambridge University's Cavendish Laboratory. In 1920 he became the first to use a direct method in determining the electric charge on the nucleus. In 1922, he became assistant director of research under Rutherford. Together they spent much of their time experimenting with the transmutation of elements, attempting to break up the nucleus of one element so that different elements could be formed.

Throughout the years of work, Chadwick and Rutherford struggled with an inconsistency. They saw that almost every element had an atomic number that was less than its atomic mass. Rutherford suggested this might be due to the existence of a particle with the mass of a proton but with a neutral charge. However, their attempts to find such a particle were in vain. But in 1932 Chadwick found the answer in the work of the Joliot-Curies, who observed that beryllium had become radioactive after being exposed to alpha particles. Chadwick showed, by using a cloud chamber filled with nitrogen, that the radiation caused the nitrogen atoms to recoil with such energy as could be imparted only by collisions with uncharged particles having approximately the mass of protons. Chadwick had proven the existence of the neutron and received the Nobel Prize in physics in 1935.

From 1935 until 1948 Chadwick held the Lyon Jones chair of physics at the University of Liverpool. From 1943-1946, he also served as head of the British mission to the Manhattan Project and was present at the first atomic test in the New Mexico desert. He was knighted in 1945 and in 1948 was elected master of Gonville and Caius College, a post from which he retired in 1959. Three years later he retired also from the United Kingdom Atomic Energy Authority, on which he had served as part-time member from 1957. Sir James Chadwick died in Cambridge, England, on July 24, 1974.

Further Reading

Pais, Abraham, Inward Bound, Oxford University Press, 1986.

Rhodes, Richard, The Making of the Bomb Simon & Schuster, 1986. □

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Chadwick, Sir James

Chadwick, Sir James (1891–1974) English physicist who discovered and named the neutron. Chadwick worked on radioactivity with Ernest Rutherford at the Cavendish Laboratory, Cambridge. In 1920, Rutherford had predicted a particle without electric charge in the nucleus of an atom, and, in 1932, Chadwick proved the neutron's existence and calculated its mass. For this, he received the 1935 Nobel Prize in physics. Chadwick also constructed Britain's first particle accelerator (1935). During World War II, he moved to the USA to head British research on the Manhattan Project.

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Chadwick, Sir James

Sir James Chadwick, 1891–1974, English physicist, grad. Manchester Univ., 1908. He worked at Manchester under Ernest Rutherford on radioactivity. He was assistant director of radioactive research in the Cavendish Laboratory, Cambridge (1923–35), professor at the Univ. of Liverpool (1935–48), and master of Gonville and Caius College, Cambridge (1948–58). For his discovery of the neutron in 1932 he received the 1935 Nobel Prize in Physics. He was knighted in 1945.

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