James Chadwick

Sir James Chadwick  (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935  Nobel Prize in physics for his discovery of the neutron in 1932. He was the head of the British scientists who worked on the Manhattan Project during World War II. He was knighted in 1945 for achievements in physics.

A graduate of the University of Manchester, where he studied under Ernest Rutherford, Chadwick was awarded an 1851 Exhibition Scholarship, and elected to study beta radiation under Hans Geiger. Using Geiger's recently developed Geiger counter, he was able to demonstrate that beta radiation produced a continuous electromagnetic spectrum, and not discrete lines as had been thought. Still in Germany when World War I broke out in Europe, he spent the war in the Ruhleben internment camp.

After the war, Chadwick followed Rutherford to the Cavendish Laboratory at the University of Cambridge, where Chadwick earned his Doctor of Philosophy degree at Gonville and Caius College, Cambridge. He was Rutherford's Assistant Director of Research at the Cavendish Laboratory for over a decade at a time when it was one of the world's foremost centres for the study of physics, attracting students like John Cockcroft, Norman Feather, and Mark Oliphant. In 1932, Chadwick pursued a line of research that led to his discovery of the neutron. He went on to measure its mass.

Chadwick left the Cavendish Laboratory in 1935 to become a professor of physics at the University of Liverpool, where he overhauled an antiquated laboratory and, by installing a cyclotron, made it an important centre for the study of nuclear physics. During the Second World War, he carried out research as part of the Tube Alloys project to build an atomic bomb. When the Quebec Agreement merged his project with the American Manhattan Project, he became head of the British Mission, and worked at the Los Alamos Laboratory. In July 1945, he viewed the Trinity nuclear test. Uncomfortable with the trend toward Big Science, Chadwick became the Master of Gonville and Caius College, Cambridge, in 1948. He retired in 1959.

Education and early life
Chadwick was born in Bollington, Cheshire, on 20 October 1891, the first child of John Joseph and Anne Mary Knowles Chadwick. He was named after his paternal grandfather. In 1895, his parents moved to Manchester, leaving him in the care of his grandparents. He went to Bollington Cross Primary School, and was offered a scholarship to Manchester Grammar School, which his family had to turn down as they could not afford the small fees that still had to be paid. Instead he attended the Central Grammar School for Boys in Manchester, rejoining his parents there. He now had two younger brothers, Harry and Hubert; a sister had died in infancy. At the age of 16, he sat for two university scholarship examinations, and was offered both.

Chadwick chose to attend Victoria University of Manchester, which he entered in 1908. Like most students, he lived at home, walking the 4 miles to the university and back each day. At the end of his first year, he was awarded a Heginbottom Scholarship to study physics. The physics department was headed by Ernest Rutherford, who assigned research projects to final-year students, and he instructed Chadwick to devise a means of comparing the amount of radioactive energy of two different sources. The idea was that they could be measured in terms of the activity of 1 g of radium, a unit of measurement which would become known as the Curie. Unfortunately, Rutherford's suggested approach was unworkable—something Chadwick knew but was afraid to tell Rutherford—so Chadwick pressed on on his own, and eventually devised the required method. The results became Chadwick's first paper, which, co-authored with Rutherford, was published in 1912. He graduated with first class honours in 1911.

Having devised a means of measuring gamma radiation, Chadwick proceeded to measure the absorption of gamma rays by various gases and liquids. This time the resulting paper was published under his name only. He was awarded his Master of Science (MSc) degree in 1912, and was appointed a Beyer Fellow. The following year he was awarded an 1851 Exhibition Scholarship, which allowed him to study and research at a university in continental Europe. He elected to go to the Physikalisch-Technische Reichsanstalt in Berlin in 1913, to study beta radiation under Hans Geiger. Using Geiger's recently developed Geiger counter, which provided more accuracy than the earlier photographic techniques, he was able to demonstrate that beta radiation produced a continuous electromagnetic spectrum, and not discrete lines as had previously been thought. The reason for this would remain unexplained for many years.

Chadwick was still in Germany at the start of World War I, and was interned in the Ruhleben internment camp near Berlin, where he was allowed to set up a laboratory in the stables and conduct scientific experiments using improvised materials such as radioactive toothpaste. With the help of Charles D. Ellis, he worked on the ionisation of phosphorus, and the photochemical reaction of carbon monoxide and chlorine. He was released after the Armistice with Germany came into effect in November 1918, and returned to his parents' home in Manchester, where he wrote up his findings over the previous four years for the 1851 Exhibition commissioners.

Rutherford gave Chadwick a part-time teaching position at Manchester, allowing him to continue research. He looked at the nuclear charge of platinum, silver, and copper, and experimentally found that this was the same as the atomic number within an error of less than 1.5 percent. In April 1919, Rutherford became director of the Cavendish Laboratory at the University of Cambridge, and Chadwick joined him there a few months later. Chadwick was awarded a Clerk-Maxwell Studentship in 1920, and enrolled as a Doctor of Philosophy (PhD) student at Gonville and Caius College, Cambridge. The first half of his thesis was his work with atomic numbers. In the second, he looked at the forces inside the nucleus. His degree was awarded in June 1921. In November he became a Fellow of Gonville and Caius College.

Cambridge
Chadwick's Clerk Maxwell studentship expired in 1923, and he was succeeded by the Russian physicist Pyotr Kapitza. The Chairman of the Advisory Council of the Department of Scientific and Industrial Research, Sir William McCormick arranged for Chadwick to become Rutherford's Assistant Director of Research. In this role, Chadwick helped Rutherford select PhD students. Over the next few years these would include John Cockcroft, Norman Feather and Mark Oliphant, who would become firm friends with Chadwick. As many students had no idea what they wanted to research, Rutherford and Chadwick would suggest topics. Chadwick would also edit all the papers produced by the laboratory.

In 1925, Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker. The two were married in August 1925. They had twin daughters, Joanna and Judy, who were born in February 1927.

In his research, Chadwick continued to probe the atomic nucleus. In 1925, the concept of spin had allowed physicists to explain the Zeeman effect, but it also created unexplained anomalies. At the time it was believed that the nucleus consisted of protons and electrons, so nitrogen, for example, with a mass number of 14, was assumed to contain 14 protons and 7 electrons. This gave it the right mass and charge, but the wrong spin.

At a conference at Cambridge on beta particles and gamma rays in 1928, Chadwick met Geiger again, who brought with him a new model of his Geiger counter, which had been improved by his post-doctoral student Walther Müller. This was something that Chadwick had not used since the war, and the new Geiger-Müller counter was potentially a major improvement over the scintillation techniques then in use, which relied on the human eye for observation. The major drawback with it was that it detected alpha, beta and gamma radiation, and radium, which the Cavendish laboratory normally used in its experiments, emitted all three and was therefore unsuitable for what Chadwick had in mind. However, polonium is an alpha emitter, and Lise Meitner sent Chadwick about 2 millicuries from Germany.

In Germany, Walter Bothe and his student Herbert Becker used polonium to bombard beryllium, producing an unusual form of radiation. Chadwick had his Australian 1851 Exhibition scholar, Hugh Webster, duplicate their results. To Chadwick, this was evidence of something that he and Rutherford had been hypothesising for years: the neutron, a theoretical nuclear particle with no electric charge. Chadwick dropped all his other responsibilities to concentrate on the neutron, frequently working late at night. He devised a simple apparatus that consisted of a cylinder containing a polonium source and beryllium target. The resulting radiation could then be directed a material such as paraffin wax and the particles displaced, which were protons, would go into a small ionisation chamber where they could be detected with an oscilloscope.

In February 1932, after only about two weeks of experimentation with neutrons, Chadwick sent a letter to Nature titled Possible Existence of a Neutron. He communicated his findings in detail in The Existence of a Neutron in May. His discovery was a milestone in understanding the nucleus. Reading Chadwick's paper, Robert Bacher and Edward Condon realised that anomalies in then-current theory like the spin of nitrogen would be resolved if the spin of the neutron was ½ and a nitrogen nucleus consisted of 7 protons and 7 neutrons. The theoretical physicists Niels Bohr and Werner Heisenberg became convinced that the neutron must be a nuclear particle, and not a proton-electron pair. Chadwick and Maurice Goldhaber, a refugee from Nazi Germany, determined the mass of the neutron experimentally, and found that it was greater than that of the proton, thereby confirming this theory. For Chadwick's discovery of the neutron, he was awarded the Hughes Medal by the Royal Society in 1932, the Nobel Prize in Physics in 1935, the Copley Medal in 1950, and the Franklin Medal in 1951.

Chadwick’s discovery of the neutron also made it possible to produce elements heavier than uranium in the laboratory, by the capture of slow neutrons followed by beta decay. Unlike the positively-charged alpha particles, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any Coulomb barrier and can therefore penetrate and enter the nuclei of even the heaviest elements such as uranium. His discovery of the neutron inspired Enrico Fermi, the Italian physicist and eventual Nobel Prize winner, to investigate the nuclear reactions brought about by collisions of nuclei with slow neutrons. The continuous spectrum of beta radiation, which Chadwick had reported in 1914, was explained by Wolfgang Pauli by recourse to another hypothetical neutral particle, which Fermi christened the neutrino.

Liverpool
Rutherford succeeded McCormick as Chairman of the Advisory Council of the Department of Scientific and Industrial Research in 1930. With the onset of the Great Depression in the United Kingdom, the government became more parsimonious with funding for science. At the same time, Ernest Lawrence's recent invention, the cyclotron, promised to revolutionise the experimental nuclear physics, and Chadwick felt that the Cavendish laboratory would fall behind unless it also acquired one. Chadwick therefore chafed under Rutherford, who clung to the belief that good nuclear physics could still be done without large, expensive items of equipment.

In March 1935, Chadwick received an offer of the Lyon Jones Chair of physics at the University of Liverpool, in Aileen's home town, in succession to Lionel Wilberforce. The university was something of a backwater, and the laboratory was so antiquated that it still ran on direct current, but Chadwick seized the opportunity, assuming the chair on 1 October 1935. The university's prestige was soon bolstered by Chadwick's Nobel Prize, which was announced in November 1935.

Chadwick set about acquiring a cyclotron for Liverpool. He started by spending £700 to refurbish the antiquated laboratories at Liverpool, so some components could be made in-house. He was able to persuade the University to provide £2,000 and obtained a grant for another £2,000 from the Royal Society. To build his cyclotron, Chadwick brought in two young experts, Bernard Kinsey and Harold Walke, who had worked with Lawrence at the University of California. A local cable manufacturer donated the copper conductor for the coils. The cyclotron's 50 ton magnet was manufactured in Trafford Park by Metropolitan-Vickers, which also made the vacuum chamber. The cyclotron was completely installed and running in July 1939. The total cost of £5,184 was more than Chadwick had received from the University and the Royal Society, so Chadwick paid the rest from his Nobel Prize money.

Tube Alloys
In January 1939, Meitner and her nephew Otto Frisch created an uproar with a paper that explained how uranium atoms, when bombarded by neutrons, broke into two roughly equal fragments, a process they called fission. They calculated that this would result in the release of about 200 MeV, implying an energy release orders of magnitude greater than chemical reactions. It was soon noted by Otto Hahn and Fritz Strassmann that if neutrons were released during fission, then a chain reaction was possible. French scientists, Pierre Joliot, Hans von Halban and Lew Kowarski, soon verified that more than one neutron was indeed emitted per fission. Neils Bohr theorised that fission was more likely to occur in the uranium-235 isotope, which made up only 0.7 percent of natural uranium. He published this in a paper co-authored with the American physicist John Wheeler.

Chadwick did not believe that there was any likelihood of another war with Germany in 1939, and took his family for a holiday on a remote lake in northern Sweden. The news of the outbreak of the Second World War therefore came as a shock. Determined not to spend another war in an internment camp, Chadwick made his way to Stockholm as fast as he could, but when he arrived there with his family, he found that all air traffic between Stockholm and London had been suspended. They made their way back to England on a tramp steamer. When he reached Liverpool, Chadwick found Joseph Rotblat, a Polish post-doctoral fellow who had come to work with the cyclotron, who was now destitute, as he was cut off from funds from Poland. Chadwick promptly hired Rotblat as a lecturer, despite his poor grasp of English.

In October 1939, Chadwick received a letter from Sir Edward Appleton the Secretary of the Department of Scientific and Industrial Research, asking for his opinion on the feasibility of an atomic bomb. Chadwick responded cautiously. He did not dismiss the possibility, but carefully went over the large number of theoretical and practical difficulties involved. Chadwick decided to investigate the properties of uranium oxide further with Rotblat. However, in March 1940, Otto Frisch and Rudolf Peierls at the University of Birmingham re-examined the theoretical issues involved in a paper that became known as the Frisch–Peierls memorandum. Instead of looking at uranium metal, they considered what would happen to a sphere of pure uranium-235, and found that not only could a chain reaction occur, but it might require as little as 1 kg of uranium-235, and unleash the energy of tons of dynamite.

As a result, a special subcommittee of the Committee for the Scientific Survey of Air Warfare (CSSAW) known as the MAUD Committee was created to investigated the matter further. It was chaired by Sir George Thomson and its original membership included Chadwick, along with Mark Oliphant, John Cockcroft and Philip Moon. While other teams investigated uranium enrichment techniques, Chadwick's team at Liverpool concentrated on determining the nuclear cross section of uranium-235. By April 1941, it had been experimentally confirmed that the critical mass of uranium-235 might be 8 kg or less. When George Pegram and Harold Urey visited Britain to see how the project, now known as Tube Alloys, was going, Chadwick was able to tell them: "I wish I could tell you that the bomb is not going to work, but I am 90 per cent sure that it will."

Manhattan Project
Owing to the danger from aerial bombardment, the Chadwicks sent Julie and Joanna to Canada as part of a government evacuation scheme. However, Chadwick was reluctant to move Tube Alloys thither, believing that the United Kingdom was a better location for the isotope separation plant, but as the enormous scope of the effort became more apparent in 1942, it became clear that even a pilot separation plant would cost over ₤1 million and strain Britain's resources, to say nothing of a full-scale plant, which was estimated to cost somewhere in the vicinity of £25 million, and would have to be built in America. But at the same time that the British became convinced that a joint project was necessary, the progress of the American Manhattan Project was such that British cooperation seemed less essential, although the Americans were eager to utilise Chadwick's talents.

The matter of cooperation had to be taken up at the highest level. In September 1943, Prime Minister Winston Churchill and President Franklin Roosevelt negotiated the Quebec Agreement, which reinstated cooperation between Britain, the United States and Canada. As a result, Chadwick, Oliphant, Peierls and Simon were summoned to the United States by the director of Tube Alloys, Sir Wallace Akers, to work with the Manhattan Project. The Quebec Agreement established a new Combined Policy Committee to direct the joint project, but the Americans disliked Akers, and so Chadwick was appointed technical advisor to the Combined Policy Committee, and the head of the British Mission.

Leaving Rotblat in charge in Liverpool, Chadwick began a tour of the Manhattan Project facilities in November 1943, except for the Hanford Site, which he was not allowed to see. Observing the work on the K-25 gaseous diffusion facility at Oak Ridge, Tennessee, he realised how wrong he had been about building the plant in wartime Britain. In early 1944, he moved to Los Alamos, New Mexico with Aileen and the twins, who now spoke with Canadian accents. For security reasons, he was given the cover name of James Chaffee.

Chadwick accepted that the Americans did not need British help, but that it could still be useful in bringing the project to an early and successful conclusion. Working closely with the director of the Manhattan Project, Major General Leslie R. Groves, Jr., he attempted to do everything he could to support the effort. He also endeavoured to place British scientists in as many parts of the project as possible in order to facilitate a post-war British project that Chadwick was committed to. Requests from Groves via Chadwick for particular scientists tended to be met with an immediate rejection by the company, ministry or university currently employing them, only to be overcome by the overriding priority accorded to Tube Alloys.

Although he had more knowledge about the project that anyone else from Britain, Chadwick had no access to the Hanford site. Lord Portal was offered a tour of Hanford in 1946. "This was the only plant to which Chadwick had been denied access in wartime, and now he asked Groves if he could accompany Portal. Groves replied that he could, but if he did then 'Portal will not see very much'." For his efforts, Chadwick received a knighthood in the New Years Honours on 1 January 1945. He considered this to be a recognition of the work of the whole Tube Alloys project.

By early 1945, Chadwick was spending most of his time in Washington, D.C., and his family relocated from Los Alamos to a house on Washington's Dupont Circle in April 1945. He was present at the meeting of the Combined Policy Committee on 4 July when Field Marshal Sir Henry Maitland Wilson gave Britain's agreement to use the atomic bomb against Japan, and at the Trinity nuclear test on 16 July, when the first atomic bomb was detonated. Inside its pit was a polonium-beryllium modulated neutron initiator, a development of the technique that Chadwick had used to discover the neutron over a decade before.

Later life
Shortly after the war ended, Chadwick was appointed to the Advisory Committee on Atomic Energy (ACAE). He also appointed the British scientific advisor to the United Nations Atomic Energy Commission. He clashed with fellow ACAE member Patrick Blackett, who disagreed with Chadwick's conviction that Britain needed to acquire its own nuclear weapons; but it was Chadwick's position that was ultimately adopted. He returned to Britain in 1946, to find a country still beset by wartime rationing and shortages.

In 1948, Chadwick accepted an offer to become the Master of Gonville and Caius College. The job was a prestigious but ill-defined one; the Master was the titular head of the College, but authority actually resided in a council of 13 fellows, of whom one was the Master. As Master, Chadwick strove to improve the academic reputation of the college. He increased the number of research fellowships from 31 to 49, and sought to bring talent into the college. This involved controversial decisions, such as hiring in 1951 the Chinese biochemist Tien-chin Tsao and the Hungarian-born economist Peter Bauer. Bauer was subsequently involved in what became known as the Peasants' Revolt, in which fellows led by Patrick Hadley voted an old friend of Chadwick's off the council and replaced him with the younger Bauer. More friends of Chadwick's were removed over the following years. He retired in November 1958.

Over the years, Chadwick received many honours, including the Medal for Merit from the United States, and Pour le Mérite from Germany. He was made a Companion of Honour in the New Year Honours on 1 January 1970 for "services to science", and went to Buckingham Palace for the investiture ceremony. He died in his sleep on 24 July 1974. His papers are held at the Churchill Archives Centre in Cambridge, and are accessible to the public.