Sir James Chadwick was born on October 20, 1891, and died on July 24, 1974. He was a British scientist who studied how matter works through experiments. In 1935, he was given the Nobel Prize in Physics for discovering the neutron, a particle found in the center of atoms. In 1941, he completed the final version of the MAUD Report, a document that influenced the United States to start researching atomic bombs. During World War II, he led the British team working on the Manhattan Project, a secret program to develop nuclear weapons. In 1945, he was honored by the British government with the title of knight for his work in nuclear physics.

Chadwick graduated from the Victoria University of Manchester in 1911. He studied under Ernest Rutherford, a scientist known as the "father of nuclear physics." At Manchester, he earned his Master of Science degree in 1913. That same year, he received an 1851 Research Fellowship from the Royal Commission for the Exhibition of 1851. He chose to study beta radiation, a type of radiation, under Hans Geiger in Berlin. Using Geiger’s new Geiger counter, a device that detects radiation, Chadwick showed that beta radiation creates a continuous range of energy levels, not separate lines as previously believed. When World War I began in 1914, Chadwick was in Germany and spent the next four years in the Ruhleben internment camp.

After the war, Chadwick joined Rutherford at the Cavendish Laboratory at the University of Cambridge. There, he earned his Doctor of Philosophy degree in 1921 under Rutherford’s guidance. He worked as Rutherford’s assistant director of research for over a decade at the Cavendish Laboratory, a leading center for physics research that attracted many students. Chadwick discovered the neutron and later measured its mass. He believed neutrons might one day help treat cancer. In 1935, he left the Cavendish Laboratory to become a professor of physics at the University of Liverpool. There, he modernized an old laboratory and built a cyclotron, a machine that helps study nuclear physics, making the university an important place for nuclear research.

Early life and education

James Chadwick was born on October 20, 1891, in Cheshire, England. He was the first child of John Joseph Chadwick, a cotton spinner, and Anne Mary Knowles, a domestic servant. He was named James after his grandfather on his father’s side. In 1895, his parents moved to Manchester, leaving him in the care of his maternal grandparents. He attended Bollington Cross Primary School and was offered a scholarship to Manchester Grammar School. However, his family could not afford the small fees required, so he attended the Central Grammar School for Boys in Manchester instead. He reunited with his parents there and had two younger brothers, Harry and Hubert. One of his sisters had died before he was born. At age 16, he took two university scholarship exams and won both.

Chadwick chose to attend Victoria University of Manchester, entering in 1908. He planned to study mathematics but enrolled in physics by mistake. Like most students, he lived at home and walked 4 miles (6.4 km) to the university daily. At the end of his first year, he received a Heginbottom Scholarship to study physics. The physics department was led by Ernest Rutherford, who assigned research projects to final-year students. Rutherford asked Chadwick to develop a method to compare the radioactive energy of two sources. The idea was to measure energy based on the activity of 1 gram (0.035 oz) of radium, a unit later called the curie. Rutherford’s suggested approach was not practical, but Chadwick did not tell him this. Instead, Chadwick created a working method, which became his first research paper. Co-authored with Rutherford, the paper was published in 1912. He graduated with First Class Honours in 1911.

After developing a way to measure gamma radiation, Chadwick studied how gamma rays were absorbed by gases and liquids. This research was published under his name alone. In 1912, he earned his M.Sc. and became a Beyer Fellow. The following year, he received an 1851 Exhibition Scholarship, allowing him to study in continental Europe. In 1913, he traveled to Berlin to study beta radiation under Hans Geiger. Using Geiger’s new Geiger counter, which was more accurate than earlier methods, Chadwick showed that beta radiation produced a continuous spectrum with peaks, not discrete lines as previously believed. During a visit to Geiger’s lab, Albert Einstein told Chadwick, “I can explain either of these things, but I can’t explain them both at the same time.” The continuous spectrum remained unexplained for many years.

Chadwick was in Germany when World War I began and was interned in the Ruhleben internment camp near Berlin. He was allowed to set up a laboratory in the stables and conduct experiments using materials like radioactive toothpaste. With help from Charles Drummond Ellis, he studied the ionization of phosphorus and the photochemical reaction of carbon monoxide and chlorine. He was released after the Armistice with Germany in November 1918 and returned to Manchester, where he wrote his findings for the 1851 Exhibition commissioners.

Rutherford gave Chadwick a part-time teaching position at Manchester, letting him continue research. He studied the nuclear charge of platinum, silver, and copper, finding it matched the atomic number within less than 1.5% error. In April 1919, Rutherford became director of the Cavendish Laboratory at the University of Cambridge, and Chadwick joined him there shortly after. In 1920, Chadwick received a Clerk Maxwell Studentship and enrolled as a Ph.D. student at Gonville and Caius College, Cambridge. His thesis covered atomic numbers in the first part and nuclear forces in the second. He earned his Ph.D. in June 1921 and became a Fellow of Gonville and Caius College in November.

Career and research

Chadwick's Clerk Maxwell Studentship ended in 1923, and he was replaced by the Russian physicist Pyotr Kapitza. Sir William McCormick, the Chairman of the Advisory Council of the Department of Scientific and Industrial Research, helped Chadwick become Rutherford's assistant director of research. In this role, Chadwick helped Rutherford choose Ph.D. students. Over the next few years, these students included John Cockcroft, Norman Feather, and Mark Oliphant, who became close friends with Chadwick. Many students did not know what research topic to choose, so Rutherford and Chadwick suggested ideas. Chadwick reviewed all the papers written by the laboratory.

In 1925, Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker. They married in August 1925, with Kapitza as the best man. The couple had twin daughters, Joanna and Judith, born in February 1927.

In his research, Chadwick studied the nucleus. In 1925, the idea of spin helped explain the Zeeman effect, but it also created problems. At the time, scientists believed the nucleus was made of protons and electrons. For example, nitrogen's nucleus, with a mass number of 14, was thought to have 14 protons and 7 electrons. This gave it the correct mass and charge, but the wrong spin.

At a conference in Cambridge in 1928, Chadwick met Geiger again. Geiger had brought a new version of his Geiger counter, improved by his student, Walther Müller. Chadwick had not used one since the war, and the new Geiger–Müller counter could be better than the methods used at Cambridge, which relied on human eyes to observe results. However, the counter detected alpha, beta, and gamma radiation, and radium, which the Cavendish Laboratory used, emitted all three, making it unsuitable for Chadwick's work. Polonium, which emits alpha particles, was useful, and Lise Meitner sent Chadwick about 2 millicuries (about 0.5 micrograms) from Germany.

When the Great Depression began in the United Kingdom, the government spent less money on science. At the same time, Lawrence's invention, the cyclotron, promised to change nuclear physics. Chadwick believed the Cavendish Laboratory would fall behind if it did not get one. He became frustrated with Rutherford, who believed good nuclear physics could still be done without expensive equipment, and refused to get a cyclotron.

Chadwick criticized large-scale science projects, especially those led by Lawrence, whom he thought focused too much on technology instead of science. When Lawrence claimed to have discovered a new particle that could provide endless energy at the Solvay Conference in 1933, Chadwick suggested the results might be due to equipment contamination. Lawrence later confirmed Chadwick was correct, while Rutherford and Oliphant found that deuterium fuses to form helium-3, causing the effect Lawrence observed. This was a major discovery, but the Oliphant–Rutherford particle accelerator was an expensive, advanced piece of equipment.

In March 1935, Chadwick received an offer for the Lyon Jones Chair of Physics at the University of Liverpool, his wife's hometown, to replace Lionel Wilberforce. The laboratory was old and outdated, still using direct current electricity, but Chadwick accepted the position, starting on 1 October 1935. The university's reputation grew after Chadwick won the Nobel Prize in November 1935. His medal was sold at auction in 2014 for $329,000.

Chadwick worked to get a cyclotron for Liverpool. He spent £700 to upgrade the old labs so some parts could be built on-site. He convinced the university to provide £2,000 and got another £2,000 from the Royal Society. To build the cyclotron, Chadwick hired two young experts, Bernard Kinsey and Harold Walke, who had worked with Lawrence at the University of California. A local cable company donated copper for the coils. The cyclotron's 50-ton magnet was made by Metropolitan-Vickers in Trafford Park, which also built the vacuum chamber. The cyclotron was fully installed and working by July 1939. The total cost of £5,184 came from Chadwick's Nobel Prize money, as the university and Royal Society provided less than needed.

At Liverpool, the Medicine and Science faculties worked closely. Chadwick was automatically on the committees of both, and in 1938, he joined a commission led by Lord Derby to study cancer treatment in Liverpool. Chadwick believed neutrons and radioactive isotopes from the 37-inch cyclotron could help study biochemical processes and might be used to fight cancer.

Discovery of the neutron

In Germany, Walther Bothe and his student, Herbert Becker, used polonium to shoot alpha particles at beryllium, creating a strange kind of radiation. Chadwick asked his Australian scholar, Hugh Webster, to repeat their experiment. This helped Chadwick and Rutherford confirm their idea about a particle called the neutron, which has no electric charge. In January 1932, Feather told Chadwick about another experiment by Frédéric and Irène Joliot-Curie. They used polonium and beryllium to knock protons out of paraffin wax, thinking the radiation was gamma rays. Rutherford and Chadwick disagreed because protons are too heavy for gamma rays to move. Neutrons, however, need little energy to do this. Ettore Majorana in Rome also concluded that the Joliot-Curies had discovered the neutron but did not realize it.

Chadwick focused on proving the neutron’s existence, working with Feather and often staying late. He built a device with a polonium source and beryllium target. The radiation from this was aimed at materials like paraffin wax, knocking out protons. These protons were detected in an ionization chamber using an oscilloscope. In February 1932, Chadwick wrote to Nature about the neutron. He later detailed his findings in Proceedings of the Royal Society A. His discovery helped scientists better understand the nucleus. Robert Bacher and Edward Condon read his paper and realized that problems in nuclear theory, such as nitrogen’s spin, could be explained if the neutron had a spin of 1/2 and if a nitrogen nucleus had seven protons and seven neutrons.

Niels Bohr and Werner Heisenberg debated whether the neutron was a basic nuclear particle like the proton and electron or a combination of a proton and electron. Heisenberg said the neutron was a new nuclear particle, but its exact nature was unclear. In his 1933 Bakerian Lecture, Chadwick estimated the neutron’s mass as about 1.0067 Da. This suggested the neutron might be a proton and electron with a binding energy of about 2 MeV. However, measuring such a small mass difference was difficult, and conflicting results arose in 1933–4. The Joliot-Curies found a large neutron mass, while Ernest Lawrence’s team found a small one. Maurice Goldhaber, a refugee from Nazi Germany, suggested using deuterons and gamma rays from thorium to measure the neutron’s mass. Chadwick and Goldhaber tested this and calculated the neutron’s mass as either 1.0084 or 1.0090 atomic units. The modern accepted value is 1.00866 Da, proving the neutron was too heavy to be a proton-electron pair.

For discovering the neutron, Chadwick received the Hughes Medal in 1932, the Nobel Prize in Physics in 1935, the Copley Medal in 1950, and the Franklin Medal in 1951. His discovery allowed scientists to create elements heavier than uranium in labs by capturing slow neutrons and using beta decay. Unlike alpha particles, which are repelled by atomic nuclei, neutrons can enter nuclei easily, inspiring Enrico Fermi’s work on nuclear reactions with slow neutrons, for which Fermi won the Nobel Prize in 1938.

In 1930, Wolfgang Pauli proposed a particle to explain the energy missing in beta radiation. He called it a neutron, but it was different from Chadwick’s neutron. Fermi later renamed it the neutrino, meaning “little neutron.” In 1934, Fermi explained beta decay as the splitting of a neutron into a proton, electron, and neutrino. The neutrino carried the missing energy, but its tiny mass and lack of charge made it hard to detect. Rudolf Peierls and Hans Bethe calculated that neutrinos could pass through Earth easily, making them hard to find. Frederick Reines and Clyde Cowan confirmed the neutrino in 1956 using a detector near a nuclear reactor.

Second World War

During the Second World War, Chadwick worked on the Tube Alloys project to develop an atom bomb. His laboratory in Manchester and the surrounding area faced bombing attacks from the Luftwaffe. When the Quebec Agreement combined his project with the American Manhattan Project, Chadwick became part of the British Mission. He worked at the Los Alamos Laboratory and in Washington, D.C. He earned the trust of project director Leslie R. Groves, Jr., which surprised many people. For his work, Chadwick was honored with a knighthood in the New Year Honours on January 1, 1945. In July 1945, he observed the Trinity nuclear test. Later, he served as the British scientific advisor to the United Nations Atomic Energy Commission. He preferred smaller scientific projects over large-scale efforts and returned to Cambridge, where he became Master of Gonville and Caius College in 1948.

In Germany, scientists Otto Hahn and Fritz Strassmann bombarded uranium with neutrons and discovered that barium, a lighter element, was among the products formed. Previously, only the same or heavier elements were produced by this process. In January 1939, Lise Meitner and her nephew Otto Frisch published a paper explaining the result. They proposed that uranium atoms bombarded with neutrons could split into two roughly equal parts, a process they called fission. They calculated that this would release about 200 MeV of energy, far greater than chemical reactions. Frisch confirmed their theory through experiments. Soon, Hahn noted that if neutrons were released during fission, a chain reaction could occur. French scientists Pierre Joliot, Hans von Halban, and Lew Kowarski confirmed that more than one neutron was emitted per fission. In a paper co-authored with American physicist John Wheeler, Niels Bohr theorized that fission was more likely to occur in uranium-235, which made up only 0.7% of natural uranium.

In 1939, Chadwick believed there was no chance of another war with Germany and took his family on a holiday in northern Sweden. He was shocked to learn that World War II had begun. To avoid being interned, he traveled to Stockholm but found air travel between Stockholm and London had stopped. He and his family returned to England on a tramp steamer. When he arrived in Liverpool, he found Joseph Rotblat, a Polish scientist, had no money because he could not access funds from Poland. Chadwick hired Rotblat as a lecturer, even though Rotblat had limited English skills.

In October 1939, Chadwick received a letter from Edward Appleton, the Secretary of the Department of Scientific and Industrial Research, asking about the possibility of building an atomic bomb. Chadwick responded cautiously, acknowledging the theoretical and practical challenges involved. He decided to study uranium oxide further with Rotblat. In March 1940, Otto Frisch and Rudolf Peierls at the University of Birmingham published the Frisch–Peierls memorandum, which suggested that a chain reaction could occur with a small amount of pure uranium-235, potentially releasing energy equivalent to tons of dynamite.

A special group called the MAUD Committee was formed to investigate the topic further. It was led by George Paget Thomson and included Chadwick, Mark Oliphant, John Cockcroft, and Philip Moon. While other teams studied uranium enrichment, Chadwick’s team in Liverpool focused on measuring the nuclear cross section of uranium-235. By April 1941, experiments confirmed that the critical mass of uranium-235 might be 8 kilograms or less. His work was complicated by frequent Luftwaffe bombings near his lab in Liverpool, which damaged windows so often they were replaced with cardboard.

In July 1941, Chadwick was asked to write the final version of the MAUD Report. When presented to President Franklin D. Roosevelt in October 1941, the report inspired the U.S. government to invest millions in developing an atom bomb. When American scientists George B. Pegram and Harold Urey visited Britain to assess the Tube Alloys project, Chadwick told them, “I wish I could tell you the bomb won’t work, but I am 90% sure it will.”

In a recent book about the Bomb project, Graham Farmelo wrote that “Chadwick did more than any other scientist to give Churchill the Bomb. … Chadwick was tested almost to the breaking point.” He struggled with stress, taking sleeping pills for most of his life. Later, Chadwick said he realized “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.” Sir Hermann Bondi suggested it was fortunate that Chadwick, not Rutherford, was the leading scientist in UK physics at the time, as Rutherford’s influence might have overshadowed Chadwick’s interest in the Bomb’s future.

Due to the danger of aerial attacks, the Chadwicks sent their twins to Canada as part of a government evacuation program. Chadwick was reluctant to move the Tube Alloys project to Canada, believing the UK was a better location for the isotope separation plant. By 1942, the project’s scale became clear: even a small pilot plant would cost over £1 million and strain Britain’s resources. A full-scale plant was estimated to cost £25 million, so it had to be built in America. At the same time, the U.S. Manhattan Project was making rapid progress, making British cooperation less essential, though Americans still valued Chadwick’s expertise.

Cooperation required high-level discussions. In September 1943, Prime Minister Winston Churchill and President Roosevelt signed the Quebec Agreement, restoring collaboration between Britain, the U.S., and Canada. Chadwick, Oliphant, Peierls, and Simon were sent to the U.S. by Sir Wallace Akers, the director of Tube Alloys, to work with the Manhattan Project. The Quebec Agreement created a Combined Policy Committee to oversee the joint effort. The Americans disliked Akers, so Chadwick was appointed technical advisor to the committee and head of the British Mission.

Leaving Rotblat in charge in Liverpool, Chadwick toured Manhattan Project facilities in November 1943, except for the Hanford Site, where plutonium was produced, which he was not allowed to visit. He became the only person, aside from Groves and his second-in-command, to access all American research and production facilities for the uranium bomb. Observing the K-25 gaseous diffusion facility at Oak Ridge, Tennessee, Chadwick realized his earlier belief that the plant could be built in wartime Britain was incorrect. The massive structure could not have been hidden from the Luftwaffe. In early 1944, he moved to Los Alamos, New Mexico.

Later life

After the war ended, Chadwick was chosen to join the Advisory Committee on Atomic Energy (ACAE). He also became the British scientific advisor to the United Nations Atomic Energy Commission. He had disagreements with another ACAE member, Patrick Blackett, who did not agree with Chadwick’s belief that Britain should develop its own nuclear weapons. However, Chadwick’s view was the one that was finally accepted. He returned to Britain in 1946, where the country was still dealing with wartime shortages and limited supplies.

At that time, Sir James Mountford, the Vice Chancellor of the University of Liverpool, wrote in his diary that he had never seen a person as physically, mentally, and spiritually exhausted as Chadwick. He described how Chadwick had faced very difficult moral choices and carried extremely heavy burdens from his scientific work.

In September 1949, Edward Teller visited England to discuss nuclear power and safety. During a dinner at the home of Sir James Chadwick and his wife in Cambridge, Teller made a negative comment about Groves. This caused Chadwick to speak more openly, explaining that the project would not have succeeded without Groves, even though he disliked the British.

In 1948, Chadwick accepted a position as Master of Gonville and Caius College, Cambridge. This role was important but not clearly defined, as the Master was the official head of the college, but real authority belonged to a group of 13 fellows, one of whom was the Master. During his time as Master, Chadwick worked to improve the college’s academic reputation. He increased the number of research fellowships from 31 to 49 and aimed to attract talented individuals. This included hiring the Chinese biochemist Tien-chin Tsao and the Hungarian-born economist Peter Bauer in 1951. These choices led to a disagreement among the college’s fellows, known as the Peasants’ Revolt, where some members removed a friend of Chadwick’s from the council and replaced him with Bauer. Over the next few years, more of Chadwick’s friends were removed from the council, and he retired in November 1958. During his leadership, Francis Crick, a Ph.D. student at Gonville and Caius College, along with Rosalind Franklin and James Watson, discovered the structure of DNA.

By the 1970s, Chadwick became weaker and rarely left his home, though he traveled to Liverpool for celebrations of his eightieth birthday. He remained an atheist throughout his life and did not adopt religious beliefs later in life. He died peacefully in his sleep on July 24, 1974, in Cambridge at the age of 82.

Commemoration

• James Chadwick's papers are stored at the Churchill Archives Centre in Cambridge and can be viewed by the public.
• The Chadwick Laboratory is located at the University of Liverpool.
• The Sir James Chadwick Chair of Experimental Physics is also at the University of Liverpool. It was named in 1991 to celebrate the 100th anniversary of his birth.
• The James Chadwick Building is home to part of the School of Chemical Engineering and Analytical Sciences at the University of Manchester, as well as the Industrial Hub for Sustainable Engineering.
• James Chadwick was described by Lorna Arnold, an official historian with the United Kingdom Atomic Energy Authority, as "a physicist, a scientist who worked in diplomacy, and a kind, thoughtful, and compassionate person."
• A crater on the Moon is named Chadwick.