James Chadwick

• 1. Introduction

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  In February 1932, after only about two weeks at the laboratory bench, James Chadwick sent a letter to the journal Nature with a deliberately tentative title: "Possible Existence of a Neutron." Three months later, the hedging was gone. His follow-up paper to the Proceedings of the Royal Society A was simply called "The Existence of a Neutron." In those few weeks of work, often late into the night, a British experimental physicist had found the particle that he and his old teacher had been chasing in theory for years.

  He was born in Cheshire in 1891, the son of a cotton spinner and a domestic servant. He almost did not become a physicist at all. As a student he meant to enroll in mathematics and signed up for physics by mistake. That accident would carry him from a German internment camp to a Nobel Prize, and eventually to a secret American mesa where his discovery was packed into the pit of the first atom bomb.

  How does a man who chafed at large, expensive machines end up at the heart of the most expensive science project in history? Why did the chargeless particle he found unlock both the treatment of cancer and the destruction of cities? And what did it cost the quiet, taciturn Chadwick to give Churchill the Bomb?

• 2. A Mistaken Enrollment

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  His family could not afford the small fees attached to a scholarship at Manchester Grammar School, so they turned the place down. The young Chadwick went instead to the Central Grammar School for Boys in Manchester. At 16 he sat two examinations for university scholarships and won both. He entered Victoria University of Manchester in 1908, intending to study mathematics, only to find himself enrolled in physics by mistake. Like most students, he lived at home and walked the 4 miles to the university and back each day.

  Ernest Rutherford headed the physics department, and he handed Chadwick a research problem: devise a way to compare the radioactive energy of two different sources. The idea was to measure them against the activity of 1 gram of radium, a unit that would come to be called the curie. Rutherford's suggested approach was unworkable. Chadwick knew it but was too afraid to say so to Rutherford, and pressed on until he found a method that worked. The result became his first paper, co-authored with Rutherford and published in 1912.

  He graduated with First Class Honours in 1911 and earned his M.Sc. in 1913, working out how to measure the absorption of gamma rays by various gases and liquids. That same year he won an 1851 Exhibition Scholarship, which sent him to study beta radiation under Hans Geiger at the Physikalisch-Technische Reichsanstalt in Berlin. There, using Geiger's recently developed counter, Chadwick showed that beta radiation did not produce discrete lines as had been thought, but a continuous spectrum with peaks in certain regions. On a visit to the laboratory, Albert Einstein told him: "I can explain either of these things, but I can't explain them both at the same time."

• 3. Toothpaste In The Stables

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  When the First World War broke out, Chadwick was still in Germany, and he was interned in the Ruhleben camp near Berlin for the next four years. He refused to let the camp end his work. He set up a laboratory in the stables and ran experiments using improvised materials, including radioactive toothpaste. With the help of Charles Drummond Ellis, he studied the ionisation of phosphorus and the photochemical reaction of carbon monoxide and chlorine.

  The Armistice came into effect in November 1918, and he was released and returned to his parents' home in Manchester. There he wrote up four years of findings for the 1851 Exhibition commissioners. Rutherford gave him a part-time teaching post at Manchester, where he measured the nuclear charge of platinum, silver, and copper, and found it matched the atomic number within an error of less than 1.5 per cent.

  In April 1919, Rutherford became director of the Cavendish Laboratory at the University of Cambridge, and Chadwick followed a few months later. Awarded a Clerk Maxwell Studentship in 1920, he enrolled as a Ph.D. student at Gonville and Caius College. His thesis joined his work on atomic numbers to a study of the forces inside the nucleus. He received his degree in June 1921, and that November became a Fellow of the college that would shape the last chapters of his life.

• 4. The Cavendish Years

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  When his studentship expired in 1923, Chadwick was succeeded by the Russian physicist Pyotr Kapitza, who would later stand as best man at his wedding. Sir William McCormick arranged for Chadwick to become Rutherford's assistant director of research, a role he held for over a decade. He helped Rutherford choose Ph.D. students, suggesting topics to those who had no idea what to research, and edited every paper the laboratory produced. The students he drew in included John Cockcroft, Norman Feather, and Mark Oliphant, who became firm friends.

  In 1925 Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker. The two married in August of that year, and in February 1927 they had twin daughters, Joanna and Judith. His domestic life settled even as the physics around him grew stranger.

  In 1925 the concept of spin let physicists explain the Zeeman effect, but it opened new anomalies. The nucleus was then thought to contain protons and electrons, so nitrogen, with a mass number of 14, was assumed to hold 14 protons and 7 electrons. That gave the right mass and charge but the wrong spin. At a 1928 Cambridge conference on beta particles and gamma rays, Chadwick met Geiger again, who brought an improved counter built with his student Walther Müller. The new Geiger-Müller counter detected alpha, beta, and gamma radiation alike, which made the lab's radium unsuitable for what Chadwick had in mind. Polonium, however, emits only alpha particles, and Lise Meitner sent him about 2 millicuries of it from Germany.

• 5. Two Weeks In 1932

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  In Germany, Walther Bothe and his student Herbert Becker had bombarded beryllium with alpha particles from polonium, producing an unusual radiation. Chadwick had his Australian 1851 Exhibition scholar, Hugh Webster, duplicate the result. To Chadwick this looked like evidence of something he and Rutherford had hypothesised for years: a nuclear particle with no electric charge. Then in January 1932, Feather pointed him to a result from Frederic and Irene Joliot-Curie, who had knocked protons from paraffin wax using polonium and beryllium and assumed the cause was gamma radiation. Rutherford and Chadwick disagreed, since protons were too heavy to be moved that way, while neutrons would need only a little energy. In Rome, Ettore Majorana reached the same verdict: the Joliot-Curies had found the neutron without knowing it.

  Chadwick dropped all his other duties to prove the particle existed, assisted by Feather and frequently working late at night. His apparatus was simple. A cylinder held a polonium source and a beryllium target, the radiation was directed at paraffin wax, and the displaced protons went into a small ionisation chamber where an oscilloscope detected them. Within about two weeks he had his answer.

  Reading the paper, Robert Bacher and Edward Condon saw that old puzzles dissolved if the neutron had a spin of one half, so a nitrogen nucleus could be seven protons and seven neutrons. Niels Bohr and Werner Heisenberg debated whether the neutron was a fundamental particle or a proton-electron pair, and Heisenberg argued it was best described as a new nuclear particle. In his 1933 Bakerian Lecture, Chadwick estimated the neutron's mass at about 1.0067, though conflicting results followed in 1933-4. The matter was settled when Maurice Goldhaber, a refugee from Nazi Germany, suggested photodisintegrating deuterons with the 2.6 MeV gamma rays of thorium C". Measuring the resulting proton's energy at 1.05 MeV, Chadwick and Goldhaber calculated the neutron's mass as either 1.0084 or 1.0090 atomic units, too large to be a proton-electron pair.

• 6. The Chargeless Key

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  For finding the neutron, Chadwick collected a remarkable run of honours: the Hughes Medal in 1932, the Nobel Prize in Physics in 1935, the Copley Medal in 1950, and the Franklin Medal in 1951. The Nobel citation read simply, "For the discovery of the neutron." His medal was later sold at auction in 2014 for 329,000 dollars.

  Because the neutron carries no charge, it does not need to overcome the Coulomb barrier that repels positively charged alpha particles. It can slip into the nuclei of even the heaviest elements, such as uranium. This made it possible to build elements heavier than uranium in the laboratory through slow-neutron capture followed by beta decay. The discovery inspired Enrico Fermi to study reactions from collisions of nuclei with slow neutrons, work that brought Fermi the Nobel Prize in 1938.

  The neutron also tied back to Chadwick's earliest research. On the 4th of December 1930, Wolfgang Pauli had proposed an undiscovered particle to explain the continuous beta spectrum Chadwick reported in 1914, since not all the energy could be accounted for and conservation of energy seemed violated. Pauli also called it a neutron, but it was not Chadwick's particle, and Fermi renamed it the neutrino, Italian for "little neutron." In 1934 Fermi proposed his theory of beta decay, in which a neutron decays into a proton, an electron, and a neutrino. Rudolf Peierls and Hans Bethe calculated that neutrinos could pass clean through the Earth, leaving little chance of detection until Frederick Reines and Clyde Cowan confirmthe particle on the 14th of June 1956.

• 7. The Liverpool Gamble

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  Chadwick was a critic of Big Science, and of Ernest Lawrence in particular, whose approach he thought careless and too focused on technology. At the 1933 Solvay Conference, when Lawrence proposed a new particle as a possible source of limitless energy, Chadwick replied that the result was more likely contamination of the equipment. Lawrence rechecked at Berkeley and found Chadwick right, while an investigation by Rutherford and Oliphant at the Cavendish showed that deuterium fuses to form helium-3, producing the effect Lawrence had seen.

  Yet Chadwick came to believe the Cavendish would fall behind without a cyclotron, Lawrence's invention. Rutherford, clinging to the conviction that good nuclear physics could be done without large, expensive equipment, turned down the request. In March 1935, Chadwick accepted the Lyon Jones Chair of Physics at the University of Liverpool, his wife's home town, succeeding Lionel Wilberforce. The laboratory was so antiquated it still ran on direct current. He took the chair on the 1st of October 1935, and his Nobel Prize was announced that November.

  Building the cyclotron took both ingenuity and his own money. He spent 700 pounds refurbishing the labs, secured 2,000 pounds from the university and another 2,000 from the Royal Society, and brought in Bernard Kinsey and Harold Walke, who had worked with Lawrence in California. A local cable maker donated the copper conductor, and the 50-ton magnet was built in Trafford Park by Metropolitan-Vickers. Installed and running by July 1939, the machine cost 5,184 pounds, more than his grants covered, so Chadwick paid the rest from his 8,243 pounds in Nobel Prize money. At Liverpool the Medicine and Science faculties worked closely, and in 1938 he joined a commission under Lord Derby on cancer treatment, anticipating that neutrons and isotopes from the 37-inch cyclotron might become a weapon in the fight against cancer.

• 8. The Bomb Years

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  Chadwick did not believe another war with Germany was likely in 1939, and he took his family to a remote lake in northern Sweden. The outbreak of war came as a shock. Determined never to spend another war interned, he raced to Stockholm, found all air traffic to London suspended, and brought his family home on a tramp steamer. Reaching Liverpool, he found Joseph Rotblat, a Polish postdoctoral fellow cut off from his funds, and promptly hired him as a lecturer despite his poor English.

  That October, Edward Appleton asked Chadwick whether an atomic bomb was feasible. He answered cautiously, neither dismissing it nor ignoring the difficulties. In March 1940, the Frisch-Peierls memorandum from Birmingham argued that a sphere of pure uranium-235, perhaps as little as 1 kilogram, could sustain a chain reaction and release the energy of tons of dynamite. The MAUD Committee, chaired by George Paget Thomson, took up the question, with Chadwick's Liverpool team determining the nuclear cross section of uranium-235. By April 1941, experiment suggested the critical mass might be 8 kilograms or less. Luftwaffe raids battered his lab so often that the windows were replaced with cardboard.

  In July 1941, Chadwick wrote the final draft of the MAUD Report. Presented by Vannevar Bush to President Franklin D. Roosevelt that October, it inspired the U.S. government to pour millions into the bomb. When George B. Pegram and Harold Urey visited Britain, Chadwick told 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." The strain was crushing. He could not sleep, turned to sleeping pills he would take for most of his remaining years, and later said he had realised "a nuclear bomb was not only possible, it was inevitable."

  The Quebec Agreement of September 1943 reinstated cooperation between Britain, the United States, and Canada, and Chadwick became technical advisor to the Combined Policy Committee and head of the British Mission. Touring Manhattan Project facilities from November 1943, he became the only man besides Leslie R. Groves and his deputy to reach all the American uranium-bomb sites, though the Hanford plutonium site stayed closed to him. Seeing the vast K-25 gaseous diffusion plant at Oak Ridge, he understood it could never have been hidden from the Luftwaffe in Britain. In early 1944 he moved to Los Alamos under the cover name James Chaffee, with twins who now spoke with Canadian accents. He earned the almost-complete trust of Groves, and his knighthood came in the New Year Honours on the 1st of January 1945. He watched the Trinity test on the 16th of July 1945, its pit holding a polonium-beryllium neutron initiator descended from the very technique he had used to find the neutron. A reporter wrote that never before had a man lived to see his own discovery materialise with such telling effect on the destiny of man.

  After the war Chadwick joined the Advisory Committee on Atomic Energy and served as British scientific advisor to the United Nations Atomic Energy Commission. He clashed with Patrick Blackett, who opposed his conviction that Britain needed its own nuclear weapons, and it was Chadwick's position that prevailed. Returning to Britain in 1946, he found a country still gripped by rationing and shortages. Sir James Mountford, the Vice Chancellor of Liverpool, wrote that he had never seen a man so physically, mentally, and spiritually tired, a man who had suffered almost insupportable agonies of responsibility from his scientific work.

  In September 1949, Edward Teller dined with the Chadwicks in Cambridge. Sir James was, as ever, taciturn, until Teller made an unflattering remark about Groves. At that Chadwick grew talkative, insisting the project would never have succeeded without Groves, even while admitting his dislike of the British.

  In 1948 he became Master of Gonville and Caius College, a prestigious but ill-defined post in which real authority lay with a council of 13 fellows. He raised the number of research fellowships from 31 to 49 and sought new talent, including the Chinese biochemist Tien-chin Tsao and the Hungarian-born economist Peter Bauer, hired in 1951. His choices stirred a revolt led by Patrick Hadley, dubbed the Peasants' Revolt, in which his allies were voted off the council. He retired in November 1958. During his mastership, Francis Crick, a Ph.D. student at the college, together with Rosalind Franklin and James Watson, worked out the structure of DNA.

  By the 1970s Chadwick was frail and seldom left his flat, though he travelled to Liverpool for his eightieth birthday celebrations. A lifelong atheist, he saw no reason to take up religion late in life. He died in his sleep on the 24th of July 1974 in Cambridge, at the age of 82. His papers rest at the Churchill Archives Centre, and a crater on the Moon carries his name.