James Chadwick

A wrong queue at Manchester

James Chadwick was born on 20 October 1891 in the small Cheshire village of Bollington, the eldest child of John Chadwick, a cotton spinner, and Anne Knowles, a domestic servant. The family had no money and almost no books. When his parents moved to Manchester for work in 1895 they left the boy with his grandparents, and he grew up quiet, careful, and so shy that he refused the small fees needed to attend Manchester Grammar School and went to a free city school instead. At sixteen he sat two scholarship exams on the same week and won both.

He arrived at the University of Manchester in 1908 intending to read mathematics. On registration day he joined the wrong queue, found himself signed up for physics, and was too shy to ask to be moved. The mistake handed him to Ernest Rutherford. By the end of his first year Chadwick had a Heginbottom Scholarship; by his fourth he had co-authored a paper with the loudest experimentalist in Europe and graduated with First Class Honours. Rutherford set him to compare two radioactive sources using a method Chadwick quickly realized was unworkable. He was, as one biographer put it, “afraid to tell Rutherford he was wrong,” so he kept his mouth shut and invented a better method on his own. That habit, of patient correction without fuss, defined his entire career.

A laboratory in the stables

In 1913 a Royal Commission for the 1851 Exhibition Scholarship sent him to Berlin to study beta radiation under Hans Geiger at the Physikalisch-Technische Reichsanstalt. Using Geiger’s new gas-filled counter he showed that beta rays did not come out in sharp spectral lines, as everyone had assumed, but in a smooth continuous spectrum. Albert Einstein, dropping in to Geiger’s lab, told him: “I can explain either of these things, but I can’t explain them both at the same time.” The continuous beta spectrum would baffle physics until Wolfgang Pauli postulated the neutrino in 1930 to balance the books.

Then in August 1914 the war broke out while Chadwick was still in Berlin. He was interned as an enemy alien at the Ruhleben camp on a former racetrack, where he spent the next four years living in a horse-box stable with five other prisoners. Otto Hahn, who would later co-discover nuclear fission, smuggled radioactive thorium and toothpaste through the wire to him. With improvised glassware Chadwick set up a tiny laboratory in the stables and, with the chemist Charles Drummond Ellis, ran experiments on the ionization of phosphorus and the photochemistry of carbon monoxide. He came out of Ruhleben in November 1918 thirty pounds lighter, with permanent digestive trouble, and with a stack of notebooks worth a doctorate.

Sir James Chadwick (20 October 1891 – 24 July 1974) was a British experimental physicist who received the Nobel Prize in Physics in 1935 for his discovery of the neutron. In 1941, he wrote the final draft of the MAUD Report, which inspired the U.S. government to begin serious atomic bomb research efforts. He was the head of the British team that worked on the Manhattan Project during World War II. He was knighted in Britain in 1945 for his achievements in nuclear physics.

Cambridge and a hunch

He went home to his parents in Manchester, picked up a teaching post under Rutherford, and followed his old chief to the Cavendish in Cambridge in 1919. By 1921 he had his PhD, by 1923 he was Rutherford’s assistant director of research, and for the next twelve years he was the quiet operational backbone of what was then the loudest physics laboratory in the world. He picked the PhD students, he edited the papers, he kept the bench running while Rutherford roared at the apparatus. He married Aileen Stewart-Brown in 1925 with Pyotr Kapitza as best man, and the couple had twin daughters, Joanna and Judith, in 1927.

What drove him through the 1920s was a hunch he and Rutherford had shared since 1920: that the nucleus contained a third particle, a neutral one, similar in mass to the proton but carrying no charge. The arithmetic of atomic masses demanded it. Nitrogen had a mass number of 14 but a nuclear charge of 7. If the nucleus were made of protons and electrons, as everyone then believed, nitrogen-14 would need fourteen protons and seven electrons, and the spins did not work out. A neutral heavy partner to the proton would resolve every puzzle. The trouble was that nobody could see it. A particle with no charge leaves no trail in a cloud chamber and triggers no detector directly. For more than a decade the neutron remained an article of faith between Rutherford and Chadwick, mentioned in Bakerian Lectures and dismissed everywhere else.

February 1932

The clue came from Germany and France. In 1930 Walther Bothe and Herbert Becker bombarded beryllium with alpha particles from polonium and found a strange neutral radiation coming out, which they assumed was an unusually energetic gamma ray. In January 1932 Frederic and Irene Joliot-Curie in Paris fired that beryllium radiation into paraffin wax and watched it knock protons out of the hydrogen with energies of several MeV. They published it as the Compton scattering of a 50 MeV gamma. Chadwick saw the paper at Cambridge, swore (a rare event), and rushed to Rutherford’s office. A gamma ray could not knock a proton that hard. A particle of about the proton’s own mass could. The Joliot-Curies had discovered the neutron and not realized it.

Chadwick dropped every other project. He set up a small chamber with a polonium source, a beryllium target, and a wax block. The recoiling protons went into an ionization detector wired to an oscilloscope. He worked through nights, with Norman Feather and Hugh Webster checking the counts. In about two weeks he had it. He applied conservation of energy and momentum to recoils off both hydrogen and nitrogen, got two equations in two unknowns, and solved for a neutral particle of mass roughly equal to the proton’s. On 17 February 1932 he sent a one-page letter to Nature titled “Possible Existence of a Neutron.” A fuller paper, “The Existence of a Neutron,” followed in May. The Hughes Medal came that autumn, the Nobel Prize in 1935. Niels Bohr and Werner Heisenberg quickly showed the neutron was not a proton-electron composite but a new fundamental particle, and the modern picture of the nucleus, with protons and neutrons held together by what would later be called the strong force, was born.

Liverpool and a cyclotron

In 1935 Chadwick left Cambridge. He had quarrelled with Rutherford over the cyclotron, the new American-style particle accelerator Ernest Lawrence was building at Berkeley. Rutherford clung to the view that good physics could still be done with desk-sized apparatus. Chadwick had seen the numbers and knew the future was megavolts. He took the Lyon Jones Chair at the University of Liverpool, found the laboratory still running on direct current, and set about building Britain’s first cyclotron. The £5,184 cost overran his grants, so he paid the difference out of his Nobel Prize money. The cyclotron started up in July 1939, six weeks before the war.

MAUD, Tube Alloys, Los Alamos

When the war began Chadwick was on holiday at a remote Swedish lake. He scrambled home through Stockholm and a tramp steamer, hired the destitute Polish refugee Joseph Rotblat into the Liverpool laboratory, and in October 1939 received the letter from Edward Appleton asking whether an atomic bomb was feasible. He answered carefully, neither yes nor no. Then the Frisch-Peierls memorandum landed in March 1940, with its terrifying estimate that one kilogram of pure uranium-235 might be enough, and the British nuclear effort began in earnest. The MAUD Committee was created; George Paget Thomson chaired it, and Chadwick was the senior experimentalist. In July 1941 it was Chadwick who wrote the final draft of the MAUD Report, the document that, on Vannevar Bush’s desk in October 1941, persuaded President Roosevelt to spend the millions that became the Manhattan Project.

He paid for it personally. Through the autumn of 1941 he could not sleep. He started on sleeping pills and would take them for the rest of his life. The German bombers came over Liverpool so often that the laboratory windows were replaced with cardboard. To visiting Americans George Pegram and Harold Urey, Chadwick said: “I wish I could tell you that the bomb is not going to work, but I am 90 per cent sure that it will.”

After the Quebec Agreement of September 1943 the British and American programs merged. Chadwick was made head of the British Mission and technical advisor to the Combined Policy Committee, and in early 1944 he and his family moved to Los Alamos under the cover name “James Chaffee.” Of every Briton on the project he was the one General Leslie Groves trusted most: the only outsider, besides Groves’s deputy, with access to every facility except the Hanford plutonium plant. He was at the meeting on 4 July 1945 when Britain agreed to the use of the bomb on Japan, and at the Trinity test on 16 July when it went off. Inside the bomb’s pit was a polonium-beryllium neutron initiator, an industrialized descendant of the apparatus he had built at Cambridge thirteen years earlier to discover the neutron. The New York Times reporter William Laurence wrote that “never before in history had any man lived to see his own discovery materialize itself with such telling effect on the destiny of man.”

Cambridge again

Chadwick was knighted in the 1945 New Year Honours. He returned to Britain in 1946 a haunted man. Sir James Mountford, the Vice Chancellor at Liverpool, wrote that he had never seen anyone “so physically, mentally and spiritually tired,” with “almost insupportable agonies of responsibility arising from his scientific work.” He served on the Advisory Committee on Atomic Energy and as the British scientific advisor to the United Nations Atomic Energy Commission, where he successfully argued, against Patrick Blackett, that Britain needed its own nuclear deterrent.

In 1948 he retreated to Cambridge to become Master of Gonville and Caius College, the post he had wanted for years. It was during his mastership that one of his graduate fellows, Francis Crick, together with James Watson and Rosalind Franklin, worked out the double-helix structure of DNA in the Cavendish across the road. He retired in 1958. A lifelong atheist who saw no reason to change his mind in old age, he died in his sleep on 24 July 1974 in Cambridge, aged eighty-two.

Chadwick's papers are held at the Churchill Archives Centre in Cambridge, and are accessible to the public. The Chadwick Laboratory at the University of Liverpool. Sir James Chadwick Chair of Experimental Physics, also at the University of Liverpool. Named in 1991 as part of celebrations of the centenary of his birth. James Chadwick Building, which houses part of the School of…

For the quantum story Chadwick is the man who closed the nucleus. After 1932 the atom had its full cast: nucleus of protons and neutrons surrounded by quantum-mechanical electrons, and physics finally had every piece it needed to write down the chemistry of the world.