John Cockcroft
Sir John Douglas Cockcroft was a British experimental physicist who shared the 1951 Nobel Prize in Physics with Ernest Walton for their splitting of the atomic nucleus, which led to the development of nuclear power and weapons.
After service on the Western Front with the Royal Field Artillery during the Great War, Cockcroft studied electrical engineering at Manchester Municipal College of Technology whilst he was an apprentice at Metropolitan Vickers Trafford Park and was also a member of their research staff. He then won a scholarship to St John's College, Cambridge, where he sat the Tripos exam in June 1924, becoming a Wrangler. Ernest Rutherford accepted Cockcroft as a research student in the Cavendish Laboratory, and Cockcroft completed his doctorate under Rutherford's supervision in 1928. With Walton and Mark Oliphant, he built what became known as a Cockcroft–Walton generator. Cockcroft and Walton used this to perform the first artificial disintegration of an atomic nucleus, a feat popularly known as "splitting the atom."
During the Second World War, Cockcroft became assistant director of scientific research in the Ministry of Supply, working on radar. He was also a member of the committee formed to handle issues arising from the Frisch–Peierls memorandum, which calculated that an atomic bomb could be technically feasible, and of the MAUD Committee which succeeded it. In 1940, as part of the Tizard Mission, he shared British technology with his counterparts in the United States. Later in the war, the fruits of the Tizard Mission came back to Britain in the form of the SCR-584 radar set and the proximity fuze, which were used to help defeat the V-1 flying bomb. In May 1944, he became director of the Montreal Laboratory, and oversaw the development of the ZEEP and NRX reactors, and the creation of the Chalk River Laboratories.
After the war Cockcroft became the director of the Atomic Energy Research Establishment at Harwell, where the low-powered, graphite-moderated GLEEP became the first nuclear reactor to operate in western Europe when it was started on 15 August 1947. This was followed by the British Experimental Pile 0 in 1948. Harwell was involved in the design of the reactors and the chemical separation plant at Windscale. Under his direction it took part in frontier fusion research, including the ZETA program. His insistence that the chimney stacks of the Windscale reactors be fitted with filters was mocked as Cockcroft's Folly until the core of one of the reactors ignited and released radionuclides during the Windscale fire of 1957.
From 1959 to 1967, he was the first master of Churchill College, Cambridge. He was also the chancellor of the Australian National University in Canberra from 1961 to 1965.
Early life and education
John Douglas Cockcroft was born on 27 May 1897 in Todmorden, England, the eldest son of John Arthur Cockcroft, a mill owner, and Annie Maude Fielden. He had four younger brothers; Eric, Philip, Keith and Lionel. His early education was at the Church of England school in Walsden from 1901 to 1908, at Todmorden Elementary School from 1908 to 1909, and at Todmorden Secondary School from 1909 to 1914, where he played football and cricket. Among the girls at the school was his future wife, Eunice Elizabeth Crabtree. In 1914, he won a County Major Scholarship, West Riding of Yorkshire, to the Victoria University of Manchester, where he studied mathematics.The Great War broke out in August 1914. Cockcroft completed his first year at Manchester in June 1915. He joined the Officers' Training Corps there, but did not wish to become an officer. During the summer break he worked at a YMCA canteen at Kinmel Camp in Wales. He enlisted in the British Army on 24 November 1915. On 29 March 1916, he joined the 59th Training Brigade, Royal Field Artillery, where he was trained as a signaller. He was then posted to B Battery, 92nd Field Artillery Brigade, one of the units of the 20th Division, on the Western Front.
Cockcroft participated in the Advance to the Hindenburg Line and the Third Battle of Ypres. He applied for a commission, and was accepted. He was sent to Brighton in February 1918 to learn about gunnery, and in April 1918, to the Officer Candidate School in Weedon Bec in Northamptonshire, where he was trained as a field artillery officer. He was commissioned as a lieutenant in the Royal Field Artillery on 17 October 1918.
After the war ended, Cockcroft was released from the Army in January 1919. He elected not to return to the Victoria University of Manchester, but to study electrical engineering at Manchester Municipal College of Technology. Because he had completed a year at Victoria University of Manchester, he was allowed to skip the first year of the course. He received his B.Sc. in June 1920. Miles Walker, the professor of electrical engineering there, persuaded him to take up an apprenticeship with Metropolitan Vickers. He obtained a 1851 Exhibition Scholarship from the Royal Commission for the Exhibition of 1851, and submitted his M.Sc. thesis, Harmonic Analysis for Alternating Currents, in June 1922.
Walker then suggested Cockcroft sit for a scholarship to St John's College, Cambridge, Walker's alma mater. Cockcroft was successful, winning a £30 scholarship and a £20 bursary awarded to undergraduates of limited means. Metropolitan Vickers gave him £50 subject to his returning after completing his degree. Walker and an aunt made up the balance of the £316 fee. As a graduate of another university, he was allowed to skip the first year of the tripos. He sat the tripos exam in June 1924, achieved a B* as a wrangler, and was awarded his B.A. degree.
Cockcroft married Elizabeth Crabtree on 26 August 1925, in a ceremony at the Bridge Street United Methodist Church in Todmorden. They had six children. The first, a boy known as Timothy, died in infancy. They subsequently had four daughters, Joan Dorothea, Jocelyn, Elisabeth Fielden and Catherine Helena; and another son, Christopher Hugh John.
Research
accepted Cockcroft as a research student in the Cavendish Laboratory on the recommendation of Miles Walker and the director of research at Metropolitan Vickers. Cockcroft enrolled as a Ph.D. student in 1924, with a Foundation Scholarship of St John's College, and a State Scholarship. Under Rutherford's supervision, he wrote his doctoral thesis, On phenomena occurring in the condensation of molecular streams on surfaces, which was published in the Proceedings of the Royal Society. He was awarded his doctorate on 6 September 1928. During this time he was an assistant to the Russian physicist Peter Kapitza, who was working on the physics of magnetic fields in extremely low temperatures. Cockcroft helped with the design and construction of helium liquefiers.In 1919, Rutherford had succeeded in disintegrating nitrogen atoms with alpha particles emitted from decaying radium atoms. This and subsequent experiments hinted at the structure of atomic nuclei. To explore it further, he needed an artificial means of creating particles with a velocity high enough to overcome the charge of the nucleus. This opened a new line of research at the Cavendish Laboratory. He assigned Cockcroft, Thomas Allibone and Ernest Walton to the problem. They built what became known as a Cockcroft–Walton accelerator. Mark Oliphant designed a proton source for them. A crucial moment came when Cockcroft read a paper by George Gamow on quantum tunnelling. Cockcroft realised that as a result of this phenomenon, the desired effect could be achieved with much lower voltages than first thought. In fact, he calculated that protons with energy of just 300,000 electronvolts would be able to penetrate a boron nucleus. Cockcroft and Walton worked on their accelerator for the next two years. Rutherford obtained a £1,000 grant from the University of Cambridge for them to buy a transformer and other equipment they needed.
Cockcroft was elected a Fellow of St John's College on 5 November 1928. He and Walton began operating their accelerator in March 1932, bombarding lithium and beryllium with high-energy protons. They expected to see gamma rays, which French scientists had reported, but none were found. In February 1932, James Chadwick demonstrated that what had been observed were actually neutrons. Cockcroft and Walton then switched to looking for alpha particles instead. On 14 April 1932, Walton bombarded a lithium target and noticed what he thought might be alpha particles. Cockcroft and then Rutherford were summoned, and confirmed that this was indeed the case. That evening, Cockcroft and Walton met at Rutherford's home and produced a letter for Nature in which they announced their results, the first artificial disintegration of an atomic nucleus, which can be described thus:
This feat was popularly known as "splitting the atom." For this accomplishment, Cockcroft and Walton were awarded the Hughes Medal in 1938, and the Nobel Prize in Physics in 1951. They went on to disintegrate carbon, nitrogen and oxygen using protons, deuterons and alpha particles. They demonstrated that they had produced radioactive isotopes, including carbon-11 and nitrogen-13.
In 1929, Cockcroft was appointed a Supervisor in Mechanical Sciences at St John's College. He was appointed a Supervisor in Physics in 1931, and in 1933 became the junior bursar, making him responsible for the upkeep of the buildings, many of which were suffering from neglect. The college gatehouse had to be partly taken down in order to repair damage done by deathwatch beetles, and Cockcroft supervised rewiring of the electrics. In 1935, Rutherford appointed him the director of research at the Mond Laboratory after Kapitza, who had returned to the Soviet Union. He supervised the installation of new cryogenic equipment, and supervised low temperature research. He was elected a Fellow of the Royal Society in 1936, and in 1939 was appointed Jacksonian Professor of Natural Philosophy.
Cockcroft and Walton were well aware of the limits of their accelerator. A much better design had been developed in the United States by Ernest Lawrence, which he called the cyclotron. The Cavendish Laboratory was able to keep ahead of the Americans despite having an inferior accelerator with clever physics, but Cockcroft pressed Rutherford to obtain a cyclotron for the Cavendish Laboratory. Rutherford baulked at the price tag, but a £250,000 gift from Lord Austin enabled a cyclotron, based on Lawrence's design, to be built, along with a new wing to house it. Cockcroft supervised the work. The cyclotron was in operation by October 1938, and the new wing was completed in 1940. Oliphant felt that the cyclotron was not big enough, and commenced construction of a larger 60-inch cyclotron at the University of Birmingham. Its construction was delayed by the outbreak of the Second World War in Europe in 1939, and it too would be obsolescent when it was completed after the war.