James Chadwick
Sir James Chadwick 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.
Chadwick graduated from the Victoria University of Manchester in 1911, where he studied under Ernest Rutherford. At Manchester, he continued to study under Rutherford until he was awarded his MSc in 1913. The same year, Chadwick was awarded an 1851 Research Fellowship from the Royal Commission for the Exhibition of 1851. He elected to study beta radiation under Hans Geiger in Berlin. Using Geiger's recently developed Geiger counter, Chadwick was able to demonstrate that beta radiation produced a continuous spectrum, and not discrete lines as had been thought. Still in Germany when World War I broke out in Europe, he spent the next four years 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 under Rutherford's supervision from Gonville and Caius College, Cambridge, in June 1921. 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. Chadwick followed his discovery of the neutron by measuring its mass. He anticipated that neutrons would become a major weapon in the fight against cancer. 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.
Early life and education
James Chadwick was born on 20 October 1891 in Cheshire, England, the first child of John Joseph Chadwick, a cotton spinner, and Anne Mary Knowles, a domestic servant. He was named James after his paternal grandfather. In 1895, his parents moved to Manchester, leaving him in the care of his maternal 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 two examinations for university scholarships, and won both of them.Chadwick chose to attend Victoria University of Manchester, which he entered in 1908. He meant to study mathematics, but enrolled in physics by mistake. Like most students, he lived at home, walking the 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 of radium, a unit of measurement which would become known as the curie. Rutherford's suggested approach was unworkable—something Chadwick knew but was afraid to tell Rutherford—so Chadwick pressed on, 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 alone. He was awarded his M.Sc. 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. In 1913, he chose to go to the Physikalisch-Technische Reichsanstalt in Berlin 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 did not produce discrete lines, as has been previously thought, but rather a continuous spectrum with peaks in certain regions. On a visit to Geiger's laboratory, Albert Einstein told Chadwick that: "I can explain either of these things, but I can't explain them both at the same time." The continuous spectrum would remain an unexplained phenomenon for many years.
Chadwick was still in Germany at the start of the First World War, 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 Drummond 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 per cent. 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 Ph.D. 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. He was awarded his degree in June 1921, and became a Fellow of Gonville and Caius College in November.
Career and research
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, he helped Rutherford select Ph.D. 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. He edited all the papers produced by the laboratory.File:The Cavendish Laboratory - geograph.org.uk - 631839.jpg|thumb|left|The original building of the Cavendish Laboratory was the home of some of the great discoveries in physics. It was founded in 1874 by the Duke of Devonshire, and its first professor was James Clerk Maxwell. The Laboratory has since moved to West Cambridge.
In 1925, Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker. The two were married in August 1925, with Kapitza as best man. The couple had twin daughters, Joanna and Judith, who were born in February 1927.
In his research, Chadwick continued to probe the 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's nucleus, 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. Geiger had brought with him a new model of his Geiger counter, which had been improved by his student, Walther Müller. Chadwick had not used one since the war, and the new Geiger–Müller counter was potentially a major improvement over the scintillation techniques then in use at Cambridge, 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.
Liverpool
With the onset of the Great Depression in the United Kingdom, the government became more parsimonious with funding for science. At the same time, Lawrence's recent invention, the cyclotron, promised to revolutionise experimental nuclear physics, and Chadwick felt that the Cavendish Laboratory would fall behind unless it also acquired one. He therefore chafed under Rutherford, who clung to the belief that good nuclear physics could still be done without large, expensive equipment, and turned down the request for a cyclotron.File:Victoria Clock Tower, Liverpool University - geograph.org.uk - 374422.jpg|thumb|left|upright|"Red brick" Victoria Building at the University of Liverpool.
Chadwick was a critic of Big Science in general, and Lawrence in particular, whose approach he considered careless and focused on technology at the expense of science. When Lawrence proposed the existence of a new and hitherto unknown particle that he claimed was a possible source of limitless energy at the Solvay Conference in 1933, Chadwick responded that the results were more likely attributable to contamination of the equipment. While Lawrence rechecked his results at Berkeley only to find that Chadwick was correct, Rutherford and Oliphant conducted an investigation at the Cavendish that found that deuterium fuses to form helium-3, thereby causing the effect that Lawrence had observed. This was another major discovery, but the Oliphant–Rutherford particle accelerator was an expensive state-of-the-art piece of equipment.
In March 1935, Chadwick received an offer of the Lyon Jones Chair of Physics at the University of Liverpool, in his wife's home town, to succeed Lionel Wilberforce. The laboratory was so antiquated that it still ran on direct current electricity, 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. His medal was sold at auction in 2014 for $329,000.
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.
At Liverpool, the Medicine and Science faculties worked together closely. Chadwick was automatically a committee member of both faculties, and in 1938 he was appointed to a commission headed by Lord Derby to investigate the arrangements for cancer treatment in Liverpool. Chadwick anticipated that neutrons and radioactive isotopes produced with the 37-inch cyclotron could be used to study biochemical processes, and might become a weapon in the fight against cancer.