James Chadwick

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James Chadwick

Chadwick.jpg
Born

October 20 1891(1891-10-20)
Cheshire, England

Died July 24 1974 (aged 82)

Cambridge, England

Citizenship United Kingdom
Field Physics
Institutions Technical University of Berlin
Liverpool University
Gonville and Caius College
Cambridge University
Alma mater Victoria University of Manchester
University of Cambridge.
Known for Discovering the neutron and being a member of the Manhattan Project
Notable prizes Nobel prize medal.svg Nobel Prize in Physics (1935)

Sir James Chadwick, CH (October 20, 1891 – July 24, 1974) was an English physicist and Nobel laureate who is best known for discovering the neutron. This discovery transformed subatomic physics and enabled scientists to produce new elements. It also led to the discovery of nuclear fission and its uses for both war and peace.

Contents

Chadwick was the leader of the British technical team that helped the United States develop the atomic bomb during World War II.

Biography

James Chadwick was born in Bollington, Cheshire, England on October 20, 1891, the son of John Joseph Chadwick and Anne Mary Knowles. He went to Bollington Cross C of E Primary School, and attended Manchester Municipal Secondary School. When he was sixteen, he won a scholarship to Manchester University. He at first intended to study mathematics, but mistakenly attended a physics orientation, and enrolled in physics instead. He at first had misgivings about his decision, but after the first year, he found the course work more interesting. He was enrolled in Ernest Rutherford's classes on electricity and magnetism, and Rutherford later assigned Chadwick a research project on the radioactive element radium.[1]

Early Research

Chadwick graduated in 1911, and continued to work under Rutherford on gamma ray (high energy x-ray) absorption, earning a masters degree in 1913. Rutherford was instrumental in obtaining a research scholarship for Chadwick that required him to work at a location other than that through which he had obtained his masters degree. He chose to study in Berlin under Hans Geiger, who had been at Manchester while Chadwick was completing his master's degree. During this period, Chadwick established the existence of continuous beta ray (high velocity electron) spectra, a result that baffled investigators at the time and that led to the discovery of the neutrino.

Interment during World War I

It was just before World War I, and as the outbreak of hostilities became imminent, Geiger warned Chadwick to return to England as soon as possible. Chadwick was sidetracked by advice from a travel company and ended up in a German prisoner of war camp until the war ended. As the time passed during his five-year confinement, Chadwick became more friendly with his captors, and he and other British captives managed to undertake some rudimentary research on fluorescence.

Work at the Cavendish Laboratory

Upon his release at the end of the war in 1918, Chadwick once again joined Rutherford, and confirmed that the charge of the nucleus was the same as the atomic number. In 1921, he was awarded a research fellowship at Gonville and Caius College, and in the following year, became Rutherford's assistant at the Cavendish Laboratory. While he was the day-to-day operational manager of the laboratory, he still found time to conduct research, the direction of which was generally suggested by Rutherford. Chadwick and Charles D. Ellis, who had been a fellow prisoner with Chadwick during the war and later pursued his studies at Trinity College and under Rutherford, investigated the transmutation of elements under bombardment by alpha particles (helium nuclei). A research group in Vienna had reported results that were at odds with those achieved at the Cavendish, the later of which were ably defended by further experimentation by Chadwick and his colleagues.

In 1925, Chadwick married Eileen Stewart-Brown. Twin daughters were born to the couple.

During the mid-1920s, Chadwick conducted experiments on the scattering of alpha particles shot into targets made of metallic elements including gold and uranium, and then by helium itself, the nucleus of which is the same mass as an alpha particle. This scattering was asymmetric, a result Chadwick explained in 1930 as a quantum phenomenon.

The neutron

As early as 1920, Rutherford had proposed the existence of an electrically neutral particle called the neutron to explain for isotopes of hydrogen. This particle was believed to be composed of an electron and a proton, but the emissions predicted by such a composition could not be detected.

In 1930, it was discovered that the bombardment of light nuclei by alpha rays emitted from polonium gave rise to penetrating rays without an electric charge. These were assumed to be gamma rays. However, when a beryllium target was used, the rays were many times more penetrating than those generated by using other target materials. In 1931, Chadwick and his co-worker, H.C. Webster, suggested that the neutral rays were actually evidence of the existence of the neutron.

In 1932, the husband-wife team of Irene Curie and Frederic Joliot showed that the emissions from beryllium were more penetrating than previous investigators had reported, but they still referred to these emissions as gamma rays. Chadwick read the report, and immediately set to work on calculating the mass of a neutral particle that could account for the latest results. He used the beryllium emissions to bombard a variety of target elements, and established that the results were consistent with impact by a neutral particle with a mass almost identical to that of the proton. This represented the experimental verification of the existence of the neutron. For this achievement, Chadwick was awarded the Nobel prize in physics in 1925.

The neutron quickly became a tool of nuclear scientists, who used it to penetrate and transform the nuclei of elements, since it suffers no repulsion from a positively charged nucleus. In this way, Chadwick prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb. For this important discovery he was awarded the Hughes Medal of the Royal Society in 1932, and subsequently the Nobel Prize for Physics in 1935. Later, he found out that a German scientist had discovered the neutron at the same time. But Hans Falkenhagen (Rostock) was afraid of publishing his results. When Chadwick learned of Falkenhagen's discovery, he offered to share the Nobel Prize with him. Falkenhagen, however, modestly refused the honor.

Chadwick’s discovery made it possible to create elements heavier than uranium in the laboratory. His discovery particularly inspired Enrico Fermi, Italian physicist and Nobel laureate, to discover nuclear reactions brought by slowed neutrons, and led Otto Hahn and Fritz Strassmann, German radiochemists in Berlin, to the revolutionary discovery of “nuclear fission,” which triggered the development of the atomic bomb.

Nuclear fission and the atomic bomb

Chadwick became professor of physics at Liverpool University in 1935. As a result of the Frisch-Peierls memorandum in 1940 on the feasibility of an atomic bomb, he was appointed to the MAUD Committee that investigated the matter further. He visited North America as part of the Tizard Mission in 1940 to collaborate with the Americans and Canadians on nuclear research. Returning to England in November 1940, he concluded that nothing would emerge from this research until after the war. In December 1940 Franz Simon, who had been commissioned by MAUD, reported that it was possible to separate the isotope uranium-235. Simon's report included cost estimates and technical specifications for a large uranium enrichment plant. Chadwick later wrote that it was at that time that he "realized that a nuclear bomb was not only possible, it was inevitable. I had then to start taking sleeping pills. It was the only remedy." Chadwick and his group generally supported a U-235 bomb, and approved of its separation by diffusion from its more plentiful U-238 isotope.[2]

He shortly afterward went to Los Alamos, the headquarters of the Manhattan Project, and, along with N. Bohr, "gave invaluable advice" to the American effort that developed the atomic bombs dropped on Hiroshima and Nagasaki.[3] Chadwick was knighted in 1945.

Later years

After World War II, Chadwick returned to his post at Liverpool. He stepped down in 1958, essentially signaling his retirement, afterwards spending a decade in North Wales until his return in 1969 to Cambridge, where he died on July 24, 1974.

Legacy

The discovery of the neutron and its properties transformed subatomic physics. It enabled scientists to produce new elements, and led to the discovery of nuclear fission and its consequences for both war and peace.

Chadwick's remarkable confinement under adverse conditions during World War I could have ended his career in physics. Rutherford, however, was a powerful enough mentor to have resurrected Chadwick from that difficult experience. Chadwick's success can be seen in light of his training under Rutherford, demonstrating a relationship similar to that of Ludwig von Helmholtz and his students such as Heinrich Hertz and Albert Michelson. While Chadwick's results were certainly his own, his story shows that a hefty element of scientific discovery lies in teamwork, both among co-workers and with senior investigators who laid the foundation for those who followed them.

See also

Notes

  1. Dahl, Per F. 1999. Heavy water and the wartime race for nuclear energy. Bristol [England]: Institute of Physics. 17.
  2. Smyth, H. D. 1947. Atomic energy for military purposes. Princeton, N.J.: Princeton University Press. 52.
  3. Smyth 214.

References

  • Dahl, Per F. 1999. Heavy water and the wartime race for nuclear energy. Bristol [England]: Institute of Physics.
  • Hendry, John. 1990. James Chadwick. Dictionary of Scientific Biography: Supplement II. New York: Charles Scribner's sons. 17:143-148.
  • Mehra, Jagdish. 2001. The historical development of quantum theory. New York: Springer. ISBN 0387951784

External links

All links retrieved April 29, 2013.


Preceded by:
J.F. Cameron
Master of Gonville and Caius College
1948-1958
Succeeded by:
Sir Nevill Mott


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