James Chadwick

principal papers
1) co author: Radiations from Radioactive substances (1930)
2) The Existence of a Neutron (4)

hardware

software

keywords
neutron, proton, electron, atom

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related subjects

Chadwick proofs the existence of the neutron in 1932 what accelerated the research in nuclear physics inmensely.

In 1913, Chadwick received a scholarship to study in Germany, placing him in Germany at the beginning of World War I. After Chadwick was detained as a civilian prisoner of war for four years. He returned to England in 1919 to carry out research at Cambridge University under Ernest Rutherford head of Cambridge University's nuclear physics lab. Rutherford oversaw Chadwick's PhD in 1921 and then made him assistant director of the lab.

Chadwick's own research focused on radioactivity. In 1919 Rutherford developed a theory on the atom's nucleaus and theorized on the existence of a neuron, a non charged particle within the atom's nucleus. But they and other researchers were finding that the neutron did not seem to be the only particle in the nucleus.

As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well -- the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. So in the helium example, there would be four protons and two electrons in the nucleus to yield a mass of 4 but a charge of only 2. Rutherford also put out the idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. There was no evidence for any of these ideas.

Chadwick kept the problem in the back of his mind while working on other things. Experiments in Europe caught his eye, especially those of Frederic and Irene Joliot-Curie. They used a different method for tracking particle radiation. Chadwick repeated their experiments but with the goal of looking for a neutral particle -- one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's. He published his findings with characteristic modesty in a first paper entitled "Possible Existence of Neutron." In 1935 he received the Nobel Prize for his discovery.

His findings were quickly accepted and Werner Heisenberg then showed that the neutron could not be a proton-electron pairing, but had to be its own unique particle -- the third piece of the atom to be found. This new idea dramatically changed the picture of the atom and accelerated discoveries in atomic physics. Physicists soon found that the neutron made an ideal "bullet" for bombarding other nuclei. Unlike charged particles, it was not repelled by similarly-charged particles and could smash right into the nucleus. Before long, neutron bombardment was applied to the uranium atom, splitting its nucleus and releasing the huge amounts of energy predicted by Einstein's equation E = mc2.

Chronology

1891 Born in Manchester
1908 Graduated Manchester University
1913 M.Sc. degree
1921 PhD in physics
1921-23 Works at Manchester under Ernest Rutherford on radioactivity.
1921-1935 Fellow of Gonville and Caius College
1923 Assistant director of radioactive research in the Cavendish Laboratory, Cambridge
1925 Married Aileen Stewart-Brown of Liverpool
1932 Discovers the neutron
1935–48 Lyon Jones Chair of Physics in the University of Liverpool
1943-46 Worked in the United States as Head of the British Mission attached to the Manhattan Project for the development of the atomic bomb.
1948 Retired from active physics.
1948–59 master of Gonville and Caius College, Cambridge
1959 Retirement
1957-62 parttime member of the United Kingdom Atomic Energy Authority.
1974 Dies in Edingburgh

Honors and awards

1927 Elected a Fellow of the Royal Society
1935 Receives the Nobel Prize for physics
1945 Knighted KBE
1950 Received the Copley Medal
1951 Franklin Medal of the Franklin Institute, Philadelphia

Honorary Fellow of the Institute of Physics
Honorary doctorate degrees from various Universities