James Chadwick Biography

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World of Scientific Discovery on James Chadwick

Chadwick was born near Manchester, England, on October 20, 1891. He entered the University of Manchester in 1908 and received his bachelor 's degree from the Honours School of Physics there in 1911. He continued his studies at Manchester under Ernest Rutherford and earned a master's degree in 1913. Chadwick's work with Rutherford involved the study of radioactivity.

In 1913, Chadwick received a scholarship for graduate study, which he took with Hans Geiger at the Physikalische Technische Reichsanstalt in Berlin. When World War I began, Chadwick was interned as a civilian prisoner of war for the duration of the conflict. After the war, Chadwick rejoined Rutherford, who had become director of the Cavendish Laboratory at Cambridge.

The research for which Chadwick was awarded the Nobel Prize for physics in 1935 involved his discovery of the neutron. By 1920, scientists had discovered two subatomic particles, the proton and the electron. They had further found that the protons were closely packed together in a central core--the nucleus --of the atom, while the electrons were located in specific energy levels outside of the nucleus.

A critical problem remained in this atomic model, however. A nucleus consisting only of protons could not adequately account for both the charge and mass of the nucleus. A helium nucleus, for example, was known to carry two positive charges and a mass of four amu (atomic mass units). One atomic mass unit is approximately equal to the mass of one proton.

One suggestion was that the nucleus contained not only protons, but enough electrons to resolve this problem. For example, a helium nucleus might consist of four protons and two electrons. The mass of an electron is so small (about 0.0055 amu) that the nuclear electrons would contribute little to the mass of the nucleus, but they would reduce the total charge to the observed +2.

Rutherford had proposed another solution. He suggested the existence of yet a third subatomic particle, one with no charge and a mass just slightly greater than that of the proton. The presence of such particles in a nucleus might account for both its mass and charge.

One problem with this suggestion was the difficulty in detecting such a particle. Lacking electrical charge, it would not be detectable by the cloud chamber, the Geiger counter, or any other particle detector whose operation depended on the movement of charged particles.

The key events that led to Chadwick's discovery of this particle occurred in the early 1930s. First, the German team of Walther Bothe and Hans Becker reported the emission of highly energetic radiation when a beryllium target was bombarded with alpha rays. The radiation behaved somewhat like gamma rays, but had some properties that were uncharacteristic of such radiation. Chadwick suspected that the radiation might consist of uncharged particles. Soon after the Bothe-Becker experiments, the French physicists Frédéric Joliot-Curie and Irène Joliot-Curie showed that the radiation the Germans had observed was able to emit protons from paraffin.

Chadwick decided to repeat the Bother-Becker and Joliot-Curie experiments in his own laboratory. He found that the new radiation was able to eject protons from a number of materials other than paraffin. He eventually concluded that the radiation was actually a beam of uncharged particles. He was able to determine that the mass of these particles was about 1.005 amu, slightly greater than the mass of the proton (mass = 1.000 amu). The particles were neutrons.

Chadwick served as Lyon Jones Professor of Physics at the University of Liverpool from 1935 to 1948. He became master of Gonville and Caius College at Cambridge in 1948 and served in that position until his retirement in 1958. He died at Cambridge on July 24, 1974.