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Artificial radioactivity.

Ever since Rutherford had brought about nuclear reactions by bombardment with subatomic particles (see 1919), physicists had been inducing more and more such reactions.

In 1934 the French physicists Frederic Joliot-Curie (1900-1958) and Irene Joliot-Curie (1897-1956), his wife, who was also the daughter of Pierre and Marie Curie (see 1897), were bombarding aluminum atoms with alpha particles.

In the process of the bombardment, the nucleus of the aluminum atom would absorb an alpha particle and give off a proton. The aluminum nucleus contains thirteen protons and fourteen neutrons, so that it is aluminum-27. By taking up an alpha particle (two protons and two neutrons) and then giving up a proton, it ended up with fourteen protons and sixteen neutrons, so that it was silicon-30, which occurs in nature.

However, after the bombardment ceased, so that alpha particles were no longer absorbed and protons were no longer given off, another form of radiation continued. The Joliot-Curies investigated and decided that in some cases the aluminum nucleus, after absorbing the alpha particle, gave off a neutron. The result was a net gain of two protons and one neutron, for a total of fifteen protons and fifteen neutrons, which made it phosphorus-30.

Phosphorus-30, however, does not occur in nature, and it is radioactive, breaking down with a halflife of just under 3 minutes (which is why it doesn't occur in nature). In breaking down, it gives off a stream of positrons (particles that had been discovered by Anderson--see 1932). Each escaping positron converts a proton to a neutron, so that phosphorus-30 becomes stable silicon-30 The Joliot-Curies were thus the first to observe radioactive breakdowns that emitted positrons and were the first to produce a radioactive isotope of an ordinarily stable element. This is called artificial radioactivity, since it comes about as the result of bombardment of nuclei in the laboratory.

Eventually it turned out that every element that possessed one or more stable types of nuclei could also possess radioactive nuclei (radioisotopes).

For the discovery of artificial radioactivity, the Joliot-Curies were awarded the Nobel Prize for chemistry in 1935.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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Next Article:Cherenkov radiation.

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