They had been interested in thorium -- a radioactive element that can fuel nuclear reactors but not be processed into weapons.

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World War Two ended. Enrico Fermi had been the first to use a neutron to produce the radioactive change of one element to another.

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When the Sun becomes more active, this isotope becomes more abundant along with beryllium, another radioactive element.

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IAEA-led team was fanning out in western Georgia in search of the remaining two of a batch of eight abandoned devices containing strontium-90, another radioactive element, which had been used in generators for communication stations.

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The device's primary source of neutron radiation was the radioactive element californium, but the stream of neutrons produced by the californium was multiplied by passing it through a lattice of highly enriched uranium U-235, whose nuclear fission released additional neutrons.

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