Wikipedia:
In 1934 the idea of chain reaction via neutron was proposed by Leó Szilárd, who patented the idea of the atomic bomb.
In 1934, French physicists Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles, and in the same year Italian physicist Enrico Fermi reported similar results when bombarding uranium with neutrons.
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