Frédéric Joliot discovered artificial radioactivity on January 11, 1934, when he bombarded aluminum with polonium alpha particles and produced a radioactive isotope of phosphorus that decayed by emitting a positron. He detected it with a Geiger–Müller counter that Wolfgang Gentner had constructed for him. Two months later, Enrico Fermi, motivated in part by an insight of his first assistant, Gian Carlo Wick, decided to see if neutrons also could produce artificial radioactivity. The transformation of a neutron into a proton in a nucleus should create an electron, so to increase their number and hence the probability of creating an electron, he bombarded various elements with intense sources of neutrons, and on March 20, 1934, with aluminum he observed the created electrons and thereby discovered neutron-induced artificial radioactivity. Less than four months later, Marie Curie died on July 4, 1934, at age sixty-six.
Aging of the Geiger-Muller counter due to particle conductance in an insulating gas
