In 1896 Henri Becquerel discovered natural radioactivity and in 1934 Frédéric Joliot and Irène Curie-Joliot discovered artificial radioactivity. Most natural radionuclides are produced through one of four radioactive decay chains, each chain fed by a long-lived and heavy parent radionuclide. The vast majority of currently known radionuclides, however, are man-made and artificially produced through a process of nuclear activation which uses bombardment of a stable nuclide with a suitable energetic particle to induce a nuclear transformation. Various particles or electromagnetic radiation generated by a variety of machines are used for this purpose, most notably neutrons from nuclear reactors for neutron activation and protons from cyclotrons for proton activation.