Confirmation of nuclear fission

On this day in 1939, physicist Otto Frisch (below left) performed an experiment in Copenhagen that provided definitive evidence for nuclear fission. Frisch had worked with his aunt, the physicist Lise Meitner (below right), to explain a previous experiment that had revealed nuclei of barium (atomic number 56) as a byproduct of collisions of neutrons and uranium nuclei (atomic number 92). Frisch and Meitner had proposed that the uranium nucleus had split in two, a process that would also release energy in accordance with Einstein’s E = mc². The Czech physicist George Placzek, who was skeptical of the atom-splitting interpretation, suggested that Frisch bombard uranium with neutrons in a cloud chamber and search for newly formed nuclei fragments. Frisch began the experiment on the afternoon of 13 January and continued taking measurements until early the next morning. Sure enough, the experiment produced pulses of light that were consistent with nuclei of the expected size and energy darting away from the uranium. Frisch coined the term fission, taking inspiration from cell division in biology. On 16 January Frisch sent out two papers that were soon published in Nature: a description of his experiment, and an explanation of his and Meitner’s fission interpretation. (Photo credits: AIP Emilio Segrè Visual Archives)