The 1938 Hahn–Strassmann experiment revealed nuclear fission when bombarded uranium produced barium.
Otto Hahn and Fritz Strassmann chemically analyzed the products formed after uranium was bombarded with neutrons. They found barium, an element much lighter than uranium. This result was unexpected because researchers had been looking for transuranium elements, not a nucleus split into roughly comparable fragments.
Lise Meitner and Otto Frisch interpreted the finding in early 1939 as the splitting of a heavy atomic nucleus. Frisch named the process “fission,” borrowing a term from biology. They also explained that the split released a large amount of energy because the combined mass of the products was slightly lower than the original uranium nucleus’s mass.
The discovery rapidly led to investigations of neutron multiplication and chain reactions. Uranium-235 can sustain a fission chain reaction with slow neutrons, while uranium-238 behaves differently. The chemical identification of barium was therefore a decisive experimental clue, while the physical interpretation came shortly afterward.