The 1938 Hahn–Strassmann experiment found barium among the products formed when uranium was bombarded with neutrons.
Otto Hahn and Fritz Strassmann chemically analyzed uranium products after neutron irradiation and detected barium, an element much lighter than uranium. Their result was startling because it indicated that the uranium nucleus had split into smaller nuclei rather than merely absorbing a neutron or emitting a small particle.
Hahn reported the finding to Lise Meitner, who was then in Sweden. Meitner and her nephew Otto Frisch interpreted the process as nuclear fission and explained that the division released substantial energy. Frisch also introduced the term “fission,” borrowing it from biology.
The discovery helped establish the physical basis for nuclear reactors and nuclear weapons. Barium was not the only possible fission product; uranium can split in several ways, producing different pairs of medium-sized nuclei and additional neutrons.