Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction

ON bombarding uranium with neutrons, Fermi and collaborators1 found that at least four radioactive substances were produced, to two of which atomic numbers larger than 92 were ascribed. Further investigations2 demonstrated the existence of at least nine radioactive periods, six of which were assigned to elements beyond uranium, and nuclear isomerism had to be assumed in order to account for their chemical behaviour together with their genetic relations. In the late 1930s, a series of experiments showed that bombarding uranium with neutrons produced several new radioactive elements, which were assumed to have atomic numbers near to that of uranium (Z = 92). This assumption followed naturally from the prevailing view of nuclear decay, which involved the emission, through tunnelling, of only small charged particles (α and β). How then did one explain the formation of an element which was, as far as could be determined, identical to barium (Z = 56), and thus much smaller than uranium? The answer came in 1939, when Meitner and Frisch proposed a process whereby the addition of a neutron would induce the uranium nucleus to split. They called this process ‘fission’, by analogy with the splitting of living cells.