It seems therefore possible that the uranium nucleus has only small stability of form, and may, after neutron capture, divide itself into two nuclei of roughly equal size (the precise ratio of sizes depending on finer structural features and perhaps partly on chance). It must be remembered, however, that the surface tension of a charged droplet is diminished by its charge, and a rough estimate shows that the surface tension of nuclei, decreasing with increasing nuclear charge, may become zero for atomic numbers of the order of 100. In the discussion of the energies involved in the deformation of nuclei, the concept of surface tension has been used 7 and its value has been estimated from simple considerations regarding nuclear forces. If the movement is made sufficiently violent by adding energy, such a drop may divide itself into two smaller drops. On account of their close packing and strong energy exchange, the particles in a heavy nucleus would be expected to move in a collective way which has some resemblance to the movement of a liquid drop. On the basis, however, of present ideas about the behaviour of heavy nuclei 6, an entirely different and essentially classical picture of these new disintegration processes suggests itself. The emission, within a short time, of a large number of charged particles may be regarded as excluded by the small penetrability of the 'Coulomb barrier', indicated by Gamov's theory of alpha decay. The formation of elements much below uranium has been considered before, but was always rejected for physical reasons, so long as the chemical evidence was not entirely clear cut. Further investigation 5, however showed that it was impossible to separate those bodies from barium (although mesothorium, an isotope of radium, was readily separated in the same experiment), so that Hahn and Strassmann were forced to conclude that isotopes of barium (Z = 56) are formed as a consequence of the bombardment of uranium (Z = 92) with neutrons.Īt first sight, this result seems very hard to understand. A body, for example, with similar properties to those of osmium was assumed to be eka-osmium (Z = 94) rather than osmium (z = 76) or ruthenium (z = 44).įollowing up an observation of Curie and Savitch 3, Hahn and Strassmann 4 found that a group of at least three radioactive bodies, formed from uranium under neutron bombardment, were chemically similar to barium and, therefore, presumably isotopic with radium. In making chemical assignments, it was always assumed that these radioactive bodies had atomic numbers near that of the element bombarded, since only particles with one or two charges were known to be emitted from nuclei. Further investigations 2 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 behavior together with their genetic relations. On bombarding uranium with neutrons, Fermi and collaborators 1 found that at least four radioactive substances were produced, to two of which atomic numbers larger than 92 were ascribed. Discovery of Fission Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction
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