The Balance of Energy in Nuclear Fission


EARLY experimental observations on the range of fission fragments in air1 suggested that the kinetic energy of fragments of nearly equal mass formed in fission (that is, symmetrical fission) was less than that of fragments of different mass (that is, in unsymmetrical fission), and very recent measurements2 have clearly confirmed this, and shown that the deficiency amounts to about 30 MeV. (32,210 micro-mass units, µM.U.). The conservation of energy in fission may be expressed by an equation of the form: where AZM's are the masses of the fissioning nucleus and of the fragments, 1 and 2, of mass number A and charge Z, and E12 is the energy released expressed in mass units. This last appears as the kinetic energy of the fragments together with the neutrons and γ-rays emitted in fission. It has been shown3 that the neutrons and γ-rays associated with symmetrical fission are not markedly greater in energy than those associated with unsymmetrical fission, and it appears that the deficiency in the kinetic energy in question must be associated with an excess in the masses of the unstable fragments. The masses of the unstable fragments formed initially in fission are not known, however, and attempts to predict them from present empirical mass formulæ have not shown the expected mass excess4.

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WALTON, G. The Balance of Energy in Nuclear Fission. Nature 192, 1062–1064 (1961).

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