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Department of Chemical Engineering and Biotechnology

A new model for nuclear fission reactions at high energies published in Physical Review C

Dr A. Zaccone

Given the insurmountable limits posed by nature on the yield of solar cells, it is likely that nuclear fission will play a major role in transitioning to a carbon-free society.

Nuclear fission was officially discovered in 1938, by chemists Hahn and Strassmann in Berlin who performed an exceptionally careful (for the time) chemical characterization of the neutron bombardment of Uranium nuclei. In reality, fission had been already discovered by Enrico Fermi and his team in Rome in 1934-1935. It was Fermi who pioneered the use of slow neutrons to induce nuclear reactions which led to the observation of transuranic elements, for which he was awarded the 1938 Nobel prize in physics. Interestingly, however, he did not correctly interpret the outcome of some reactions his team performed in 1935 where fission was certainly one of the outcomes. German chemist Ida Noddack and her husband published a correct interpretation of some of those results being fission in Angewandte Chemie, but this remained largely ignored by the physicists also because there was still a firm belief in the community that only alpha particles could be generated in nuclear reactions. The situation drastically changed with the careful experiments of Hahn and Strassmann. The theoretical interpretation of fission of unstable nuclei, in terms of the liquid drop model (see the figure) was provided by Lise Meitner, a former collaborator of Hahn, together with her nephew Otto Frisch, in her exile in Sweden escaping, as a Jew, from the racial laws in Nazi Germany. The first theory of the rate and temperature dependence of nuclear fission reactions was put forward as early as 1939 by Niels Bohr and John A. Wheeler. Their theory uses a transition-state argument, well known to all chemists and chemical engineers, that was already being used to establish the temperature dependence of the rates of chemical reactions. Their model however neglects the effect of "viscous" dissipation in the deformation of the unstable nucleus which is especially important for large nuclei (this is made clear by the analogy with fission of a liquid drop). This problem was fixed by Kramers in a remarkable 1940 paper where he derived expressions for reaction rates in the presence of dissipation and mentioned the possible application to nuclear fission. This line of research to include dissipation in the description of nuclear fission has been intensively pursued in the last decades.

In a recent paper published in Physical Review C (the flagship journal for nuclear physics published by the American Physical Society) CEB Dr Alessio Zaccone, working with PartII B students Chris Eccles and Sanil Roy, who were brave enough to embark in nuclear fission studies for the PartII B project, and PhD student Thomas Gray, has shown that the well established Kramers theory which predicts the fission time (or its inverse, the rate) breaks down at sufficiently high values of the nuclear temperature. This is the case, for example, of the fission of unstable heavy nuclei formed from the nuclear fusion of two parent nuclei. The compound nucleus formed in this way has a higher excitation energy than in standard fission reactions, and the experimentally measured fission time as a function of temperature has been shown in this paper to be better described by the new model formulated by Dr Zaccone and his students, which correctly describes the transition between an Arrhenius-type dependence of the fission time on temperature, and a free-diffusion type dependence in the limit of extremely high temperatures where the normalized fission barrier becomes small. Future work will clarify potential applications of the new theory in various sectors of nuclear physics and nuclear engineering.

Temperature dependence of nuclear fission time in heavy-ion fusion-fission reactions
Chris Eccles, Sanil Roy, Thomas H. Gray, and Alessio Zaccone
Phys. Rev. C 96, 054611 – Published 28 November 2017

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