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SUMMARY:The statistical model of nuclear fission: from Bohr-Wheeler to hea
 vy-ion fusion-fission reactions - Dr Alessio Zaccone
DTSTART:20180223T103000Z
DTEND:20180223T113000Z
UID:TALK99127@talks.cam.ac.uk
CONTACT:Jo Boyle
DESCRIPTION:The first theory of the rate and temperature dependence of nuc
 lear fission reactions was put forward as early as 1939 by Niels Bohr and 
 John A. Wheeler. Their theory uses a transition-state argument\, well know
 n especially to physical chemists\, that was already being used to rationa
 lise the temperature dependence of the rates of chemical reactions since t
 he 1930's. Their model however relies on equilibrium statistical mechanics
 \, and neglects two important\, if not essential\, aspects of fission. The
  first aspect is the nonequilibrum decay process of the parent nucleus whi
 ch can be described as the decay of a thermodynamically metastable state. 
 The second aspect is the effect of "viscous" dissipation in the deformatio
 n of the unstable nucleus which is especially important for large nuclei. 
 These problems were addressed by Kramers in a remarkable 1940 paper where 
 he derived expressions for generic decays of metastable states in the pres
 ence of dissipation. This line of research to include dissipation in the d
 escription of nuclear fission has been intensively pursued in the last dec
 ades. After briefly outlining the fundamentals of theoretical modelling of
  nuclear fission of heavy nuclei from the point of view of statistical mod
 els\, I will discuss some recent developments in the regime of high-excita
 tion energy\, which is encountered for example in heavy-ion induced fusion
 -fission reactions. In this regime\, the exp(U/T) Kramers-Arrhenius depend
 ence of fission time on nuclear temperature is shown to break down dramati
 cally. A recent model [Eccles\, Roy\, Gray\, Zaccone\, Phys. Rev. C 96\, 0
 54611 (2017)] solves this problem and leads to the correct temperature dep
 endence also in the high energy regime. An application to the example of h
 eavy-ion induced fusion-fission of Thorium will be presented. 
LOCATION:Department of Engineering - LT6
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