Excitation energy partition in fission

M Albertsson,B.G Carlsson,T Døssing,P Möller,J Randrup,S Åberg

doi:10.1016/j.physletb.2020.135276

M Albertsson, B.G Carlsson + Show 4 more

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https://doi.org/10.1016/j.physletb.2020.135276

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Abstract

The transformation of an atomic nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. As in previous studies, it is assumed that the division of mass and charge is frozen in at a critical neck radius of c0=2.5fm. In order to also determine the energetics, we follow the system further until scission occurs at a smaller neck radius, at which point the shapes of the proto-fragments are extracted. The statistical energy available at scission is then divided on the basis of the respective microscopic level densities. This approach takes account of important (and energy-dependent) finite-size effects. After the fragments have been fully accelerated and their shapes have relaxed to their equilibrium forms, they undergo sequential neutron evaporation. The dependence of the resulting mean neutron multiplicity on the fragment mass, ν¯(A), including the dependence on the initial excitation energy of the fissioning compound nucleus, agrees reasonably well with observations, as demonstrated here for 235U(n,f), and the sawtooth appearance of ν¯(A) can be understood from shell-structure effects in the level densities.

Highlights

Even 80 years after its discovery [1,2], nuclear fission remains a fertile topic for experimental and theoretical research [3,4,5,6] and improvements in instrumentation, modeling, and computation have enabled a renaissance in the field.In their seminal paper [7], Bohr and Wheeler described fission as an evolution of the nuclear shape subject to both conservative forces from the potential energy of deformation and dissipative forces resulting from the coupling to the residual system
We demonstrate that a consistent use of the appropriate microscopic level densities in the distorted proto-fragments at scission provides a reasonable description of the sawtooth appearance of the fragment-mass dependence of the mean neutron multiplicity ν (A)
For the purpose of elucidating the importance of structure effects for the degree of excitation of the primary fission fragments, we have augmented the recently developed level-density guided Metropolis shape evolution treatment [20] with shape-dependent microscopic level densities for the nascent proto-fragments which are distorted relative to their equilibrium shapes

Summary

Introduction

Even 80 years after its discovery [1,2], nuclear fission remains a fertile topic for experimental and theoretical research [3,4,5,6] and improvements in instrumentation, modeling, and computation have enabled a renaissance in the field In their seminal paper [7], Bohr and Wheeler described fission as an evolution of the nuclear shape subject to both conservative forces from the potential energy of deformation and dissipative forces resulting from the coupling to the residual system. Extension of the potential energy to include both neutron and proton degrees of freedom has yielded a good reproduction of the observed odd-even staggering in the fragment charge distributions [17,18]

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