Abstract
The time evolution of the nuclear density of the fissioning system 240Pu during the scission process is obtained from the time-dependent superfluid local-density approximation (TDSLDA) to the density functional theory. A nuclear energy density functional based on the Skyrme force Skm* is used. The duration of the scission process Δt as well as the neck radius (rmin) of the ‘just-before scission’ configuration and the minimum separation (dmin) of the inner surfaces of the fragments in the ’immediately-after scission’ configuration were extracted in order to calculate the multiplicity of the scission neutrons (Vsc) using a phenomenological dynamical scission model (DSM). We find that Vsc=1.347, i.e. half of the prompt fission neutrons measured in the reaction 239Pu(nth; f) are released at scission. After scission, the fragments are left excited and with some extra deformation energy (mainly the heavy one). In this way we can account for the evaporation of the other half and for the emission of γ rays.
Highlights
The most important feature of nuclear fission is that it is accompanied by emission of prompt fission neutrons (PFN)
While the 1st component depends on the excitation and extra deformation with which the fission fragments are born [1], the 2nd depends on the dynamical evolution of the system during the scission process [2]
To determine when the scission process starts and when it ends we analyze the evolution of the moments of the density distribution, in particular we look for the sharp decrease of the total quadrupole and for the oscillation of the fragment octupoles
Summary
The most important feature of nuclear fission is that it is accompanied by emission of prompt fission neutrons (PFN). There are three parameters in the dynamical scission model: the nuclear shapes just before (↵i) and immediatelly after (↵ f ) scission and the duration ∆T of the transition between these two shapes Since these quantities were unknown, in the past we have used educated guesses. Among the many nuclear energy density functionals we choose the one based on the Skyrme force SkM* [4] that reproduces the fission barriers in 240Pu. Among the many nuclear energy density functionals we choose the one based on the Skyrme force SkM* [4] that reproduces the fission barriers in 240Pu We extract these three quantities and calculate the multiplicity of the neutrons released at scission and emitted immediately after (i.e., during the acceleration of the fragments). The ai f coefficients are used to calculate the scission neutron multiplicity ⌫sc in a simple and intuitive way It is given by the sum of the probabilities Piem that a neutron occupying a given bound-state i is emitted: Piem. The nuclear shapes involved are described by Cassini ovals [5] with two parameters: ↵ (the overall deformation) and ↵1 that fixes the mass asymmetry
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