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Theses

Fission des Noyaux Lourds et Superlourds

Abstract : This thesis deals with the study of fusion-fission dynamics from a theoretical as well as from an experimental point of view. In the case of highly excited nuclei, we describe the symmetric fission (only present at high energy) by a one-dimensional macroscopic model based on the resolution of the Langevin equation coupled to the Master equations governing light-particle evaporation. A nice agreement between theoretical predictions and experimental results for pre-scission neutron multiplicities was then obtained. At lower energy the possible coexistence of symmetric and asymmetric fission modes, called multimodal fission, has to be taken into account. Our theoretical description is then extended to a multidimensional treatment of the Langevin equation. The influence of different theoretical prescriptions (shell and pairing corrections, friction, evaporation) on fission probabilities and times as well as on pre-scission particle multiplicities is studied. In a second part we analyse the experi\-mental data obtained at the U400 cyclotron of the JINR, Dubna. The $^{227}$Pa compound nucleus was synthesized by bombarding a $^{209}$Bi target with a $^{18}$O beam. The spectrome\-ter CORSET which consists of Micro Channel Planes is used to detect the fission fragments and the multidetector DeMoN allows to measure the neutrons emitted in coincidence. A kinematic reconstruction permits to obtain the mass and velocity distributions of the fission fragments and a source analysis gives access to the pre-scission neutrons. It is in particular interesting to study the variation of the pre-scission multiplicity with the mass asymmetry of the fragments. The confrontation between theory and experiment allows us to point out the assets but also the drawbacks of the model and to propose in certain cases (description of friction, evaporation theory) possible improvements (temperature dependence of transport coefficients, nuclear shape parametrization).
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https://tel.archives-ouvertes.fr/tel-00001529
Contributor : Christelle Schmitt <>
Submitted on : Friday, August 2, 2002 - 8:24:08 AM
Last modification on : Thursday, April 23, 2020 - 2:26:21 PM
Long-term archiving on: : Tuesday, September 11, 2012 - 5:40:21 PM

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  • HAL Id : tel-00001529, version 1

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Christelle Schmitt. Fission des Noyaux Lourds et Superlourds. Physique Nucléaire Théorique [nucl-th]. Université Louis Pasteur - Strasbourg I, 2002. Français. ⟨tel-00001529⟩

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