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Section efficace de fission du 242Pu : progrès théoriques et expérimentaux

Carole Chatel 1, 2, 3 
2 LEPH - Laboratoire d'Etudes de PHysique
SPRC - Service de Physique des Réacteurs et du Cycle : DEN/DER
3 ACEN - Aval du cycle et Energie Nucléaire
CENBG - Centre d'Etudes Nucléaires de Bordeaux Gradignan
Abstract : Present PhD aims to study the neutron fission cross section of the 242Pu that is classified as fertile isotope. This work relies on two complementary branches of nuclear physics, meaning the experimental and the theoretical aspect. This PhD takes place within a collaboration between the CEA (LEPh) of Cadarache and the CENBG (ACEN group) of Bordeaux-Gradignan. When a fission occurs, a heavy nucleus starts to deform from its spheroid equilibrium shape. The minimum of nuclear potential fluctuates as a function of the nucleus main deformation (i.e. the elongation), drawing a double-humped curve with two wells: the first one corresponds to the point of normal equilibrium shape and the second one to the fission isomer deformation. A heavy nucleus cross section overlays several energetic regions including the resolved resonance range (RRR), the unresolved resonance range (URR) and the neutron continuum. To model a cross section, the most physical and mathematically correct formalism must be applied as a function of the energy range involved. The theoretical part of this work aims to analyze and model the fission cross section over the various pre-cited energy domains. Hence, in the RRR, approximations of R-Matrix theory are used with the in-house developed computer code CONRAD. The excited states in the fission isomer well, namely the class-II states, are here described explicitly. In the URR, Hauser-Feshbach theory associated to Coupled Channels Optical Model calculations was used to represent the average resonant fission cross section. Once more, the class-II states have been identified and a more accurate data file is proposed. The present work ends with a careful study of the neutron continuum above the URR. Both the URR and the continuum were analyzed with the TALYS-ECIS-06 system of codes. Our goal was to explain the unknown origin of a sizeable structure lying right after the fission threshold at around 1.1 MeV of neutron energy. Prior to the present investigation, this rough structure was not correctly modeled, as well as its origin properly described. Since current experimental fission cross section database still shows 10 to 15% of discrepancies, a new precise measurement is advertised. For a high quality cross section measurement, several choices have to be made: the type of beam (Time Of Flight method or quasi mono-energetic neutrons), the type of detectors to be selected (one for measuring the number of fission, the other for characterizing the neutron flux) and finally the cross section to be used as a reference to quantify the neutron flux. For a fission cross section measurement, the reference commonly chosen is the 235U(n,f) reaction that belongs to the category of secondary standards. The detectors classically selected are fission chambers. It was decided to set up an uncorrelated measurement with photovoltaic cells as fission detectors and to use the diffusion cross section H(n,n)p as reference. The latter is described as a primary standard and involves the proton recoil technique. However, no detector is suitable yet to cover the energy range spanning the broad resonant structure to be investigated. The experimental part of this work was driven around the development of a new detector: so called the Gaseous Proton Recoil Telescope. This detector carries the particularity to be a miniaturized Time Projection Chamber (TPC). Indeed, any particle track can be reconstructed in 3 dimensions, thanks to the Micromegas segmented detection plane and the electron drift velocity. The first task of my work was to determine the best operating range for this brand new detector. Once this was achieved, we paid attention to the device intrinsic efficiency since a precise measurement requires a detector with a 100% of intrinsic efficiency. The work was initially carried out with a 3α source and then the final results were validated with a direct proton beam (at the AIFIRA accelerator of CENBG).
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Submitted on : Thursday, August 4, 2022 - 7:19:08 PM
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Carole Chatel. Section efficace de fission du 242Pu : progrès théoriques et expérimentaux. Physique Nucléaire Théorique [nucl-th]. Aix-Marseille Université (AMU), 2021. Français. ⟨tel-03746124⟩

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