Littérature scientifique sur le sujet « Prompt γ-spectroscopy »

Experiments to perform precision spectroscopy of fast neutron induced fission were carried out during the ν-Ball experimental campaign at the ALTO facility of IJC Laboratory Orsay. Low energy fission of 232Th(n,f), 238U(n,f) and spontaneous fission of 252Cf were studied using this hybrid highresolution spectrometer and calorimeter. New observables such as γ-ray multiplicity distributions correlated with specific fission fragments are presented and discussed. A new method using fast-timing techniques to detect prompt fission neutrons in coincidence with prompt fission γ-rays is described.

Inelastic reactions ((n,xn) for x ≥ 1) play a key role in reactor cores as they influence the slowing down of the neutrons. A reactor neutron energy spectrum depends thus on this process which is in strong competition with elastic scattering and fission; a nice example is the case of 238U. Inelastic scattering (x = 1) impacts keff and radial power distribution in the nuclear reactor. For several years, it has been shown that the knowledge of the inelastic cross sections in nuclear databases is not good enough to accurately simulate reactor cores and a strong demand for new measurements has emerged with very tight objectives (only a few percent) for the uncertainties on the cross section. To bypass the well-known experimental difficulty to detect neutrons, the prompt γ-ray spectroscopy method is a powerful but indirect way to obtain inelastic cross sections. Our collaboration has used this method for more than ten years and have produced a lot of (n,n′γ) cross sections for nuclei from 7Li to 238U. In this article, we will first discuss the issues of the prompt γ-ray spectroscopy regarding the control of all the uncertainties involved in the (n,n′γ) cross section estimation. Secondly, we will focus on the role of theoretical modeling which, in certain cases, is crucial to reach the objectives of a few percent uncertainty on the (n,n′) cross sections.

At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of 235U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&FATIMA mixed array of HPGe and LaBr3(Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.

GRAPhEME is a γ-spectrometer developed by CNRS/IPHC Strasbourg (France), in collaboration with EC-JRC Geel (Belgium) and IFIN-HH Bucharest (Romania). With its 6 High Purity Planar Germanium detectors and one fission chamber, GRAPhEME, installed at the EC-JRC GELINA facility, was optimized for measurements of accurate (n, xnγ) cross sections on actinides. The experimental methodology is based on the prompt γ-ray spectroscopy coupled to time-of-flight measurements. In this paper, we present an overview of fifteen years of experiments with GRAPhEME at EC-JRC GELINA facility, illustrated by main achievements to highlight the performances reached by our spectrometer. Beyond the experimental work, a close collaboration with theoreticians has emerged allowing the use of the data produced with GRAPhEME to test and constraint nuclear reaction codes like TALYS, CoH and EMPIRE. In a near future, GRAPhEME will be available to start measurement campaigns at the new neutron beam facility SPIRAL2/NFS. There, studies of (n, 2n) and (n, 3n) reactions will be possible and will complete the work done at EC-JRC GELINA on (n, n ) reactions. Despite the amount of cross section data provided by GRAPhEME up to now, the prompt γ-ray spectroscopy method presents some weaknesses that our collaboration tries to overcome. This goes through new calculation schemes based on theoretical modeling constrained on experimental data to infer the total (n, xn) cross section, new instrument to measure conversion electrons but also by being proactive in dissemination activities to make the nuclear structure community aware of our needs about new accurate nuclear structure information on actinides.

Dans le contexte du développement des réacteurs de génération IV, des données nucléaires précises sont requises. Dans ce travail de thèse, on s’intéresse en particulier à la section efficace de diffusion inélastique (n, n’) pour les noyaux d’233U et d’238U. L'analyse des données de GRAPhEME, dispositif combinant les méthodes de la spectroscopie-γ prompte et du temps de vol, ont permis d’obtenir, pour la première fois, 12 sections efficaces 233U(n, n’γ). Une modélisation des sections efficaces de ces deux isotopes (233U et 238U) a été réalisée avec le code de réaction nucléaire TALYS. Dans le cas de l’238U, l’implémentation de nouveaux modèles permet un meilleur accord calcul/mesure pour les sections efficaces (n, n’γ). Il a été montré cependant que cela n’influe pas sur la section efficace (n, n’). Enfin, une évaluation des incertitudes des sections efficaces de réaction pour l’238U menée avec CONRAD compare différentes méthodes afin de propager les incertitudes expérimentales aux paramètres des modèles via l’inférence Bayésienne
In the context of the development of generation IV nuclear reactors, precise nuclear data are needed. In this work, we study in particular inelastic scattering cross section for 233U and 238U nuclei. From the analysis of data from GRAPhEME, an experimental device combining prompt γ- spectroscopy and time of flight, we obtained for the first time 12 233U(n, n’γ) reaction cross sections. Cross sections modelling has been performed for both isotopes (233U et 238U) with the nuclear reaction code TALYS. In the 238U case, the implementation of new model in the code highlighted better agreement calculation/experiment for (n, n’γ) reaction cross sections. Despite this result, no change in the total inelastic scattering cross section has been seen. Finally, a cross sections uncertainties evaluation has been done with CONRAD, comparing different methods of propagating experimental uncertainties to model parameters via the Bayesian inference

La loi « Bataille » fait obligation au CNRS de développer des recherches sur le retraitement des déchets nucléaires. C’est dans ce cadre que notre équipe GRACE (Groupe de Recherche sur l’Aval du Cycle Electronucléaire) de l’IPHC a entrepris des mesures nécessaires à la mise au point des réacteurs hybrides. Le champ de recherche s’est élargi puisqu’il inclut maintenant des études visant à développer le cycle de Thorium. GRACE s’est plus précisément attaché à mesurer des sections efficaces de réactions (n,xng) mal connues jusqu’à présent ou pour lesquelles aucune mesure n’existe. Les mesures ont été effectuées auprès du faisceau « blanc » de Gelina (IRMM Geel, Euratom). La technique de temps de vol a été appliquée. La spectroscopie g en ligne, utilisée pour étudier les réactions (n,xng), requiert avant tout de mesurer l’énergie des rayons g avec une bonne résolution. A cause de l’existence d’un flash g créé en même temps que le faisceau, il faut aussi un temps mort très faible, de façon à pouvoir détecter les neutrons de haute énergie. GRACE a réussi à concilier ces deux impératifs en mettant au point une nouvelle méthode basée sur la digitalisation et le traitement numérique du signal. Des mesures de section efficaces (n,xng) sur le 206,207,208Pb ont été réalisées ainsi avec succès à Geel, à l’aide des détecteurs coaxiaux de gros volume et à une distance de vol de 200 m. Le travail présenté dans ce mémoire a consisté à adapter la méthode pour rendre possible la spectroscopie g en ligne sur des cibles radioactives. Pour cela, la nouvelle mesure s’est déroulée sur une piste de vol de 30 m, et des détecteurs germanium planaires ont été utilisés. Dans un premier temps, et afin de s’assurer du bon fonctionnement du nouveau dispositif expérimental, des mesures de sections efficaces (n,n’g) sur une cible de plomb naturel ont été effectuées et les résultats sont comparés à ceux de la mesure à 200 m et aux calculs du code TALYS. Ensuite, les sections efficaces inélastiques partielles sur le 182,186W ont été mesurées entre le seuil de la réaction et 8 MeV. Enfin, une cible de 235U enrichie à 93 ,2 % a été bombardée et trois transitions dues à la réaction 235U(n,2ng)234U ont été analysées avec succès dont celle de l’état 8+ vers l’état 6+ jamais mesurée avant. Ce travail est une étape essentielle pour utiliser cette nouvelle méthode afin de mesurer des réactions (n,2ng) sur l’233U
The “Bataille” law obliged the CNRS to develop researches on the reprocessing of the nuclear waste. It is in this frame that our group GRACE (Groupe de Recherche sur l’Aval du Cycle Electronucléaire) of the IPHC began researches that contribute to the development of the hybrid reactors. However, the field of research widened because it now includes studies to develop the Thorium cycle. GRACE has undertaken measurements of (n,xng) reactions cross sections badly known or for which no measurement exists yet. The experiments were performed at the “white” neutron beam generated by GELINA facility in Geel, Belgium. The time of flight technique was applied. The g spectroscopy used for these measurements requires the detection of g rays with a good energy resolution. Because of the existence of a g flash created at the same time as the neutron beam, a short dead time is also required, to be able to detect the high energy neutrons. GRACE managed to conciliate these two imperatives by finalizing a new method based on the digital treatment of the signal. Using large HPGe coaxial detectors, (n,xng) cross sections measurements on the 206,207,208Pb nuclei were successfully realized at a 200 m flight path. The work presented in this thesis consists in adapting the method to highly radioactive targets. For that reason, the new measurement took place on a 30 m flight path and planar germanium detectors were used. In order to check the correct functioning of the new experimental method, (n,n’g) cross section measurements on a natural lead target were done and the results found were compared with the experiments at 200 m and with the theoretical calculations of the TALYS code. After that, the partial inelastic scattering cross sections on the 182,186W were performed from the threshold up to 8 Mev. Finally, a 93,2% enriched 235U target was bombarded and three transitions due to the 235U(n,2ng)234U reaction were successfully analysed including the one from the 8+ to the 6+ state, never measured before. This work is an essential step for using this new method in order to measure the (n,2ng) reaction cross sections on the highly radioactive 233U isotope