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The conference proceedings will be published (on-line) at the EPJ Web of Conferences.; Neutron-induced capture cross sections of short-lived nuclei are of great importance in many areas: fundamental nuclear physics, nuclear astrophysics and nuclear applications, such as nuclear reactors. However, very often the high radioactivity of the samples makes the direct measurement of these cross sections extremely difficult. The surrogate reaction method [1] is an indirect way of determining neutron-induced cross sections for reactions that proceed through a compound nucleus. In this method, the decaying nucleus of the desired reaction is produced via a transfer or an inelastic scattering reaction (figure 1). This technique presents the advantage that the target material can be stable or less radioactive than the material required for a neutron-induced measurement. Nevertheless, a significant question in the use of the surrogate reaction method lies in the difference between the spin and parity population in the neutron-induced and surrogate reactions. The CENBG collaboration has successfully applied this technique to determine the neutron-induced fission cross sections of several short-lived nuclei such as 233Pa[2], 242;243Cm and 241Am[3]. The results are in very good agreement with neutron-induced data. We are currently investigating whether this powerful technique can also be used to determine radiative capture cross sections. For this purpose, we have studied the transfer reactions 174Yb(3He,pg)176Lu and 174Yb(3He,4Heg)173Yb as surrogates for the 175Lu(n,g) and 172Yb(n,g) reactions, respectively. The g probabilities obtained in our experiment show clear discrepancies with already existing neutroninduced data. In this contribution we will present the experimental set-up and the data analysis. Then, we will focus on the origin of these discrepencies. Perspectives for the surrogate method applied to capture cross sections will be discussed.

The modelling of the neutron cross sections consists in their interpretation in three different energy ranges: the first one is the Resolved Resonance Range (RRR) at low energy, the second is the Unresolved Resonance Range (URR), and the third one concerns the high energies. For this, we generally use the Reich-Moore approximation of the R-Matrix formalism, the average R-Matrix formalism and optical model calculations. One of the main challenges of such a work is to study the consistency of the average parameters obtained by these different calculations. With the ESTIMA and SPRT methods, we provide a set of parameters for partial s and p waves (strength functions, effective potential scattering radius, mean-level spacing and reduced neutron width). But, in order to analyse accurately the URR domain, we need more information than the parameters associated to the orbital moments L = 0a ndL = 1. This work describes the link between the average R-Matrix formalism and the optical model, especially concerning the transmission coefficients, total and shape elastic cross sections. Based on these two models, we propose a generalization of the SPRT method for L > 1, and we obtain a new set of parameters for the URR domain in terms of SLJ and R ∞ JL .

This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence 4.0; International audience; The feasibility of retrieving accurate fission observables with a Ge-detector array around a fissile target placed in a cold neutron beam was tested. In three measurement campaigns performed at ILL with the EXILL setup, 235U and 241Pu targets were placed in the high flux cold neutron beam available at the PF1B neutron guide. Gamma-rays following fission were detected by an array of 16 Ge detectors. In the following study, part of data was analyzed as a proof of principle. A set of yields belonging to the Kr-Ba pair were extracted using a gamma-gamma coincidence technique. Preliminary results were compared to the predictions of two phenomenological models: GEF and FIFRELIN.

In spite of the huge amount of fission yield data available in different libraries, more accurate values are still needed for nuclear energy applications and to improve our understanding of the fission process. Thus measurements of fission yields were performed at the mass spectrometer Lohengrin at the Institut Laue-Langevin in Grenoble, France. The mass separator Lohengrin is situated at the research reactor of the institute and permits the placement of an actinide layer in a high thermal neutron flux. It separates fragments according to their atomic mass, kinetic energy and ionic charge state by the action of magnetic and electric fields. Coupled to a high resolution ionization chamber the experiment was used to investigate the mass and isotopic yields of the light mass region. Almost all fission yields of isotopes from Th to Cf have been measured at Lohengrin with this method. To complete and improve the nuclear data libraries, these measurements have been extended in this work to the heavy mass region for the reactions 235U(nth,f), 239Pu(nth,f) and 241Pu(nth,f). For these higher masses an isotopic separation is no longer possible. So, a new method was undertaken with the reaction 239Pu(nth,f) to determine the isotopic yields by spectrometry. These experiments have allowed to reduce considerably the uncertainties. Moreover the ionic charge state and kinetic energy distributions were specifically studied and have shown, among others, nanosecond isomers for some masses, Les rendements de fission sont des données très importantes pour les applications nucléaires. Elles représentent par exemple une partie des données d'entrée des différents codes de simulation. De même les modèles théoriques qui tentent de reproduire ces distributions ont besoin de valeurs expérimentales auxquelles ils peuvent se comparer. Par le passé le spectromètre de masse Lohengrin a permis, grâce à sa très bonne résolution en masse et en énergie, de déterminer avec une grande précision les rendements des produits de fission légers pour de nombreux systèmes fissionants. Cependant la méthode expérimentale utilisée dans cette région de la fission, détection par chambre à ionisation, ne permet pas la séparation des isotopes dans la région des produits de fission lourds. Pour cette raison un nouveau dispositif expérimental par spectrométrie a été mis en place au cours de cette thèse. Il a été validé par comparaison des précédentes mesures dans la région des produits de fission légers et permet de déterminer les rendements isotopiques que ce soit pour les produits de fission légers comme pour les produits de fission lourds. Ainsi au cours de ces travaux, afin de compléter et d'améliorer les bibliothèques de donn ées, les rendements en masse des produits de fission lourds pour les réactions 235U(nth,f), 239Pu(nth,f) et 241Pu(nth,f) ont été mesurés pour la première fois sur le spectromètre Lohengrin. Ils ont permis d'améliorer considérablement les incertitudes associées. De la même manière les rendements isotopiques des produits de fission lourds pour le 239Pu(nth,f) ont été déterminés par spectrométrie . De plus les distributions en charge ionique et en énergie cinétique des produits de fission ont été particulièrement étudiées. Elles ont entre autres mis en évidence la présence d'isomères nanosecondes pour certains noyaux.