Spectroscopie gamma en ligne de noyaux légers riches en neutrons produits par fragmentation de faisceau radioactif et mesures de temps de vie des niveaux excités dans les noyaux proches de 68^{68}Ni

This thesis deals with studies of nuclei far from the valley of stability produced at GANIL by projectile fragmentation at intermediate energies. It consists of two parts. The first one is dedicated to the study of very light exotic nuclei around N=14. This is the first time that online gamma-ray spectroscopy combined with the projectile fragmentation was used with radioactive incident beams at GANIL. The advantages and the limitations of this method were established. 40 different nuclei have been produced and studied at the same time. A strong dependence of the population of excited states on the type of projectile was observed. New information was obtained on the structure of the isotopes 14,15B, 17,18,19,20C, 18,19,20,21,22N, 22,23,24O, 24,25,26F and 29Ne. The level schemes obtained from this study have been compared with shell-model predictions. In particular, the energy of 1588(20) keV found for the first 2+ excited state in 20C, as well as the non-existence of a bound state in 24O, show that the protonneutron interaction plays an important role in the structure of these nuclei. In the second part, an experiment is presented concerning the neutron-rich isomer nuclei around 68Ni produced by the LISE spectrometer. The fast-timing method was applied for the first time for the study of nuclei produced by projectile fragmentation. Subnanosecond half-lives of several levels in 67,69,90Ni and 71,72Cu were measured simultaneously and with high precision. These results have allowed us to test the shell model predictions for several E2 transitions and their associated B(E2) transition probabilities.Cette thèse entre dans le cadre de l'étude des noyaux loin de la vallée de stabilité produits au GANIL par fragmentation du projectile aux énergies intermédiaires. Elle comprend deux parties. La première concerne l'étude des noyaux exotiques très légers ayant des nombres de neutrons proches de 14. C'est la première fois que la méthode spectroscopie gamma en-ligne par fragmentation du projectile est utilisée avec des faisceaux incidents radioactifs au GANIL. Ainsi les performances et les limitations de cette méthode ont pu être établies. 40 noyaux différents ont été produits et étudiés en même temps. Une forte dépendance dans les peuplements des états excités en fonction du type de projectile a été observée. Des informations nouvelles sur la structure ont pu être obtenues pour les isotopes de 14,15B, 17,18,19,20C, 18,19,20,21,22N, 22,23,24O, 24,25,26F, 29Ne. Les schémas de niveaux établis ont été comparés aux prédictions du modèle en couches. En particulier l'énergie de 1588(20) keV trouvée pour le premier état excité 2+ dans 20C ainsi que l'inexistence d'un état lié dans 24O a montré que l'interaction proton-neutron joue un rôle très important dans la structure de ces noyaux. Dans la deuxième partie, une expérience concernant les noyaux isomères riches en neutrons autour de 68Ni produits par le spectromètre LISE est présentée. C'est la première fois que la méthode "fast timing" est appliquée à l'étude des noyaux produits par la fragmentation du projectile. Des temps de vie entre quelques dizaines de picosecondes et quelques nanosecondes ont été mesurés avec une grande précision pour plusieurs niveaux dans les noyaux 67,69,90Ni et 71,72Cu. Ces résultats ont permis de vérifier les prédictions du modèle en couches pour plusieurs transitions de type E2 et leurs probabilités de transitions B(E2) associées

Spectroscopie gamma en ligne de noyaux légers riches en neutrons produits par fragmentation de faisceau radioactif et mesures de temps de vie des niveaux excités dans les noyaux proches du 68Ni

CAEN-BU Sciences et STAPS (141182103) / SudocSTRASBOURG-Bib.Central Recherche (674822133) / SudocSudocFranceF

Neutron Inelastic Cross Section Measurements for Sodium

Inelastic scattering cross sections for sodium are of interest to the development of sodium cooled fast reactors. A recent OECD-NEA subgroup analysed the sensitivity of reactor parameters to cross sections and accordingly determined target uncertainties for the nuclear data. Comparing these target uncertainties with the current status of nuclear data uncertainties and covariance data resulted in a list of target priorities. Among these features sodium inelastic scattering for which a target uncertainty of about 5% was established, approximately two to three times as good as the uncertainty for current evaluated data files for this isotope (see OECD-NEA High Priority Request List, http://www.nea.fr/html/dbdata/hprl/hprlview.pl?ID=448). At IRMM, the GAINS gamma-array for inelastic neutron scattering was developed with the purpose of measuring cross sections with uncertainties at or below the target uncertainties for nuclides like 23Na using the (n,n'gamma)-technique. Measurements were performed at the GELINA facility at a 200 m flight path with eight high purity germanium detectors. The sample was an 80 mm diameter metallic disk prepared at IRMM by cutting, pressing, rolling and punching. For the experimental work, a careful review was made of the gamma-efficiency calibrations and the flux normalization in order to investigate in detail the corrections and the final uncertainties that may realistically be achieved. An elaborate account will be presented of the data analysis and checks that have been made and implications for earlier work by our group will be discussed. First results will be shown for the gamma-production cross sections of the main transitions in the energy range from threshold to 10 MeV.JRC.DG.D.5-Nuclear physic

( n , x n ) cross sections on 56,57 Fe

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(n,xn

In the context of improvement of nuclear data bases for future nuclear reactor researches, we study (n,xn) reactions experimentally with the help of the (n,xn γ) technique. The experiments are performed at the GELINA facility which delivers a pulsed, white neutron beam. Several measurement campaigns have been performed on 235U, 232Th, 182,183,184,186W and 238U isotopes. A compilation of all these experimental cross sections (mostly still preliminary) compared to theoretical predictions will be discussed

Measurement of (n,xngamma) Reactions of Interest for the New Nuclear Reactors

The design of Generation IV nuclear reactors and the research of new fuel cycles require knowledge of cross sections of different nuclear reactions. Our research is focused on cross section measurements of (n,xngamma) reactions occurring in these new reactors. The aim is to measure unknown cross sections and to reduce uncertainty on present data relative to reactions and isotopes present in transmutation or regeneration processes. The current work consists of studying 232Th(n,n'gamma) and 235U(n,xngamma) reactions in the fast neutron energy domain (up to 20 MeV). The experiments are performed at GELINA which delivers a pulsed, white neutron beam at IRMM, Belgium. The time characteristics of the beam enable us to measure neutron energies with the time of flight (TOF) technique. The neutron induced reactions (in this case inelastic scattering and (n,2n) reactions) are identified by online prompt gamma spectroscopy with an experimental setup including 4 HPGe detectors. A double layered fission chamber is used to monitor the incident neutron flux. The experimental setup and analysis methods will be presented and a comparison between the obtained cross sections and the TALYS predictions will be discussed. This work is a first step in the preparation of the measurement of 233U(n,xngamma) reactions, which are completely unknown at this stage although of very high importance in the 232Th regeneration process.JRC.DG.D.5-Nuclear physic

Measurement of (n,xngamma) Reactions of Interest for New Nuclear Reactors

Our presented research is focused on cross section measurements of (n,xn gamma) reactions in the framework of Generation IV nuclear reactors studies. Indeed the development of new fast reactors or the investigations concerning new fuel cycles require the improvement of nuclear data bases over a wide range of energies, nuclei and reactions. One of the challenges of new measurements, in this field, is the accuracy level that they can reach and which is required by the Nuclear Data High Priority List produced by the NEA. Our collaboration has developed an experimental set up based on the prompt gamma ray spectroscopy method, using the GELINA facility of IRMM at Geel (Belgium), which produces a pulsed, white neutron beam. The results concerning 232Th(n,xn gamma) and 235U(n,xn gamma) reactions cross sections measurement, in the fast neutron energy domain (up to 20 MeV), are presented and compared with existing experimental data but also with theoretical TALYS calculations. All these investigations are performed in the framework of ANDES program (7th framework program, EURATOM).JRC.D.5-Nuclear physic

Measurement of 235U(n,n'g) and 235U(n,2n) reaction cross sections

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(n,xnγ) cross sections on actinides versus reaction code calculations

The experimental setup GRAPhEME (GeRmanium array for Actinides PrEcise MEasurements) has been used at GELINA (EC-JRC, Geel, Belgium) to perform (n,xn γ) cross sections measurements. GRAPhEME has been especially designed to take into account the specific difficulties generated by the use of actinides samples. This work takes place in the context of new nuclear data measurements for nuclear reactor applications. Considering the very tight accuracy requested for new experimental data, special care has been paid to quantify as accurately as possible all the uncertainties from the instruments and the analysis procedure. From the precise (n,xn γ) cross sections produced with GRAPhEME, the use of model calculations is required to obtain (n,xn) cross sections. Beyond the measurements, extensive work on theoretical models is necessary to achieve a better evaluation of the (n,xn) processes. In this paper, we will discuss the final step of the 238U data analysis and present some recent results obtained on 232Th compared to TALYS modellings. A new measurement campaign on 233U has started recently, a first assessment of the recorded data will be presented