Search

To probe the neutron inelastic scattering off 232Th, an experiment took place at the EC-JRC Geel conducted with the experimental setup GRAPhEME to detect emitted γ-rays. The prompt γ-ray spectroscopy method was used and 70 experimental 232Th(n, n’γ) cross sections were obtained from the experimental data. Combining these cross sections, nuclear-structure data available in databases and hypotheses to complete the latter, neutron inelastic level production cross sections in 232Th and the total inelastic cross section were calculated. For the first time, the total inelastic cross section of an actinide nucleus was derived on the total neutron energy range from experimental data only. Comparisons of (n, n’) cross section data with evaluated data reveal a good agreement between them all above 300 keV of neutron energy. TALYS calculations are compatible but lower than the evaluated data.

International audience; The production of useful and high-quality nuclear data requires measurements with high precision and extensive information on uncertainties and possible correlations. Analytical treatment of uncertainty propagation can become very tedious when dealing with a high number of parameters. Even worse, the production of a covariance matrix, usually needed in the evaluation process, will require lenghty and error-prone formulas. To work around these issues, we propose using random sampling techniques in the data analysis to obtain final values, uncertainties and covariances and for analyzing the sensitivity of the results to key parameters. We demonstrate this by one full analysis, one partial analysis and an analysis of the sensitivity to branching ratios in the case of (n,n’γ) cross section measurements.

International audience; The necessary improvement of evaluated nuclear data for nuclear applications development is possible through new and high-quality measurements, often combined with appropriate nuclear-reaction modelling. In particular, improving inelastic cross-section evaluations requires new and high-quality data. We measure (n, n’γ) cross-sections using prompt γ-ray spectroscopy and neutron energy determination by time-of-flight. To extract, from these partial data, the total inelastic cross-section, we rely on theoretical model as well as nuclear structure data such as γ ray emission probabilities. This structure information, tabulated in databases, comes with uncertainty. This directly affects the precision of our results, regardless of how good the measurement is. In this paper, we will present the issue of limited precision structure data and its impact on nuclear reaction data quality in the case of neutron inelastic scattering measurements. We will also discuss how to foresee and mitigate the issue.

The extended dataset of 56Fe(n,n’γ) cross sections measured by our group more than a decade ago at GELINA (Geel Linear Accelerator) was used in many recent evaluations like ENDF, JEFF and CIELO. Despite the special measures we took to ensure reliability and accuracy, concerns were raised by various groups with regard to several features of this dataset (absolute normalization and/or shape) and therefore the 56Fe(n,inl) cross section is still under the evaluation by the International Nuclear Data Evaluation Network (INDEN). Consequently, a new experiment is now under preparation aiming to take advantage of the numerous experimental improvements of the GAINS (Gamma Array for Inelastic Neutron Scattering) setup implemented over the years. While γ spectroscopy combined with the time-of-flight method will remain the main technique involved, several other experimental details will differ substantially.

GAINS (Gamma Array for Inelastic Neutron Scattering), currently installed at the GELINA (Geel Electron Linear Accelerator) neutron source of the EC-JRC (European Commission-Joint Research Centre), is one of the best instruments available worldwide for measuring high resolution, low uncertainty neutron inelastic cross-section data relevant for both fundamental research and nuclear physics applications (like Generation IV facilities). It features 12 HPGe detectors coupled with a 235U fission chamber for neutron flux monitoring. The paper presents a short history of the GAINS development over the years and reviews some of the main experimental results together with plans for future experiments.

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.

This paper reports partial results of a (n, n’γ) measurement on nickel. The inelastic channel was measured using the Gamma Array for Inelastic Neutron Scattering (GAINS) spectrometer at the 100-m measurement cabin of the Geel Electron Linear Accelerator (GELINA) neutron source of the European Commission’s Joint Research Centre (EC-JRC) in Geel, Belgium. Using γ spectroscopy, we were able to extract angle-integrated production cross sections for several γ rays but we report here only the results for the main transition in 58Ni. We discuss however in detail the observed discrepancy between our data and other experiments (especially the work of Voss et al.). We also shortly comment on the quality of the neutron-target optical model potential in describing the inelastic data in this mass region. The calculations were performed using the talys 1.9 code in the default settings.

A neutron inelastic scattering experiment was performed at the GELINA (Geel Electron LINear Accelerator) neutron source of the European Commission Joint Research Centre Geel (EC-JRC Geel) with the aim of determining the reaction cross sections for the stable isotopes of natural titanium. A 235U fission chamber was used to monitor neutrons with energies up to 20 MeV. The GAINS (Gamma Array for Inelastic Neutron Scattering) spectrometer was employed to detect the γ rays resulting from the decay of the excited nuclei. We determined the γ-ray production cross sections of the first transitions in the 46,48,49,50Ti isotopes. The experimental values were compared with previous reported results and also with theoretical calculations performed with the TALYS 1.8 code using the default input parameters. Uncertainties of around 5% were obtained for the strongest observed transitions.

During an under barrier Rutherford scattering of a proton on a high Z nucleus, in the vicinity of the target, the proton acquires a large potential energy. This energy makes its beta decay possible. Simplified estimates of the decay rates are provided for use in possible experimental attempts to observe the phenomenon. The problem rises a number of fundamental aspects that have a chance to be experimentally challenged.

233U is the fissile nucleus produced in 232T h/233U fuel cycle which has been proposed as asafer and cleaner alternative to the 238U/239P u cycle. The accurate knowledge of the neutroncapture cross-section of this isotope is needed with high-precision for design and developmentof this fuel cycle. The only two reliable experimental data for the capture cross-section of233U show discrepancies up to 10%. These differences may be due to systematic uncertaintiesassociated with the detector efficiency, dead-time effects, background subtraction and signalpile-up caused by the α-activity of the sample. A special experimental set-up for simultaneousmeasurement of fission and capture cross sections of radioactive fissile nuclei was designed,assembled and optimized at CENBG in the frame of this work. The measurement will be per-formed at the Gelina neutron time-of-flight facility at IRMM, where neutron cross sectionscan be measured over a wide energy range with high energy resolution. The fission detectorconsists of a multi-plate high-efficiency ionization chamber (IC). The γ-rays produced in cap-ture reactions are detected by an array of six C6 D6 scintillators surrounding the IC. In thesemeasurements the radiative capture γ-rays are hidden in large background of fission γ-rays thatrepresents a challenging issue. The latter has then to be subtracted by detecting fission eventswith a very well known efficiency (VETO method). An accurate determination of this efficiencyis rather difficult. In this work we have thoroughly investigated the prompt-fission-neutronsmethod for the IC efficiency measurement, providing new insights on this method. Thanks tothis study the IC efficiency was determined with a very low uncertainty. Using a 252Cf source,several parameters (gas pressure, high voltage and the distance between the electrodes) havebeen studied to determine the behaviour of the IC in order to find the ideal operation point:a good energy separation between α-particles and fission fragments (FF) and a good timingresolution. A good α-FF separation has been obtained with a highly radioactive 233U target.Also, the pulse-shape discrimination between γ-rays and neutrons in the C6D6 detectors wasobserved at Gelina under realistic experimental conditions. To conclude, the experimentalset-up and the VETO method have been carefully checked and validated, opening the way tofuture measurements of the capture and fission cross sections of 233U., 233 U est le noyau fissile produit dans le cycle du combustible 232 T h/233 U qui a été proposé comme une alternative plus sûre et plus propre du cycle 238 U/239 P u. La connaissance précise de la section efficace de capture de neutrons de cet isotope est requise avec une haute précision pour la conception et le développement de réacteurs utilisant ce cycle du combustible. Les deux seuls jeux de données expérimentales fiables pour la section efficace de capture de l’233 U montrent des écarts important allant jusqu’à 20%. Ces différences peuvent être dues à desincertitudes systématiques associées à l'efficacité du détecteur, la correction du temps mort, la soustraction du bruit de fond et le phénomène d’empilement de signaux causé par la forteactivité α de l’échantillon. Un dispositif expérimental dédié a la mesure simultanée des sections efficaces de fission et de capture des noyaux fissiles radioactifs a été conçu, assemblé et optimiséau CENBG dans le cadre de ce travail. La mesure sera effectuée à l’installation de temps de vol de neutrons Gelina de l’IRMM, où les sections efficaces neutroniques peuvent être mesurées sur une large gamme d’énergie avec une haute résolution énergétique. Le détecteur de fission se compose d’une chambre à ionisation (CI) multi-plaque de haute efficacité. Les rayons γ produits dans les réactions de capture sont détectés par un ensemble de six scintillateurs C6 D6entourant la CI. Dans ces mesures, les rayons γ de la capture radiative sont masqués parle grand nombre de rayons γ de fission, ce qui représente le problème le plus délicat. Ces γ parasites doivent être soustraits par la détection des événements de fission avec une efficacité très bien connue (méthode de VETO). Une détermination précise de cette efficacité est assezdifficile. Dans ce travail, nous avons soigneusement étudié la méthode des neutrons prompts de fission pour la mesure de l'efficacité de la CI, apportant un éclairage nouveau sur la méthode, ce qui a permi d’obtenir une excellente précision sur l'efficacité de détection des fission d’une sourcede 252 Cf. Avec cette même source, plusieurs paramètres (pression du gaz, haute tension et la distance entre les électrodes) ont été étudiés afin de déterminer le comportement de la CI et detrouver le point de fonctionnement idéal : une bonne séparation énergétique entre les particulesα et les fragments de fission (FF) et une bonne résolution temporelle. Une bonne séparationα-FF a également été obtenue avec une cible d’233 U très radioactive. De plus, l’analyse deforme de signaux entre les rayons γ et les neutrons dans les détecteurs C6 D6 a été observée àGelina dans des conditions expérimentales réalistes. Pour conclure, le dispositif expérimentalet la méthode de VETO ont été soigneusement vérifiés et validés, ouvrant la voie à la mesure future des sections efficaces de capture et fission d’233 U .