We report on high-resolution laser spectroscopy of ^{255}Fm (T_{1/2}=20  h), one of the heaviest nuclides available from reactor breeding. The hyperfine structures in two different atomic ground-state transitions at 398.4 nm and 398.2 nm were probed by in-source laser spectroscopy at the RISIKO mass separator in Mainz, using the perpendicularly illuminated laser ion source and trap (PI-LIST) high-resolution ion source. Experimental results were combined with hyperfine fields from various atomic ab initio calculations, in particular using multiconfiguration Dirac-Hartree-Fock theory, as implemented in grasp18. In this manner, the nuclear magnetic dipole and electric quadrupole moments were derived to be μ=-0.75(5)  μ_{N} and Q_{s}=+5.84(13)  eb, respectively. The magnetic moment indicates occupation of the ν7/2[613] Nilsson orbital, while the large quadrupole moment confirms strong, stable prolate deformation consistent with systematics in the heavy actinides. Comparisons with available expectation values from nuclear theory show good agreement, providing a stringent benchmark for the used theoretical models. These results revise earlier data and establish ^{255}Fm as a reference isotope for future high-resolution studies.

Nuclear physics influences are present throughout the Universe at every scale. Over the past decades, significant efforts have been dedicated to various fields related to nucleosynthesis and stellar evolution. These include both experimental and theoretical nuclear physics, as well as ground-and space-based astronomical observations and astrophysical modeling. Despite numerous successes, major challenges and unresolved questions persist. Concerning nuclear physics, despite the remarkable efforts of experimentalists in studying unstable targets, it remains unlikely that we will be able to measure the structure and interaction properties of all astrophysically relevant nuclei in the near future. Therefore, further progress must rely on theoretical approaches. The necessary ingredients (properties of cold and hot nuclei, nuclear level densities, optical potentials, photon strength functions, fission properties, β-strength functions) should ideally be derived from global, universal and microscopic models. New progress based on mean-field models are described and their impact on nuclear reaction rates of astrophysical relevance discussed and on nucleosynthesis illustrated.

The present contribution gives an overview of the photon strength function models developed to compensate for the lack of data for experimentally non-accessible nuclei. Phenomenological and microscopic approaches are presented in light of their respective strengths. Since the microscopic models included in the TALYS code are based on the quasi-particle random phase approximation (QRPA) method, the scope of application of this approach is presented. The QRPA-based strength functions, obtained using effective, covariant or Skyrme or Gogny interactions, reproduce the majority of experimental data with a variable level of precision which is briefly discussed. Some alternatives or updates to the gamma-ray strength functions based on QRPA methods are proposed. Returning to the definition of the photon strength function, particular attention is paid to the differences between the processes of photon absorption and the de-excitation of nuclei. As perspectives, preliminary theoretical results including probabilities of transition between excited states are presented.

Nuclear reaction models, and in particular compound nucleus reactions, require the knowledge of nuclear level densities (NLDs), among other ingredients. For decades, analytical expressions have been used in nuclear reaction codes, due to the freedom they offer to the user to modify their associated parameters in order to fit cross sections. The development of computational resources has opened a new era, roughly 20 years ago, by allowing calculation of NLDs from more microscopic approaches and their use in reaction codes through tables stored in databases. During this 20 year period, several approaches have been developed to improve step by step the physical description of NLDs. We review some of these efforts and show where we are now and what we foresee as future improvements.

We report on a microscopic modeling of the first order of multistep direct emission based on one phonon excitations, given by the QRPA model, and an effective in-medium nucleon nucleon interaction, described within the JLM folding model. The results of our coupled channels framework for deformed target are illustrated in the case of low energy discrete state excitations in 152Sm. Pre-equilibrium predictions and their impact on inclusive and exclusive (n,xn) cross sections and fission cross sections are discussed for actinides. We discuss the importance of i) collective excitations to describe the neutron emission spectra and of ii) spin distribution of the residual nucleus formed after the neutron pre-equilibrium emission, that is a key ingredient to model the residual nucleus decay.

The quantum-mechanical nuclear-shell structure determines the stability and limits of the existence of the heaviest nuclides with large proton numbers Z ≳ 100 (refs. 1-3). Shell effects also affect the sizes and shapes of atomic nuclei, as shown by laser spectroscopy studies in lighter nuclides4. However, experimental information on the charge radii and the nuclear moments of the heavy actinide elements, which link the heaviest naturally abundant nuclides with artificially produced superheavy elements, is sparse5. Here we present laser spectroscopy measurements along the fermium (Z = 100) isotopic chain and an extension of data in the nobelium isotopic chain (Z = 102) across a key region. Multiple production schemes and different advanced techniques were applied to determine the isotope shifts in atomic transitions, from which changes in the nuclear mean-square charge radii were extracted. A range of nuclear models based on energy density functionals reproduce well the observed smooth evolution of the nuclear size. Both the remarkable consistency of model prediction and the similarity of predictions for different isotopes suggest a transition to a regime in which shell effects have a diminished effect on the size compared with lighter nuclei.

One of the main methods used to microscopically describe collective states in atomic nuclei is the quasiparticle random-phase approximation (QRPA). However, due to its high computational cost, systematic studies covering the full nuclear chart are rare. In this work we show the first results of our systematic large-scale QRPA calculations. We do this by means of the quasiparticle finite-amplitude method (QFAM), which significantly reduces computation times. We use two kinds of interactions, the covariant DD-PC1 and a novel chiral interaction.

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.

We report on high-resolution laser spectroscopy studies on 249-253Cf with spectral linewidths in the order of 100 MHz carried out at the RISIKO mass separator at Mainz University. In total three atomic ground-state transitions were investigated and the hyperfine parameters for the odd-A isotopes and isotope shift for all examined isotopes have been determined from the measured spectra. The isotope shift measurements allowed tracking of changes in mean-squared charge radii across the deformed nuclear shell closure at N = 152, whereby shape discontinuities were not observed. Experimental hyperfine coupling constants of the atomic ground state were combined with relativistic many-body atomic calculations to extract the nuclear magnetic-dipole moment of 249Cf with improved precision to mu I(249Cf) = -0.395(17 )mu N, whereas mu I(251Cf) = -0.571(24 )mu N and mu I(253Cf) = -0.731(35 )mu N were derived for the first time. Additionally, the spectroscopic quadrupole moments QS(249Cf) = 6.27(33) eb and QS(253Cf) = 5.53(51) eb were extracted.

The necessary improvement of evaluated nuclear databases for appplication will be achieved with improvement of models and new, precise data. In particular, the effect of inelastic neutrons scattering can be of importance for reactors. In order to test the models, we performed measurement of (n, n’γ) and (n, 2nγ) cross-sections on 183 W. These data will help constrain the calculation codes and ensure a better evaluation of the total (n, x n) cross section. The experimental setup and the data analysis method will be presented. The preliminary experimental results for the 183 W isotope will be compared to predictions from Talys nuclear reaction code.

Microscopic modeling of inelastic scattering to the continuum is applied to neutron induced reaction on spherical and axially deformed even-even targets. The spin distributions of the residual compound nucleus formed after the fast inelastic process are calculated with two microscopic models and compared to the prescription usually associated to the semi-classical exciton model. As the semi-classical exciton model does not account for angular momentum conservation, it is often assumed that it is the same as the compound nucleus spin distribution, but this forgets the dynamics of the reaction. It is found that microscopic approaches drastically reduce the average spin value in comparison to what was previously assumed. This strongly impacts (n,n’ γ) as well as isomer production cross sections when high spin levels are involved. New spin cut-off parameters are deduced from the microscopic calculations that can be used as an alternative to previous prescriptions which neglect the reaction dynamics when they are applied in the context of pre-equilibrium emission process.

TALYS is a software package for the simulation of nuclear reactions below 200 MeV. It is used worldwide for the analysis and prediction of nuclear reactions and is based on state-of-art nuclear structure and nuclear reaction models. A general overview of the implemented physics and capabilities of TALYS is given. The general nuclear reaction mechanisms described are the optical model, direct reactions, compound nucleus model, pre-equilibrium reactions and fission. The most important nuclear structure models are those for masses, discrete levels, level densities, photon strength functions and fission barriers. A wide variety of nuclear reactions simulated with TALYS will be demonstrated, ranging from low-energy neutron cross sections, astrophysics, high-energy charged particle reactions and other reactions. TALYS is a nuclear reaction software which aims to give a complete description of nuclear reaction observables, and to be an important link between fundamental nuclear physics and applications.

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.

In the context of the development of Gen. IV nuclear reactors, the GIF (Generation IV. International Forum) has selected six innovative technologies. Among them, one can highlight the concept of breeding for 232 Th/ 233 U and 238 U/ 239 Pu fuel cycles. But those nuclei, crucial for such cycles, suffer from a lack of precise knowledge (nuclear structure, reaction cross sections). In particular, it has been demonstrated that neutron inelastic scattering reaction cross sections are not known with sufficient precision for the isotopes 238 U and 239 Pu, and not known at all experimentally for 233 U. In order to perform simulations of innovative reactor cores for the development of those technologies, the knowledge of the reaction cross section has to be improved which implies that new measurements have to be done. The GRAPhEME (GeRmanium array for Actinides PrEcise MEasurements) experimental setup, developed by the IPHC laboratory from CNRS and installed at the EC-JRC-Geel GELINA facility is a powerful tool to answer this need [1, 2]. Combining the prompt γ-ray spectroscopy and the time-of-flight methods, it measures partial (n, xnγ) reaction cross sections. This paper reports on the improvements made on the GRAPhEME setup and data analysis methodology to tackle the challenge of (n, xnγ) cross section measurements on high activity actinides. Results obtained so far on 233 U are presented compared to TALYS calculations.

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.

Today's development of nuclear installations rely on numerical simulation for which the main input are evaluated nuclear data. Inelastic neutron scattering (n, xn) is a reaction of importance because it modifies the neutron population, the neutron energy distribution and may create new isotopes. The study of this reaction on tungsten isotopes is interesting because it is a common structural material. Additionally, tungsten isotopes are a good testing field for theories. The IPHC group started an experimental program with the GRAPhEME setup installed at the neutron beam facility GELINA to measure (n, xn γ) reaction cross sections using prompt gamma spectroscopy and neutron energy determination by time-of-flight. The obtained experimental data provide constraints on nuclear reaction mechanisms models for 182,184,186 W. Indeed, to reproduce correctly the experimental (n, n' γ) cross-sections, the reaction codes must include accurate models of the reaction mechanism, nuclear de-excitation process and use correct nuclear structure information.

The γ-ray strength function (γSF) is a nuclear quantity that governs photoabsorption in (γ, n) and photoemission in (n, γ) reactions. Within the framework of the γ-ray strength function method, we use (γ, n) cross sections as experimental constraints on the γSF from the Hartree-Fock-Bogolyubov plus quasiparticle-random phase approximation based on the Gogny D1M interaction for E1 and M1 components. The experimentally constrained γSF is further supplemented with the zero-limit M1 and E1 strengths to construct the downward γSF with which (n, γ) cross sections are calculated. We investigate (n, γ) cross sections in the context of astrophysical applications over the nickel and barium isotopic chains along the s-process path.

Today's development of nuclear installations rely on numerical simulation for which the main input are evaluated nuclear data. Inelastic neutron scattering (n, xn) is a reaction of importance because it modifies the neutron population, the neutron energy distribution and may create new isotopes. The study of this reaction on tungsten isotopes is interesting because it is a common structural material. Additionally, tungsten isotopes are a good testing field for theories. The IPHC group started an experimental program with the GRAPhEME setup installed at the neutron beam facility GELINA to measure (n, xn γ) reaction cross sections using prompt gamma spectroscopy and neutron energy determination by time-of-flight. The obtained experimental data provide constraints on nuclear reaction mechanisms models for W. Indeed, to reproduce correctly the experimental (n, n' γ) cross-sections, the reaction codes must include accurate models of the reaction mechanism, nuclear de-excitation process and use correct nuclear structure information.

Premiere partie d’un ensemble de deux articles exposant le processus d’evaluation des donnees nucleaires, cet ensemble presente l’etat actuel de la connaissance theorique des phenomenes de physique nucleaire mis en jeu. Lors de l’evaluation, la connaissance theorique et experimentale est condensee et synthetisee dans des fichiers informatiques utilises par les codes de simulation. Apres un survol du contenu des fichiers evalues, nous decrivons les differentes methodes utilisees pour l’evaluation des donnees nucleaires. Nous exposons particulierement leur modelisation. Des exemples tires de la pratique quotidienne de l’evaluation sont presentes.