Towards Nuclear Data Evaluations Based on Many Body Theory

S. Hilaire,A. J. Koning,S. Goriely 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

The increasing need for cross sections far from the valley of stability poses a challenge for nuclear reaction models. So far, predictions of cross sections have relied on more or less phenomenological approaches, depending on parameters adjusted to available experimental data or deduced from systematic relations. While such predictions are expected to be reliable for nuclei not too far from the experimentally known regions, it is clearly preferable to use more fundamental approaches, based on sound physical bases, when dealing with very exotic nuclei. Thanks to the high computer power available today, all the ingredients required to model a nuclear reaction can now be (and have been) microscopically (or semi-microscopically) determined starting from the information provided by a nucleon-nucleon effective interaction. This concerns the nuclear masses, the optical model potential, the total nuclear level densities, the photon strength functions, as well as the fission barriers. All these nuclear model ingredients, traditionnaly given by phenomenological expressions, now have a microscopic counterpart implemented in the TALYS nuclear reaction code. We are thus now able to perform fully microscopic cross section calculations. We will discuss both the quality of these ingredients and the impact of using them instead of the usually adopted phenomenological parameters.

Improving the Description of Collective Effects within the Combinatorial Model of Nuclear Level Densities

S. Hilaire,M. Girod,S. Goriely 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

The combinatorial model of nuclear level densities has now reached a level of accuracy comparable to that of the best global analytical expressions without suffering from the limits imposed by the statistical hypothesis on which the latter expressions rely. In particular, it provides naturally, non Gaussian spin distribution as well as non equipartition of parities which are known to have a significant impact on cross section predictions at low energies [1, 2]. Our first global model developed in Ref. 1 suffered from deficiencies, in particular in the way the collective effects -both vibrational and rotational - were treated. We have recently improved this treatment using simultaneously the single particle levels and collective properties predicted by a newly derived Gogny interaction [3] , therefore enabling a microscopic description of energy-dependent shell, pairing and deformation effects. In addition, for deformed nuclei, the transition to sphericity is coherently taken into account on the basis of a temperature-dependent Hartree-Fock calculation which provides at each temperature the structure properties needed to build the level densities. This new method is described and shown to give promising preliminary results with respect to available experimental data.

Towards Improved Evaluation of Neutron-Induced Fission Cross Section

S. Goriely,S. Hilaire,A. J. Koning,R. Capote 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

Mean-field calculations can nowadays provide all the nuclear ingredients required to describe the fission path from the equilibrium deformation up to the nuclear scission point. The information obtained from microscopic mean-field models has been included in reaction codes to improve the predictions of neutron-induced fission cross section. The nuclear inputs concern not only the details of the energy surface along the fission path, but also the coherent estimate of the nuclear level density derived within the combinatorial approach on the basis of the same single-particle properties, in particular at the fission saddle points. The predictive power of such a microscopic approach is tested. It is also shown that the various inputs can be tuned to reproduce at best experimental data in one unique coherent framework, so that it is now possible to make reliable and accurate fission cross-section calculations on the basis of microscopic models, but also to use such approaches to estimate the corresponding modeling uncertainties for nuclei, energy ranges or reaction channels for which no data exist.

Nuclear Structure Properties with the Gogny Force

S. Hilaire,M. Girod,S. Goriely 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

The increasing need for nuclear data far from the valley of stability requires information on nuclei which cannot be accessed experimentally or for which almost no experimental data is known. Consequently, the use of microscopic approaches to predict properties of such poorly known nuclei is necessary as a first step to improve the quality of nuclear data evaluations. Within this context, large scale mean field calculations from proton to neutron drip-lines have been performed using the Hartree-Fock-Bogoliubov method based on the Gogny nucleon-nucleon effective interaction. This extensive study has shown the ability of the method to reproduce bulk nuclear structure data available experimentally. This includes nuclear masses, radii, matter densities, deformations, moment of inertia as well as collective mode (low energy and giant resonances). In particular, the first mass table based on a Gogny-Hartree-Fock-Bogolyubov calculation including an explicit and coherent account of all the quadrupole correlation energies is presented. The rms deviation with respect to essentially all the available mass data is 798 keV. Nearly 8000 nuclei have been studied under the axial symmetry hypothesis and going beyond the mean-field approach. The corresponding properties are made available to the nuclear scientific community on an internet web site for every individual nucleus. The content and original feature of this nuclear data library is also presented.

KAPPA DISTRIBUTION MODEL FOR HARD X-RAY CORONAL SOURCES OF SOLAR FLARES

Oka, M.,Ishikawa, S.,Saint-Hilaire, P.,Krucker, S.,Lin, R. P. IOP Publishing 2013 The Astrophysical journal Vol.764 No.1

<P>Solar flares produce hard X-ray emission, the photon spectrum of which is often represented by a combination of thermal and power-law distributions. However, the estimates of the number and total energy of non-thermal electrons are sensitive to the determination of the power-law cutoff energy. Here, we revisit an 'above-the-loop' coronal source observed by RHESSI on 2007 December 31 and show that a kappa distribution model can also be used to fit its spectrum. Because the kappa distribution has a Maxwellian-like core in addition to a high-energy power-law tail, the emission measure and temperature of the instantaneous electrons can be derived without assuming the cutoff energy. Moreover, the non-thermal fractions of electron number/energy densities can be uniquely estimated because they are functions of only the power-law index. With the kappa distribution model, we estimated that the total electron density of the coronal source region was similar to 2.4 x 10(10) cm(-3). We also estimated without assuming the source volume that a moderate fraction (similar to 20%) of electrons in the source region was non-thermal and carried similar to 52% of the total electron energy. The temperature was 28 MK, and the power-law index delta of the electron density distribution was -4.3. These results are compared to the conventional power-law models with and without a thermal core component.</P>

An Intrusive Method for the Uncertainty Propagation

P. Dossantos-Uzarralde,V. Nimal,G. Dejonghe,M. Sancandi,R. Andre,S. Hilaire 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

Models of physical processes like particle scattering often require adjusted parameters to fit experimental data. These parameters are basically uncertain and this feature spreads through the model down to the solution. In this study, an intrusive method of uncertainty propagation, based on Galerkin projection over chaos polynomials, is proposed for optical model calculations. This provides a way to evaluate the uncertainty of the solution induced by the uncertain parameters of the Wood-Saxon potential used form. We employ generalized polynomial chaos expansions (PCE) to express the random response of the optical model and obtain a set of deterministic coupled equations for the expansion coefficients by Galerkin projection. We justify the use of the Cowell method to solve this system in a decoupled fashion. Several moments of the solution are re-built. We provide an illustration of these method for the n+Y^(89) system.