The (γSF) Method - Determination of Radiative Neutron Capture Cross Sections for Unstable Nuclei

H. Utsunomiya,S. Goriely,H. Akimune,H. Harada,F. Kitatani,S. Goko,H. Toyokawa,K. Yamada 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

An indirect method of determining radiative neutron-capture cross sections for unstable nuclei which we refer to as the γ-ray strength function (γSF) method is outlined and applied to Zr and Sn isotopes. Photoneutron cross sections for ^(91,92,94,96)Zr and ^(117,116)Sn near neutron threshold show the presence of extra γ-ray strengths known as giant M1 and pygmy E1 resonances on top of the low-energy γSF of GDR. Based on the γSF method, we present (n,γ) cross sections for two radioactive nuclei, a long-lived fission product, ^(93)Zr with T^(1/2) = 1.5 × 10^6 y and an s-process branching point nucleus, ^(95)Zr with T^(1/2) = 64.0 d.

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.

HFB Mass Models for Nucleosynthesis Applications

S. Goriely,N. Chamel,J. M. Pearson 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

The latest developments made in deriving microscopic mass models are presented. Mass models play a fundamental role by providing a large part of the nuclear structure properties required for a proper description of reaction and β-decay rates in nuclear astrophysics. Some recent progress made in estimating the nuclear structure properties, and most of all nuclear masses, in the framework of the Skyrme-Hartree-Fock-Bogolyubov (HFB) mean field model is detailed. The rms deviation obtained with our latest Skyrme forces with respect to essentially all the available mass data falls to 0.58 MeV, the best value ever found within the mean-field framework. Since our Skyrme forces are also constrained by the properties of pure neutron matter, including its stability against ferromagnetic instabilities, our new models are particularly well-suited for application to astrophysical problems involving a neutron-rich environment, such as the elucidation of the r-process of nucleosynthesis, and the description of supernova cores and neutron-star crusts. The impact of the mass predictions on the reaction rates for exotic neutron-rich nuclei is highlighted and the influence on nucleosynthesis calculations illustrated.

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.

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.

Impact of the Phonon Coupling on the Dipole Strength and Radiative NeutronCapture

A. Avdeenkov,S. Goriely,S. Kamerdzhiev 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.23

The E1 strength functions and radiative capture cross sections for several compound Sn isotopes, including unstable ^(132)Sn and ^(150)Sn, have been calculated using the self-consistent microscopic theory. In addition tothe standard RPA or QRPA approaches, the method includes the quasiparticle-phonon coupling and the single-particle continuum. The resultsobtained show that the phonon contribution significantly affects the pygmy dipole resonance, which is of particular relevance for a proper description of the radiative neutron capture. The impact of the phonon coupling on the pygmy dipole resonance and the radiativeneutron capture cross sections increases with the (N-Z) difference. For example, in the (0 - 10) MeV interval the fulltheory gives 17% of the energy-weighted sum rule for ^(150)Sn and 2.8% for ^(124)Sn, whereas the continuum QRPA approach gives 5.1% and 1.7%, respectively. These results indicate the importance the self-consistent calculation can have, especially when applied to neutron-rich nuclei of astrophysical interest. The comparison with the widely-used phenomenological Generalized Lorentzian approach shows that the (Q)RPA approach gives an increase in the neutron capture cross section by a factor of 2 for ^(132)Sn and a factor of 10 for ^(150)Sn and that the inclusion of the phonon coupling still increases these cross sections even furhter, by a factor of 2 - 3.