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Séminaires 2013 (physique nucléaire)

 Mercredi 11 Décembre 2013
Excitation-energy sorting in fission
Beatriz JURADO and Karl-Heinz SCHMIDT (Centre d’Études Nucléaires de Bordeaux-Gradignan)
Recent experiments have shown that at low excitation energy the nuclear level density follows rather well a "constant-temperature" behaviour in which the inverse logarithmic slope of the level density (which is also called "temperature") is nearly independent of the excitation energy. This temperature is roughly proportional to the inverse of the nucleus surface. In fission, on the way to scission, the nascent fragments form a system of two nuclei with different temperatures in thermal contact. Because of the invariance of the nucleus temperature with excitation energy, each fragment simulates the action of a thermal reservoir to the other one. This leads to energy sorting, i.e. the flow of thermal energy from the light (hotter) to the heavy (colder) fragment until the excitation energy in the light fragment is practically exhausted. The excitation-energy sorting process explains in a transparent way why an increase of excitation energy in the fissioning nucleus leads to an increase of the number of neutrons emitted by the heavy fission fragment only. This observation remained unexplained over decades. It also explains complex features observed in the even-odd staggering of fission-fragment Z distributions that have been puzzle since many years.

Mercredi 4 Décembre 2013
Phase transitions in compressed baryonic matter with strangeness
Adriana RADUTA (IFIN-HH, Bucharest-Magurele, POB-MG6, Roumanie)
The phase diagram of a system constituted of neutrons, protons and Lambda- hyperons in thermal equilibrium is evaluated in the mean-field approximation. It is shown that this simple system exhibits a complex phase diagram with first- and second-order phase transitions. Due to the generic presence of attractive and repulsive couplings, the existence of phase transitions involving strangeness appears independent of the specific interaction model. In addition we will show under which conditions a phase transition towards strange matter at high density exists, which is expected to persist even within a complete treatment including all the different strange and nonstrange baryon states. The impact of this transition on the composition of matter in the inner core of neutron stars is discussed.

Mercredi 27 Novembre 2013
Pairing properties of finite-systems within models restoring the good particle number symmetry
Danilo GAMBACURTA (Ganil, Caen)
Pairing correlations in nuclear systems play a crucial role in several aspects, i.e. binding energies and odd-even effects, superfluid phenomena and pair transfer mechanisms, just to say few of them. On the theoretical side, the standard description of these features is done by using BCS or HFB models which allow to describe in a simple way pairing effects. However, due to the explicit breaking of the particle number, these theories present some limitations which can be cured by using particle number projection techniques. In this talk we will show the merits of these techniques in the case of the Richardson pairing model and their efficiency in the study of different properties. We will start by introducing a beyond BCS-HFB method where the effect of four quasi-particle states is included perturbatively and by restoring subsequently the particle number, an extremely good description of ground state properties is achieved [1]. We will then show the need of restoring the good particle number also in the study of excited states and in the description of pair transfer probabilities in superfluid systems [2]. After that, a full Variation After Projection at finite temperature (VAP-FT) approach which allows a proper description of the thermodynamics properties of finite systems is applied to analyze and discuss the issue of the pairing phase transition in finite systems [3]. Finally, the VAP-FT method will be employed to analyze the influence of pairing correlations on the specific heat for the (161,162)Dy and (171,172)Yb isotopes [4].
[1] D. Lacroix and D. Gambacurta Phys. Rev. C 86, 014306 (2012)
[2] D. Gambacurta and D. Lacroix Phys. Rev. C 85, 064320 (2012)
[3] D. Gambacurta and D. Lacroix Phys. Rev. C 85, 044321 (2012)
[4] D. Gambacurta, D. Lacroix and N. Sandulescu, Phys. Rev. C, 88, 034324 (2013)

Mercredi 20 Novembre 2013
Microscopic description of nuclear motion : From giant resonances to fusion reactions
Guillaume SCAMPS (Ganil, Caen)
Pairing correlations play a major role in nuclear structure. In the present contribution, we would like to discuss the effect of pairing in collective motions and nuclear reactions like particle emission, transfer and fusion. The Time-Dependent Hartree-Fock Bogolyubov (TDHFB) theory has been developed to incorporate pairing theory in a fully microscopic transport theory in the TDHF +BCS simplified version. The application of this theory on the transfer reaction shows a strong effect of pairing which increase the probability to transfer one and two particles. For collective motion, a systematic study of the isoscalar (IS) and isovector (IV) quadrupole responses of nuclei along all the chart is done. It is found that the mean-energy of the collective high energy state globally reproduces the experimental IS and IV collective energy.

Mercredi 13 Novembre 2013
Strongly interacting matter near unitarity
Hans-Werner HAMMER (TU Darmstadt)
Particles with strong interactions characterized by a large scattering length have universal low-energy properties that do not depend on the details of the short-distance dynamics. Such systems can be realized in experiments with ultracold atoms close to a Feshbach resonance, but also occur in nuclear and particle physics. I will discuss signatures of universal physics in few- and many-body systems near the unitary limit of large scattering length.

Vendredi 8 novembre 2013
Neutral-current neutrino-nucleus reactions at finite temperature in supernova environments
Alan A. DZHIOEV and A. I. VDOVIN (Bogoliubov Laboratory of Theoretical Physics, JINR, Dubna)
We study thermal effcts on inelastic neutrino scattering and neutrino-antineutrino pair emission by hot nuclei. The studies are relevant for core collapse supernova simulations, as the considered processes may play an important role in the energy transport during the collapse. The reaction rates and cross sections are calculated for the sample nuclei 56Fe and 82Ge at temperatures typical for the supernova environment. The nuclear structure part of the problem is described within a quasiparticle random-phase approximation based on a phenomenological Hamiltonian with a BCS pairing force and separable multipole-multipole residual interactions in a particle-hole channel. Thermal effects are treated by applying the thermo field dynamics formalism [1,2]. For inelastic neutrino scattering, in agreement with the earlier studies [3], we observe a significant thermal enhancement in the cross section for low-energy neutrinos. We also find that temperature rise increases the neutrino pair emission rates from thermally excited nuclei and shifts the spectrum of emitted neutrinos to higher energies [4].
[1] A. A. Dzhioev, A. I. Vdovin et al, Phys. Rev. C81 015804 (2010).
[2] A. A. Dzhioev, A. I. Vdovin et al, Phys. At. Nucl. 74 1162 (2011) ; arXiv:1012.2543.
[3] A. Juodagalvis, K. Langanke et al, Nucl. Phys. A747 87 (2005).
[4] A. A. Dzhioev, A. I. Vdovin, to be published in Phys. At. Nucl. (2013) ; arXiv:1307.4011.

Mercredi 6 Novembre 2013
Proton-neutron pairing and alpha-like quartet correlations in atomic nuclei
Nicolae SANDULESCU (National Institute of Physics and Nuclear Engineering, Bucharest)
In nuclei with N=Z the isovector proton-neutron pairing is expected to coexist in equal amount with the neutron-neutron and proton-proton pairing, as a consequence of isospin invariance of the nuclear forces. The most common approach employed to treat the isovector pairing in these nuclei is the generalized BCS model. In this model, however, the proton-neutron pairs do not coexist with the like-particle pairs. This fact indicates the need for a more general formalism which goes beyond the BCS based approximations. One of such formalisms, proposed recently in Refs.[1-3], consists in describing the isovector pairing correlations in terms of alpha-like quartets built by two neutrons and two protons coupled to a total isospin T=0. We shall show that this formalism is able to describe with very high precision the isovector pairing correlations in the ground state of self-conjugate nuclei as well as in nuclei with N>Z. Using the quartet formalism we shall demonstrate that the isovector proton-neutron pairing has a significant contribution to the Wigner energy, i.e., the extra binding energy observed in the nuclei close to the N=Z line.
[1] N. Sandulescu, D. Negrea, J. Dukelsky, C. W. Johnson, Phys. Rev. C 85, 061303 (R) (2012).
[2] N. Sandulescu, D. Negrea, C. W. Johnson, Phys. Rev. C 86, 041302(R)(2012).
[3] M. Sambataro and N. Sandulescu, submitted to Phys. Rev. C.

Mercredi 30 octobre 2013
Nuclear Clustering in the Energy Density. Functional Approach
Jean-Paul EBRAN (CEA/DAM/DIF, F-91297 Arpajon)
Clusterization, i.e. the property of a system to have its elementary degrees of freedom aggregate into composite ones, is a generic phenomenon found at every scale of physics. In nuclear physics, clusterization leads to the existence of additional bound sub-structures of nucleons in the nucleus itself that contrast with the more common quantum liquid like behavior of atomic nuclei with nucleons roughly homogeneously distributed in the nuclear volume. If nuclear clustering is an essential feature of the structure of the lightest nuclei, its origin is not clearly established. To unravel this issue, a unified treatment of both the quantum liquid and cluster aspects of nuclei is needed. However, most of the approaches dedicated to the description of clusters in nuclei assume a priori their existence and are therefore more concerned about the clusters properties than their origin itself. On the other hand, clusters involve large nucleonic excitations across major shells that prevent nuclear configuration interaction from tackling them. Are self consistent mean field approaches a relevant tool to investigate nuclear clustering ? That is the question that will be discussed in the presentation.

Mercredi 16 octobre 2013
On the nature and the size of the σ meson
Miguel ALBALADEJO (Institut de Physique Nucléaire, Université Paris-Sud 11, Orsay)
In this seminar the nature of the σ or f0(600) resonance is discussed by evaluating its quadratic scalar radius, <r2>s σ. This allows one to have a quantitative estimate for the size of this resonance. We obtain that the s resonance is a compact object with <r2>sσ = (0.19 ± 0.02)i (0.06 ± 0.02) fm2. Within our approach, employing unitary chiral perturbation theory, the σ is a dynamically generated resonance that stems from the pion-pion interactions. Given its small size we conclude that the two pions inside the resonance are merged. A four-quark picture is then more appropriate. However, when the pion mass increases, for pion masses somewhat above 400 MeV, the picture of a two-pion molecule is the appropriate one. The σ is then a spread π π bound state. These results are connected with other recent works that support a non standard nature of the σ as well, while fulfilling strong QCD constraints, as well as with lattice QCD. We offer a detailed study of the low-energy S-wave pi pi scattering data from where we extract our values for the threshold parameters of S-wave pi pi phase shifts, the O(p4) chiral perturbation theory low energy constants as well as the σ pole position. From the comparison with other accurate determinations in the literature we obtain the average values for the I = 0 S-wave pi pi threshold parameters, a00 = 0.220 ± 0.003, b00 = 0.279 ± 0.003 M-2pi, and for the real and imaginary parts of the σ pole position in sqrt(s), 458 ± 14 - (261 ± 17)i MeV. The quark mass dependence of the size of the σ, its mass and width are considered too. The latter agree accurately with a previous lattice QCD calculation. The fact that the mass of this resonance tends to follow the threshold of two pions is a clear indication that the σ is a dynamically generated meson-meson resonance.

Mercredi 9 octobre 2013
An impurity in a Fermi sea on a narrow Feshbach resonance : A variational study of the polaronic and dimeronic branches
Christian TREFZGER (Laboratoire Kastler Brossel, ENS, Paris )
I will present the problem of a single impurity of mass M immersed in a Fermi sea of particles of mass m. The impurity and the fermions interact through a s-wave broad or narrow Feshbach resonance, so that the Feshbach length R* naturally appears in the system. Using simple variational ansätze, limited to at most one pair of particle-hole excitations of the Fermi sea, we determine the phase diagram for the polaronic and dimeronic branches. Furthermore, for the polaronic branch we determine ground state properties other than its energy, namely the polaron quasiparticle residue Z, and the impurity-to-fermion pair correlation function G(x). From the properties of G(x) we show that the polaron has a diverging rms radius and exhibits Friedel-like oscillations. In the weakly attractive limit, we obtain analytical results which reveal that G(x) exhibits an intriguing multiscale structure.

Mercredi 17 Juillet 2013
Challenges in Nuclear Astrophysics
Carlos BERTULANI (Texas A & M University-Commerce )
Eighty years of nuclear science has allowed us to infer the origin of the chemical elements out of which our bodies and the Earth are made. We now believe that the lightest elements were created in nuclear reactions in the first three minutes after the big bang, and all the rest were made in nuclear reactions inside the stars and distributed throughout interstellar space via stellar winds and giant stellar explosions. I will show how a new generation of theoretical developments and experiments can shed light on the complex nuclear processes that control the evolution of stars and stellar explosions.

Mardi 25 Juin 2013
Fusion, breakup and elastic scattering of weakly bound nuclei
Paulo GOMES (Instituto de Fisica, UFF, Brazil)
In the last years one has asked whether the complete fusion induced by light weakly bound projectiles, particularly halo nuclei, is enhanced or suppressed when compared with the situation where there is no-breakup process. There are two kinds of effects to be investigated. The first kind are static effects, caused by the longer tail of the optical potential, owing to the low binding energies of the weakly bound and specially halo nuclei. The second kind are the dynamical effects, which are due to the strong coupling between the elastic channel and the continuum states representing the breakup channel. Recent systematic results have shown that the dynamical effects due to breakup and transfer processes enhance the fusion cross section at sub- barrier energies and suppress it at energies above the barrier for stable and neutron-halo nuclei, although for proton-halo systems the behavior is not very clear. Also, the elastic scattering of weakly bound nuclei does not show the usual energy dependence of the optical model known as Threshold Anomaly (TA), but rather a different behavior called Breakup Threshold Anomaly (BTA). In this talk I will discuss the systematic results and the main open aspects in the field.

Mercredi 17 Avril 2013
Soft dipole Lambda mode in double-Lambda hypernuclei
Futoshi MINATO (JAEA Tokai, Japon)
In this talk, I will introduce a newly predicted dipole motion of Lambda hypernuclei. A Lambda hypernucleus is a nucleus containing at least one Lambda hyperon. A Lambda hyperon getting into a nucleus induces various changes in its nuclear structure, e.g. , a shrinkage of interval between clusters and a change from deformed to spherical shape. The presence of a Lambda hyperon also affects nuclear vibrational excitations significantly [1]. To study these effects, we apply the Random Phase Approximation to ^18_Lambda LambdaO in the framework of the Skyrme-Hartree-Fock with Lambda hyperon degree of freedom, and we observe a single dipole mode with a sizable strength emerging at lower energy region [2]. It turned out to be the dipole motion of the Lambda hyperons oscillating against the core nucleus out of phase. This dipole motion cannot be observed in a normal nucleus and it is similar to the soft dipole motion in halo nuclei, where weakly bound valence neutrons oscillate against the core nucleus. We further discuss the soft dipole Lambda mode systematically from light to heavy hypernuclei with a schematic approach [3].
[1] K. Hagino, J.M. Yao, F. Minato, Z.P. Li and M. Thi Win, to be published in Nucl. Phys. A
[2] F. Minato and K. Hagino, Phys. Rev. C80, 024316/1-10 (2012)
[3] F. Minato and K. Hagino, to be submitted

Mercredi 20 Mars 2013
Unifying fission, quasifission and the extra push
Neil ROWLEY (Groupe Théorie, IPN Orsay)
In the collision of two heavy nuclei, when Z1Z2 is not too large, crossing the Coulomb barrier leads to fusion into an excited compound nucleus. This object rapidly cools, by emitting neutrons, protons and alpha- particles, yielding a range of relatively long-lived evaporation residues (ER). The fusion-ER cross section then depends only on the probability of being captured by the potential barrier, though this probability is strongly influenced by the rotational and vibrational modes of the target and projectile. Indeed for sufficiently high-quality data, one can extract a ``barrier distribution" that provides a clear fingerprint of the nature of these collective modes. For heavier compound nuclei, fission can compete with evaporation and the cross sections for these two processes must then be added to recover the capture dynamics. However, if Z1Z2 ≥ 1600, a new process intervenes that complicates the picture by preventing compound-nucleus formation, even if the barrier is crossed. This phenomenon is known as quasifission (QF), since it is a binary process in which the kinetic energy of the two separating fragments is heavily damped (like fission) but their masses are similar to those in the entrance channel (unlike fission). This imposes severe limits on the creation of the superheavy elements. When QF comes into play, more energy (an ``extra push") may be required to obtain fusion, though with a much reduced probability. The greatly differing aspects of heavy-nucleus, exit-channel dynamics can be explained through the same static and dynamic deformations that give rise to the entrance-channel barrier distributions, though now they play an additional, decisive role via the properties of the neck that forms between the two participants.

Mercredi 13 Mars 2013
A new formulation to calculate general HFB matrix elements through the Pfaffian
Takahiro MIZUSAKI (Senshu University)
A new formula is presented for the calculation of matrix elements between multi-quasiparticle Hartree-Fock-Bogoliubov (HFB) states[1]. The formula is expressed in terms of the Pfaffian, and is derived by using Fermion coherent states with Grassmann numbers. It turns out that the formula corresponds to an extension of the generalized Wick’s theorem and simplifies the combinatorial complexity resulting from practical applications of the generalized Wick’s theorem by unifying the transition density and the transition pairing tensor in HFB theory. The resultant formula is simpler and more compact than the traditional description of matrix elements of general many-body operators. In addition, through the derivation of our new formula, we found that the Pfaffian version of the Lewis Carroll formula corresponds to a relation suggested by Balian and Brezin for HFB theory in 1969.
[1]M. Oi and T. Mizusaki, Phys. Lett. B 707, 305 (2012), T. Mizusaki and M. Oi, Phys. Lett. B 715, 219 (2012)

Mercredi 6 Mars 2013
Predicting exotic nuclei
Vittorio SOMÀ (TU Darmstadt)
As ab-initio calculations of atomic nuclei enter the A=40-100 mass range, one of the biggest challenges is to provide accurate predictions for the vast majority of open-shell (degenerate) isotopes. The Gorkov-Green’s function approach, starting from realistic nuclear interactions, allows for first-principle calculations of truly open-shell systems, extending the scope of ab-initio methods to hundreds of nuclei and giving access to full isotopic and isotonic chains in the medium-mass region. The main output of the formalism is the single-particle spectral function, which describes processes involving the addition or knockout of a nucleon and provides a theoretical optical potential for elastic scattering. Binding energies, as well as all one-body observables can be consistently computed from the single-particle propagator. In this talk I will present the most recent developments of the Gorkov-Green’s function method. These include, in particular, benchmarks that assess the quality of the calculations, implementation of three-body forces and the first applications

Mercredi 27 Février 2013
Spectroscopy of light nuclei with the multiparticle-multihole Gogny energy density functional
For the first time, a rather complete set of observables — 2n and 2p separation energies, binding energies, magnetic and spectroscopic quadrupole moments as well as electromagnetic transition probabilities — are calculated within the multiparticle-multihole configuration mixing approach [1,2] and compared to experimental data. The D1S Gogny interaction has been used. This study focuses on even-even nuclei with 10≤N,Z≤18 and a systematic investigation of the properties of 1+1, 2+1, 2+2, 3+1, 3+2 and 4+1 states has been carried out. A very satisfactory agreement is found with experiment for excitation energies, magnetic and spectroscopic quadrupole moments and B(M1) transition probabilities. Systematic smaller values than experiment are found for B(E2) transition probabilities. However, the experimental trend for B(E2 ; 2+1 —> 0+1) are quite well reproduced along isotopic and isotonic chains.
[1] N. Pillet et al., Phys. Rev. C 78 (2008) 024305.
[2] N. Pillet et al., Phys. Rev. C 85 (2012) 044315.

Mercredi 20 Février 2013
Equation of state for ultracold fermions
Xavier LEYRONAS (Laboratoire de Physique Statistique, ENS )
The field of ultra cold fermions in two internal states ("spin-one half" fermions) will be introduced. These systems are particularly rich since one can experimentally tune the strength of the interaction, or "polarize" the system. Then I will present my work on the calculation of the equation of state in various situations. These calculations will be compared to accurate experiments on ultracold Li6 gases performed at ENS and MIT.

Mercredi 13 Février 2013
Diffusion élastique de nucléons par un noyau à couches fermées dans l’approximation HF+RPA avec l’interaction de Gogny
Nous étudions la diffusion élastique de nucléons par un noyau sphérique. Le potentiel optique, régissant l’interaction entre le nucléon projectile et le noyau cible, est déterminé à l’approximation Hartree-Fock plus RPA (HF+RPA). Nous reprenons l’approche de la structure nucléaire développée par N. Vinh Mau et A. Bouyssy [1]. L’interaction de Gogny est utilisée de manière cohérente afin de générer le terme Hartree-Fock et le terme RPA du potentiel optique microscopique. Le potentiel obtenu est non-local et dépend de l’énergie incidente du projectile.L’équation intégro-différentielle de Schroedinger est résolue aussi bien pour les états liés que pour les états du continuum, sans aucune procédure de localisation.
[1] N. Vinh Mau, Theory of nuclear structure (IAEA, Vienna, 1970) p. 931.

Lundi 28 Janvier 2013 et Mercredi 30 Janvier 2013
The Unitary Correlation OperatorMethod in Nuclear Structure
Panagiota PAPAKONSTANTINOU (Groupe Théorie, IPN Orsay)
Interaction-induced correlations play a subtle yet crucial role in nuclear structure. Although their signatures are often elusive, such correlations underliemany of the complications of the nuclear manybody problem. The purpose of this series of two lectures is to present an intuitive and versatile method to deal with correlations in nuclear systems, namely the Unitary Correlation Operator Method (UCOM) in the form worked out by H.Feldmeier et al.(*) over a decade ago. Applications will be discussed, with some emphasis on further prospects for predictive nuclear-response theory throughout the nuclear chart.
The first lecture will focus on the concept of correlations, some historical aspects, the ideas behind the UCOM, and basic formalism. The second lecture will deal with the UCOM in more detail : selected applications in nuclear structure and response within various many-body methods, some limitations, and relations with other modern attempts at ab initio large-scale calculations using renormalized Hamiltonians or purely phenomenological approaches.
Each lecture is scheduled to last one hour, with extra time to allow for questions and discussion. An effort will be made to maintain a mixed level, so that beginners can follow while the experienced ones can find the material rewarding.
(*)For a related review see R.Roth et al., Prog. Part. Nucl. Phys. 65 (2010) 50-93.
1er cours : Lundi 28 Janvier 2013, 14h00-15h30
Part I : Interaction-induced correlations - concepts, signatures, introduction to the UCOM.
2ème cours : Mercredi 30 Janvier 2013, 11h00-12h30
Part II : UCOM and nuclear structure - applications to the response of heavy nuclei ; connections with other methods.



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