E. Khan

Université Paris-Sud 11, Orsay, Île-de-France, France

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Publications (107)263.68 Total impact

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    ABSTRACT: The isoscalar monopole response has been measured in the unstable nucleus ^{68}Ni using inelastic alpha scattering at 50A MeV in inverse kinematics with the active target MAYA at GANIL. The isoscalar giant monopole resonance (ISGMR) centroid was determined to be 21.1±1.9 MeV and indications for a soft monopole mode are provided for the first time at 12.9±1.0 MeV. Analysis of the corresponding angular distributions using distorted-wave-born approximation with random-phase approximation transition densities indicates that the L=0 multipolarity dominates the cross section for the ISGMR and significantly contributes to the low-energy mode. The L=0 part of this low-energy mode, the soft monopole mode, is dominated by neutron excitations. This demonstrates the relevance of inelastic alpha scattering in inverse kinematics in order to probe both the ISGMR and isoscalar soft modes in neutron-rich nuclei.
    Physical Review Letters 07/2014; 113(3):032504. · 7.73 Impact Factor
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    ABSTRACT: The magic nature of the $^{54}$Ca nucleus is investigated in the light of the recent experimental results. We employ both HFB and HF+BCS methods using Skyrme-type SLy5, SLy5+T and T44 interactions. The evolution of the single-particle spectra is studied for the N=34 isotones: $^{60}$Fe, $^{58}$Cr, $^{56}$Ti and $^{54}$Ca. An increase is obtained in the neutron spin-orbit splittings of $p$ and $f$ states due to the effect of the tensor force which also makes $^{54}$Ca a magic nucleus candidate. QRPA calculations on top of HF+BCS are performed to investigate the first $J^{\pi}$=$2^{+}$ states of the calcium isotopic chain. A good agreement for excitation energies is obtained when we include the tensor force in the mean-field part of the calculations. The first $2^{+}$ states indicate a subshell closure for both $^{52}$Ca and $^{54}$Ca nuclei. We confirm that the tensor part of the interaction is quite essential in explaining the neutron subshell closure in $^{52}$Ca and $^{54}$Ca nuclei.
    Physical Review C 06/2014; 89(6):064322. · 3.72 Impact Factor
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    ABSTRACT: The framework of nuclear energy density functionals is applied to a study of the formation and evolution of cluster states in nuclei. The relativistic functional DD-ME2 is used in triaxial and reflection-asymmetric relativistic Hartree-Bogoliubov calculations of relatively light $N = Z$ and neutron-rich nuclei. The role of deformation and degeneracy of single-nucleon states in the formation of clusters is analysed, and interesting cluster structures are predicted in excited configurations of Be, C, O, Ne, Mg, Si, S, Ar and Ca $N = Z$ nuclei. Cluster phenomena in neutron-rich nuclei are discussed, and it is shown that in neutron-rich Be and C nuclei cluster states occur as a result of molecular bonding of $\alpha$-particles by the excess neutrons, and also that proton covalent bonding can occur in $^{10}$C.
    06/2014;
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    ABSTRACT: We study the finite temperature Hartree-Fock-BCS approximation for selected Sn nuclei with zero-range Skyrme forces. Hartree Fock BCS approximation allows for a straightforward interpretation of the results since it involves $u$ and $v$'s which are not matrices as in HFB. Pairing transitions from superfluid to the normal state are studied with respect to the temperature. The temperature dependence of the nuclear radii and neutron skin are also analyzed. An increase of proton and neutron radii is obtained in neutron rich nuclei especially above the critical temperature. Using different Skyrme energy functionals, a correlation between the effective mass in symmetric nuclear matter and the critical temperature is found. The temperature dependence of the nucleon effective mass is also investigated, showing that proton and neutron effective masses display different behavior below and above the critical temperature.
    06/2014;
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    ABSTRACT: The role of saturation for cluster formation in atomic nuclei is analyzed by considering three length-scale ratios and performing deformation-constrained self-consistent mean-field calculations. The effect of clusterization in deformed light systems is related to the saturation property of the internucleon interaction. The formation of clusters at low nucleon density is illustrated by expanding the radius of O16 in a constrained calculation. A phase diagram shows that the formation of clusters can be interpreted as a hybrid state between the crystal and the liquid phases. In the hybrid cluster phase the confining potential attenuates the delocalization generated by the effective nuclear interaction.
    Physical Review C 03/2014; 89:031303. · 3.72 Impact Factor
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    ABSTRACT: The role of superfluidity on the symmetry energy and on the incompressibility is studied in nuclear matter and finite nuclei employing Hartree-Fock-Bogoliubov modeling based on several types of pairing interactions (surface, mixed and isovector-density dependent). It is observed that, while pairing has only a marginal effect on the symmetry energy and on the incompressibility at saturation density, the effects are significantly larger at lower densities.
    European Physical Journal A 01/2014; 50(2). · 2.04 Impact Factor
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    ABSTRACT: The isoscalar monopole response is studied in doubly magic 208Pb, 100,132Sn nuclei using the Skyrme HF+RPA model. A low-energy strength is predicted and corresponds to almost pure single-particle excitations. These pure single-particle excitations allow to analyse the splitting of the corresponding spinorbit partners. A good agreement with the spin-orbit splitting data is found in the case of 208Pb. The experimental width of the giant monopole resonance may hinder the measurement of the soft monopole mode.
    European Physical Journal A 10/2013; 49(124):1-8. · 2.04 Impact Factor
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    ABSTRACT: The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.
    10/2013;
  • E. Khan, J. Margueron
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    ABSTRACT: Background: The determination of the density dependence of the nuclear incompressibility can be investigated using the isoscalar giant monopole resonance.Purpose: The importance of the so-called crossing density at subsaturation density is underlined.Methods: The measurements of the isoscalar giant monopole resonance (GMR), also called the breathing mode, are analyzed with respect to their constraints on the quantity M_{c}, e.g., the density dependence of the nuclear incompressibility around the so-called crossing density ρ_{c}=0.1 fm^{−3}.Results: The correlation between the centroid of the GMR, E_{GMR}, and M_{c} is shown to be more accurate than the one between E_{GMR} and the incompressibility modulus at saturation density, K_{∞}, giving rise to an improved determination on the nuclear equation of state. The relationship between M_{c} and K_{∞} is given as a function of the skewness parameter Q_{∞} associated with the density dependence of the equation of state. The large variation of Q_{∞} among different energy density functionals directly impacts the knowledge of K_{∞}: A better knowledge of Q_{∞} is required to deduce more accurately K_{∞}. Using the local density approximation, a simple and accurate expression relating E_{GMR} and the quantity M_{c} is derived and successfully compared to the fully microscopic predictions.Conclusions: The measurement of the GMR constrains the slope of the incompressibility M_{c} at the crossing density rather than the incompressibility modulus at the saturation density.
    Physical Review C 09/2013; 88(3). · 3.72 Impact Factor
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    ABSTRACT: The incompressibility of finite fermionic systems is investigated using analytical approaches and microscopic models. The incompressibility of a system is directly linked to the zero-point kinetic energy of constituent fermions, and this is a universal feature of fermionic systems. In the case of atomic nuclei, this implies a constant value of the incompressibility in medium-heavy and heavy nuclei. The evolution of nuclear incompressibility along Sn and Pb isotopic chains is analyzed using global microscopic models, based on both non-relativistic and relativistic energy functionals. The result is an almost constant incompressibility in stable nuclei and systems not far from stability, and a steep decrease in nuclei with pronounced neutron excess, caused by the emergence of a soft monopole mode in neutron-rich nuclei.
    Physical Review C 06/2013; 87:064311. · 3.72 Impact Factor
  • E. Khan, J. Margueron
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    ABSTRACT: The measurements of the isoscalar giant monopole resonance (GMR), also called the breathing mode, are analyzed with respect to their constraints on the quantity $M_c$, e.g. the density dependence of the nuclear incompressibility around the so-called crossing density $\rho_c$=0.1 fm$^{-3}$. The correlation between the centroid of the GMR, $E_\mathrm{GMR}$, and $M_c$ is shown to be more accurate than the one between $E_\mathrm{GMR}$ and the incompressibility modulus at saturation density, $K_\infty$, giving rise to an improved determination on the nuclear equation of state. The relationship between $M_c$ and $K_\infty$ is given as a function of the skewness parameter $Q_\infty$ associated to the density dependence of the equation of state. The large variation of $Q_\infty$ among different energy density functionnals directly impacts the knowledge of $K_\infty$: a better knowledge of $Q_\infty$ is required to deduce more accurately $K_\infty$. Using the Local Density Approximation, a simple and accurate expression relating $E_\mathrm{GMR}$ and the quantity $M_c$ is derived and successfully compared to the fully microscopic predictions.
    04/2013;
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  • E. Khan
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    ABSTRACT: Recent advances on the dipole and monopole responses of the atomic nucleus are discussed. The soft dipole response is diversified and its structure depends on the nucleus. The giant dipole resonance is involved in astrophysical processes such as the r-process and in the propagation of ultra-high-energy cosmic rays. A soft monopole response is predicted. The giant monopole resonance is related to the determination of the nuclear incompressibility, but an alternative quantity may be better extracted from measurements.
    Physica Scripta Volume T. 01/2013;
  • J. Margueron, E. Khan
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    ABSTRACT: Based on a microscopic description of superfluidity in overflowing nuclear systems, it is shown that continuum coupling plays an important role in the suppression, the persistence, and the reentrance of pairing. In such systems, the structure of the drip-line nucleus determines the suppression and the persistence of superfluidity. The reentrance of pairing with increasing temperature leads to additional critical temperatures between the normal and superfluid phases.
    Physical Review C 12/2012; 86(6). · 3.72 Impact Factor
  • E. Khan
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    ABSTRACT: The role of superfluidity in incompressibility is studied in nuclear matter and finite nuclei. Pairing has a weak effect on nuclear matter incompressibility at saturation density, but the effects are strong at lower densities. The pairing effect on the centroid energy of the isoscalar Giant Monopole Resonance (GMR) is also evaluated by using a microscopic constrained-Hartree-Fock Bogoliubov approach. The measurement of the GMR in isotopic chains including unstable nuclei is useful for the determination of nuclear incompressibility.
    Physica Scripta Volume T. 10/2012;
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    ABSTRACT: We compute electron-capture rates for 54,56Fe and Ge isotopes using a self-consistent microscopic approach. The single-nucleon basis and the occupation factors in the target nucleus are calculated in the finite-temperature Skyrme Hartree-Fock model, and the Jπ=0±, 1±, 2± charge-exchange transitions are determined in the finite-temperature random-phase approximation (RPA). The scheme is self-consistent; i.e., both the Hartree-Fock and the RPA equations are based on the same Skyrme functional. Several interactions are used in order to provide a theoretical uncertainty on the electron-capture rates for different astrophysical conditions. Comparing electron-capture rates obtained either with different Skyrme sets or with different available models indicates that differences up to one to two orders of magnitude can arise.
    Physical Review C 09/2012; 86(3). · 3.72 Impact Factor
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    ABSTRACT: Using the framework of nuclear energy density functionals we examine the conditions for single-nucleon localization and formation of cluster structures in finite nuclei. We propose to characterize localization by the ratio of the dispersion of single-nucleon wave functions to the average inter-nucleon distance. This parameter generally increases with mass and describes the gradual transition from a hybrid phase in light nuclei, characterized by the spatial localization of individual nucleon states that leads to the formation of cluster structures, toward the Fermi liquid phase in heavier nuclei. Values of the localization parameter that correspond to a crystal phase cannot occur in finite nuclei. Typical length and energy scales in nuclei allow the formation of liquid drops, clusters, and halo structures.
    Physical Review C 07/2012; 87(4). · 3.72 Impact Factor
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    ABSTRACT: Nucleonic matter displays a quantum-liquid structure, but in some cases finite nuclei behave like molecules composed of clusters of protons and neutrons. Clustering is a recurrent feature in light nuclei, from beryllium to nickel. Cluster structures are typically observed as excited states close to the corresponding decay threshold; the origin of this phenomenon lies in the effective nuclear interaction, but the detailed mechanism of clustering in nuclei has not yet been fully understood. Here we use the theoretical framework of energy-density functionals, encompassing both cluster and quantum liquid-drop aspects of nuclei, to show that conditions for cluster formation can in part be traced back to the depth of the confining nuclear potential. For the illustrative example of neon-20, we show that the depth of the potential determines the energy spacings between single-nucleon orbitals in deformed nuclei, the localization of the corresponding wavefunctions and, therefore, the degree of nucleonic density clustering. Relativistic functionals, in particular, are characterized by deep single-nucleon potentials. When compared to non-relativistic functionals that yield similar ground-state properties (binding energy, deformation, radii), they predict the occurrence of much more pronounced cluster structures. More generally, clustering is considered as a transitional phenomenon between crystalline and quantum-liquid phases of fermionic systems.
    Nature 07/2012; 487(7407):341-4. · 38.60 Impact Factor
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    ABSTRACT: In this proceeding we report on alpha particle emission through the nuclear break-up in the reaction 40Ca on a 40Ca target at 50A MeV. It is observed that alpha particles are emitted to the continuum with very specific angular distribution during the reaction. The alpha particle properties seem to be compatible with an alpha cluster in the daughter nucleus that is perturbed by the short range nuclear attraction of the collision partner and emitted as described by a time-dependent theory. This mechanism offers new possibilities to study alpha particle properties in the nuclear medium.
    International Journal of Modern Physics E 05/2012; 20(04). · 0.63 Impact Factor
  • E.khan, N.sandulescu
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    ABSTRACT: Finite temperature Hartree-Fock-Bogoliubov calculations are performed in finite nuclei using Skyrme and zero-range, density-dependent pairing interactions. The value of the critical temperature follows approximatively the empirical rule Tc ≃ 0.5 ΔT=0. The specific heat is calculated within the FT-HFB framework. Preliminary results obtained with the FT-QRPA are discussed.
    International Journal of Modern Physics E 05/2012; 20(02). · 0.63 Impact Factor

Publication Stats

1k Citations
263.68 Total Impact Points

Institutions

  • 2005–2014
    • Université Paris-Sud 11
      • Institut de Physique Nucléaire (IPN)
      Orsay, Île-de-France, France
  • 2013
    • Yildiz Technical University
      • Department of Physics
      İstanbul, Istanbul, Turkey
  • 2000–2012
    • Institut de Physique Nucléaire de Lyon
      Lyons, Rhône-Alpes, France
  • 2002–2010
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2007
    • University of Zagreb
      • Department of Physics (VEF)
      Zagrabia, Grad Zagreb, Croatia