arXiv subjects

C. Simenel

Publications and source records attributed to C. Simenel.

At least 55 records · Page 3Linked to original sources

arXiv2015

Dynamics of quasifission

Quasifission is the primary reaction mechanism that limits the formation of superheavy nuclei and consequently an important ingredient for choosing the best target-projectile combinations for the heavy element searches. Quasifission is characterized by nuclear contact-times that are much longer than those found in deep-inelastic reactions, resulting in a substantial mass and charge transfer. In this manuscript we employ the fully microscopic time-dependent Hartree-Fock (TDHF) theory to study quasifission. New results are presented for the $^{48}$Ca+$^{249}$Bk system.

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arXiv2015

Shape evolution and collective dynamics of quasifission in TDHF

Background: At energies near the Coulomb barrier, capture reactions in heavy-ion collisions result either in fusion or in quasifission. The former produces a compound nucleus in statistical equilibrium, while the second leads to a reseparation of the fragments after partial mass equilibration without formation of a compound nucleus. Extracting the compound nucleus formation probability is crucial to predict superheavy-element formation cross-sections. It requires a good knowledge of the fragment angular distribution which itself depends on quantities such as moments of inertia and excitation energies which have so far been somewhat arbitrary for the quasifission contribution. Methods: We investigate the evolution of the nuclear density in time-dependent Hartree-Fock (TDHF) calculations leading to quasifission. Our main goal is to extract ingredients of the formula used in the analysis of experimental angular distributions. These include the moment-of-inertia and temperature. We study the dependence of these quantities on various initial conditions of the reaction process. Results: The evolution of the moment of inertia is clearly non-trivial and depends strongly on the characteristics of the collision. The temperature rises quickly when the kinetic energy is transformed into internal excitation. Then, it rises slowly during mass transfer. Conclusions: Fully microscopic theories are useful to predict the complex evolution of quantities required in macroscopic models of quasifission.

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arXiv2015

Reduced quasifission competition in fusion reactions forming neutron-rich heavy elements

Measurements of mass-angle distributions (MADs) for Cr + W reactions, providing a wide range in the neutron-to-proton ratio of the compound system, (N/Z)CN, have allowed for the dependence of quasifission on the (N/Z)CN to be determined in a model-independent way. Previous experimental and theoretical studies had produced conflicting conclusions. The experimental MADs reveal an increase in contact time and mass evolution of the quasifission fragments with increasing (N/Z)CN, which is indicative of an increase in the fusion probability. The experimental results are in agreement with microscopic time-dependent Hartree-Fock calculations of the quasifission process. The experimental and theoretical results favor the use of the most neutron-rich projectiles and targets for the production of heavy and superheavy nuclei.

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arXiv2014

Interplay between quantum shells and orientation in quasi-fission

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in $^{40}$Ca$+^{238}$U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed $^{238}$U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of $^{238}$U produce quasi-fission fragments in the magic $Z=82$ region, whilst collisions with the side are the only one which may result in fusion.

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arXiv2014

Dissipative dynamics in quasi-fission

Quasi-fission is the primary reaction mechanism that prevents the formation of superheavy elements in heavy-ion fusion experiments. Employing the time-dependent density functional theory approach we study quasi-fission in the systems $^{40,48}$Ca+$^{238}$U. Results show that for $^{48}$Ca projectiles the quasi-fission is substantially reduced in comparison to the $^{40}$Ca case. This partly explains the success of superheavy element formation with $^{48}$Ca beams. For the first time, we also calculate the repartition of excitation energies of the two fragments in a dynamic microscopic theory. The system is found in quasi-thermal equilibrium only for reactions with $^{40}$Ca. The differences between both systems are interpreted in terms of initial neutron to proton asymmetry of the colliding partners.

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arXiv2014

Microscopic study of the effect of intrinsic degrees of freedom on fusion

Fusion cross-sections are computed for the $^{40}$Ca$+^{40}$Ca system over a wide energy range with two microscopic approaches where the only phenomenological input is the Skyrme energy density functional. The first method is based on the coupled-channels formalism, using the bare nucleus-nucleus potential calculated with the frozen Hartree-Fock technique and the deformation parameters of vibrational states computed with the time-dependent Hartree-Fock (TDHF) approach. The second method is based on the density-constrained TDHF method to generate nucleus-nucleus potentials from TDHF evolution. Both approaches incorporate the effect of couplings to internal degrees of freedoms in different ways. The predictions are in relatively good agreement with experimental data.

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arXiv2014

Challenges in description of heavy-ion collisions with microscopic time-dependent approaches

Important efforts have been dedicated in the past few years to describe near-barrier heavy-ion collisions with microscopic quantum theories like the time-dependent Hartree-Fock approach and some of its extensions. However, this field is still facing important challenges such as the description of cluster dynamics, the prediction of fragment characteristics in damped collisions, and sub-barrier fusion by quantum tunnelling. These challenges are discussed and possible approaches to solve them are presented.

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arXiv2014

Formation and dynamics of fission fragments

Although the overall time-scale for nuclear fission is long, suggesting a slow process, rapid shape evolution occurs in its later stages near scission. Theoretical prediction of the fission fragments and their characteristics are often based on the assumption that the internal degrees of freedom are equilibrated along the fission path. However, this adiabatic approximation may break down near scission. This is studied for the symmetric fission of $^{258,264}$Fm. The non-adiabatic evolution is computed using the time-dependent Hartree-Fock method, starting from an adiabatic configuration where the fragments have acquired their identity. It is shown that dynamics has an important effect on the kinetic and excitation energies of the fragments. The vibrational modes of the fragments in the post-scission evolution are also analyzed.

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arXiv2014

Energy dependence of potential barriers and its effect on fusion cross-sections

Couplings between relative motion and internal structures are known to affect fusion barriers by dynamically modifying the densities of the colliding nuclei. The effect is expected to be stronger at energies near the barrier top, where changes in density have longer time to develop than at higher energies. Quantitatively, modern TDHF calculations are able to predict realistic fusion thresholds. However, the evolution of the potential barrier with bombarding energy remains to be confronted with the experimental data. The aim is to find signatures of the energy dependence of the barrier by comparing fusion cross-sections calculated from potentials obtained at different bombarding energies with the experimental data. This comparison is made for the $^{40}$Ca+$^{40}$Ca and $^{16}$O+$^{208}$Pb systems. Fusion cross-sections are computed from potentials calculated with the density-constrained TDHF method. The couplings decrease the barrier at low-energy in both cases. A deviation from the Woods-Saxon nuclear potential is also observed at the lowest energies. In general, fusion cross-sections around a given energy are better reproduced by the potential calculated at this energy. The coordinate-dependent mass plays a crucial role for the reproduction of sub-barrier fusion cross-sections. Effects of the energy dependence of the potential can be found in experimental barrier distributions only if the variation of the barrier is significant in the energy-range spanned by the distribution. It appears to be the case for $^{16}$O+$^{208}$Pb but not for $^{40}$Ca+$^{40}$Ca. These results show that the energy dependence of the barrier predicted in TDHF calculations is realistic. This confirms that the TDHF approach can be used to study the couplings between relative motion and internal degrees of freedom in heavy-ion collisions.

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arXiv2013

Microscopic approach to coupled-channels effects on fusion

Background: Near-barrier fusion can be strongly affected by the coupling between relative motion and internal degrees of freedom of the collision partners. The time-dependent Hartree-Fock (TDHF) theory and the coupled-channels (CC) method are standard approaches to investigate this aspect of fusion dynamics. However, both approaches present limitations, such as a lack of tunnelling of the many-body wave function in the former and a need for external parameters to describe the nucleus-nucleus potential and the couplings in the latter. Method: A method combining both approaches is proposed to overcome these limitations. CC calculations are performed using two types of inputs from Hartree-Fock (HF) theory: the nucleus-nucleus potential calculated with the frozen HF method, and the properties of low-lying vibrational states and giant resonances computed from the TDHF linear response. Results: The effect of the couplings to vibrational modes is studied in the $^{40}$Ca$+^{40}$Ca and $^{56}$Ni$+^{56}$Ni systems. This work demonstrates that the main effect of these couplings is a lowering of the barrier, in good agreement with the fusion thresholds predicted by TDHF calculations. Conclusions: As the only phenomenological inputs are the choice of the internal states of the nuclei and the parameters of the energy density functional used in the HF and TDHF calculations, the method presented in this work has a broad range of possible applications, including studies of alternative couplings or reactions involving exotic nuclei.

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arXiv2013

Probing quantum many-body dynamics in nuclear systems

Quantum many-body nuclear dynamics is treated at the mean-field level with the time-dependent Hartree-Fock (TDHF) theory. Low-lying and high-lying nuclear vibrations are studied using the linear response theory. The fusion mechanism is also described for light and heavy systems. The latter exhibit fusion hindrance due to quasi-fission. Typical characteristics of quasi-fission, such as contact time and partial symmetrisation of the fragments mass in the exit channel, are reproduced by TDHF calculations. The (multi-)nucleon transfer at sub-barrier energies is also discussed.

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arXiv2013

Microscopic study of $^{16}$O+$^{16}$O fusion

We perform a study of $^{16}$O+$^{16}$O fusion at above and below the interaction barrier energies using three-dimensional time-dependent Hartree-Fock (TDHF) calculations at above barrier energies and density-constrained TDHF calculations for the entire energy range. We discuss the variations of the experimental data at above the barrier energies. Calculations reasonably reproduce the observed energy-dependent broad oscillations in the fusion excitation functions. These oscillations result from overcoming $L$-dependent fusion barriers. The role of the coupling to low-lying octupole states is also discussed.

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arXiv2013

Structure and direct decay of Giant Monopole Resonances

We study structure and direct decay of the Giant Monopole Resonance (GMR) at the RPA level using the Time-Dependent Energy Density Functional method in the linear response regime in a few doubly-magic nuclei. A proper treatment of the continuum, through the use of large coordinate space, allows for a separation between the nucleus and its emitted nucleons. The microscopic structure of the GMR is investigated with the decomposition of the strength function into individual single-particles quantum numbers. A similar microscopic decomposition of the spectra of emitted nucleons by direct decay of the GMR is performed. In this harmonic picture of giant resonance, shifting every contribution by the initial single-particle energy allows to reconstruct the GMR strength function. The RPA residual interaction couples bound 1-particle 1-hole states to unbound ones, allowing for the total decay of the GMR. In this article, we then intend to get an understanding of the direct decay mechanism from coherent one-particle-one-hole superpositions, while neglecting more complex configurations. Time-dependent beyond mean-field approaches should be use, in the future, to extend this method.

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arXiv2011

Investigation of the role of shell structure in quasi-fission mass distributions

Systematic measurements of mass-ratio distributions for fission following collisions of $^{48}$Ti projectiles with even-even target nuclei from $^{144}$Sm to $^{208}$Pb have been made at sub-barrier energies. They show the presence of quasifission, and depend strongly on target nucleus deformation and the fissility of the composite nucleus. A new framework to analyse systematic mass-ratio measurements allows direct comparison with the trends expected from shell structure, independent of assumptions or fits. This indicates that quasi-fission mass distributions show trends consistent with low energy mass-asymmetric fission of the same actinide elements.

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arXiv2011

Novel insights into transfer processes in the reaction 16O+208Pb at sub-barrier energies

The collision of the doubly-magic nuclei $^{16}$O+$^{208}$Pb is a benchmark in nuclear reaction studies. Our new measurements of back-scattered projectile-like fragments at sub-barrier energies show show that transfer of 2 protons ($2p$) is much more probable than $α$-particle transfer. $2p$ transfer probabilities are strongly enhanced compared to expectations for the sequential transfer of two uncorrelated protons; at energies around the fusion barrier absolute probabilities for two proton transfer are similar to those for one proton transfer. This strong enhancement indicates strong $2p$ pairing correlations in $^{16}$O, and suggests evidence for the occurrence of a nuclear supercurrent of two-proton Cooper pairs in this reaction, already at energies well below the fusion barrier.

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arXiv2010

A new inverse quasifission mechanism to produce neutron-rich transfermium nuclei

Based on time-dependent Hartree-Fock theory, a new inverse quasifission mechanism is proposed to produce neutron-rich transfermium nuclei, in collision of prolate deformed actinides. Calculations show that collision of the tip of one nucleus with the side of the other results in a nucleon flux toward the latter. The role of nucleon evaporation and impact parameter, as well as the collision time are discussed.

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arXiv2008

Gamow-Teller strengths in 24Na using the 24Mg(t,3He) reaction at 115 AMeV

Gamow-Teller transitions from 24Mg to 24Na were studied via the (t,3He) reaction at 115 AMeV using a secondary triton beam produced via fast fragmentation of 150 AMeV 16O ions. Compared to previous (t,3He) experiments at this energy that employed a primary alpha beam, the secondary beam intensity is improved by about a factor of five. Despite the large emittance of the secondary beam, an excitation-energy resolution of ~200 keV is achieved. A good correspondence is found between the extracted Gamow-Teller strength distribution and those available from other charge-exchange probes. Theoretical calculations using the newly developed USDA and USDB sd-shell model interactions reproduce the data well.

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arXiv2008

On the extraction of weak transition strengths via the (3He,t) reaction at 420 MeV

Differential cross sections for transitions of known weak strength were measured with the (3He,t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn and 208Pb. Using this data, it is shown the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

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