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V. E. Oberacker

Publications and source records attributed to V. E. Oberacker.

At least 37 records · Page 2Linked to original sources
arXiv2013

Microscopic analysis of sub-barrier fusion enhancement in $^{132}$Sn+$^{40}$Ca vs. $^{132}$Sn+$^{48}$Ca}

We provide a theoretical analysis of recently measured fusion cross sections which show a surprising enhancement at low $E_\mathrm{c.m.}$ energies for the system $^{132}$Sn+$^{40}$Ca as compared to the more neutron-rich system $^{132}$Sn+$^{48}$Ca. Dynamic microscopic calculations are carried out on a three-dimensional lattice with a time-dependent density-constrained density functional theory. There are no adjustable parameters, the only input is the Skyrme effective NN interaction. Heavy-ion potentials $V(R)$, coordinate-dependent mass parameters $M(R)$, and total fusion cross sections $σ(E_\mathrm{c.m.})$ are calculated for both systems. We are able to explain the measured fusion enhancement in terms of the {\it narrower width} of the ion-ion potential for $^{132}$Sn+$^{40}$Ca, while the barrier heights and positions are approximately the same in both systems.

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arXiv2012

Single-particle dissipation in TDHF studied from a phase-space perspective

We study dissipation and relaxation processes within the time-dependent Hartree-Fock approach using the Wigner distribution function. On the technical side we present a geometrically unrestricted framework which allows us to calculate the full six-dimensional Wigner distribution function. With the removal of geometrical constraints, we are now able to extend our previous phase-space analysis of heavy-ion collisions in the reaction plane to unrestricted mean-field simulations of nuclear matter on a three-dimensional Cartesian lattice. From the physical point of view we provide a quantitative analysis on the stopping power in TDHF. This is linked to the effect of transparency. For the medium-heavy $^{40}$Ca+$^{40}$Ca system we examine the impact of different parametrizations of the Skyrme force, energy-dependence, and the significance of extra time-odd terms in the Skyrme functional.

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arXiv2012

Dynamic Microscopic Theory of Fusion Using DC-TDHF

The density-constrained time-dependent Hartree-Fock (DC-TDHF) theory is a fully microscopic approach for calculating heavy-ion interaction potentials and fusion cross sections below and above the fusion barrier. We discuss recent applications of DC-TDHF method to fusion of light and heavy systems.

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arXiv2012

Microscopic DC-TDHF study of heavy-ion potentials and fusion cross sections

We study heavy-ion fusion reactions at energies near the Coulomb barrier, in particular with neutron-rich radioactive ion beams. Dynamic microscopic calculations are carried out on a three-dimensional lattice using the Density-Constrained Time-Dependent Hartree-Fock (DC-TDHF) method. New results are presented for the $^{132}$Sn+$^{40}$Ca system which are compared to $^{132}$Sn+$^{48}$Ca studied earlier. Our theoretical fusion cross-sections agree surprisingly well with recent data measured at HRIBF. We also study the near- and sub-barrier fusion of $^{24,16}$O on $^{12}$C which is important to determine the composition and heating of the crust of accreting neutron stars.

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arXiv2012

Microscopic study of Ca$+$Ca fusion

We investigate the fusion barriers for reactions involving Ca isotopes $\mathrm{^{40}Ca}+\mathrm{^{40}Ca}$, $\mathrm{^{40}Ca}+\mathrm{^{48}Ca}$, and $\mathrm{^{48}Ca}+\mathrm{^{48}Ca}$ using the microscopic time-dependent Hartree-Fock theory coupled with a density constraint. In this formalism the fusion barriers are directly obtained from TDHF dynamics. We also study the excitation of the pre-equilibrium GDR for the $\mathrm{^{40}Ca}+\mathrm{^{48}Ca}$ system and the associated $γ$-ray emission spectrum. Fusion cross-sections are calculated using the incoming-wave boundary condition approach. We examine the dependence of fusion barriers on collision energy as well as on the different parametrizations of the Skyrme interaction.

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arXiv2012

Phase space structure and dynamics within the time-dependent Hartree-Fock approach

We study the equilibration and relaxation processes within the time-dependent Hartree-Fock approach using the Wigner distribution function. On the technical side we present a geometrically unrestricted framework which allows us to calculate the full six-dimensional Wigner distribution function. With the removal of geometrical constraints, we are now able to extend our previous phase-space analysis of heavy-ion collisions in the reaction plane to unrestricted mean-field simulations of nuclear matter on a three-dimensional Cartesian lattice. From the physical point of view we provide a quantitative analysis on the stopping power in TDHF. This is linked to the effect of transparency. For the medium-heavy $^{40}$Ca+$^{40}$Ca system we examine the impact of different parametrizations of the Skyrme force, energy-dependence, and the significance of extra time-odd terms in the Skyrme functional. For the first time, transparency in TDHF is observed for a heavy system, $^{24}$Mg+$^{208}$Pb.

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arXiv2012

Dynamic microscopic study of pre-equilibrium giant resonance excitation and fusion in the reactions $^{132}$Sn+$^{48}$Ca and $^{124}$Sn+$^{40}$Ca

We study pre-equilibrium giant dipole resonance excitation and fusion in the neutron-rich system $^{132}$Sn+$^{48}$Ca at energies near the Coulomb barrier, and we compare photon yields and total fusion cross sections to those of the stable system $^{124}$Sn+$^{40}$Ca. The dynamic microscopic calculations are carried out on a three-dimensional lattice using both the Time-Dependent Hartree-Fock method and the Density Constrained TDHF method. We demonstrate that the peak of the GDR excitation spectrum occurs at a substantially lower energy than expected for an equilibrated system, thus reflecting the very large prolate elongation of the dinuclear complex during the early stages of fusion. Our theoretical fusion cross-sections for both systems agree reasonably well with recent data measured at HRIBF.

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arXiv2011

Microscopic Composition of Ion-Ion Interaction Potentials

We present a new method to calculate the total ion-ion interaction potential in terms of building blocks which we refer to as "single-particle interaction potentials". This allows also to compose the separate contributions from neutrons and protons to the interaction potentials. The method is applied to nuclear collisions via the use of time-dependent Hartree-Fock theory.

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arXiv2011

Microscopic Calculation of Heavy-Ion Potentials Based on TDHF

We discuss the implementation and results of a recently developed microscopic method for calculating ion-ion interaction potentials and fusion cross-sections. The method uses the TDHF evolution to obtain the instantaneous many-body collective state using a density constraint. The ion-ion potential as well as the coordinate dependent mass are calculated from these states. The method fully accounts for the dynamical processes present in the TDHF time-evolution and provides a parameter-free way of calculating fusion cross-sections.

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arXiv2011

Static and Dynamic Chain Structures in the Mean-Field Theory

We give a brief overview of recent work examining the presence of $α$-clusters in light nuclei within the Skyrme-force Hartree-Fock model. Of special significance are investigations into $α$-chain structures in carbon isotopes and $^{16}$O. Their stability and possible role in fusion reactions are examined in static and time-dependent Hartree-Fock calculations. We find a new type of shape transition in collisions and a centrifugal stabilization of the $4α$ chain state in a limited range of angular momenta. No stabilization is found for the $3α$ chain.

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arXiv2011

Localization in light nuclei

We investigate the presence of spatial localization in nuclei using a method that maps the nucleon same-spin pair probability and is based on the density-matrix. The method is used to study spatial localization of light nuclei within the Hartree-Fock approximation. We show that the method provides an alternative tool for studying spatial localization in comparison to the localization observed from maxima in the nuclear mass density.

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arXiv2010

Microscopic study of the $^{132,124}$Sn+$^{96}$Zr reactions: dynamic excitation energy, energy-dependent heavy-ion potential, and capture cross section

We study reactions between neutron-rich $^{132}$Sn nucleus and $^{96}$Zr within a dynamic microscopic theory at energies in the vicinity of the ion-ion potential barrier peak, and we compare the properties to those of the stable system $^{124}$Sn+$^{96}$Zr. The calculations are carried out on a three-dimensional lattice using the density-constrained Time-Dependent Hartree-Fock method. In particular, we calculate the dynamic excitation energy $E^*(t)$ and the quadrupole moment of the dinuclear system, $Q_{20}(t)$, during the initial stages of the heavy-ion collision. Capture cross sections for the two reactions are analyzed in terms of dynamic effects and a comparison with recently measured data is given.

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arXiv2010

Entrance Channel Dynamics of Hot and Cold Fusion Reactions Leading to Superheavy Elements

We investigate the entrance channel dynamics for the reactions $\mathrm{^{70}Zn}+\mathrm{^{208}Pb}$ and $\mathrm{^{48}Ca}+\mathrm{^{238}U}$ using the fully microscopic time-dependent Hartree-Fock (TDHF) theory coupled with a density constraint. We calculate excitation energies and capture cross-sections relevant for the study of superheavy formations. We discuss the deformation dependence of the ion-ion potential for the $\mathrm{^{48}Ca}+\mathrm{^{238}U}$ system and perform an alignment angle averaging for the calculation of the capture cross-section. The results show that this parameter-free approach can generate results in good agreement with experiment and other theories.

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arXiv2010

Microscopic Study of the triple-$α$ Reaction

We present a microscopic dynamical study of the reactions involving three $^{4}$He clusters. We show that the much discussed triple-$α$ linear chain configuration of $^{12}$C is formed with a certain lifetime and subsequently decays into a triangular configuration of $^{12}$C and then to a configuration near the ground state. Time-dependent Hartree-Fock (TDHF) theory coupled with a density constraint is used to study the properties of these configurations. We find a sequence of dynamical transitions analogous to the suggested astrophysical mechanism for the formation of $^{12}$C nuclei.

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arXiv2010

Microscopic Description of Nuclear Fission Dynamics

We discuss possible avenues to study fission dynamics starting from a time-dependent mean-field approach. Previous attempts to study fission dynamics using the time-dependent Hartree-Fock (TDHF) theory are analyzed. We argue that different initial conditions may be needed to describe fission dynamics depending on the specifics of the fission phenomenon and propose various approaches towards this goal. In particular, we provide preliminary calculations for studying fission following a heavy-ion reaction using TDHF with a density contraint. Regarding prompt muon-induced fission, we also suggest a new approach for combining the time-evolution of the muonic wave function with a microscopic treatment of fission dynamics via TDHF.

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arXiv2009

Microscopic Calculation of Pre-Compound Excitation Energies for Heavy-Ion Collisions

We introduce a microscopic approach for calculating the excitation energies of systems formed during heavy-ion collisions. The method is based on time-dependent Hartree-Fock (TDHF) theory and allows the study of the excitation energy as a function of time or ion-ion separation distance. We discuss how this excitation energy is related to the estimate of the excitation energy using the reaction $Q$-value, as well as its implications for dinuclear pre-compound systems formed during heavy-ion collisions.

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arXiv2008

Center-of-mass motion and cross-channel coupling in time-dependent Hartree-Fock theory

We provide a discussion of issues related to the center-of-mass motion and cross-channel coupling in applications of the time-dependent Hartree-Fock (TDHF) theory to heavy-ion collisions. We find that the entrance channel dynamics of a heavy-ion collision as described by TDHF does not seem to be significantly influenced by these effects, whereas the long-time evolution may be less reliable.

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