37 research outputs found
Nuclear Quantum Many-Body Dynamics: From Collective Vibrations to Heavy-Ion Collisions
A summary of recent researches on nuclear dynamics with realistic microscopic
quantum approaches is presented. The Balian-V\'en\'eroni variational principle
is used to derive the time-dependent Hartree-Fock (TDHF) equation describing
the dynamics at the mean-field level, as well as an extension including
small-amplitude quantum fluctuations which is equivalent to the time-dependent
random-phase approximation (TDRPA). Such formalisms as well as their practical
implementation in the nuclear physics framework with modern three-dimensional
codes are discussed. Recent applications to nuclear dynamics, from collective
vibrations to heavy-ion collisions are presented. A particular attention is
devoted to the interplay between collective motions and internal degrees of
freedom. For instance, the harmonic nature of collective vibrations is
questioned. Nuclei are also known to exhibit superfluidity due to pairing
residual interaction. Extensions of the theoretical approach to study such
pairing vibrations are now available. Large amplitude collective motions are
investigated in the framework of heavy-ion collisions leading, for instance, to
the formation of a compound system. How fusion is affected by the internal
structure of the collision partners, such as their deformation, is discussed.
Other mechanisms in competition with fusion, and responsible for the formation
of fragments which differ from the entrance channel (transfer reactions,
deep-inelastic collisions, and quasi-fission) are investigated. Finally,
studies of actinide collisions forming, during very short times of few
zeptoseconds, the heaviest nuclear systems available on Earth, are presented.Comment: 49 pages. Review article to be published in EPJA. v.3: minor changes
in Eqs. 2.50, A.13 and B.3
Particle number fluctuations and correlations in transfer reactions obtained using the Balian-V\'en\'eroni variational principle
The Balian-V\'en\'eroni (BV) variational principle, which optimizes the
evolution of the state according to the relevant observable in a given
variational space, is used at the mean-field level to determine the particle
number fluctuations in fragments of many-body systems. For fermions, the
numerical evaluation of such fluctuations requires the use of a time-dependent
Hartree-Fock (TDHF) code. Proton, neutron and total nucleon number fluctuations
in fragments produced in collisions of two 40Ca are computed for a large range
of angular momenta at a center of mass energy E_cm=128 MeV, well above the
fusion barrier. For deep-inelastic collisions, the fluctuations calculated from
the BV variational principle are much larger than standard TDHF results, and
closer to mass and charge experimental fluctuations. For the first time,
correlations between proton and neutron numbers are determined within a quantum
microscopic approach. These correlations are shown to be larger with exotic
systems where charge equilibration occurs.Comment: Accepted for publication in Phys. Rev. Lett. New version with more
detailed comparison with experimental data and prediction for exotic beam
Effect of shell structure on the fission of sub-lead nuclei
Fission of atomic nuclei often produces mass asymmetric fragments. However,
the origin of this asymmetry was believed to be different in actinides and in
the sub-lead region [A. Andreyev {\it et al.}, Phys. Rev. Lett. {\bf 105},
252502 (2010)]. It has recently been argued that quantum shell effects
stabilising pear shapes of the fission fragments could explain the observed
asymmetries in fission of actinides[G. Scamps and C. Simenel, Nature {\bf 564},
382 (2018)]. This interpretation is tested in the sub-lead region using
microscopic mean-field calculations of fission based on the Hartree-Fock
approach with BCS pairing correlations. The evolution of the number of protons
and neutrons in asymmetric fragments of mercury isotope fissions is interpreted
in terms of deformed shell gaps in the fragments. A new method is proposed to
investigate the dominant shell effects in the pre-fragments at scission. We
conclude that the mechanisms responsible for asymmetric fissions in the
sub-lead region are the same as in the actinide region, which is a strong
indication of their universality.Comment: Accepted as a rapid communication by Phys. Rev.
Actinide collisions for QED and superheavy elements with the time-dependent Hartree-Fock theory and the Balian-V\'en\'eroni variational principle
Collisions of actinide nuclei form, during very short times of few zs
( s), the heaviest ensembles of interacting nucleons available on
Earth. Such collisions are used to produce super-strong electric fields by the
huge number of interacting protons to test spontaneous positron-electron pair
emission (vacuum decay) predicted by the quantum electrodynamics (QED) theory.
Multi-nucleon transfer in actinide collisions could also be used as an
alternative way to fusion in order to produce neutron-rich heavy and superheavy
elements thanks to inverse quasifission mechanisms. Actinide collisions are
studied in a dynamical quantum microscopic approach. The three-dimensional
time-dependent Hartree-Fock (TDHF) code {\textsc{tdhf3d}} is used with a full
Skyrme energy density functional to investigate the time evolution of
expectation values of one-body operators, such as fragment position and
particle number. This code is also used to compute the dispersion of the
particle numbers (e.g., widths of fragment mass and charge distributions) from
TDHF transfer probabilities, on the one hand, and using the Balian-Veneroni
variational principle, on the other hand. A first application to test QED is
discussed. Collision times in U+U are computed to determine the
optimum energy for the observation of the vacuum decay. It is shown that the
initial orientation strongly affects the collision times and reaction
mechanism. The highest collision times predicted by TDHF in this reaction are
of the order of zs at a center of mass energy of 1200 MeV. According to
modern calculations based on the Dirac equation, the collision times at
GeV are sufficient to allow spontaneous electron-positron pair
emission from QED vacuum decay, in case of bare uranium ion collision. A second
application of actinide collisions to produce neutron-rich transfermiums is
discussed. A new inverse quasifission mechanism associated to a specific
orientation of the nuclei is proposed to produce transfermium nuclei ()
in the collision of prolate deformed actinides such as Th+Cf.
The collision of the tip of one nucleus with the side of the other results in a
nucleon flux toward the latter. The probability distributions for transfermium
production in such a collision are computed. The produced nuclei are more
neutron-rich than those formed in fusion reactions, thus, leading to more
stable isotopes closer to the predicted superheavy island of stability. In
addition to mass and charge dispersion, the Balian-Veneroni variational
principle is used to compute correlations between and distributions,
which are zero in standard TDHF calculations.Comment: Proceeding of the FUSION11 conferenc
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.Comment: 8 pages, 7 figure
Nuclear quantum many-body dynamics
A summary of recent researches on nuclear dynamics with realistic microscopic quantum approaches is presented. The Balian-Vénéroni variational principle is used to derive the time-dependent Hartree-Fock (TDHF) equation describing the dynamics at the mean-field level, as well as an extension including small-amplitude quantum fluctuations which is equivalent to the time-dependent random-phase approximation (TDRPA). Such formalisms as well as their practical implementation in the nuclear physics framework with modern three-dimensional codes are discussed. Recent applications to nuclear dynamics, from collective vibrations to heavy-ion collisions are presented. Particular attention is devoted to the interplay between collective motions and internal degrees of freedom. For instance, the harmonic nature of collective vibrations is questioned. Nuclei are also known to exhibit superfluidity due to pairing residual interaction. Extensions of the theoretical approach to study such pairing vibrations are now available. Large amplitude collective motions are investigated in the framework of heavy-ion collisions leading, for instance, to the formation of a compound system. How fusion is affected by the internal structure of the collision partners, such as their deformation, is discussed. Other mechanisms in competition with fusion, and responsible for the formation of fragments which differ from the entrance channel (transfer reactions, deep-inelastic collisions, and quasi-fission) are investigated. Finally, studies of actinide collisions forming, during very short times of few zeptoseconds, the heaviest nuclear systems available on Earth, are presented
Dynamical description of the fission process using the TD-BCS theory
The description of fission remains a challenge for nuclear microscopic
theories. The time-dependent Hartree-Fock approach with BCS pairing is applied
to study the last stage of the fission process. A good agreement is found for
the one-body observables: the total kinetic energy and the average mass
asymmetry. The non-physical dependence of two-body observables with the initial
shape is discussed.Comment: 4 pages, 3 figures, Submitted as AIP Conference Proceeings for
NSD2015, Portoroz, Sloveni
Collision dynamics of two U atomic nuclei
Collisions of actinide nuclei form, during very short times of few
s, the heaviest ensembles of interacting nucleons available on Earth. Such very
heavy ions collisions have been proposed as an alternative way to produce heavy
and superheavy elements. These collisions are also used to produce super-strong
electric fields by the huge number of interacting protons to test spontaneous
positron-electron () pair emission predicted by the quantum
electrodynamics theory. The time-dependent Hartree-Fock theory which is a fully
microscopic quantum approach is used to study collision dynamics of two
U atomic nuclei. In particular, the role of nuclear deformation on
collision time and on reaction mechanisms such as nucleon transfer is
emphasized. These calculations are pessimistic in terms of transfermium
elements () production. However, the highest collision times
( s at 1200 MeV) should allow experimental signature of
spontaneous emission. Surprisingly, we also observe ternary fission
due to purely dynamical effects.Comment: Accepted for publication in Physical Review Lette
Microscopic approaches for nuclear Many-Body dynamics: applications to nuclear reactions
These lecture notes are addressed to PhD student and/or researchers who want
a general overview of microscopic approaches based on mean-field and applied to
nuclear dynamics. Our goal is to provide a good description of low energy
heavy-ion collisions. We present both formal aspects and practical applications
of the time-dependent Hartree-Fock (TDHF) theory. The TDHF approach gives a
mean field dynamics of the system under the assumption that particles evolve
independently in their self-consistent average field. As an example, we study
the fusion of both spherical and deformed nuclei with TDHF. We also focus on
nucleon transfer which may occur between nuclei below the barrier. These
studies allow us to specify the range of applications of TDHF in one hand, and,
on the other hand, its intrinsic limitations: absence of tunneling below the
Coulomb barrier, missing dissipative effects and/or quantum fluctuations.
Time-dependent mean-field theories should be improved to properly account for
these effects. Several approaches, generically named "beyond TDHF" are
presented which account for instance for pairing and/or direct nucleon-nucleon
collisions. Finally we discuss recent progresses in exact ab-initio methods
based on the stochastic mean-field concept.Comment: 55 pages. Lecture given at the "Joliot Curie" school, Maubuisson,
september 17-22, 2007. A french version is available at
http://www.cenbg.in2p3.fr/heberge/EcoleJoliotCurie/coursannee/cours/CoursSimenel.pd