1,396 research outputs found
Clusters and halos in light nuclei
The fermionic molecular dynamics approach uses Gaussian wave packets as
single-particle basis states. Many-body basis states are Slater determinants
projected on parity, angular momentum and total linear momentum. The
wave-packet basis is very flexible - FMD contains harmonic oscillator shell
model and Brink-type cluster states as special cases. The parameters of the
wave packets are obtained by variation. A realistic effective interaction
derived from the Argonne V18 interaction by means of the unitary correlation
operator method is employed. We discuss the fully microscopic calculation of
the 3He(alpha,gamma)7Be capture reaction within the FMD approach. The model
space contains frozen cluster configurations at large distances and polarized
configurations in the interaction region. The polarized configurations are
essential for a successful description of the 7Be bound state properties and
for the S- and D-wave scattering states. The calculated cross section agrees
well with recent measurements regarding both the absolute normalization and the
energy dependence. We also discuss the structure of the cluster states,
including the famous Hoyle state, in 12C. From the two-body densities we
conclude that the Hoyle state has a spatially extended triangular alpha-cluster
structure, whereas the third 0+ state features a chain-like obtuse triangle
structure. We also calculate the N hbar Omega decomposition of our wave
functions to illuminate the challenges of no-core shell model calculations for
these cluster states.Comment: 7 pages, Proceedings of Horizons of Innovative Theories, Experiments,
and Supercomputing in Nuclear Physics (HITES 2012
Microscopic calculation of the 3He(alpha,gamma)7Be and 3H(alpha,gamma)7Li capture cross sections using realistic interactions
The radiative capture cross sections for the 3He(alpha,gamma)7Be and
3H(alpha,gamma)7Li reactions are calculated in the fully microscopic fermionic
molecular dynamics approach using a realistic effective interaction that
reproduces the nucleon-nucleon scattering data. At large distances bound and
scattering states are described by antisymmetrized products of 4He and 3He/3H
ground states. At short distances the many-body Hilbert space is extended with
additional many-body wave functions needed to represent polarized clusters and
shell-model-like configurations. Properties of the bound states are described
well, as are the scattering phase shifts. The calculated S factor for the
3He(alpha,gamma)7Be reaction agrees very well with recent experimental data
both in absolute normalization and energy dependence. In case of the
3H(alpha,gamma)7Li reaction the calculated S factor is larger than available
experimental data by about 15%.Comment: 4 pages, 5 figures, published versio
Clusters, Halos, And S-Factors In Fermionic Molecular Dynamics
In Fermionic Molecular Dynamics antisymmetrized products of Gaussian wave
packets are projected on angular momentum, linear momentum, and parity. An
appropriately chosen set of these states span the many-body Hilbert space in
which the Hamiltonian is diagonalized. The wave packet parameters - position,
momentum, width and spin - are obtained by variation under constraints. The
great flexibility of this basis allows to describe not only shell-model like
states but also exotic states like halos, e.g. the two-proton halo in 17Ne, or
cluster states as they appear for example in 12C close to the \alpha-breakup
threshold where the Hoyle state is located. Even a fully microscopic
calculation of the 3He(\alpha,\gamma)7Be capture reaction is possible and
yields an astrophysical S-factor that compares very well with newer data. As
representatives of numerous results these cases will be discussed in this
contribution, some of them not published so far. The Hamiltonian is based on
the realistic Argonne V18 nucleon-nucleon interaction.Comment: Presented at HIAS 2013, 8.-12. April 2013, Canberr
The Hoyle state and its relatives
The Hoyle state and other resonances in the continuum above the 3 alpha
threshold in 12C are studied in a microscopic cluster model. Whereas the Hoyle
state is a very sharp resonance and can be treated reasonably well in bound
state approximation, the other higher lying states require a proper treatment
of the continuum. The model space consists of an internal region with 3 alpha
particles on a triangular grid and an external region consisting of the 8Be
ground state and excited (pseudo)-states of 8Be with an additional alpha. The
microscopic R-matrix method is used to match the many-body wave function to the
asymptotic Coulomb behavior of bound states, Gamow states and scattering
states. 8Be-alpha phase shifts are analyzed and resonance properties like radii
and transition strengths are investigated.Comment: 7 pages, Talk given at SOTANCP3, 3rd International Workshop on State
of the Art in Nuclear Cluster Physics, Yokohama, Japan, May 26-30, 201
Towards Microscopic Ab Initio Calculations of Astrophysical S-Factors
Low energy capture cross sections are calculated within a microscopic
many-body approach using an effective Hamiltonian derived from the Argonne V18
potential. The dynamics is treated within Fermionic Molecular Dynamics (FMD)
which uses a Gaussian wave-packet basis to represent the many-body states. A
phase-shift equivalent effective interaction derived within the Unitary
Correlation Operator Method (UCOM) that treats explicitly short-range central
and tensor correlations is employed. As a first application the
3He(alpha,gamma)7Be reaction is presented. Within the FMD approach the
microscopic many-body wave functions of the 3/2- and 1/2- bound states in 7Be
as well as the many-body scattering states in the 1/2+, 3/2+ and 5/2+ channels
are calculated as eigenstates of the same microscopic effective Hamiltonian.
Finally the S-factor is calculated from E1 transition matrix elements between
the many-body scattering and bound states. For 3He(alpha,gamma)7Be the S-factor
agrees very well, both in absolute normalization and energy dependence, with
the recent experimental data from the Weizmann, LUNA, Seattle and ERNA
experiments. For the 3H(alpha,gamma)7Li reaction the calculated S-factor is
about 15% above the data
Nuclear Structure in the Framework of the Unitary Correlation Operator Method
Correlations play a crucial role in the nuclear many-body problem. We give an
overview of recent developments in nuclear structure theory aiming at the
description of these interaction-induced correlations by unitary
transformations. We focus on the Unitary Correlation Operator Method (UCOM),
which offers a very intuitive, universal and robust approach for the treatment
of short-range correlations. We discuss the UCOM formalism in detail and
highlight the connections to other methods for the description of short-range
correlations and the construction of effective interactions. In particular, we
juxtapose UCOM with the Similarity Renormalization Group (SRG) approach, which
implements the unitary transformation of the Hamiltonian through a very
flexible flow-equation formulation. The UCOM- and SRG-transformed interactions
are compared on the level of matrix elements and in many-body calculations
within the no-core shell model and with Hartree-Fock plus perturbation theory
for a variety of nuclei and observables. These calculations provide a detailed
picture of the similarities and differences as well as the advantages and
limitations of unitary transformation methods.Comment: 72 pages, 31 figure
Long Range Tensor Correlations in Charge and Parity Projected Fermionic Molecular Dynamics
Within the framework of Fermionic Molecular Dynamics a method is developed to
better account for long range tensor correlations in nuclei when working with a
single Slater determinant. Single-particle states with mixed isospin and broken
parity build up an intrinsic Slater determinant which is then charge and parity
projected. By minimizing the energy of this many-body state with respect to the
parameters of the single-particle states and projecting afterwards on angular
momentum ground state energies are obtained that are systematically lower than
corresponding Hartree-Fock results. The realistic Argonne V18 potential is used
and short range correlations are treated with the Unitary Correlation Operator
Method. Comparison with exact few-body calculations shows that in He about
one fifth of the correlation energy due to long-range correlations are
accounted for. These correlations which extend over the whole nucleus are
visualized with the isospin and spin-isospin density of the intrinsic state.
The divergence of the spin-isospin density, the source for pion fields, turns
out to be of dipole nature.Comment: 12 pages, 4 figure
Short-range correlations in nuclei with similarity renormalization group transformations
Realistic nucleon-nucleon interactions induce
short-range correlations in nuclei. To solve the many-body problem unitary
transformations like the similarity renormalization group (SRG) are often used
to soften the interactions.
Two-body densities can be used to illustrate how the SRG
eliminates short-range correlations in the wave function. The short-range
information can however be recovered by transforming the density operators.
The many-body problem is solved for He in the no core
shell model (NCSM) with SRG transformed AV8' and chiral N3LO interactions. The
NCSM wave functions are used to calculate two-body densities with bare and SRG
transformed density operators in two-body approximation.
The two-body momentum distributions for AV8' and N3LO
have similar high-momentum components up to relative momenta of about
, dominated by tensor correlations, but differ in their
behavior at higher relative momenta. The contributions of many-body
correlations are small for pairs with vanishing pair momentum but not
negligible for the momentum distributions integrated over all pair momenta.
Many-body correlations are induced by the strong tensor force and lead to a
reshuffling of pairs between different spin-isospin channels.
When using the SRG it is essential to use transformed
operators for observables sensitive to short-range physics. Back-to-back pairs
with vanishing pair momentum are the best tool to study short-range
correlations.Comment: 13 pages, 9 figures, submitted to Phys. Rev.
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