306 research outputs found
Microscopic three-cluster study of light exotic nuclei
I develop a microscopic three-cluster model for exotic light nuclei. I use
the hyperspherical formalism, associated with the Generator Coordinate Method.
This model is well adapted to halo nuclei, since the long-range part of the
radial wave functions is accurately reproduced. The core wave functions are
described in the shell model, including excited states. This technique provides
large bases, expressed in terms of projected Slater determinants. Matrix
elements involve seven-dimension integrals, and therefore require long
calculation times. I apply the model to 11Li, 14Be, 15B, and 17N described by
two neutrons surrounding a 9Li, 12Be, 13B and 15N core, respectively. The 17Ne
(as 15O+p+p) and 15Ne (as 13O+p+p) mirror nuclei are briefly discussed. I
present the spectra and some spectroscopic properties, such as r.m.s. radii or
E2 transition probabilities. I also analyze the importance of core excitations.Comment: Accepted for publication in Phys. Rev.
Low-energy He scattering in a microscopic model
A microscopic version of the Continuum Discretized Coupled Channel (CDCC)
method is used to investigate He scattering on Al, Ni,
Sn, and Pb at energies around the Coulomb barrier. The He
nucleus is described by an antisymmetric 6-nucleon wave function, defined in
the Resonating Group Method. The He continuum is simulated by
square-integrable positive-energy states. The model is based only on well known
nucleon-target potentials, and is therefore does not depend on any adjustable
parameter. I show that experimental elastic cross sections are fairly well
reproduced. The calculation suggests that breakup effects increase for high
target masses. For a light system such as He+Al, breakup effects
are small, and a single-channel approximation provides fair results. This
property is explained by a very simple model, based on the sharp-cut-off
approximation for the scattering matrix. I also investigate the He-target
optical potentials, which confirm that breakup channels are more and more
important when the mass increases. At large distances, polarization effects
increase the Coulomb barrier, and provide a long-tail absorption component in
the imaginary part of the nucleus-nucleus interaction.Comment: 11 pages, 11 figures, accepted at Phys. Rev.
An R-matrix package for coupled-channel problems in nuclear physics
We present an -matrix Fortran package to solve coupled-channel problems in
nuclear physics. The basis functions are chosen as Lagrange functions, which
permits simple calculations of the matrix elements. The main input are the
coupling potentials at some nucleus-nucleus distances, specified by the
program. The program provides the collision matrix and, optionally, the
associated wave function. The present method deals with open and closed
channels simultaneously, without numerical instability associated with closed
channels. It can also solve coupled-channel problems for non-local potentials.
Long-range potentials can be treated with propagation techniques, which
significantly speed up the calculations. We first present an overview of the
-matrix theory, and of the Lagrange-mesh method. A description of the
package and its installation on a UNIX machine is then provided. Finally, five
typical examples are discussed.Comment: 28 pages, 6 figures, accepted for publication at Computer Physics
Communication
9Be scattering with microscopic wave functions and the CDCC method
We use microscopic 9Be wave functions defined in a alpha+alpha+n multicluster
model to compute 9Be+target scattering cross sections. The parameter sets
describing 9Be are generated in the spirit of the Stochastic Variational Method
(SVM), and the optimal solution is obtained by superposing Slater determinants
and by diagonalizing the Hamiltonian. The 9Be three-body continuum is
approximated by square-integral wave functions. The 9Be microscopic wave
functions are then used in a Continuum Discretized Coupled Channel (CDCC)
calculation of 9Be+208Pb and of 9Be+27Al elastic scattering. Without any
parameter fitting, we obtain a fair agreement with experiment. For a heavy
target, the influence of 9Be breakup is important, while it is weaker for light
targets. This result confirms previous non-microscopic CDCC calculations. One
of the main advantages of the microscopic CDCC is that it is based on
nucleon-target interactions only; there is no adjustable parameter. The present
work represents a first step towards more ambitious calculations involving
heavier Be isotopes.Comment: Phys. Rev. C in pres
Elastic scattering of 17F, 17O and 19F on a heavy target in microscopic continuum discretized coupled-channels method
Background: Microscopic description of the projectile, based on an effective
NN interaction, in a microscopic CDCC (MCDCC) model [PRL 111, 082701 (2013)]
has been successfully applied to the 7Li+208Pb scattering.
Purpose: The MCDCC method is applied to the low energy elastic scattering of
17F, 17O and 19F on 58Ni and 208Pb targets. The goal of the calculations is
twofold - to test the adequacy and the accuracy of the MCDCC model for the
heavier projectiles, and to study the contribution of various channels to the
elastic scattering cross sections.
Methods: The elastic scattering cross sections are calculated using the MCDCC
method. The nucleon-target optical potential is folded with the projectile
densities resulting from an effective NN interaction, which includes central
nuclear, spin-orbit and Coulomb terms. Discretization of the continuum is
achieved via the pseudo state method. Coupled equations are solved using the
R-matrix method on a Lagrange mesh.
Results: For the test case of 17F at 10 MeV/nucleon, the cross sections are
weakly sensitive to the choice of the effective NN interaction, three different
energy dependent optical nucleon-target potentials provide a similar reasonable
agreement with data. Just below the Coulomb barrier, the MCDCC significantly
underestimates the cross sections at larger angles. The coupling to continuum
is not significant in most of the assessed cases.
Conclusions: The MCDCC is very satisfactory in the sense, that it includes
the microscopic properties of the projectile in a reaction model. Well above
the Coulomb barrier, the cross sections are in a good agreement with data. The
reasons for the discrepancy between the data and the calculated cross sections
at the lower energies, which is also observed in a traditional CDCC, are
unclear
The R-matrix theory
The different facets of the -matrix method are presented pedagogically in
a general framework. Two variants have been developed over the years: The
"calculable" -matrix method is a calculational tool to derive scattering
properties from the Schr\"odinger equation in a large variety of physical
problems. It was developed rather independently in atomic and nuclear physics
with too little mutual influence. The "phenomenological" -matrix
method is a technique to parametrize various types of cross sections. It was
mainly (or uniquely) used in nuclear physics. Both directions are explained by
starting from the simple problem of scattering by a potential. They are
illustrated by simple examples in nuclear and atomic physics. In addition to
elastic scattering, the -matrix formalism is applied to transfer and
radiative-capture reactions. We also present more recent and more ambitious
applications of the theory in nuclear physics.Comment: 93 pages, 26 figures. Rep. Prog. Phys., in pres
Microscopic description of Li in the and elastic scattering at high energies
We employ a microscopic continuum-discretized coupled-channels reaction
framework (MCDCC) to study the elastic angular distribution of the
Li nucleus colliding with C and Si targets at
=350 MeV. In this framework, the Li projectile is described
in a microscopic cluster model and impinges on non-composite targets. The
diagonal and coupling potentials are constructed from nucleon-target
interactions and Li microscopic wave functions. We obtain a fair
description of the experimental data, in the whole angular range studied, when
continuum channels are included. The inelastic and breakup angular
distributions on the lightest target are also investigated. In addition, we
compute LiC MCDCC elastic cross sections at energies much higher
than the Coulomb barrier and we use them as reference calculations to test the
validity of multichannel eikonal cross sections.Comment: 9 Pages, 6 Figure
Coulomb breakup of 22C in a four-body model
Breakup cross sections are determined for the Borromean nucleus 22C by using
a four-body eikonal model, including Coulomb corrections. Bound and continuum
states are constructed within a 20C + n + n three-body model in hyperspherical
coordinates. We compute continuum states with the correct asymptotic behavior
through the R-matrix method. For the n+ n potential, we use the Minnesota
interaction. As there is no precise experimental information on 21C, we define
different parameter sets for the 20C + n potentials. These parameter sets
provide different scattering lengths, and resonance energies of an expected
3/2+ excited state. Then we analyze the 22C ground-state energy and rms radius,
as well as E1 strength distributions and breakup cross sections. The E1
strength distribution presents an enhancement at low energies. Its amplitude is
associated with the low binding energy, rather than with a three-body
resonance. We show that the shape of the cross section at low energies is
sensitive to the ground-state properties. In addition, we suggest the existence
of a low-energy 2+ resonance, which should be observable in breakup
experiments
Toward a microscopic description of reactions involving exotic nuclei
We propose an extension of the Continuum Discretized Coupled Channels (CDCC)
method, where the projectile is described by a microscopic cluster model. This
microscopic generalization (MCDCC) only relies on nucleon-target interactions,
and therefore presents an important predictive power. Core excitations can be
included without any further parameter. As an example we investigate the
\lipb elastic scattering at and 35 MeV. The Li nucleus is
known to present an cluster structure, and is well described by the
Resonating Group Method. An excellent agreement is obtained for the \lipb
elastic cross sections, provided that breakup channels are properly included.
We also present an application to inelastic scattering, and discuss future
applications of the MCDCC.Comment: 5 pages, 3 figures, Physical Review Letters (2013) in pres
- …